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1.
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2.
  • Halldin, Sven, et al. (författare)
  • Energy, water and carbon exchange in a boreal forest landscape - NOPEX experiences
  • 1999
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-9:SI, s. 5-29
  • Forskningsöversikt (refereegranskat)abstract
    • The role of the land surface in controlling climate is still underestimated and access to information from the boreal-forest zone is instrumental to improve this situation. This motivated the organisation of NOPEX (Northern hemisphere climate-Processes land-surface Experiment) in the southern part of the European boreal zone. This paper summarises results from NOPEX in its first phase, dealing with spring- and summertime conditions. Two concentrated field efforts (CFE1 on 27 May-23 June 1994, CFE2 on 18 April-14 July 1995) were carried out with coordinated measurements of energy, water, and CO2 budgets at 13 ground-based sites and at various airborne platforms. Flux aggregation was a central issue in the heterogeneous, patchy NOPEX landscape. It is shown that simple land-use-weighted averaging of fluxes from fields/forests/lakes agree well with regional fluxes. Momentum fluxes can be parameterised over the whole area with a roughness length of approximately 1.5 m, whereas fluxes of sensible heat and other scalars depend on the averaging scale, Local measurements of soil moisture can be classified and meaningful averages can be deduced with a 1 km resolution. Lakes play an important role and differs in both diurnal and annual cycles compared to the forests and fields. Multiannual data from an agricultural and a forest site has allowed quantification and modelling of seldom occuring phenomena. One unexpected result was that the Norunda Common forest acted as a source and not a sink of CO2. The successful completion of CFE1-2 and a pilot winter campaign (CFE3) will lead NOPEX into its final phase, devoted to wintertime processes. Measurements and model results reside in SINOP. the System for Information in NOPEX, open for NOPEX participants. Data from CFE1 and CFE2 are released on CD as an integrated part of this Special Issue. (C) 1999 Elsevier Science B.V. All rights reserved.
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3.
  • Lankreijer, Harry, et al. (författare)
  • Evaporation and storage of intercepted rain analysed by comparing two models applied to a boreal forest
  • 1999
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-99, s. 595-604
  • Tidskriftsartikel (refereegranskat)abstract
    • Rainfall and throughfall were measured during the summer of 1995. Rainfall interception is often simulated by a version of the well-known Rutter-Gash analytical model. In this study this model was compared to a model based on an exponential saturation equation. The concept of the ‘minimum method' for deriving canopy storage capacity and free throughfall coefficient by the Leyton-analysis, is compared to the concept of maximum storage capacity by reversing the models. Measured evaporation rate during rain events was found to be lower than simulated by the Penman equation using different known formulations for aerodynamic resistance. The concept of a high internal canopy resistance and decoupling of the canopy from the atmosphere should be analysed further in order to explain low evaporation during rainfall.
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4.
  • Lundin, Lars-Christer, et al. (författare)
  • Continuous long-term measurements of soil-plant-atmosphere variables at a forest site
  • 1999
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-99, s. 53-73
  • Tidskriftsartikel (refereegranskat)abstract
    • It is a major challenge in modem science to decrease the uncertainty in predictions of global climate change. One of the largest uncertainties in present-day global climate models resides with the understanding of processes in the soil-vegetation-atmosphere-transfer (SVAT) system. Continuous, long-term data are needed in order to correctly quantify balances of water, energy and CO2 in this system and to correctly model it. It is the objective of this paper to demonstrate how a combined system of existing sensor, computer, and network technologies could be set up to provide continuous and reliable long-term SVAT-process data from a forested site under almost all environmental conditions. The Central Tower Site (CTS) system was set up in 1993-1994 in a 25 m high boreal forest growing on a highly heterogeneous till soil with a high content of stones and blocks. It has successfully monitored relevant states and fluxes in the system, such as atmospheric fluxes of momentum, heat, water vapour and CO2, atmospheric profiles of temperature, water vapour, CO2, short-and long-wave radiation, heat storage in soil and trees, sap-dow and a variety of ecophysiological properties, soil-water contents and tensions, and groundwater levels, rainfall and throughfall. System uptime has been more than 90% for most of its components during the first 5 years of operation. Results from the first 5 years of operation include e.g., budgets for energy, water and CO2, information on important but rarely occurring events such as evaporation from snow-covered canopies, and reactions of the forest to extreme drought. The carbon budget shows that the forest may be a sink of carbon although it is still growing. The completeness of the data has made it possible to test the internal consistency of SVAT models. The pioneering set-up at the CTS has been adopted by a large number of SVAT-monitoring sites around the world. Questions concerning tower maintenance, long-term calibration plans, maintenance of sensors and data-collection system, and continuous development of the computer network to keep it up to date are, however, only partly of interest as a research project in itself. It is thus difficult to get it funded from usual research-funding agencies. The full value of data generated by the CTS system can best be appreciated after a decade or more of continuous operation. Main uses of the data would be to evaluate how SVAT models handle the natural variability of climate conditions, quantification of water. carbon and energy budgets during various weather conditions, rind development of new parameterisation schemes in global and regional climate models. 
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5.
  • Lundin, Lars-Christer, et al. (författare)
  • System of information in NOPEX : retrieval, use, and query of climate data
  • 1999
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-99, s. 31-51
  • Tidskriftsartikel (refereegranskat)abstract
    • The uncertainty in climate predictions caused by improper understanding of the role of the land-surface is underestimated and easy access to data from a series of landscape types around the globe would improve this. Such data exist from a series of large-scale land-surface experiments but access to them has been difficult. It is the objective of this paper to demonstrate how the System for Information in NOPEX (SINOP) could be set up to provide a combination of data archive and tool for executing various time-limited and long-term field activities. Setting up and running SINOP involved both technical and psychological issues. The major technical problems were associated with (i) the uninterrupted flow of large data volumes, (ii) data homogeneity, and (iii) the exploding technology development. The psychological and organisational problems were more difficult to tackle than the technical problems. Funding agencies assumed somebody else would take care of data archiving and documentation, academic organisations have difficulties to compete with the private market for database managers, many individual scientists were unwilling to deliver their datasets and, especially, unwilling to document them. It is suggested that changes in attitudes from scientists, academic organisations, and publishers are needed to give credit for the publication of good datasets and for the production of good documentation about them. CDs incorporating a subset of SINOP with well-documented datasets from NOPEX operations in 1994 and 1995 are published together with this NOPEX Special Issue. The CDs include climate variables, such as radiation, fluxes of heat, momentum, and water vapour, and various energy storage terms as well as hydrological variables from 13 sites within the central-Swedish NOPEX region, at the southern boundary of the boreal zone. The publication of these data is seen as a step towards giving data-set owners proper and citeable credit for their work.
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6.
  • Motovilov, Yuri G., et al. (författare)
  • Validation of a distributed hydrological model against spatial observations
  • 1999
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-99, s. 257-277
  • Tidskriftsartikel (refereegranskat)abstract
    • In connection with climate change studies a new hydrologic field has evolved - regional hydrological modelling or hydrologic macro modelling, which implies repeated application of a model everywhere within a region using a global set of parameters. The application of a physically based distributed hydrological model ECOMAG to river basins within the NOPEX southern region with this purpose in mind is presented. The model considers the main processes of the land surface hydrological cycle: infiltration, evapotranspiration, heat and water regime of the soil, snowmelt, formation of surface, subsurface and river runoff and groundwater. The spatial integration of small and meso-scale non-homogeneity of the land surface is a central issue both for the definition of fundamental units of the model structure and for determination of representative values for model validation. ECOMAG is based on a uniform hydrological (or landscape) unit representation of the river basin, which reflects topography, soil, vegetation and land use. As a first step the model was calibrated using standard meteorological and hydrological data for 7 years from regular observation networks for three basins. An additional adjustment of the soil parameters was performed using soil moisture and groundwater level data from five small experimental basins. This step was followed by validation of the model against runoff for 14 years from six other drainage basins, and synoptic runoff and evapotranspiration measurements performed during two concentrated field efforts (CFEs) of the NOPEX project in 1994 and 1995. The results are promising and indicate directions for further research. 
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7.
  • 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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8.
  • 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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9.
  • Arnqvist, Johan, et al. (författare)
  • Examination of the mechanism behind observed canopy waves
  • 2016
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 218, s. 196-203
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, we document the existence of wave-like motions above a forest canopy using data taken from a 138 m high tower placed within a forest Characteristics of the waves are examined in relation to their possible effects on wind energy. It is shown that when the wave signal is relatively clean, the phase lag between horizontal and vertical velocity is close to 90, which limits the contribution of the waves to the downward momentum flux. Numerical solutions of the linear wave equations agree with measurements in terms of wave period and the vertical shape of the wave amplitude. Linear analysis show that shear instability is the cause of unstable wave growth, and that the fastest growing unstable wave number typically has a period of 10-100 s. In addition to the shear instability, the linear analysis predicts that under certain conditions instabilities of the Holmboe kind can develop over forests.
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10.
  • Aubinet, M., et al. (författare)
  • Direct advection measurements do not help to solve the night-time CO2 closure problem: Evidence from three different forests
  • 2010
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 150:5, s. 655-664
  • Tidskriftsartikel (refereegranskat)abstract
    • The ADVEX project involved conducting extensive advection measurements at three sites, each with a different topography. One goal of the project was to measure the [CO2] balance under night-time conditions, in an attempt to improve NEE estimates. Four towers were arranged in a square around a main tower, with the sides of the square about 100 m long. Equipped with 16 sonic anemometers and [CO2] sampling points, the towers were installed to measure vertical and horizontal advection of [CO2]. Vertical turbulent fluxes were measured by an eddy covariance system at the top of the main tower. The results showed that horizontal advection varied greatly from site to site and from one wind sector to another, the highest values being reached when there were large friction velocities and fairly unstable conditions. There was less variation in vertical advection, the highest values being reached when there were low friction velocities and stable conditions. The night-time NEE estimates deduced from the mass balance were found to be incompatible with biologically driven fluxes because (i) they varied strongly from one wind sector to another and this variation could not be explained in terms of a response of the biologic flux to climate, (ii) their order of magnitude was not realistic and (iii) they still showed a trend vs. friction velocity. From a critical analysis of the measurement and data treatment we concluded that the causes of the problem are related to the representativeness of the measurement (control volume size, sampling resolution) or the hypotheses underlying the derivation of the [CO2] mass balance (ignoring the horizontal turbulent flux divergence). This suggests that the improvement of eddy flux measurements by developing an advection completed [CO2] mass balance at night would be practically difficult. (C) 2010 Elsevier B.V. All rights reserved.
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11.
  • Axelsson, SRJ, et al. (författare)
  • Tree-heights derived from radar profiles over boreal forests
  • 1999
  • Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-9, s. 427-435
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, results from helicopter-borne NOPEX-measurements over a boreal forest using a multi-band radar profiler are presented. Comparisons are made between tree-height profiling responses at three different frequency bands (5, 14 and 35 GHz). Values derived from radar profiles are also compared with maximum tree-height, and stem volume per hectare derived from ground-based measurements made in circular plots below the Eight track. The range response of a radar profiler and its relationship to the foliage back-scattering and stem volume is modelled and discussed. (C) 1999 Elsevier Science B.V. All rights reserved.
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12.
  • 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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13.
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14.
  • Beldring, S, et al. (författare)
  • Distribution of soil moisture and groundwater levels at patch and catchment scales
  • 1999
  • Ingår i: AGRICULTURAL AND FOREST METEOROLOGY. - : ELSEVIER SCIENCE BV. - 0168-1923. ; 98-9, s. 305-324
  • Tidskriftsartikel (refereegranskat)abstract
    • This study is a contribution to the northern hemisphere climate processes land surface experiment (NOPEX). Its purpose is to investigate the spatial variability of groundwater levels and soil moisture content at different scales in a landscape dominated b
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15.
  • 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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16.
  • Björklund, Jesper, et al. (författare)
  • Disentangling the multi-faceted growth patterns of primary Picea abies forests in the Carpathian arc
  • 2019
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 271, s. 214-224
  • Tidskriftsartikel (refereegranskat)abstract
    • © 2019 Elsevier B.V. A tree's radial growth sequence can be thought of as an aggregate of different growth components such as age and size limitations, presence or absence of disturbance events, continuous impact of climate variability and variance induced by unknown origin. The potentially very complex growth patterns with prominent temporal and spatial variability imply that our understanding of climate-vegetation feedbacks essentially benefits from the expansion of large tree ring networks into data-poor regions, and our ability to disentangle growth constraints by comparing ring series at multiple scales. In this study, we analyze Central-Eastern Europe's most substantial assemblage of primary Norway spruce forests found in the Carpathian arc. The vast data set, >10,000 tree-ring series, is stratified along a prominent gradient in climate response space over four separate landscapes. We integrated curve intervention detection and dendroclimatic standardization to decompose tree growth variance into climatic, disturbance and residual components to explore the behavior of the components over increasingly larger spatial hierarchies. We show that the residual variance of unknown origin is the most prominent variance in individual Carpathian spruce trees, but at larger spatial hierarchies, climate variance dominates. The variance induced by climate was further explored with common correlation analyses, growth response to extreme climate years and forward modeling of tree growth to identify leading modes of climate response, and potentially non-linear and mixed climate response patterns. We find that the climatic response of the different forest landscapes overall can be described as an asymptotic response to June and July temperatures, most likely intermixed with influence from winter precipitation. In the collection of landscapes, Southern Romania stands out as being the least temperature sensitive and most likely exhibiting the most complicated mixed temperature and moisture limitation.
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17.
  • Boudreault, Louis-Etienne, et al. (författare)
  • A LiDAR method of canopy structure retrieval for wind modeling of heterogeneous forests
  • 2015
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 201, s. 86-97
  • Tidskriftsartikel (refereegranskat)abstract
    • The difficulty of obtaining accurate information about the canopy structure is a current limitation towards higher accuracy in numerical predictions of the wind field in forested terrain. The canopy structure in computational fluid dynamics is specified through the frontal area density and this information is required for each grid point in the three-dimensional computational domain. By using raw data from aerial LiDAR scans together with the Beer-Lambert law, we propose and test a method to calculate and grid highly variable and realistic frontal area density input. An extensive comparison with ground-based measurements of the vertically summed frontal area density (or plant area index) and tree height was used to optimize the method, both in terms of plant area index magnitude and spatial variability. The resolution of the scans was in general low (<2.5 reflections m(-2)). A decrease of the resolution produced an increasing systematic underestimation of the spatially averaged tree height, whereas the mean plant area index remained insensitive. The gridded frontal area density and terrain elevation were used at the lower boundary of wind simulations in a 5 km x 5 km area of a forested site. The results of the flow simulations were compared to wind measurements using a vertical array of sonic anemometers. A good correlation was found for the mean wind speed of two contrasting wind directions with different influences from the upstream forest. The results also predicted a high variability on the horizontal and vertical mean wind speed, in close correlation with the canopy structure. The method is a promising tool for several computational fluid dynamics applications requiring accurate predictions of the near-surface wind field. (C) 2014 The Authors. Published by Elsevier B.V.
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18.
  • Bylund, Helena (författare)
  • Larval crowding during an insect outbreak reduces herbivory pressure on preferred shrubs in a warmer environment
  • 2018
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 263, s. 180-187
  • Tidskriftsartikel (refereegranskat)abstract
    • With warming climate many species are predicted to shift their distributions toward the poles. However, climate change models developed to predict species distributions do not always incorporate interactions between them. The northerly shift of the boreal forest and associated dwarf shrub communities will be directly affected by warming. But warming will also indirectly affect plant communities via impacts on the intensity and frequency of associated insect outbreaks. We present a general model exploring plant host herbivory in response to the balance between insect crowding, host consumption and climate. We examined how these factors dictate the feeding preference of Epirrita autumnata larvae during an outbreak on dwarf shrub vegetation in Sub-arctic Fennoscandia. Data were collected from an outdoor experiment investigating future climate change scenarios (elevated CO2 and temperature) on the dwarf shrub community that included deciduous (Vaccinium myrtillus) and evergreen species (V. vitis-idaea and Empetrum nigrum). We observed that larval crowding was independent of treatment under outbreak conditions. We also tested and confirmed model predictions that larvae would prefer monospecific stands of either deciduous shrubs or its evergreen competitors. For current climate conditions, larvae had a preference to consume more deciduous shrubs in mixed stands. However, at elevated temperature bilberry consumption and herbivore pressure was lower, particularly in mixed stands. Our results show that during future warming, E. autumnata herbivory could promote the success of thermophile deciduous species and possible northward migration. Insect behaviour and preferences should therefore be considered when predicting future vegetation movements responding to warming.
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19.
  • 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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20.
  • 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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21.
  • Chi, Jinshu (författare)
  • Adjustment of CO2 flux measurements due to the bias in the EC150 infrared gas analyzer
  • 2019
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 276
  • Tidskriftsartikel (refereegranskat)abstract
    • During the Regional Approaches to Climate Change (REACCH) program, eddy covariance monitoring over agricultural fields were used to estimate annual carbon and water budgets in the inland Pacific Northwest. Here, we assess the effect of a bias in the high-frequency CO2 concentration measurements using the Campbell Scientific EC150 infra-red gas analyzer on the CO2 fluxes and field-scale carbon balances. The bias stems from using a lower frequency temperature measurement to calculate the CO2 density, which misses higher frequency temperature fluctuations. To generate the bias adjustment, data were collected over four similar agricultural sites as part of the Long-Term Agroecosystem Research network for multiple months using the same four instrument sets used in the REACCH project. The difference between the high-frequency and low-frequency CO2 fluxes were regressed against the kinematic heat fluxes to generate a correction equation for each instrument set, which were applied to the historical REACCH data to determine the effect of the bias on the measured and gap-filled flux values. The re-calculated positive biases in the measured fluxes were 40 gC-CO(2)m(-2) yr(-1) to 126 gC-CO(2)m(-2) yr-1, indicating greater losses to the atmosphere than initially estimated. Once gap-filled, three out of fourteen site-years switched from weak carbon sinks to weak carbon sources. When the carbon exported via harvest was included in the budget calculation the bias correction still impacted the source/sink strength but did not change the sign of the carbon balance. Overall, the total net ecosystem exchange decreased between 300-470 gC-CO(2)m(-2) per site (29-46%) over the 4 crop-years from the bias adjustment process.
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22.
  • 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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23.
  • Chi, Jinshu, et al. (författare)
  • The carbon balance of a managed boreal landscape measured from a tall tower in northern Sweden
  • 2019
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 274, s. 29-41
  • Tidskriftsartikel (refereegranskat)abstract
    • Boreal forests exchange large amounts of carbon dioxide (CO2) with the atmosphere. A managed boreal landscape usually comprises various potential CO2 sinks and sources across forest stands of varying age classes, clear-cut areas, mires, and lakes. Due to this heterogeneity and complexity, large uncertainties exist regarding the net CO2 balance at the landscape scale. In this study, we present the first estimate of the net CO2 exchange over a managed boreal landscape (∼68 km2) in northern Sweden, based on tall tower eddy covariance measurements. Our results suggest that from March 1, 2016 to February 28, 2018, the heterogeneous landscape was a net CO2 sink with a 2-year mean uptake of −87 ± 6 g C m−2 yr−1. Due to an earlier and warmer spring and sunnier autumn, the landscape was a stronger CO2 sink during the first year (−122 ± 8 g C m−2) compared to the second year (−52 ± 9 g C m−2). Footprint analysis shows that 87% of the CO2 flux measurements originated from forests, whereas mires, clear-cuts, lakes, and grassland contributed 11%, 1%, 0.7%, and 0.2%, respectively. Altogether, the CO2 sink strength of the heterogeneous landscape was up to 38% lower compared to the sink strength of a mature stand surrounding the tower. Overall, this study suggests that the managed boreal landscape acted as a CO2 sink and advocates tall tower eddy covariance measurements to improve regional carbon budget estimates.
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24.
  • Cienciala, E, et al. (författare)
  • Canopy transpiration from a boreal forest in Sweden during a dry year
  • 1997
  • Ingår i: AGRICULTURAL AND FOREST METEOROLOGY. - : ELSEVIER SCIENCE BV. - 0168-1923. ; 86:3-4, s. 157-167
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • Estimation of areal evapotranspiration is crucial for the parameterization of the soil-vegetation-atmosphere interface in climate models and for the assessment of land-use changes on water resources. Present knowledge on how areal forest evapotranspiratio
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25.
  • 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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26.
  • 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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27.
  • 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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28.
  • Drobyshev, Igor, et al. (författare)
  • Forest fire activity in Sweden: Climatic controls and geographical patterns in 20th century
  • 2012
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 154-155, s. 174-186
  • Tidskriftsartikel (refereegranskat)abstract
    • We used Swedish county-scale forest fire statistics to quantify modern fire activity, identify its main temporal and geographical patterns, and evaluate statistical performance of six fire-related weather indices as proxy of fire activity in Sweden over 1942–1975 and 1996–2008, the periods with available countyscale fire statistics. The analyzed indices were monthly precipitation, SPI, MDC, PDSI, calibrated PDSI, and DI, a Drought Index calculated as a ratio between actual and equilibrium evapotranspiration. The modern fire cycle (FC) in the northern part of Sweden varies between 2 × 103 and 3 × 104 years, whereas in southern Sweden the FC is somewhat shorter (103–2 × 104 years). No temporal trend in average FC was evident at the country scale between the two periods. Significant and negative values of a Mantel test, obtained on county data for both periods (r = −0.494, p = 0.001 for 1942–1975 and r = −0.281 and p = 0.015 for 1996–2008) indicated the presence of a geographical pattern in annual forest fire activity. Over 1942–1975, PCA revealed that the central and northern counties formed one group with synchronized fire activity, and the southern and south–western counties formed another group. This pattern became less evident during the more recent period (1996–2008). Over 1996–2008, the analysis showed little synchronicity in annual fire activity across different parts of the country. The geographical position of a county had a clear effect on seasonal pattern of forest fires. In southern Sweden, the peak in the number of fires and the burnt area was in April–May, during a relatively short dry period immediately following the snowmelt. In northern Sweden, fires in the second half of fire season dominated the total annual area burnt. Analyzed indices differed considerably in their predictive power in respect to counties’ records of annual area burnt. Calibrated PDSI was a superior proxy of fire activity for the southern region (R2 = 60.8% in regression against total annual area burnt for respective provinces), and DIlate (Drought Index for the first half of the growing season) was superior for the northern counties (R2 = 73.3%). Predictive power of the indices was much higher for the recent period (1996–2002), with R2 values staying within 81.2 and 97.8%. Even if modern levels of forest fire activity in Sweden are very low from historical perspective, there is a strong spatiotemporal association between fire activity and climatic variability at regional scales, which provides a basis for modeling of the future fire hazard.
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29.
  • Drobyshev, Igor, et al. (författare)
  • Reconstruction of a 253-year long mast record of European beech reveals its association with large scale temperature variability and no long-term trend in mast frequencies
  • 2014
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 192, s. 9-17
  • Tidskriftsartikel (refereegranskat)abstract
    • Synchronous production of large seed crops, or mast years (MYs), is a common feature of many Fagus species, which is closely linked to the dynamics of forest ecosystems, including regeneration of canopy trees and changes in animal population densities. To better understand its climatic controls and check for the presence of long-term temporal trends in MY frequencies, we reconstructed MY record of the European beech (Fagus sylvatica L.) for the southern Swedish province of Halland over 1753-2006. We used superimposed epoch analysis (SEA) to relate MY (a) to summer temperature fields over the European subcontinent and (b) to the patterns of 500 mb geopotential heights over the 35-75 degrees N. For the MY reconstruction, we used newly developed regional beech ring-width chronology (1753-2006), an available summer temperature reconstruction, and a discontinuous historical MY record. A Monte Carlo experiment allowed identification of the thresholds in both growth and summer temperature anomalies, indicative of historical MYs, which were verified by dividing data into temporally independent calibration and verification sub-periods.MYs were strongly associated with both the 500 mb height anomalies and average summer temperatures during two years preceding a MY: a mast year (t) followed a cold summer two years (t-2) prior to the mast year and a warm summer one year prior (t-1) to the mast year. During t-2 years, the geographical pattern of 500 mb height anomalies exhibited a strong height depression in the region centered in the Northern Sea and extending toward eastern North America and statistically significant (p<0.05) temperature anomalies covering predominantly southern Scandinavia (area below 60 N) and British Isles. A year immediately preceding a mast year (t-1) was characterized by a strong regional high pressure anomaly centered in southern Scandinavia with significant temperature anomalies extended mostly over southern Scandinavia and Germany.The long-term mean MY return interval was 6.3 years, with 50 and 90% probabilities of MY occurrence corresponding to 6 and 15 years, respectively. Periods with intervals significantly shorter than the long-term mean were observed around 1820-1860 and 1990-2006 (means 3.9 and 3.2 years, respectively). However, the difference in return intervals between two sub-periods themselves was not significant.Geographically large and temporally rapid changes in atmospheric circulation among years, responsible for summer temperature conditions in the Northern Europe, are likely primary environmental drivers of masting phenomenon. However, decadal and centurial variability in MY intervals is difficult to relate directly to temperature variability, suggesting the presence of conditions "canceling" would-be MYs. Long-term MY reconstruction demonstrates high variability of reproductive behavior in European beech and indicates that a period with shorter MY intervals at the end of 20th may be not unique in a multi-century perspective. (C) 2014 Elsevier B.V. All rights reserved.
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30.
  • 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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31.
  • Drollinger, Simon (författare)
  • Interannual and seasonal variability in carbon dioxide and methane fluxes of a pine peat bog in the Eastern Alps, Austria
  • 2019
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 275, s. 69-78
  • Tidskriftsartikel (refereegranskat)abstract
    • Intact peat bogs are carbon dioxide (CO2) sinks and methane (CH4) sources. Facing drought and drainage, they may turn into CO2 sources and decreased CH4 sources. Information on the CO2 and CH4 exchange of alpine peat bogs in Central Europe has been missing so far. Here, we present data from two years of CO2 and CH4 exchange between an alpine low-shrub pine bog in the Eastern Alps and the atmosphere using the eddy covariance method. The annual net CO2 ecosystem exchange of the peatland differs substantially between the two measurement years, with -24 +/- 13 g C m(-2) yr(-1) for the drought affected first year and -84 +/- 13 g C m(-2) yr(-1) for the more humid second year. We found ecosystem respiration (R-eco) to depend on variations in soil temperature and soil moisture, and gross primary production (GPP) to be strongly linked to net radiation and daylength. The summer drought in 2015 shifted the peatland from a C sink to a C source, as increases in R-eco clearly exceeded enhanced GPP. Annual CH4 emission was 4.40 +/- 2.40 g C m(-2) yr(-1) during the drought-affected year and 5.24 +/- 2.57 g C m(-2) yr(-1) during the wetter year. Summer CH4 fluxes contribute 44% to the annual balance, followed by autumn (27%), spring (20%) and winter season fluxes (9%). CH4 fluxes most strongly depend on soil temperatures, soil moisture effects increase at smaller time-scales. Annual CH4 emissions are low compared to other temperate bogs, which most likely is the result of the ongoing degradation, indicated by a shift in vegetation composition. Net flux of both greenhouse gases was positive in the first year (+75 g CO2-eq m(-2)) and negative in the second year (-110 g CO2-eq m(-2)). Our results indicate that drought events and seasonal and interannual variations in temperature and precipitation strongly affect the C cycle of alpine peat bogs.
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32.
  • Eckersten, Henrik, et al. (författare)
  • Management and spatial resolution effects on yield and water balance at regional scale in crop models
  • 2019
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 275, s. 184-195
  • Tidskriftsartikel (refereegranskat)abstract
    • Due to the more frequent use of crop models at regional and national scale, the effects of spatial data input resolution have gained increased attention. However, little is known about the influence of variability in crop management on model outputs. A constant and uniform crop management is often considered over the simulated area and period. This study determines the influence of crop management adapted to climatic conditions and input data resolution on regional-scale outputs of crop models. For this purpose, winter wheat and maize were simulated over 30 years with spatially and temporally uniform management or adaptive management for North Rhine-Westphalia ((similar to)34 083 km(2)), Germany. Adaptive management to local climatic conditions was used for 1) sowing date, 2) N fertilization dates, 3) N amounts, and 4) crop cycle length. Therefore, the models were applied with four different management sets for each crop. Input data for climate, soil and management were selected at five resolutions, from 1 x 1 km to 100 x 100 km grid size. Overall, 11 crop models were used to predict regional mean crop yield, actual evapotranspiration, and drainage. Adaptive management had little effect (< 10% difference) on the 30-year mean of the three output variables for most models and did not depend on soil, climate, and management resolution. Nevertheless, the effect was substantial for certain models, up to 31% on yield, 27% on evapotranspiration, and 12% on drainage compared to the uniform management reference. In general, effects were stronger on yield than on evapotranspiration and drainage, which had little sensitivity to changes in management. Scaling effects were generally lower than management effects on yield and evapotranspiration as opposed to drainage. Despite this trend, sensitivity to management and scaling varied greatly among the models. At the annual scale, effects were stronger in certain years, particularly the management effect on yield. These results imply that depending on the model, the representation of management should be carefully chosen, particularly when simulating yields and for predictions on annual scale.
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33.
  • Eriksson, Helena, et al. (författare)
  • Estimating LAI in deciduous forest stands
  • 2005
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 129:1-2, s. 27-37
  • Tidskriftsartikel (refereegranskat)abstract
    • The leaf area index (LAI), was estimated in deciduous forests of southern Sweden using different estimation techniques. The effective LAI (L-e) was estimated optically by measurements with the gap fraction instrument LAI-2000 and corrections for the aggregation of leaves were achieved by using the TRAC (Tracing Radiation and Architecture of Canopies) instrument. Two methods for correcting the contribution of the woody material were tested. The aim was to find the most reliable estimation technique, and therefore the measurements were compared with LAI estimated from litter fall collection. Optically estimated L-e was correlated with litter trap LAI, indicating that L-e can be used as an estimate of LAI in deciduous forests of the same characteristics as tested here. The litter fall collection technique involves knowledge about the specific leaf area (SLA). Since, we found that SLA varied between leaves picked from top positioned branches and leaves picked from low positioned branches as well as between different species, we suggest some caution when interpreting the values.
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34.
  • 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.
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35.
  • Feigenwinter, Christian, et al. (författare)
  • Comparison of horizontal and vertical advective CO2 fluxes at three forest sites
  • 2008
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 148:1, s. 12-24
  • Tidskriftsartikel (refereegranskat)abstract
    • Extensive field measurements have been performed at three CarboEurope-Integrated Project forest sites with different topography (Renon/Ritten, Italian Alps, Italy; Wetzstein, Thuringia, Germany; Norunda, Uppland, Sweden) to evaluate the relevant terms of the carbon balance by measuring CO2 concentrations [CO2] and the wind field in a 3D multi-tower cube setup. The same experimental setup (geometry and instrumentation) and the same methodology were applied to all the three experiments. It is shown that all sites are affected by advection in different ways and strengths. Everywhere, vertical advection (F-VA) occurred only at night. During the day, F-VA disappeared because of turbulent mixing, leading to a uniform vertical profile of [CO2]. Mean F-VA was nearly zero at the hilly site (wetzstein) and at the flat site (Norunda). However, large, momentary positive or negative contributions occurred at the flat site, whereas vertical non-turbulent fluxes were generally very small at the hilly site. At the slope site (Renon), F-VA was always positive at night because of the permanently negative mean vertical wind component resulting from downslope winds. Horizontal advection also occurred mainly at night. It was positive at the slope site and negative at the flat site in the mean diurnal course. The size of the averaged non-turbulent advective fluxes was of the same order of magnitude as the turbulent flux measured by eddy-covariance technique, but the scatter was very high. This implies that it is not advisable to use directly measured quantities of the non-turbulent advective fluxes for the estimation of net ecosystem exchange (NEE) on e.g. an hourly basis. However, situations with and without advection were closely related to local or synoptic meteorological conditions. Thus, it is possible to separate advection affected NEE estimates from fluxes which are representative of the source term. However, the development of a robust correction scheme for advection requires a more detailed site-specific analysis of single events for the identification of the relevant processes. This paper presents mean characteristics of the advective CO2 fluxes in a first site-to-site comparison and evaluates the main problems for future research.
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36.
  • Feigenwinter, Christian, et al. (författare)
  • Spatiotemporal evolution of CO2 concentration, temperature, and wind field during stable nights at the Norunda forest site
  • 2010
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 150:5, s. 692-701
  • Tidskriftsartikel (refereegranskat)abstract
    • Unusually high CO2 concentrations were frequently observed during stable nights in late summer 2006 at the CarboEurope-Integrated Project (CEIP) forest site in Norunda, Sweden. Mean CO2 concentrations in the layer below the height of the eddy-covariance measurement system at 30 m reached up to 500 mu mol mol(-1) and large vertical and horizontal gradients occurred, leading to very large advective fluxes with a high variability in size and direction. CO2 accumulation was found to build up in the second part of the night, when the stratification in the canopy sub-layer turned from stable to neutral. Largest vertical gradients of temperature and CO2 were shifted from close to the ground early in the night to the crown space of the forest late at night, decoupling the canopy sub-layer from the surface roughness layer. At the top of the canopy at 25 m CO2 concentrations up to 480 mu mol mol(-1) were observed at all four tower locations of the 3D cube setup and concentrations were still high (>400 mu mol mol(-1)) at the 100 m level of the Central tower. The vertical profiles of horizontal advective fluxes during the nights under investigation were similar and showed largest negative horizontal advection (equivalent to an additional CO2-sink) to occur in the crown space of the forest, and not, as usually expected, close to the ground. The magnitude of these fluxes was sometimes larger than 50 mu mol m(-2) s(-1)and they were caused by the large horizontal CO2 concentration gradients with maximum values of up to 1 mu mol mol(-1) m(-1). As a result of these high within canopy CO2 concentrations, the vertical advection also became large with frequent changes of direction according to the sign of the mean vertical wind component, which showed very small values scattering around zero. Inaccuracy of the sonic anemometer at such low wind velocities is the reason for uncertainty in vertical advection, whereas for horizontal advection, instrument errors were small compared to the fluxes. The advective fluxes during these nights were unusually high and it is not clear what they represent in relation to the biotic fluxes. Advection is most likely a scale overlapping process. With a control volume of about 100 m x 100 m x 30 m and the applied spatial resolution of the sensors, we obviously miss relevant information from processes in the mesoscale as well as in the turbulent scale. (C) 2009 Elsevier B.V. All rights reserved.
  •  
37.
  • Gioli, B, et al. (författare)
  • Comparison between tower and aircraft-based eddy covariance fluxes in five European regions
  • 2004
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 127:1-2, s. 1-16
  • Tidskriftsartikel (refereegranskat)abstract
    • Airborne eddy covariance measurements provide a unique opportunity to directly measure surface energy, mass and momentum fluxes at the regional scale. This offers the possibility to complement the data that are obtained by the ground-based eddy covariance networks and to validate estimates of the surface fluxes that can be obtained by means of satellite products and models. The overall accuracy and the reliability of airborne eddy covariance measurements have already been assessed in the past for different platforms. More recently an international collaboration between several research laboratories and a European aeronautical manufacturer led to the development of a new small environmental research aircraft, called the Sky Arrow ERA (Environmental Research Aircraft). This aircraft has been used in the framework of the European Research Project RECAB (Regional Assessment and Modelling of the Carbon Balance in Europe), that is part of the CarboEurope projects cluster, to measure surface mass and energy exchange at five different European locations. An extensive comparison between airborne and ground-based flux data at seven flux measurement sites, showed the overall matching between airborne and tower data. While friction velocity and latent heat flux estimates made by airborne and tower data were comparable at all sites and under whatever conditions, substantial and consistent underestimation of CO2 (28% on average) and sensible heat fluxes (35% on average) was observed. Differences in the aircraft and tower footprint and flux divergence with height explained most of the discrepancies. (C) 2004 Elsevier B.V. All rights reserved.
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38.
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39.
  • Gottschalk, L, et al. (författare)
  • Scale aggregation - comparison of flux estimates from NOPEX
  • 1999
  • Ingår i: AGRICULTURAL AND FOREST METEOROLOGY. - : ELSEVIER SCIENCE BV. - 0168-1923. ; 98-9, s. 103-119
  • Tidskriftsartikel (refereegranskat)abstract
    • The NOPEX two concentrated field efforts (CFEs) (June 1994 and April-July 1995) provide high quality data sets for the Boreal environment. The analysis of these data with traditional meteorological and hydrological approaches allow estimations of fluxes o
  •  
40.
  • Granda, Elena, et al. (författare)
  • Aged but withstanding : Maintenance of growth rates in old pines is not related to enhanced water-use efficiency
  • 2017
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 243, s. 43-54
  • Tidskriftsartikel (refereegranskat)abstract
    • Growth of old trees in cold-limited forests may benefit from recent climate warming and rising atmospheric CO2 concentrations (c(a)) if age-related constraints do not impair wood formation. To test this hypothesis, we studied old Mountain pine trees at three Pyrenean high-elevation forests subjected to cold-wet (ORD, AIG) or warmer drier (PED) conditions. We analyzed long-term trends (1450-2008) in growth (BAI, basal area increment), maximum (MXD) and minimum (MID) wood density, and tree-ring carbon (delta C-13) and oxygen (delta O-18) isotope composition, which were used as proxies for intrinsic water-use efficiency (iWUE) and stomatal conductance (g(s)), respectively. Old pines showed positive (AIG and ORD) or stable (PED) growth trends during the industrial period (since 1850) despite being older than 400 years. Growth and wood density covaried from 1850 onwards. In the cold-wet sites (AIG and ORD) enhanced photosynthesis through rising c(a) was likely responsible for the post-1850 iWUE improvement. However, uncoupling between BAI and iWUE indicated that increases in iWUE were not responsible for the higher growth but climate warming. A reduction in g(s) was inferred from increased delta O-18 for PED trees from 1960 onwards, the wannest site where the highest iWUE increase occurred (34%). This suggests that an emergent drought stress at warm-dry sites could trigger stomatal closure to avoid excessive transpiration. Overall, carbon acquisition as lasting woody pools is expected to be maintained in aged trees from cold and high-elevation sites where old forests constitute unique long-term carbon reservoirs.
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41.
  • Granier, A., et al. (författare)
  • Evidence for soil water control on carbon and water dynamics in European forests during the extremely dry year: 2003
  • 2007
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 143:1-2, s. 123-145
  • Tidskriftsartikel (refereegranskat)abstract
    • The drought of 2003 was exceptionally severe in many regions of Europe, both in duration and in intensity. In some areas, especially in Germany and France, it was the strongest drought for the last 50 years, lasting for more than 6 months. We used continuous carbon and water flux measurements at 12 European monitoring sites covering various forest ecosystem types and a large climatic range in order to characterise the consequences of this drought on ecosystems functioning. As soil water content in the root zone was only monitored in a few sites, a daily water balance model was implemented at each stand to estimate the water balance terms: trees and understorey transpiration, rainfall interception, throughfall, drainage in the different soil layers and soil water content. This model calculated the onset date, duration and intensity of the soil water shortage (called water stress) using measured climate and site properties: leaf area index and phenology that both determine tree transpiration and rainfall interception, soil characteristics and root distribution, both influencing water absorption and drainage. At sites where soil water content was measured, we observed a good agreement between measured and modelled soil water content. Our analysis showed a wide spatial distribution of drought stress over Europe, with a maximum intensity within a large band extending from Portugal to NE Germany. Vapour fluxes in all the investigated sites were reduced by drought, due to stomatal closure, when the relative extractable water in soil (REW) dropped below ca. 0.4. Rainfall events during the drought, however, typically induced rapid restoration of vapour fluxes. Similar to the water vapour fluxes, the net ecosystem production decreased with increasing water stress at all the sites. Both gross primary production (GPP) and total ecosystem respiration (TER) also decreased when REW dropped below 0.4 and 0.2, for GPP and TER, respectively. A higher sensitivity to drought was found in the beech, and surprisingly, in the broadleaved Mediterranean forests; the coniferous stands (spruce and pine) appeared to be less drought-sensitive. The effect of drought on tree growth was also large at the three sites where the annual tree growth was measured. Especially in beech, this growth reduction was more pronounced in the year following the drought (2004). Such lag effects on tree growth should be considered an important feature in forest ecosystems, which may enhance vulnerability to more frequent climate extremes.
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42.
  • 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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43.
  • Greiser, Caroline, et al. (författare)
  • Monthly microclimate models in a managed boreal forest landscape
  • 2018
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 250-251, s. 147-158
  • Tidskriftsartikel (refereegranskat)abstract
    • The majority of microclimate studies have been done in topographically complex landscapes to quantify and predict how near-ground temperatures vary as a function of terrain properties. However, in forests understory temperatures can be strongly influenced also by vegetation. We quantified the relative influence of vegetation features and physiography (topography and moisture-related variables) on understory temperatures in managed boreal forests in central Sweden. We used a multivariate regression approach to relate near-ground temperature of 203 loggers over the snow-free seasons in an area of ∼16,000 km2 to remotely sensed and on-site measured variables of forest structure and physiography. We produced climate grids of monthly minimum and maximum temperatures at 25 m resolution by using only remotely sensed and mapped predictors. The quality and predictions of the models containing only remotely sensed predictors (MAP models) were compared with the models containing also on-site measured predictors (OS models). Our data suggest that during the warm season, where landscape microclimate variability is largest, canopy cover and basal area were the most important microclimatic drivers for both minimum and maximum temperatures, while physiographic drivers (mainly elevation) dominated maximum temperatures during autumn and early winter. The MAP models were able to reproduce findings from the OS models but tended to underestimate high and overestimate low temperatures. Including important microclimatic drivers, particularly soil moisture, that are yet lacking in a mapped form should improve the microclimate maps. Because of the dynamic nature of managed forests, continuous updates of mapped forest structure parameters are needed to accurately predict temperatures. Our results suggest that forest management (e.g. stand size, structure and composition) and conservation may play a key role in amplifying or impeding the effects of climate-forcing factors on near-ground temperature and may locally modify the impact of global warming.
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44.
  • 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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45.
  • 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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46.
  • Grelle, Achim (författare)
  • Improved trace gas flux estimation through IRGA sampling optimization
  • 2009
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 149, s. 623 – 638-
  • Tidskriftsartikel (refereegranskat)abstract
    • We examine the theoretical and practical aspects of improving the sampling methods of spectroscopic trace gas sensors of Eddy covariance flux measurement systems. Theory is developed based on non-ideal ventilation devices and existing equations for tube flow and attenuation of non-reactive trace gases and temperature. Model results indicate an optimum design exists which can be expressed in relation to intake tube diameter and which depends upon the ventilation device employed. Field experiment results (employing modified open path IRGAs) show that the use of short intake tubes can reduce flux losses by trace gas signal attenuation while minimizing the adjustments required for density fluctuations, with additional benefits of increased data capture under adverse environmental conditions. (C) 2008 Elsevier B.V. All rights reserved.
  •  
47.
  • Grelle, A, et al. (författare)
  • Seasonal variation of boreal forest surface conductance and evaporation
  • 1999
  • Ingår i: AGRICULTURAL AND FOREST METEOROLOGY. - : ELSEVIER SCIENCE BV. - 0168-1923. ; 98-9, s. 563-578
  • Tidskriftsartikel (refereegranskat)abstract
    • Long term measurements (June 1994 to December 1996) of evaporation were made in a boreal forest in central Sweden. Fluxes were measured continuously with eddy-correlation systems from a 100 m tower. Surface conductance and potential evaporation were estim
  •  
48.
  • Grewling, L., et al. (författare)
  • Variation in Artemisia pollen seasons in Central and Eastern Europe
  • 2012
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 160, s. 48-59
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper aims to address some gaps in current knowledge by studying temporal and spatial variations in Artemisia pollen counts (2000-2009) at 13 sites located in different biogeographical areas of Central and Eastern Europe. Analysis showed that start dates of Artemisia pollen seasons are greatly dependent on temperature during June and July, with hot summer temperatures having a tendency to delay summer flowering. However, this relationship is not linear and the rate at which seasons become later increases when mean minimum June-July temperatures reach a threshold of about 13 degrees C. No explanation for variations in pollen season intensity could be found. The geographical location or amount of urbanisation did not influence, either positively or negatively, the seasonal pollen index. Second peaks in Artemisia pollen seasons can be described as the pollen seasons of late flowering Artemisia species, and mainly occurred in the geographical area south of the Carpathian Mountains. These second peaks can significantly influence the seasonal pollen index, contributing over 50% to the season's total Artemisia pollen recorded at one site. (C) 2012 Elsevier B.V. All rights reserved.
  •  
49.
  • Groenendijk, M., et al. (författare)
  • Assessing parameter variability in a photosynthesis model within and between plant functional types using global Fluxnet eddy covariance data
  • 2011
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 151:1, s. 22-38
  • Tidskriftsartikel (refereegranskat)abstract
    • The vegetation component in climate models has advanced since the late 1960s from a uniform prescription of surface parameters to plant functional types (PFTs) PFTs are used in global land-surface models to provide parameter values for every model grid cell With a simple photosynthesis model we derive parameters for all site years within the Fluxnet eddy covariance data set We compare the model parameters within and between PFTs and statistically group the sites Fluxnet data is used to validate the photosynthesis model parameter variation within a PFT classification Our major result is that model parameters appear more variable than assumed in PFTs Simulated fluxes are of higher quality when model parameters of individual sites or site years are used A simplification with less variation in model parameters results in poorer simulations This indicates that a PFT classification Introduces uncertainty in the variation of the photosynthesis and transpiration fluxes Statistically derived groups of sites with comparable model parameters do not share common vegetation types or climates A simple PFT classification does not reflect the real photosynthesis and transpiration variation Although site year parameters give the best predictions the parameters are generally too specific to be used in a global study The site year parameters can be further used to explore the possibilities of alternative classification schemes (C) 2010 Elsevier B V All rights reserved
  •  
50.
  • Hadden, David, et al. (författare)
  • Changing temperature response of respiration turns boreal forest from carbon sink into carbon source
  • 2016
  • Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 223, s. 30-38
  • Tidskriftsartikel (refereegranskat)abstract
    • Seventeen years (1997-2013) of carbon dioxide (CO2) fluxes were measured in a boreal forest stand in northern Sweden using the eddy covariance technique. During the measurement period the forest turned from a net carbon sink into a net carbon source. The net ecosystem exchange (NEE) was separated using values from periods of darkness into the gross components of total ecosystem respiration (TER) and gross primary productivity (GPP), which was calculated as GPP=-NEE + TER. From the gross components we could determine that an increase in TER during the autumn (September to end of November) and spring (March to end of May) periods resulted in the forest becoming a net source of CO2, We observed no increase in the GPP from the eddy covariance measurements. This was further supported by measurements of tree growth rings. The increased TER was attributed to a change in the forest's temperature response at lower temperatures (-5 to 10 degrees C) rather than to a temperature increase. This study shows that changes in ecosystem functioning can have a larger impact on the carbon balance than climate warming per se. (C) 2016 Elsevier B.V. All rights reserved.
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