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Sökning: L773:1873 2240 OR L773:0168 1923 > (2020-2024)

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1.
  • Ahmed, Mukhtar (författare)
  • Simulation of evapotranspiration and yield of maize: An Inter-comparison among 41 maize models
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 333
  • Tidskriftsartikel (refereegranskat)abstract
    • Accurate simulation of crop water use (evapotranspiration, ET) can help crop growth models to assess the likely effects of climate change on future crop productivity, as well as being an aid for irrigation scheduling for today's growers. To determine how well maize (Zea mays L.) growth models can simulate ET, an initial inter-comparison study was conducted in 2019 under the umbrella of AgMIP (Agricultural Model Inter-Comparison and Improvement Project). Herein, we present results of a second inter-comparison study of 41 maize models that was conducted using more comprehensive datasets from two additional sites -Mead, Nebraska, USA and Bushland, Texas, USA. There were 20 treatment-years with varying irrigation levels over multiple seasons at both sites. ET was measured using eddy covariance at Mead and using large weighing lysimeters at Bushland. A wide range in ET rates was simulated among the models, yet several generally were able to simulate ET rates adequately. The ensemble median values were generally close to the observations, but a few of the models sometimes performed better than the median. Many of the models that did well at simulating ET for the Mead site did poorly for drier, windy days at the Bushland site, suggesting they need to improve how they handle humidity and wind. Additional variability came from the approaches used to simulate soil water evaporation. Fortunately, several models were identified that did well at simulating soil water evaporation, canopy transpiration, biomass accumulation, and grain yield. These models were older and have been widely used, which suggests that a larger number of users have tested these models over a wider range of conditions leading to their improvement. These revelations of the better approaches are leading to model improvements and more accurate simulations of ET.
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2.
  • Bader, Martin K.-F., et al. (författare)
  • Less pronounced drought responses in ring-porous than in diffuse-porous temperate tree species
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier. - 0168-1923 .- 1873-2240. ; 327
  • Tidskriftsartikel (refereegranskat)abstract
    • Tree species differ in their physiological responses to drought, but the underlying causes are often unclear. Here we explored responses of radial growth to centennial drought events and sap flow (Fs) to seasonal drought in four mixed forests on either moist or drier sites in northwestern Switzerland. While the diffuse-porous species (Fagus sylvatica, Prunus avium, Tilia platyphyllos) showed marked growth reductions in 1976 and 2003, two known marker years for severe drought, growth of the two ring-porous species (Quercus petraea and Fraxinus excelsior) was less severely affected. During a dry early to midsummer, diffuse-porous species strongly reduced Fs at the two drier sites but not (or less so) at the two moister sites. Regardless of soil moisture availability, the deep- rooting, ring-porous trees invariably down-regulated Fs to 60–70% of their maxima in response to vapour pressure deficit (VPD) and maintained similar fluxes across sites, irrespective of upper soil moisture conditions. A generalised additive model of normalised Fs as a function of VPD and soil matric potential yielded a drought- sensitivity ranking of Fs led by the two insensitive ring-porous species followed by the diffuse-porous trees (ordered by increasing sensitivity: Fraxinus excelsior < Quercus petraea < Prunus avium < Acer pseudoplatanus < Fagus sylvatica < Tilia platyphyllos). In conclusion, ring-porous tree species exhibited stronger VPD-driven stomatal control over Fs, and tree-ring formation was less sensitive to severe drought than in their neighbouring diffuse-porous species. The Fs regulation explained the greater drought tolerance of the ring-porous trees.
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3.
  • Bhalerao, Rishikesh P. (författare)
  • Differences between four sympatric subtropical tree species in the interactive effects of three environmental cues on leaf-out phenology
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 327
  • Tidskriftsartikel (refereegranskat)abstract
    • Climatic warming is currently changing the spring phenology of extratropical trees, and this has several important effects on the trees and ecosystems. The major climatic cues regulating the spring phenology are winter chilling, spring forcing, and photoperiod. The interactions between these three remain largely unstudied because most studies concentrate on the effects of one cue, or maximally two, at a time. We studied the effects and interactions of chilling duration, forcing temperature, and forcing photoperiod simultaneously in four subtropical tree species. The main emphasis in our experiments was on the interaction of chilling duration and forcing temperature. The existence of this interaction was suggested in the 'Vegis theory', put forward decades ago but largely forgotten since. We also introduced a novel method for testing the theory experimentally. We found support for the Vegis theory in two of the four species examined. In the other two species the leaf-out timing was largely controlled by spring forcing. The effects of photoperiod were generally minor. Our results show that there are major differences between sympatric subtropical tree species in their phenological responses to environmental cues. These differences need to be addressed in the development of process-based tree phenology models. Our results further suggest that different subtropical trees respond differently to climatic warming because of differences related to the Vegis theory. This hypothesis remains to be tested in further studies.
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4.
  • Campbell, David, et al. (författare)
  • Gap-filling eddy covariance methane fluxes: Comparison of machine learning model predictions and uncertainties at FLUXNET-CH4 wetlands
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 308
  • Tidskriftsartikel (refereegranskat)abstract
    • Time series of wetland methane fluxes measured by eddy covariance require gap-filling to estimate daily, seasonal, and annual emissions. Gap-filling methane fluxes is challenging because of high variability and complex responses to multiple drivers. To date, there is no widely established gap-filling standard for wetland methane fluxes, with regards both to the best model algorithms and predictors. This study synthesizes results of different gap-filling methods systematically applied at 17 wetland sites spanning boreal to tropical regions and including all major wetland classes and two rice paddies. Procedures are proposed for: 1) creating realistic artificial gap scenarios, 2) training and evaluating gap-filling models without overstating performance, and 3) predicting halfhourly methane fluxes and annual emissions with realistic uncertainty estimates. Performance is compared between a conventional method (marginal distribution sampling) and four machine learning algorithms. The conventional method achieved similar median performance as the machine learning models but was worse than the best machine learning models and relatively insensitive to predictor choices. Of the machine learning models, decision tree algorithms performed the best in cross-validation experiments, even with a baseline predictor set, and artificial neural networks showed comparable performance when using all predictors. Soil temperature was frequently the most important predictor whilst water table depth was important at sites with substantial water table fluctuations, highlighting the value of data on wetland soil conditions. Raw gap-filling uncertainties from the machine learning models were underestimated and we propose a method to calibrate uncertainties to observations. The python code for model development, evaluation, and uncertainty estimation is publicly available. This study outlines a modular and robust machine learning workflow and makes recommendations for, and evaluates an improved baseline of, methane gap-filling models that can be implemented in multi-site syntheses or standardized products from regional and global flux networks (e.g., FLUXNET).
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5.
  • Chi, Jinshu, et al. (författare)
  • Forest floor fluxes drive differences in the carbon balance of contrasting boreal forest stands
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 306
  • Tidskriftsartikel (refereegranskat)abstract
    • The forest floor provides an important interface of soil-atmosphere CO2 exchanges but their controls and contributions to the ecosystem-scale carbon budget are uncertain due to measurement limitations. In this study, we deployed eddy covariance systems below- and above-canopy to measure the spatially integrated net forest floor CO2 exchange (NFFE) and the entire net ecosystem CO2 exchange (NEE) at two mature contrasting stands located in close vicinity in boreal Sweden. We first developed an improved cospectra model to correct below-canopy flux data. Our empirical below-canopy cospectra models revealed a greater contribution of large- and small-scale eddies in the trunk space compared to their distribution in the above-canopy turbulence cospectra. We found that applying the above-canopy cospectra model did not affect the below-canopy annual CO2 fluxes at the sparse pine forest but significantly underestimated fluxes at the dense mixed spruce-pine stand. At the mixed spruce-pine stand, forest floor respiration (Rff) was higher and photosynthesis (GPPff) was lower, leading to a 1.4 times stronger net CO2 source compared to the pine stand. We further found that drought enhanced Rff more than GPPff, leading to increased NFFE. Averaged across the six site-years, forest floor fluxes contributed 82% to ecosystem-scale respiration (Reco) and 12% to gross primary production (GPP). Since the annual GPP was similar between both stands, the considerable difference in their annual NEE was due to contrasting Reco, the latter being primarily driven by the variations in NFFE. This implies that NFFE acted as the driver for the differences in NEE between these two contrasting stands. This study therefore highlights the important role of forest floor CO2 fluxes in regulating the boreal forest carbon balance. It further calls for extended efforts in acquiring high spatiotemporal resolution data of forest floor fluxes to improve predictions of global change impacts on the forest carbon cycle.
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6.
  • De Pauw, Karen, et al. (författare)
  • Urban forest microclimates across temperate Europe are shaped by deep edge effects and forest structure
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 341
  • Tidskriftsartikel (refereegranskat)abstract
    • The urban heat island (UHI) causes strong warming of cities and their urban forests worldwide. Especially urban forest edges are strongly exposed to the UHI effect, which could impact urban forest biodiversity and functioning. However, it is not known to what extent the UHI effect alters edge-to-interior microclimatic gradients within urban forests and whether this depends on the forests' structure.Here we quantified gradients of air temperature, relative air humidity and vapour pressure deficits (VPD) along urban forest edge-to-interior transects with contrasting stand structures in six major cities across Europe. We performed continuous hourly microclimate measurements for two consecutive years and analysed the magnitude and depth of edge effects, as well as forest structural drivers of microclimatic variation.Compared to edge studies in rural temperate forests, we found that edge effects reached deeper into urban forests, at least up to 50 m. Throughout the year, urban forest edges were warmer and drier compared to forest interiors, with the largest differences occurring during summer and daytime. Not only maximum, but also mean and minimum temperatures were higher at the urban forest edge up to large edge distances (at least 85 m). Denser forests with a higher plant area index buffered high air temperatures and VPDs from spring to autumn.We conclude that urban forest edges are unique ecotones with specific microclimates shaped by the UHI effect. Both forest edges and interiors showed increased buffering capacities with higher forest canopy density. We advocate for the conservation and expansion of urban forests which can buffer increasingly frequent and intense climate extremes. To this end, urban forest managers are encouraged to aim for multi-layered dense forest canopies and consider edge buffer zones of at least 50 m wide.
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7.
  • Díaz-Calafat, Joan, et al. (författare)
  • From broadleaves to conifers : The effect of tree composition and density on understory microclimate across latitudes
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 341
  • Tidskriftsartikel (refereegranskat)abstract
    • Forest canopies buffer the macroclimate and thus play an important role in mitigating climate-warming impacts on forest ecosystems. Despite the importance of the tree layer for understory microclimate buffering, our knowledge about the effects of forest structure, composition and their interactions with macroclimate is limited, especially in mixtures of conifers and broadleaves. Here we studied five mixed forest stands along a 1800 km latitudinal gradient covering a 7°C span in mean annual temperature. In each of these forests we established 40 plots (200 in total), in which air and soil temperatures were measured continuously for at least one year. The plots were located across gradients of forest density and broadleaved proportions (i.e. from open to closed canopies, and from 100% conifer to 100% broadleaved tree dominance). Air minimum, mean and maximum temperature offsets (i.e. difference between macroclimate and microclimate) and soil mean temperature offsets were calculated for the coldest and warmest months. Forest structure, and especially forest density, was the key determinant of understory temperatures. However, the absolute and relative importance of the proportion of broadleaves and forest density differed largely between response variables. Forest density ranged from being independent of, to interacting with, tree species composition. The effect of these two variables was independent of the macroclimate along our latitudinal gradient. Temperature, precipitation, snow depth and wind outside forests affected understory temperature buffering. Finally, we found that the scale at which the overstory affects soil microclimate approximated 6-7 m, whereas for air microclimate this was at least 10 m. These findings have implications for biodiversity conservation and forest management in a changing climate, as they facilitate the projection of understory temperatures in scenarios where both forest structure and macroclimate are dynamic. This is especially relevant given the global importance of ongoing forest conversion from conifers to broadleaves, and vice versa.
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8.
  • Dox, Inge, et al. (författare)
  • Wood growth phenology and its relationship with leaf phenology in deciduous forest trees of the temperate zone of Western Europe
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier. - 0168-1923 .- 1873-2240. ; 327
  • Tidskriftsartikel (refereegranskat)abstract
    • Wood growth phenology of temperate deciduous trees is less studied than leaf phenology, hindering the understanding of their interaction. In order to describe the variability of wood growth and leaf phenology across locations, species and years, we performed phenological observations of both xylem formation and leaf development in three typical temperate forest areas in Western Europe (Northern Spain, Belgium and Southern Norway) for four common deciduous tree species (Fagus sylvatica L., Betula pendula Roth., Populus tremula L. and Quercus robur L.) in 2018, 2019 and 2020, with only beech and birch being studied in the final year. The earliest cambial reactivation in spring occurred at the Belgian stands while the end of cambial activity and wood growth cessation generally occurred first in Norway. Results did not show much consistency across species, sites or years and lacked general patterns, except for the end of cambial activity, which occurred generally first in birch. For all species, the site variation in phenophases (up to three months) was substantially larger than the inter-annual variability (up to six weeks). The timeline of bud-burst and cambium reactivation, as well as of foliar senescence and cessation of wood growth, were variable across species even with the same type of wood porosity. Our results suggest that wood growth and leaf phenology are less well connected than previously thought. Linear models showed that temperature is the dominant driver of wood growth phenology, but with climate zone also having an effect, especially at the start of the growing season. Drought conditions, on the other hand, have a larger effect on the timing of wood growth cessation. Our comprehensive analysis represents the first large regional assessment of wood growth phenology in common European deciduous tree species, providing not only new fundamental insights but also a unique dataset for future modelling applications.
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9.
  • Drobyshev, Igor (författare)
  • Trends and patterns in annually burned forest areas and fire weather across the European boreal zone in the 20th and early 21st centuries
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 306
  • Tidskriftsartikel (refereegranskat)abstract
    • Fire remains one of the main natural disturbance factors in the European boreal zone and understanding climatic forcing on fire activity is important for projecting effects of climate change on ecosystem services in this region. We analyzed records of annually burned areas in 16 administrative regions of the European boreal zone (countries or administrative units within countries) and fire weather variability to test for their spatio-temporal patterns over the 1901-2017 period.Over the 1992-2017 period, the region exhibited large variability in forest fire activity with the fire cycles varying from similar to 1600 (St. Petersburg region) to similar to 37000 years (Finland). The clustering of administrative units in respect to their burned area, suggested the presence of sub-regions with synchronous annual variability in burned areas. Large fire years (LFYs) in each of the clusters were associated with the development of the high pressure cell over or in immediate proximity of the regions in question in July, indicating climatic forcing of LFYs. Contingency analysis indicated that there was no long-term trend in the synchrony of LFYs observed simultaneously in several administrative units. We documented a trend towards higher values of Monthly Drought Code (MDC) for the months of April and May in the western (April) and northern (April and May) sections. The significant positive correlation between biome-wide fire activity index and June SNAO (Summer North Atlantic Oscillation) (r = 0.53) pointed to the importance of large-scale atmospheric circulation, in particular the summer European blocking pattern, in controlling forest fires across EBZ. The forest fire activity of the European boreal zone remains strongly connected to the annual climate variability. Higher frequency of strongly positive SNAO states in the future will likely synchronize years with a large area burned across the European boreal zone.
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10.
  • Greiser, Caroline, 1987-, et al. (författare)
  • Higher soil moisture increases microclimate temperature buffering in temperate broadleaf forests
  • 2024
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 345
  • Tidskriftsartikel (refereegranskat)abstract
    • Forest canopies can buffer the understory against temperature extremes, often creating cooler microclimates during warm summer days compared to temperatures outside the forest. The buffering of maximum temperatures in the understory results from a combination of canopy shading and air cooling through soil water evaporation and plant transpiration. Therefore, buffering capacity of forests depends on canopy cover and soil moisture content, which are increasingly affected by more frequent and severe canopy disturbances and soil droughts. The extent to which this buffering will be maintained in future conditions is unclear due to the lack of understanding about the relationship between soil moisture and air temperature buffering in interaction with canopy cover and topographic settings. We explored how soil moisture variability affects temperature offsets between outside and inside the forest on a daily basis, using temperature and soil moisture data from 54 sites in temperate broadleaf forests in Central Europe over four climatically different summer seasons. Daily maximum temperatures in forest understories were on average 2 °C cooler than outside temperatures. The buffering of understory temperatures was more effective when soil moisture was higher, and the offsets were more sensitive to soil moisture on sites with drier soils and on sun-exposed slopes with high topographic heat load. Based on these results, the soil–water limitation to forest temperature buffering will become more prevalent under future warmer conditions and will likely lead to changes in understory communities. Thus, our results highlight the urgent need to include soil moisture in models and predictions of forest microclimate, understory biodiversity and tree regeneration, to provide a more precise estimate of the effects of climate change.
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11.
  • Grelle, Achim, et al. (författare)
  • Affordable relaxed eddy accumulation system to measure fluxes of H2O, CO2, CH4 and N2O from ecosystems
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 307
  • Tidskriftsartikel (refereegranskat)abstract
    • The relaxed eddy accumulation (REA) technique is frequently applied to measure fluxes of a large variety of atmospheric tracers above ecosystems. It is often the method of choice since the eddy covariance (EC) technique is limited to a few tracers due to the lack of fast response analysers, high financial costs and in some cases high power consumption. REA avoids the need for a fast response analyser by collecting air from up-drafts and down-drafts into separate reservoirs. After collecting the air over a predefined time period, trace gas concentrations in the reservoirs are analysed by a slow response analyser and the average fluxes can be calculated.We developed and tested a REA system that is capable of measuring CO2, CH4, N2O and H2O fluxes simultaneously with only one gas analyser (Picarro G2805). This system is compatible with virtually any gas analyser and thus supports the flux analysis of a wide range of other air constituents such as isotopes, aerosols and volatile organic compounds. Furthermore, the modular design and rugged casing makes the sampling system robust and portable, and with its 12 V DC operation it is suitable for a wide range of field campaigns. The performance of the REA system was tested during the growing seasons of 2018 and 2020 on a grassland on organic soil in central Sweden.The system has worked reliably during several months in the Nordic climate, covering ambient temperatures between -20°C and +30°C. Measured fluxes of CO2 and H2O agree well with fluxes measured independently by an EC system. The similarity in the technology and the determined detection limits made us confident that the REA system even captures fluxes of CH4 and N2O well.
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12.
  • Grelle, Achim, et al. (författare)
  • From source to sink : recovery of the carbon balance in young forests
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier. - 0168-1923 .- 1873-2240. ; 330
  • Tidskriftsartikel (refereegranskat)abstract
    • We analyzed ecosystem carbon fluxes from eddy-covariance measurements in five young forests in southernSweden where the previous stand had been harvested by clear-cutting or wind-felled: three stands with Norwayspruce (Picea abies (L.) Karst.), one with Scots pine (Pinus sylvestris) and one with Larch (Larix x eurolepis A.Henry). One of the spruce stands had the stumps harvested, one was fertilized and one without any specialtreatments. These stands returned from positive (sources) to negative (sinks) annual carbon fluxes 8–13 yearsafter disturbance, depending on site productivity and management. This corresponds to approximately 15% ofthe rotation periods at these sites. Extrapolation in combination with chronosequence data suggests thatconventionally regenerated stands reach a neutral carbon balance after approximately 30% of the rotationperiod. The lowest carbon emissions and shortest recovery time was observed in a stand where the stumps of thetrees, in addition to the stems and logging residues, were removed after harvest. This stand not only returned to acarbon sink within this time period but the total carbon gains since disturbance also equaled the total losses afteronly 11 years. These results stress that production stands in southern Sweden are carbon sources during arelatively small part of the rotation period, and that this part can be considerably shortened by measures thatincrease productivity or reduce the amount of woody debris left after disturbance.
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13.
  • Hrad, M., et al. (författare)
  • Determination of methane emissions from biogas plants, using different quantification methods
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier B.V.. - 0168-1923 .- 1873-2240. ; 326
  • Tidskriftsartikel (refereegranskat)abstract
    • Reliable and comparable quantification methods are needed for assessing the effectiveness of the biogas production and utilisation process in mitigating methane (CH4) emissions as well as improving the database for emission inventories. The objective of this study was to compare and validate CH4 emissions quantified at two agricultural biogas plants, for up to three days, using diverse on-site (two teams) and off-site methods (three teams), including differential absorption lidar (DIAL), tracer gas dispersion (TDM) and inverse dispersion modelling (IDM). For plant 1, with a constant combined heat and power (CHP) load, the average emission factor varied from 0.3% CH4 (on-site approaches) to 1.2% CH4 (off-site approaches). On-site approaches underestimated overall emissions due to many small (unquantified) CH4 leakages. All methods observed comparable average emission factors for plant 2, ranging between 1.9 and 2.2% CH4. In this case, the majority of emissions emanated from just a few sources. However, correcting the significant influence of the varying CHP load during the measurement campaign revealed significant differences between TDM and IDM (DIAL did not participate). It was demonstrated that TDM and IDM could recover the emission rate from a known point source (controlled release of CH4 via a small diffuser) within an accuracy of 93 ± 15% (TDM) and 92 ± 17% (IDM) under favourable and similar conditions. 
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14.
  • Kardol, Paul (författare)
  • Net neutral carbon responses to warming and grazing in alpine grassland ecosystems
  • 2020
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 280
  • Tidskriftsartikel (refereegranskat)abstract
    • In natural grasslands, effects of warming on net ecosystem CO2 exchange (NEE) may interact with grazing. Yet, the effects of these two main drivers of terrestrial carbon cycling are typically studied in isolation, limiting our understanding of how NEE would be affected under different global change scenarios. Here, we report results of a warming experiment using infrared heaters combined with summer and winter grazing for 7-years in a Tibetan alpine grassland. We found that regardless of warming summer grazing decreased soil carbon sink (i.e., increased annual mean net biome productivity (NBP) indicated by a negative value of NBP), and warming also reduced soil carbon sink under no-grazing only during 3-years of summer grazing. However, warming and grazing did not change soil carbon sink during 4-years of winter grazing. Interactive effects between warming and grazing on annual NBP varied with year and grazing season. Overall, both warming and grazing did not alter annual mean NBP under 7-years of the rotational grazing system in summer and winter grasslands because of offsetting effects on annual mean gross primary productivity and ecosystem respiration. Annual mean soil temperature explained 58% of the variation of annual mean NBP during summer grazing, whereas seasonal mean soil moisture explained 48 and 44% of its variation during winter grazing and the two season grazing system, respectively. Together, our results suggest that rotational grazing between summer and winter alpine grasslands would result in net neutral climate feedback based on the response of annual NBP under future warming. The two season grazing system not only supports animal production but also realizes balance in the soil carbon budget under future warming.
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15.
  • Klosterhalfen, Anne, et al. (författare)
  • Two-level eddy covariance measurements reduce bias in land-atmosphere exchange estimates over a heterogeneous boreal forest landscape
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 339
  • Tidskriftsartikel (refereegranskat)abstract
    • Estimates of land-atmosphere exchanges of carbon, energy, water vapor, and other greenhouse gases based on the eddy covariance (EC) technique rely on the fundamental assumption that the flux footprint area is homogeneous. We investigated the impact of source area heterogeneity on flux estimates in single-level EC measurements over a managed boreal forest landscape. For this purpose, we compared single-level measurements with those from a two-level approach consisting of concurrent EC measurements at 60 and 85 m above the ground. This two-level set-up provided a unique opportunity to obtain nearly congruent diel footprint areas by combining data from the higher and lower levels during day- and nighttime, respectively. We found that the variation in the averaged footprint area between day- and nighttime was reduced by up to 89% in the two-level approach compared to the single-level data at the higher level (85 m). Considering spring, summer, and fall months, the resulting relative potential bias in flux observations due to landscape heterogeneity was highest at short time steps (≤ daily) ranging between 35% and 325% for half-hourly data. During winter months, when stable atmospheric regimes prevailed during day and night, the footprints within the diel course nearly overlapped also at a given single level and hence no improvement of flux estimates was found. The absolute cumulated sums for the study period (excluding winter months) of gross primary production, ecosystem respiration, latent heat, and sensible heat flux were underestimated by about 28%, 52%, 5%, and 3%, respectively, whereas that of net ecosystem CO2 exchange was overestimated by about 109% in the single-level approach. Overall this study suggests that footprint heterogeneity may introduce considerable bias in single-level flux estimates — particularly at short time scales — with large implications for model-data fusion studies, site comparisons, and up- or downscaling of land-atmosphere exchange processes.
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16.
  • Martinez-Garcia, Eduardo, et al. (författare)
  • Overstory dynamics regulate the spatial variability in forest-floor CO2 fluxes across a managed boreal forest landscape
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier. - 0168-1923 .- 1873-2240. ; 318
  • Tidskriftsartikel (refereegranskat)abstract
    • The forest-floor represents an important interface for various carbon dioxide (CO2) fluxes, however, our knowledge of their variability and drivers across a managed boreal forest landscape is limited. Here, we used a three-year (2016-2018) data set of biometric- and chamber-based flux measurements to investigate the net forest-floor CO2 exchange (NEff) and its component fluxes across 50 forest stands spanning different soil types, tree species, and age classes within a 68 km2 boreal catchment in Sweden. We found that the forest-floor acted as a net CO2 source with the 10th-90th percentile (used hereafter for describing reported variations) ranging from 149 to 399 g C m- 2 yr-1. Among the key landscape attributes, stand age strongly affected most NEff component fluxes, whereas tree species and soil type effects were weak and absent, respectively. Specifically, forest-floor net CO2 emissions increased with stand age due to declining understory gross and net primary production, ranging between 77-275 and 49-163 g C m- 2 yr- 1, respectively. Furthermore, we observed higher understory production rates in pine than in spruce stands. Across the 50 stands, the total forest-floor respiration ranged between 340 and 549 g C m- 2 yr-1 and its spatial variation was primarily regulated by its autotrophic components, i.e., understory and tree root respiration, which displayed divergent increasing and decreasing age-related trends, respectively. Furthermore, heterotrophic soil respiration remained within a relatively narrow range (154-290 g C m- 2 yr- 1), possibly owing to compensating gradients in forest-floor properties. We further identified tree biomass as the major driver of the landscape-scale variations of CO2 fluxes, likely attributable to modulating effects on forest-floor resource availability and growing conditions. This implies that tree growth responses to forest management and global change will be particularly important for regulating magnitudes and spatial variations of forest-floor CO2 fluxes in boreal forests.
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17.
  • Meeussen, Camille, et al. (författare)
  • Microclimatic edge-to-interior gradients of European deciduous forests
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 311
  • Tidskriftsartikel (refereegranskat)abstract
    • Global forest cover is heavily fragmented. Due to high edge-to-surface ratios in small forest patches, a large proportion of forests is affected by edge influences involving steep microclimatic gradients. Although forest edges are important ecotones and account for 20% of the global forested area, it remains unclear how biotic and abiotic drivers affect forest edge microclimates at the continental scale. Here we report soil and air temperatures measured in 225 deciduous forest plots across Europe for two years. Forest stands were situated along a latitudinal gradient and subject to a varying vegetation structure as quantified by terrestrial laser scanning. In summer, the average offset of air and soil temperatures in forest edges compared to temperatures outside the forest amounted to -2.8 degrees C and -2.3 degrees C, respectively. Edge-to-interior summer temperature gradients were affected by the macroclimate and edge structure. From the edge onwards, larger offsets were observed in dense forest edges and in warmer, southern regions. In open forests and northern Europe, altered microclimatic conditions extended deeper into the forest and gradients were steeper. Canopy closure and plant area index were important drivers of summer offsets in edges, whereas in winter also the forest-floor biomass played a key role. Using high-resolution maps, we estimated that approximately 10% of the European broadleaved forests would be affected by altered temperature regimes. Gradual transition zones between forest and adjacent lands are valuable habitat types for edge species. However, if cool and moist forest interiors are desired, then (i) dense and complex forest edges, (ii) an undisturbed forested buffer zone of at least 12.5 m deep and (iii) trees with a high shade casting ability could all contribute to an increased offset. These findings provide important guidelines to mitigate edge influences, to protect typical forest microclimates and to adapt forest management to climate change.
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18.
  • Messori, Gabriele, et al. (författare)
  • Atmospheric jet stream variability reflects vegetation activity in Europe
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier. - 0168-1923 .- 1873-2240. ; 322
  • Tidskriftsartikel (refereegranskat)abstract
    • Jet streams are a key component of the climate system, whose dynamics couple closely to regional climate variability. Yet, the link between jet stream variability and vegetation activity has received little attention. Here, we leverage our understanding of the mid-latitude jet stream dynamics over the Euro-Atlantic sector to probe climate-vegetation interactions across Europe. We link indices related to the meridional location of the jet and the large-scale zonal wind speed with remotely-sensed vegetation greenness anomalies during locally-defined growing seasons. Correlations between greenness anomalies and jet latitude anomalies point to a control of the jet stream's variability on vegetation activity over large parts of Europe. This potential control is mediated by the jet latitude anomalies' correlations with temperature, soil moisture and downward surface solar radiation. The sign and strength of these correlations depend on location and time of the year. Furthermore, jet stream variability modulates conditions at the onset and end of the growing season. The link between jet latitude anomalies and vegetation greenness is not only specific to the climate zone, but also to the landclass and subperiod within the growing season. It is thus important to use a locally-defined growing season for interpreting the atmospheric controls on regional vegetation phenology. Results consistent with the correlation analysis emerge when focussing on local high or low greenness months only or on zonal wind speed anomalies, confirming the relevance of jet variability for vegetation activity.
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19.
  • Mäki, Mari, et al. (författare)
  • Heterotrophic and rhizospheric respiration in coniferous forest soils along a latitudinal gradient
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 317
  • Tidskriftsartikel (refereegranskat)abstract
    • Northern forest soils are a major carbon (C) reservoir of global importance. To estimate how the C balance in these soils will change, the roles of tree roots and soil microbes in C balance should first be decoupled. This study determined how the activity of heterotrophs and tree roots together with root-associated microbes in the rhizosphere varies in coniferous forest soils in boreal, hemiboreal, and temperate climates along a latitudinal gradient using a trenching approach. We created experimental plots without living tree roots, measured soil respiration (CO2 efflux) from these and from unmanipulated plots using the chamber technique, and partitioned the efflux into root-rhizosphere (RR) and heterotrophic (RH) respiration. The share of RR in ecosystem gross primary production (GPP) decreased from north to south in the Scots pine (Pinus sylvestris L.) and the Norway spruce (Picea abies (L.) Karst.) forests, with the exception of a mixed site, where the share of RR in GPP varied strongly between the years. RR per ground area and per root biomass were mainly independent of climate within the gradient. RH per ground area increased from north to south with temperature, while RH per soil C did not change with temperature. Soil moisture did not significantly affect the respiration components in the northernmost site, whereas soil moisture was positively connected with RH and negatively with RR in other Scots pine sites and positively connected with RR in pure Norway spruce stands. The dynamic ecosystem model LPJ-GUESS was able to capture the seasonal dynamics of RH and RR at the sites, but overall accuracy varied markedly between the sites, as the model underestimated RH in the southern site and RR elsewhere. Our study provides knowledge about the nature of soil respiration components. The valuable insights can be used in more accurate land-ecosystem modelling of forest ecosystems.
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20.
  • Nyasulu, Maganizo Kruger, 1988-, et al. (författare)
  • African rainforest moisture contribution to continental agricultural water consumption
  • 2024
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 346
  • Tidskriftsartikel (refereegranskat)abstract
    • Precipitation is essential for food production in Sub-Saharan Africa, where more than 80 % of agriculture is rainfed. Although ∼40 % of precipitation in certain regions is recycled moisture from Africa's tropical rainforest, there needs to be more knowledge about how this moisture supports the continent's agriculture. In this study, we quantify all moisture sources for agrarian precipitation (African agricultural precipitationshed), the estimates of African rainforest's moisture contribution to agricultural precipitation, and the evaporation from agricultural land across the continent. Applying a moisture tracking model (UTRACK) and a dynamic global vegetation model (LPJmL), we find that the Congo rainforest (>60 % tree cover) is a crucial moisture source for many agricultural regions. Although most of the rainforest acreage is in the DRC, many neighboring nations rely significantly on rainforest moisture for their rainfed agriculture, and even in remote places, rainforest moisture accounts for ∼10–20 % of agricultural water use. Given continuous deforestation and climate change, which impact rainforest areas and resilience, more robust governance for conserving the Congo rainforest is necessary to ensure future food production across multiple Sub-Saharan African countries.
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21.
  • Peichl, Matthias (författare)
  • Assessing the relative importance of sunshine, temperature, precipitation, and spring phenology in regulating leaf senescence timing of herbaceous species in China
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 313
  • Tidskriftsartikel (refereegranskat)abstract
    • In temperate and boreal climate, the timing of plant's growth cessation in autumn highly affects plant carbohydrate reserves and ecosystem productivity. However, the abiotic and biotic factors regulating plant's autumn phenology remain poorly understood, especially on herbaceous plants inhabiting temperate grassland ecosystems. This study collected 10' 533 of ground phenological observations conducted by professionals on 41 herbaceous species (26 for forb species and 15 for graminoid species) in 201 stations spread over China. With multiple regression analysis and contribution decomposition, we assessed the total and separate effect of preseason temperature, precipitation, sunshine hours, and spring phenology on leaf senescence and explored their relationships with species, plant functional types, and geographical gradients. Results show that leaf senescence has significantly delayed for 31% of the site-species. Sunshine hours, temperature, precipitation, and the leaf-out date primarily explained the variation of leaf senescence for 36.6%, 31.7%, 22.0%, and 9.8% of the investigated site-species, respectively. Further analysis demonstrated that sunshine hours were the foremost factor in controlling leaf senescence for 60.1% of graminoid species (n = 15) and temperature for 42.3% of forb species (n = 26). This suggests that light and heat primarily determine the autumn phenology of herbaceous plants, but there are great differences among species/functional types. Spatially with increasing altitude, we observed an increasing total explanation rate of four factors and a growing individual explanation rate of precipitation to leaf senescence. Overall, we not only highlight the key effects of sunshine hours and temperature on the autumn phenology of herbaceous plants but also emphasize the dependence of phenological responses on species, functional-types, and geographical gradients, which implies the complexity of modeling future autumn phenology of herbaceous species at regional and global scale.
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22.
  • Peng, Haijun (författare)
  • Temporal variations of carbon and water fluxes in a subtropical mangrove forest: Insights from a decade-long eddy covariance measurement
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 343
  • Tidskriftsartikel (refereegranskat)abstract
    • Mangroves, highly efficient ecosystems in sequestering CO2, are strongly impacted by climate change. The lack of long-term observation in mangroves hinders the evaluation of seasonal and inter-annual variability in carbon and water fluxes and their responses to various environmental drivers. In this study, we measured net ecosystem CO2 exchange and evapotranspiration between the atmosphere and subtropical mangroves using the eddy covariance technique over a decade (2010-2019) in southern China. This mangrove forest acted as a strong CO2 sink, with annual net ecosystem production (NEP) ranging from 622.5 to 832.8 g C m(-2) year(-1). The annual evapotranspiration (ET) varied between 934.6 and 1004.9 mm year(-1). During the study period, ET consistently remained higher in the wet season (May to October) compared to the dry season, while NEP did not exhibit consistent seasonal variation. Path analysis indicated that during the dry season, NEP was primarily influenced by global solar radiation and vapor pressure deficit. However, in the wet season, NEP was regulated by a combination of global solar radiation, vapor pressure deficit, air temperature, and tidal inundation time. Additionally, the promoting effect of global solar radiation on NEP decreased in the wet season, while the inhibitory influences of higher temperature and vapor pressure deficit on NEP intensified during the period. Unlike NEP, the dominant factors affecting ET (global solar radiation, air temperature, and vapor pressure deficit) and their intensities remained relatively consistent during both seasons. Furthermore, the relative importance of global solar radiation on NEP and ET increased over the decade, while the influence of tidal inundation time diminished. This study not only improves the understanding of the response of subtropical mangroves to climate change but also provides a valuable benchmark dataset to validate the interannual variability of mangrove carbon and water fluxes estimated from the models.
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23.
  • Rannik, Üllar, et al. (författare)
  • Impact of coordinate rotation on eddy covariance fluxes at complex sites
  • 2020
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 287
  • Tidskriftsartikel (refereegranskat)abstract
    • The choice of coordinate system to calculate eddy covariance fluxes becomes particularly relevant at complex measurement sites. The traditional way is to perform double rotation (DR) of the coordinate system i.e., to calculate turbulent fluxes in a coordinate system that is aligned with the flow streamlines within the flux averaging period (e.g., Kaimal and Finnigan, 1994). The second approach, the so-called planar-fitted (PF) coordinate system, averages the flow over a longer period of time, in practice a month or more. The PF method allows to derive an intercept coefficient of the vertical wind speed which can be attributed to the offset of the sonic anemometer or the average vertical flow related to meteorological conditions. We evaluated the variants of the PF methods using data from a variety of sites ranging from complex urban and forest sites to nearly ideal forest and peatland sites. At complex sites, we found that the intercept of the vertical wind speed derived from the PF method is a function of wind direction, time of day and/or stability. The sector-wise PF (SPF) method frequently led to insignificant statistical relationships. We tested a continuous PF (CPF) method where the relationship establishing the coordinate frame was represented as the continuous function in the form of Fourier series. The method enabled to obtain the PF with lower uncertainty as compared to the SPF method, by selecting necessary number of harmonics for each site based on confidence intervals of estimated parameters. Therefore, we recommend to use the CPF method in cases when the number of observations in some wind direction interval is low or the obtained SPF is insignificant due to large variance in measurements. We also showed that significant systematic difference can exist in cumulative turbulent fluxes between the DR and PF methods over a longer period of time. Derived vertical advection of carbon dioxide exhibited large variability with wind direction due to topography at complex sites and therefore, without considering horizontal advection, cannot be used to improve the net ecosystem exchange estimation during nocturnal, low turbulence conditions.
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24.
  • Schiestl-Aalto, Pauliina, et al. (författare)
  • Disaggregating the effects of nitrogen addition on gross primary production in a boreal Scots pine forest
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 301
  • Tidskriftsartikel (refereegranskat)abstract
    • Adding nitrogen to boreal forest ecosystems commonly increases gross primary production (GPP). The effect of nitrogen addition on ecosystem GPP is convoluted due to the impacts of and interactions among leaf scale photosynthetic productivity, canopy structure, site fertility, and environmental constraints. We used a unique controlled nitrogen fertilisation experiment combined with eddy covariance measurements and the calibration of a LUE-based (light use efficiency) photosynthetic production model in order to reveal differences in photosynthetic capacity due to nitrogen addition. A systematically designed soil moisture survey was conducted to characterise the within-site spatial heterogeneity and validate the difference of water stress between fertilised and control sites. The canopy photosynthetic light responses and environmental constraints were evaluated using an inverse modelling approach. We found that nitrogen fertilisation elevated ecosystem GPP by 24% according to model simulations. This was caused by increases in ecosystem light interception (through an increase in leaf area index (LAI)) and LUE by 7% and 17%, respectively. Nitrogen addition increased canopy potential LUE for both low and high photosynthetic photon flux density (PPFD) conditions. The calculations of leaf area and light interception indicated that the understorey vegetation contributed 9% of ecosystem GPP in the fertilised site and 7% in the control site when assuming a same LUE for trees and shrubs. The constraint arising from atmospheric water demand, rather than soil water stress, was the dominating control of the intra- and inter-annual GPP variations. The uncertainty propagated from soil moisture data is negligible for GPP predictions, but influential in the inference on the severity of the drought. This study demonstrates the combination of the controlled field experiment with the inverse modelling approach provides a powerful tool to quantitatively describe and disaggregate N addition effects on forest ecosystem GPP.
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25.
  • Schlyter, Fredrik (författare)
  • The Last Trees Standing: Climate modulates tree survival factors during a prolonged bark beetle outbreak in Europe
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 322
  • Tidskriftsartikel (refereegranskat)abstract
    • Plant traits are an expression of strategic tradeoffs in plant performance that determine variation in allocation of finite resources to alternate physiological functions. Climate factors interact with plant traits to mediate tree survival. This study investigated survival dynamics in Norway spruce (Picea abies) in relation to tree-level morphological traits during a prolonged multi-year outbreak of the bark beetle, Ips typographus, in Central Europe. We acquired datasets describing the trait attributes of individual spruce using remote sensing and field surveys. We used nonlinear regression in a hypothesis-driven framework to quantify survival probability as a function of tree size, crown morphology, intraspecific competition and a growing season water balance. Extant spruce trees that persisted through the outbreak were spatially clustered, suggesting that survival was a nonrandom process. Larger diameter trees were more susceptible to bark beetles, reflecting either life history tradeoffs or a dynamic interaction between defense capacity and insect aggregation behavior. Competition had a strong negative effect on survival, presumably through resource limitation. Trees with more extensive crowns were buffered against bark beetles, ostensibly by a more robust photosynthetic capability and greater carbon reserves. The outbreak spanned a warming trend and conditions of anomalous aridity. Sustained water limitation during this period amplified the consequences of other factors, rendering even smaller trees vulnerable to colonization by insects. Our results are in agreement with prior research indicating that climate change has the potential to intensify bark beetle activity. However, forest outcomes will depend on complex cross-scale interactions between global climate trends and tree-level trait factors, as well as feedback effects associated with landscape patterns of stand structural diversity.
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26.
  • Sieber, Petra, et al. (författare)
  • Albedo on cropland: Field-scale effects of current agricultural practices in Northern Europe
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 321
  • Tidskriftsartikel (refereegranskat)abstract
    • Agricultural land use and management affect land surface albedo and thus the climate. Increasing the albedo of cropland could enhance reflection of solar radiation, counteracting the radiative forcing (RF) of greenhouse gases (GHGs) and local warming. However, knowledge is lacking on how agricultural practices affect albedo under local conditions, and on the benefits of individual practices. In this study, field measurements were made in 15 paired plots at a site in Northern Europe to determine albedo, net shortwave irradiance and RF impacts under various common crops, cultivation intensities and tillage practices. Field data for 2019-2020 were compared with satellite-based albedo for the surrounding region in 2010-2020. At regional level, different combinations of soil type, yearly weather and agricultural practices led to great variability in the albedo of individual crops, despite similar pedo-climatic conditions. At field level within years, albedo differences were determined mainly by crop type, species-specific phenology and post-harvest management. Annual albedo was higher with perennial ley (0.20-0.22) and winter-sown crops (0.18-0.22) than with spring-sown crops (0.16-0.18) and bare soil (0.13). Barley had the highest albedo among winter and spring cereals. In summer, when increased albedo could alleviate local heat stress, oats reduced net shortwave irradiance at the surface by 0.8-5.8 Wm(-2) compared with other cereals, ley, peas or rapeseed. Delayed or reduced tillage gave high local cooling potential (up to-13.6 Wm(-2)) in late summer. Potential benefits for global mean climate as GWP(100 )per hectare and year reached-980 kg CO(2)e for avoiding black fallow,-578 kg CO(2)e for growing a winter-sown variety and-288 kg CO(2)e for delayed tillage. Thus realistic albedo increases on cropland could have important effects on local temperatures and offset a substantial proportion of the RF deriving from field-scale GHG emissions on short time-scales.
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27.
  • Stecher Justiniano Pinto, Guilherme Alexandre, et al. (författare)
  • Effects of human-related and biotic landscape features on the occurrence and size of modern forest fires in Sweden
  • 2020
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 291
  • Tidskriftsartikel (refereegranskat)abstract
    • The influence of landscape features on the occurrence and size of forest fires in Northern Europe has not been well-studied. In this paper, we analyzed the impact of human-related landscape properties (road and human population density), biotic features (amount of firebreak area and vegetation zone) and fire weather indices (Buildup Index, BUI and Initial Spread Index, ISI) on the occurrence and size of forest fires in Sweden from 1998 to 2017. To analyze the environmental controls of fire occurrence and fire size under different levels of climatological fire hazard, we divided the data into two subsets: (1) large fire years (LFY), defined as the years with the total amount of burned area being higher than the dataset-wide average (2002, 2003, 2006, 2008 and 2014), and (2) the remaining years (nLFY). Our analytical approach was based on spatial models using Integrated Nested Laplace Approximations (INLA). Models built on both LFY and nLFY subsets suggested a strong human influence on fire occurrence: road density, the number of firebreaks, and population density, all were positively associated with fire occurrence, suggesting an important role of human-related ignitions. The southernmost vegetation zones in Sweden (boreo-nemoral and nemoral) exhibited the highest fire occurrence (LFY), a pattern potentially related to a higher population density in combination with weather more conducive for fires in this part of the country. The patterns that emerged from the fire size models pointed to the climate as the main factor controlling fire size, irrespective of the type of years analyzed. Road density, number of firebreaks and population density showed a negative association with fire size, possibly indicating higher efficiency of fire suppression in the areas with higher human presence. Vegetation zones were selected as an informative predictor, indicating that the fire activity varies across the zones, with those in mid-Sweden being the most prone to large events. The ISI correlated strongly and positively with fire size in both subsets (LFY and nLFY), pointing to the role of weather conditions favorable for fire spread, primarily that of surface fires. The BUI showed a weak negative correlation to fire size, indicating that dryness of organic horizon, specifically its deeper layers, is less relevant for predicting fire size. Contemporary fire activity in Sweden is driven by a combination of human-related ignitions, and weather conditions controlling fire spread with a moderate effect of vegetation composition and generally efficient fire suppression. Human-related landscape features (roads and population density) play a major role in shaping ignition patterns, whereas climate (ISI) and vegetation properties appear informative as predictors of fire size, even under a modern fire suppression effort.
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28.
  • Vanneste, Thomas, et al. (författare)
  • Contrasting microclimates among hedgerows and woodlands across temperate Europe
  • 2020
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 281
  • Tidskriftsartikel (refereegranskat)abstract
    • Hedgerows have the potential to facilitate the persistence and migration of species across landscapes, mostly due to benign microclimatic conditions. This thermal buffering function may become even more important in the future for species migration under climate change. Unfortunately, there is a lack of empirical studies quantifying the microclimate of hedgerows, particularly at broad geographical scales. Here we monitored sub-canopy temperatures using 168 miniature temperature sensors distributed along woodland-hedgerow transects, and spanning a 1600-km macroclimatic gradient across Europe. First, we assessed the variation in the temperature offset (that is, the difference between sub-canopy and corresponding macroclimate temperatures) for minimum, mean and maximum temperatures along the woodland-hedgerow transects. Next, we linked the observed patterns to macroclimate temperatures as well as canopy structure, overstorey composition and hedgerow characteristics. The sub-canopy versus macroclimate temperature offset was on average 0.10 degrees C lower in hedgerows than in woodlands. Minimum winter temperatures were consistently lower by 0.10 degrees C in hedgerows than in woodlands, while maximum summer temperatures were 0.80 degrees C higher, albeit mainly around the woodland-hedgerow ecotone. The temperature offset was often negatively correlated with macroclimate temperatures. The slope of this relationship was lower for maximum temperatures in hedgerows than in woodlands. During summer, canopy cover, tree height and hedgerow width had strong cooling effects on maximum mid-day temperatures in hedgerows. The effects of shrub height, shrub cover and shade-casting ability, however, were not significant. To our knowledge, this is the first study to quantify hedgerow microclimates along a continental-scale environmental gradient. We show that hedgerows are less efficient thermal insulators than woodlands, especially at high ambient temperatures (e.g. on warm summer days). This knowledge will not only result in better predictions of species distribution across fragmented landscapes, but will also help to elaborate efficient strategies for biodiversity conservation and landscape planning.
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29.
  • Vernay, Antoine, et al. (författare)
  • Partitioning gross primary production of a boreal forest among species and strata: A multi-method approach
  • 2024
  • Ingår i: AGRICULTURAL AND FOREST METEOROLOGY. - 0168-1923 .- 1873-2240. ; 345
  • Tidskriftsartikel (refereegranskat)abstract
    • We compared three methods of estimating gross primary production (GPP) of a boreal forest dominated by spruce and pine with the goals of 1) converging on the best estimate and 2) disaggregating the GPP among the two canopy species and the understory stratum. The three methods were: 1) eddy covariance (EC), 2) a soil-vegetation-atmosphere transfer model, APES, driven by meteorological data, and 3) an ecophysiological approach (Iso/SF) based on sap flux and phloem delta C-13, where sap flux is used to estimate stomatal conductance and delta C-13 is used to estimate intrinsic water-use efficiency (WUEi). The EC and APES methods agreed rather well, which was expected because APES was developed to predict eddy covariance data. The Iso/SF method, which is based on independent data, yielded lower estimates. This was partly because it excluded understory vegetation from the GPP estimate. We also found that the measured sap flux/transpiration estimates for spruce in Iso/SF were much lower than those from APES. In contrast, the absolute values for Scots pines were very similar between the two methods, especially in the summer. In both species, the seasonal dynamics match well among all methods. This multi-method approach allowed us to detect possible problems in the spruce sap-flux measurements, but successfully upscaled pine data from ecophysiological traits to stand and ecosystem functioning.
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30.
  • Vico, Giulia, et al. (författare)
  • Photosynthetic capacity, canopy size and rooting depth mediate response to heat and water stress of annual and perennial grain crops
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 341
  • Tidskriftsartikel (refereegranskat)abstract
    • Perennial grain crops are promoted as an alternative to annual staple crops to reduce negative environmental effects of agriculture and support a variety of ecosystem services. While perennial grains have undergone extensive testing, their vulnerability to projected future warmer and drier growing conditions remains unclear. To fill this gap, we compared leaf temperature and gas exchange rates of annual wheat and different perennial wheat ideotypes using a multi-layer process-based eco-hydrological model. The model combines leaf-level gas exchange, optimality principles regulating stomatal conductance, energy balance, radiative and momentum transfer inside the canopy, as well as soil water balance. Wheat ideotypes are parameterized based on an extensive review of field data. When compared with annual wheat, perennial wheat ideotypes with high leaf area index had between 12% and 39% higher canopy transpiration and net CO2 assimilation, depending on their photosynthetic capacity and water status. Differences in leaf temperature and instantaneous water use efficiency between annual wheat and the perennial ideotypes were moderate (-0.5 to +0.4 & DEG;C and -6 to +2%, respectively). Low soil water availability did not alter the ranking of ideotypes in terms of canopy temperature and gas exchanges. During a prolonged dry down, cumulated water use was higher and canopy temperature lower in perennial than annual ideotypes, thanks to the deeper roots, whereas cumulated net CO2 fixation depended on the specific traits and air temperature. Leaf-specific and whole plant characteristics interacted with hydro-meteorological conditions in defining the perennial's vulnerability envelopes to potential heat and water stress. These findings underline the importance of plant characteristics, and particularly leaf area and rooting depth, in defining the suitability of perennial grain crops under future climates.
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31.
  • Wachiye, Sheila, et al. (författare)
  • Soil greenhouse gas emissions from a sisal chronosequence in Kenya
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 307
  • Tidskriftsartikel (refereegranskat)abstract
    • Sisal (Agave sisalana) is a climate-resilient crop grown on large-scale farms in semi-arid areas. However, no studies have investigated soil greenhouse gas (GHGs: CO2, N2O and CH4) fluxes from these plantations and how they relate to other land cover types. We examined GHG fluxes (Fs) in a sisal chronosequence at Teita Sisal Estatein southern Kenya. The effects of stand age on Fs were examined using static GHG chambers and gas chromatography for a period of one year in seven stands: young stands aged 1–3 years, mature stands aged 7–8 years, and old stands aged 13–14 years. Adjacent bushland served as a control site representing the surrounding land use type. Mean CO₂ fluxes were highest in the oldest stand (56 ± 3 mg C m-2 h-1) and lowest in the 8-year old stand (38 ± 3 mg C m-2 h-1), which we attribute to difference in root respiration between the stand. All stands had 13–28% higher CO₂ fluxes than bushland (32 ± 3 mg C m-2 h-1). CO2 fluxes in the wet season were about 70% higher than dry season across all sites. They were influenced by soil water content (WS) and vegetation phenology. Mean N2O fluxes were very low (<5 μg N m-2 h-1) in all sites due to low soil nitrogen (N) content. About 89% of CH4 fluxes were below the detection limit (LOD ± 0.02 mg C m-2 h-1). Our results imply that sisalplantations have higher soil CO2 emissions than the surrounding land use type, and the seasonal emissions were largely driven by WS and the vegetation status. Methane and nitrous oxide are of minor importance. Thus, soil GHG fluxes from sisal plantations are a minor contributor to agricultural GHG emissions in Kenya.
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32.
  • Wallach, Daniel, et al. (författare)
  • Multi-model evaluation of phenology prediction for wheat in Australia
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 298
  • Tidskriftsartikel (refereegranskat)abstract
    • Predicting wheat phenology is important for cultivar selection, for effective crop management and provides a baseline for evaluating the effects of global change. Evaluating how well crop phenology can be predicted is therefore of major interest. Twenty-eight wheat modeling groups participated in this evaluation. Our target population was wheat fields in the major wheat growing regions of Australia under current climatic conditions and with current local management practices. The environments used for calibration and for evaluation were both sampled from this same target population. The calibration and evaluation environments had neither sites nor years in common, so this is a rigorous evaluation of the ability of modeling groups to predict phenology for new sites and weather conditions. Mean absolute error (MAE) for the evaluation environments, averaged over predictions of three phenological stages and over modeling groups, was 9 days, with a range from 6 to 20 days. Predictions using the multi-modeling group mean and median had prediction errors nearly as small as the best modeling group. About two thirds of the modeling groups performed better than a simple but relevant benchmark, which predicts phenology by assuming a constant temperature sum for each development stage. The added complexity of crop models beyond just the effect of temperature was thus justified in most cases. There was substantial variability between modeling groups using the same model structure, which implies that model improvement could be achieved not only by improving model structure, but also by improving parameter values, and in particular by improving calibration techniques.
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33.
  • Wang, Chunyu, et al. (författare)
  • Modelling water and energy fluxes with an explicit representation of irrigation under mulch in a maize field
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 326
  • Tidskriftsartikel (refereegranskat)abstract
    • Globally, water-saving irrigation plays a vital role in agricultural ecosystems to achieve sustainable food pro-duction under climate change. Irrigation under mulch (IUM) system has been widely used in modern agricultural ecosystems due to its high water use efficiency, but it remains unclear how each component of the water and energy processes responds to this agricultural management practice. Current modeling approaches are inade-quate in investigating the impacts of IUM management on water-energy balance, which have shown more complicated than non-mulched management. Therefore, this study provided an explicit simulation of water and energy fluxes in IUM system using a process-oriented ecosystem model-CoupModel and the three years of the eddy covariance (EC) measurements. Based on Monte Carlo and the multiple model performance evaluation criteria, most of the model sensitive parameters were well constrained and 32 potentially important parameters, e.g., iscovevap, the fraction of mulch coverage, were identified to characterize the impacts of plastic mulching on energy balance and water transport. After proper calibration, the coefficient of determination (R2) for measured and simulated soil temperature (T) and soil water content (SWC) was 0.79 and 0.60, respectively, and the R2 for T and SWC during the validation period were 0.91 and 0.71, respectively. Furthermore, we found that there was a strong coupling between the parameters of the water and energy processes, which would restrict the simulation results due to the correlation between the parameters and the evaluation indices. This study presented a sys-tematic model parameters calibration in the agricultural ecosystem implemented with IUM and provided with a more comprehensive understanding of the water and energy balance in cropland. These results would help agricultural model development with more detailed considerations of the water-saving management.
  •  
34.
  • Wechsung, F., et al. (författare)
  • The upper homeostatic range for the temperature–yield response of irrigated US wheat down revised from a theoretical and experimental perspective
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier. - 0168-1923 .- 1873-2240. ; 307
  • Tidskriftsartikel (refereegranskat)abstract
    • High temperature (HT) and drought (D) have detrimental effects on growth and phenology that result in reductions of the yields of agricultural crops. Nevertheless, homeostatic ranges of tolerance exist. Recent analysis of survey data and simulations suggest US wheat (Triticum aestivum L.) yields remain relatively stable under irrigation in the range of 35°– 40°C. Applying analogue statistical procedures on experimental data and simulations from the Hot Serial Cereal Experiment (HSC) we demonstrate that failed incorporation of the corresponding phenological acceleration due to HT and a low interannual temperature variability lead to this result. Here, we incorporate the phenological effect into the used binned temperature exposure yield model by rescaling (normalizing) the absolute seasonal temperature counts to a maximum season length. The application to observed and simulated HSC data with a wide temperature range reveals that the suggested upper homeostatic response limit of US wheat yields to HT requires a down–revision. HT stress can be reduced by transpirational cooling. However, the effect is currently not sufficient to expand the homeostatic range of irrigated wheat markedly beyond 25°C taking our analysis of the HSC experiment. 
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35.
  • Yuan, Kunxiaojia, et al. (författare)
  • Causality guided machine learning model on wetland CH4 emissions across global wetlands
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier. - 0168-1923 .- 1873-2240. ; 324
  • Tidskriftsartikel (refereegranskat)abstract
    • Wetland CH4 emissions are among the most uncertain components of the global CH4 budget. The complex nature of wetland CH4 processes makes it challenging to identify causal relationships for improving our understanding and predictability of CH4 emissions. In this study, we used the flux measurements of CH4 from eddy covariance towers (30 sites from 4 wetlands types: bog, fen, marsh, and wet tundra) to construct a causality-constrained machine learning (ML) framework to explain the regulative factors and to capture CH4 emissions at sub -seasonal scale. We found that soil temperature is the dominant factor for CH4 emissions in all studied wetland types. Ecosystem respiration (CO2) and gross primary productivity exert controls at bog, fen, and marsh sites with lagged responses of days to weeks. Integrating these asynchronous environmental and biological causal relationships in predictive models significantly improved model performance. More importantly, modeled CH4 emissions differed by up to a factor of 4 under a +1C warming scenario when causality constraints were considered. These results highlight the significant role of causality in modeling wetland CH(4 )emissions especially under future warming conditions, while traditional data-driven ML models may reproduce observations for the wrong reasons. Our proposed causality-guided model could benefit predictive modeling, large-scale upscaling, data gap-filling, and surrogate modeling of wetland CH4 emissions within earth system land models.
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36.
  • Zhang, Shengmin, et al. (författare)
  • Retention forestry amplifies microclimate buffering in boreal forests
  • 2024
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 350
  • Tidskriftsartikel (refereegranskat)abstract
    • Retention forestry is increasingly adopted as an alternative to clearcutting practices and involves retaining structural and compositional complexity (e.g., living and dead trees) from preharvest to postharvest. Past studies have examined the role of retention forestry in supporting various ecosystem functions and biodiversity, whilst its microclimate buffering capacity has been largely neglected. We investigated the microclimates and the underlying mechanisms of retention forests relative to clearcuts and old forests in a boreal forest landscape in central Sweden. We found that both air temperature and vapour pressure deficit (VPD) differed significantly between the forest types. Old forests consistently exhibited the most buffered forest microclimates, followed by retention forests, while clearcuts displayed the lowest. Basal area and canopy cover were identified as the key determinants influencing air temperature and VPD across the forest types. Retention practices can also impact a stand’s microclimates. Specifically, maintaining diverse tree species had the potential to lower the stand’s maximum temperature, given its positive association with canopy cover. Large volumes of lying deadwood were found to be negatively correlated with both basal area and canopy cover, likely contributing to increased maximum temperatures. Furthermore, standing deadwood directly lowered the maximum temperature within forest stands. Finally, edge effects were observed in the retention forests, with south-facing edges experiencing significantly higher maximum temperature and VPD compared to north-facing edges and forest interiors. These south-facing edge effects were positively associated with the difference in lying deadwood volumes between forest edges and interiors. Our findings support the positive influence of retention practices on a stand’s microclimate buffering, achieved through preserving diverse tree species, standing deadwood, and implementing measures to prevent severe wind-induced tree mortality, particularly in south-facing edges (e.g. creating southfacing buffer zones). Forest managers and policy makers can utilize these results to minimize the climate-change impacts on below-canopy biodiversity and functioning.
  •  
37.
  • Zhao, Peng (författare)
  • Age-related response of forest floor biogenic volatile organic compound fluxes to boreal forest succession after wildfires
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 308
  • Tidskriftsartikel (refereegranskat)abstract
    • The amplification of global warming in the Northern regions results in a higher probability of wildfires in boreal forests. On the forest floor, wildfires have long-term effects on vegetation composition as well as soil and its microbial communities. A large variety of biogenic volatile organic compounds (BVOCs) such as isoprene, monoterpenes, sesquiterpenes have been observed to be emitted from soil and understory vegetation of boreal forest floor. Ultimately, the fire-induced changes in the forest floor affect its BVOC fluxes, and the recovery of the forest floor determines the quantity and quality of BVOC fluxes. However, the effects of wildfires on forest floor BVOC fluxes are rarely studied. Here we conducted a study of the impacts of post-fire succession on forest floor BVOC fluxes along a 158-year fire chronosequence in boreal Scots pine stands near the northern timberline in north-eastern Finland throughout a growing season. We determined the forest floor BVOC fluxes and investigated how the environmental and ground vegetation characteristics, soil respiration rates, and soil microbial and fungal biomass are associated with the BVOC fluxes during the post-fire succession. The forest floor was a source of diverse BVOCs. Monoterpenes (MTs) were the largest group of emitted BVOCs. We observed forest age-related differences in the forest floor BVOC fluxes along the fire chronosequence. The forest floor BVOC fluxes decreased with the reduction in ground vegetation coverage resulted from wildfire, and the decreased fluxes were also connected to a decrease in microbial activity as a result of the loss of plant roots and soil organic matter. The increase in BVOC fluxes was associated with the recovery of aboveground plant coverage and soils. Our results suggested taking into consideration the implications of BVOC flux variations on the atmospheric chemistry and climate feedbacks.
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38.
  • Zhao, Peng, et al. (författare)
  • Long-term nitrogen addition raises the annual carbon sink of a boreal forest to a new steady-state
  • 2022
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 324
  • Tidskriftsartikel (refereegranskat)abstract
    • The boreal forest is an important global carbon (C) sink. Since low soil nitrogen (N) availability is commonly a key constraint on forest productivity, the prevalent view is that increased N input enhances its C sink-strength. This understanding however relies primarily on observations of increased aboveground tree biomass and soil C stock following N fertilization, whereas empirical data evaluating the effects on the whole ecosystem-scale C balance are lacking. Here we use a unique long-term experiment consisting of paired forest stands with eddy covariance measurements to explore the effect of ecosystem-scale N fertilization on the C balance of a managed boreal pine forest. We find that the annual C uptake (i.e. net ecosystem production, NEP) at the fertilized stand was 16 +/- 2% greater relative to the control stand by the end of the first decade of N addition. Subsequently, the ratio of NEP between the fertilized and control stand remained at a stable level during the following five years with an average NEP to N response of 7 & PLUSMN; 1 g C per g N. Our study reveals that this non-linear response of NEP to long-term N fertilization was the result of a cross-seasonal feedback between the N-induced increases in both growing-season C uptake and subsequent winter C emission. We further find that one decade of N addition altered the sensitivity of ecosystem C fluxes to key environmental drivers resulting in divergent responses to weather patterns. Thus, our study highlights the need to account for ecosystem-scale responses to perturbations to improve our understanding of nitrogen-carbon-climate feedbacks in boreal forests.
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39.
  • Zignol, Francesco, et al. (författare)
  • The understory microclimate in agroforestry now and in the future-a case study of Arabica coffee in its native range
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 340
  • Tidskriftsartikel (refereegranskat)abstract
    • Climate change is having a major impact on crop production and food security worldwide, and particularly so for smallholder farmers. As agroforestry is common with smallholder farmers, it is important to not only model the macroclimate, but also the microclimate that crops experience below the canopies. However, there are few highresolution spatiotemporal climate projections for forest understories, because of constraints related to the lack of i) development of models for downscaling global climate projections, ii) high-resolution gridded datasets of environmental factors influencing microclimate, and iii) spatially replicated in-situ microclimate measurements. We focused on a landscape in southwestern Ethiopia where Arabica coffee originated, and, in the present day, is commonly grown as a shade crop. We first examined the relative contribution of in-situ field measurements vs. GIS-derived estimates of vegetation and topographic features in explaining in-situ microclimate. Second, we used a statistical downscaling approach to obtain past and future microclimate maps at 30-meter spatial resolution for the part of the landscape that is covered by trees. Predictive models using in-situ variables performed equal to models with GIS variables, indicating that remote sensing data might substitute for in-situ field measurements. Vegetation and topographic features were both important in explaining microclimatic variation. Our spatiotemporal projections of the microclimate indicate that coffee farming might have to relocate to higher altitudes due to increasing temperatures, that vegetation might buffer the macroclimate at middle altitudes to some extent, and that decreasing trends in relative humidity at the beginning of the wet season might become problematic for coffee production. Taken together, our findings demonstrate that we can rely on remote sensing data to create microclimate maps in landscapes where in-situ field measurements are challenging, and we suggest how these microclimate projections can be used as a tool to promote climate-resilient agriculture at the local and landscape levels.
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40.
  • Zuccarini, Paolo, et al. (författare)
  • Drivers and dynamics of foliar senescence in temperate deciduous forest trees at their southern limit of distribution in Europe
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier. - 0168-1923 .- 1873-2240. ; 342
  • Tidskriftsartikel (refereegranskat)abstract
    • Research on autumn phenology is very important for understanding and simulating the future growth of temperate deciduous forests. This is especially needed at the southern edge of the temperate zone, where climate change impacts are particularly intense. We studied foliar senescence timing for mature stands of Fagus sylvatica L., Populus tremula L., Betula pendula Roth, Quercus petraea (Matt.) Liebl. and Quercus robur L. at the southern edge of European temperate forests. First, we analysed long-term series (1997–2019) of senescence timing in southern France. Then, we compared a more detailed four-year dataset (2017–2020) of senescence dynamics and its correlations to meteorological conditions for stands in northern Spain and Belgium, with the latter area representing the core distribution of the species. In contrast to other temperate regions, no significant delaying trends in the timing of autumn phenology were detected in the long-term in southern temperate forests, which were characterized by high inter-site variability. In the short-term (4 years), species-specific correlations with meteorological conditions were found, with, for example, the senescence of F. sylvatica being affected by temperature while the senescence of Q. petraea was affected by both temperature and precipitation. Autumn dynamics differed between the core and southern areas of distribution of the species in the study period. In particular, while late season conditions affected senescence timing in the southern populations, the senescence dynamics of the core populations was mostly influenced by the legacy of spring – early summer conditions and, overall, more affected by precipitation than southern populations. Our data fill important knowledge gaps on the functioning of temperate deciduous forests at the southern limit of distribution in Europe.
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41.
  • Agbohessou, Yélognissè, et al. (författare)
  • To what extent are greenhouse-gas emissions offset by trees in a Sahelian silvopastoral system?
  • 2023
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923. ; 343
  • Tidskriftsartikel (refereegranskat)abstract
    • To assess the extent to which trees in a semi-arid silvopastoral system (SPS) can offset the greenhouse-gas (GHG) emissions of the system's livestock, this study used two process-based models (STEP-GENDEC-N2O and DynACof) to simulate 9 years of agricultural activity and resulting emissions in a SPS that has been operating in sahelian Senegal. STEP-GENDEC-N2O simulated soil N2O and CO2 fluxes, plus growth of the herbaceous layer, while DynACof focused on the tree layer. Outputs from the models included simulated time series of vegetative growth, water fluxes, and emissions. This output was validated through the use of published data, and measurements that were made at the SPS. Overall, the outputs from STEP-GENDEC-N2O agreed well with validation data for water fluxes, soil N, soil C, herbaceous biomass, and N2O emissions. Good agreement was also found between the measured fluxes of the SPS ecosystem, and the simulated values that were generated by combining STEP-GENDEC-N2O's simulations (of the herbaceous layer's heterotrophic respiration, autotrophic respiration, and gross primary productivity (GPP)) with DynACof's simulations of the tree layer's autotrophic respiration and GPP. Among the insights gained from the simulations was that in this SPS's sandy soils, nitrification was the dominant process that leads to N2O emissions. Our results show that the trees, at their current density (81 ha−1) offset 18 % to 41 % of the GHG emissions from livestock. With further development, the model set-up can be used for estimating the GHG offset at other tree densities, and will be useful for guiding future policies regarding climate-change adaptation and mitigation in the management of the Sahel's SPSs.
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42.
  • Carrasco-Molina, Tania, et al. (författare)
  • Validation and parametrization of the soil moisture index for stomatal conductance modelling and flux-based ozone risk assessment of Mediterranean plant species
  • 2024
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923. ; 354
  • Tidskriftsartikel (refereegranskat)abstract
    • The Mediterranean region chronically experiences high levels of tropospheric ozone (O3) that can affect the health of vegetation. However, limiting plant growing conditions, such as low soil moisture, may restrict the stomatal phytotoxic ozone dose (POD) absorbed by vegetation, modulating O3 detrimental effects. Atmospheric chemistry transport models that estimate POD for O3 risk assessment of effects on vegetation species, such as the European Monitoring and Evaluation Programme (EMEP), have adopted the soil moisture index (SMI) to consider the influence of soil moisture on POD. The objectives of this study were the parametrization and validation of the SMI effect on stomatal conductance (gs) for improving the POD estimation and O3 risk assessment for different vegetation species under water-limiting growing conditions, using field data collected in Italy and Spain and a literature review. The modelled SMI from EMEP proved to be a good indicator of soil moisture dynamics across sites and years, although it showed a general tendency to overestimate soil moisture availability for plants, particularly in the driest seasons. New parametrizations derived for modelling SMI effects on gs under Mediterranean conditions proposed in this study stress the importance of using species-specific parameters for species showing contrasting water-saving strategies in contrast of the current approach of using a simple relation between SMI and gs for all the species. Furthermore, gs modelling parametrizations based on soil water potential (SWP) were found to be more suitable than SMI for local scale estimation of POD under water-limiting conditions. Further consideration of rooting depth and distribution will be required in the future to determine the soil depth at which the soil moisture should be measured in POD modelling, since these features represent one of the most important uncertainties affecting the estimation of POD that could not be addressed with the present database.
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43.
  •  
44.
  • Fan, H. W., et al. (författare)
  • Spatial synchrony in delta O-18 time-series from a tree-ring network are driven by synchronous hydroclimate variability in the transitional zone of the Asian summer monsoon
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 311
  • Tidskriftsartikel (refereegranskat)abstract
    • Owing to the increased need to assess global forest behavior under changing climatic conditions within a longterm context, spatial coherence in tree-ring records is of widespread interest. Spatial networks of tree growth have been widely discussed from regional to global scales, but few studies have reported stable oxygen isotopes in networks of tree rings (delta O-18(tree)), thus impeding a comprehensive hydroclimatic interpretation of delta O-18(tree) records over large spatial scales, i.e., isoscapes. In this study, a network of four annually resolved delta O-18(tree) chronologies (1805-2016) was developed for Southwest China, the transitional zone of the Asian summer monsoon, to test spatial coherence in delta O-18(tree) variations and to explore the climatic factors driving synchrony at different spatial scales. The quantitative analyses show that the four chronologies exhibit consistent annual variations, with the first principal component (PC1) accounting for 70% of the total variance, which uniformly respond to local relative humidity (RH) and precipitation delta O-18 during the monsoon season. This confirms the spatial coherence in delta O-18(tree) records. Furthermore, significant inter-site correlations of local RH and precipitation delta O-18 are also detected, revealing that it is the regionally homogeneous year-to-year variations in the hydroclimate that should be responsible for spatial coherence. For a larger spatial perspective, spatial correlation analyses reveal that the most prominent correlations between delta O-18(tree) and hydroclimate fields occur in the Indochina Peninsula, in which delta O-18(tree) changes are synchronized with those in Southwest China and the synchrony temporally varies with the Indian summer monsoon (ISM) intensity, indicating that coherent delta O-18(tree) variations are linked to ISM-related moisture transport. In addition, delta O-18(tree) records have shown strong in-phase relationships with the natural variabilities of the ISM and El Nino-Southern Oscillation (ENSO) across interannual and interdecadal timescales over the last 150 years, suggesting that spatial coherence in delta O-18(tree) records can be utilized to reflect the long-term history of large-scale atmospheric circulation.
  •  
45.
  • Helbig, Manuel, et al. (författare)
  • Integrating continuous atmospheric boundary layer and tower-based flux measurements to advance understanding of land-atmosphere interactions
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 307
  • Forskningsöversikt (refereegranskat)abstract
    • The atmospheric boundary layer mediates the exchange of energy, matter, and momentum between the land surface and the free troposphere, integrating a range of physical, chemical, and biological processes and is defined as the lowest layer of the atmosphere (ranging from a few meters to 3 km). In this review, we investigate how continuous, automated observations of the atmospheric boundary layer can enhance the scientific value of co-located eddy covariance measurements of land-atmosphere fluxes of carbon, water, and energy, as are being made at FLUXNET sites worldwide. We highlight four key opportunities to integrate tower-based flux measurements with continuous, long-term atmospheric boundary layer measurements: (1) to interpret surface flux and atmospheric boundary layer exchange dynamics and feedbacks at flux tower sites, (2) to support flux footprint modelling, the interpretation of surface fluxes in heterogeneous and mountainous terrain, and quality control of eddy covariance flux measurements, (3) to support regional-scale modeling and upscaling of surface fluxes to continental scales, and (4) to quantify land-atmosphere coupling and validate its representation in Earth system models. Adding a suite of atmospheric boundary layer measurements to eddy covariance flux tower sites, and supporting the sharing of these data to tower networks, would allow the Earth science community to address new emerging research questions, better interpret ongoing flux tower measurements, and would present novel opportunities for collaborations between FLUXNET scientists and atmospheric and remote sensing scientists.
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46.
  • Islam, Md Rafikul, et al. (författare)
  • Projected effects of climate change and forest management on carbon fluxes and biomass of a boreal forest
  • 2024
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923. ; 349
  • Tidskriftsartikel (refereegranskat)abstract
    • Boreal forests are key to global carbon (C) sequestration and storage. However, the potential impacts of climate change on these forests could be profound. Nearly 70 % of the European boreal forests are intensively managed, but our understanding of the combined effects of forest management and climate change on the forest's integral role as a C sink is still limited. In this study, we aim to fill this gap with simulations of the process-based dynamic global vegetation model LPJ-GUESS. We evaluated the effects of four forest management options under two different climate scenarios (RCP 4.5 and RCP 8.5), at a southern boreal forest stand in Sweden. These options were compared against a baseline without clear-cut or management interventions. We found that the projected increase in temperatures (+2 to +4 °C) during the latter part of the 21st century will reduce the net C sink strength, particularly in the unmanaged forest. The standing biomass C for reforestations was projected to be 57–67 % lower in 2100 than in the old forest in 2022. The study also revealed that the C sequestration potential of replanted pine forests may surpass that of 200-years old forests in the far future (2076–2100). The study did not detect statistically significant differences in overall net C exchange between the clear-cut with subsequent reforestation options and the baseline, even though specific reforestation strategies, such as pine plantations, enhanced the overall net C sink by 7–20 % relative to the baseline during 2022–2100. These findings underscore the profound influence of forest management on the net C budget, surpassing that of climate change scenarios alone. By adopting pertinent reforestation strategies, C uptake could be augmented, with concurrently improved forest productivity, resulting in favourable outcomes for the forest's critical role in C sequestration and storage amidst a changing climate.
  •  
47.
  • Kelly, Julia, et al. (författare)
  • Modelling and upscaling ecosystem respiration using thermal cameras and UAVs: Application to a peatland during and after a hot drought
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 300
  • Tidskriftsartikel (refereegranskat)abstract
    • Field-based thermal infrared cameras provide surface temperature information at very high spatial and temporal resolution and could complement existing phenological camera and spectral sensor networks. Since temperature is one of the main drivers of ecosystem respiration (ER), field-based thermal cameras offer a new opportunity to model and upscale ER in unprecedented detail. We present such an approach based on manual chamber CO2 flux measurements and thermal imagery from a tower-based camera and from Unmanned Aerial Vehicle (UAV) flights. Data were collected over two growing seasons, including the hot drought of 2018, for the two main vegetation microforms (hummock and hollow) of a hemi-boreal peatland in Sweden. Thermal imagery proved suitable for modelling ER in this ecosystem: ER model accuracies were similar when air, soil or surface temperature measurements were used as input. Our findings allowed us to upscale ER using UAV-derived thermal images and we present maps of ER at sub-decimeter resolution (<7 cm). The significantly different ER measured for each microform highlighted the importance of modelling their ER separately. Not accounting for these differences and the microforms' spatial distribution across the peatland led to a bias in upscaled ER of up to 18%. As a result of the severity and timing of the hot drought in 2018, we observed reductions in the ER of both microforms, but more so for hummocks (-48%) than for hollows (-15%), and modelled ER leveled off at high temperatures. These findings indicate that peatland carbon loss during hot droughts may be lower than expected and strongly relates to vegetation composition. The presented upscaling approach offers a new method to analyse how ER varies across a peatland or within a flux-tower footprint, and to interpret biases that occur when using coarse resolution satellite data to upscale chamber or tower-based flux measurements.
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48.
  • Kelly, Julia, et al. (författare)
  • Wildfire impacts on the carbon budget of a managed Nordic boreal forest
  • 2024
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923. ; 351
  • Tidskriftsartikel (refereegranskat)abstract
    • Wildfire is one of the most important disturbances affecting boreal forests. Most previous research on boreal forest fires has occurred in North American forests which have different fire regimes, tree species and are less intensively managed than their Eurasian counterparts. Recent extreme fire years have highlighted the vulnerability of the Nordic boreal forest to climatic shifts that are increasing forest fire frequency and severity. The Ljusdal fire (2018) was one of the largest wildfires in recorded history in Sweden. We established eddy covariance flux towers to track the impacts of this fire on the carbon balance of two Pinus sylvestris sites subject to different fire severities and forest management strategies 1–4 years post-fire. The ‘SLM’ site was a mature stand that experienced low-severity fire (trees survived) followed by salvage-logging and reseeding, whilst the ‘HY’ site was 10 years old when it experienced high-severity fire (all trees killed) then was replanted with seedlings. During the study period, both sites were net carbon sources at the annual scale. It took up to 4 years after the fire until the first day of net CO2 uptake was recorded at each site. We estimated that it will take 13 years (8, 21; mean ± 95 % confidence intervals) after the fire until the sites reach a neutral annual carbon balance. It will take up to 32 years (19, 53) at HY and 46 years (31, 70) at SLM to offset the carbon lost during and after the fire and salvage-logging. In addition, our measurements showed that more carbon was emitted in the first 4 years after the fire compared to the carbon lost from combustion during the fire. Quantifying carbon fluxes during the initial years after fire is therefore crucial for estimating the net impact of wildfire on the carbon budget of boreal forests.
  •  
49.
  • Lammirato, C., et al. (författare)
  • Measuring frequency and accuracy of annual nitrous oxide emission estimates
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 310
  • Tidskriftsartikel (refereegranskat)abstract
    • Accurate estimates of cumulative N2O fluxes from agricultural soil are essential for quantifying global N2O emissions and for identifying effective mitigation strategies. This study focuses on the short term temporal variability of N2O fluxes, and on how the accuracy of annual cumulative estimates is affected by different (simulated) measuring frequencies of an automatic system based on the closed chamber method. Fluxes were measured with high temporal resolution (24 per day) for approximately one year on agricultural soil in the southwest of Sweden. The short-term temporal variability of N2O fluxes was considerable: the predictive power of measured fluxes decreased to negligible levels in a time frame of 4-5 h, and large intraday flux ranges were observed frequently, particularly in days with soil temperatures below 0 degrees C. A time of day well suited for consistently providing good estimates of the mean daily flux could not be identified. Consequently, the accuracy of annual emission estimates strongly depended on the measuring frequency. Multiple measurements per day were necessary for consistently quantifying annual emission estimates with accuracy. Accuracy improved with measuring frequency increasing up to four times per day, and higher frequencies generated negligible further improvements. Based on this study the following recommendations are made with regard to measuring frequency: i) measure four times per day for maximum accuracy and ii) measure twice per day for a good compromise between accuracy and the number of plots that can be monitored.
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50.
  • Li, Xinxi, et al. (författare)
  • Increasing importance of precipitation in spring phenology with decreasing latitudes in subtropical forest area in China
  • 2021
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 304-305
  • Tidskriftsartikel (refereegranskat)abstract
    • Climate warming has significantly advanced plant spring phenology in temperate and boreal biomes in the northern hemisphere. However, the response of subtropical forest phenology to climate change remains largely unclear. This study aimed to determine the spatiotemporal patterns of spring photosynthetic phenology in subtropical forests in China over the period 2002-2017 and explore its underlying mechanism in response to the changes of different climate variables. We applied four methods to extract the start of the photosynthetic period (SOP) from a solar–induced chlorophyll fluorescence (SIF) data set during the period 2002 to 2017, and determined correlations between SOP and environmental factors using partial correlation analyses. Overall, the SOP was advanced by 6.8 days. Furthermore, we found that the SIF-based SOP is highly correlated with the flux data–based photosynthetic onset dates, demonstrating that SIF can be a useful index in characterizing the photosynthetic phenology in subtropical forests. Interestingly, based on partial correlation analysation temperature dominated the SOP in the northern subtropical forest, but the importance of precipitation increased with decreasing latitudes, and the primary climatic control of SOP in southern monsoon evergreen forests is precipitation. These results suggested that the predicted increase in temperature and shift in precipitation regimes under ongoing climate change might potentially largely affect the photosynthetic phenology, and thus affect the carbon and water cycles in subtropical forests.
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