| 1. |
- Arnqvist, Johan, et al.
(author)
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Examination of the mechanism behind observed canopy waves
- 2016
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 218, s. 196-203
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Journal article (peer-reviewed)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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| 2. |
- Boudreault, Louis-Etienne, et al.
(author)
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A LiDAR method of canopy structure retrieval for wind modeling of heterogeneous forests
- 2015
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 201, s. 86-97
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Journal article (peer-reviewed)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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| 3. |
- Bylund, Helena
(author)
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Larval crowding during an insect outbreak reduces herbivory pressure on preferred shrubs in a warmer environment
- 2018
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 263, s. 180-187
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Journal article (peer-reviewed)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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| 4. |
- Chi, Jinshu
(author)
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Adjustment of CO2 flux measurements due to the bias in the EC150 infrared gas analyzer
- 2019
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 276
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Journal article (peer-reviewed)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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| 5. |
- Chi, Jinshu, et al.
(author)
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The carbon balance of a managed boreal landscape measured from a tall tower in northern Sweden
- 2019
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 274, s. 29-41
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Journal article (peer-reviewed)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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| 6. |
- Drollinger, Simon
(author)
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Interannual and seasonal variability in carbon dioxide and methane fluxes of a pine peat bog in the Eastern Alps, Austria
- 2019
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 275, s. 69-78
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Journal article (peer-reviewed)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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| 7. |
- Eckersten, Henrik, et al.
(author)
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Management and spatial resolution effects on yield and water balance at regional scale in crop models
- 2019
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 275, s. 184-195
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Journal article (peer-reviewed)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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| 8. |
- Granda, Elena, et al.
(author)
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Aged but withstanding : Maintenance of growth rates in old pines is not related to enhanced water-use efficiency
- 2017
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 243, s. 43-54
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Journal article (peer-reviewed)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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| 9. |
- Greiser, Caroline, et al.
(author)
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Monthly microclimate models in a managed boreal forest landscape
- 2018
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 250-251, s. 147-158
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Journal article (peer-reviewed)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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| 10. |
- Hadden, David, et al.
(author)
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Changing temperature response of respiration turns boreal forest from carbon sink into carbon source
- 2016
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 223, s. 30-38
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Journal article (peer-reviewed)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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| 11. |
- Hadden, David, et al.
(author)
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The impact of cultivation on CO2 and CH4 fluxes over organic soils in Sweden
- 2017
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 243, s. 1-8
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Journal article (peer-reviewed)abstract
- Cultivated organic soils are large contributors to global greenhouse gas emissions. We measured carbon dioxide (CO2) and methane (CH4) fluxes from two organic soil plots in central Sweden between 2012 and 2016 using the eddy covariance technique. One site was cultivated whilst the other was set aside with no cultivation occurring. The aim was to establish the impact that cultivation had on the carbon balance. Over the five year period the set aside field acted as a net carbon source whilst the cultivated site was seen to be a small carbon sink. The set aside was further seen to act as a small methane sink over a 36 month period, which reduced the CO2 source by ca. 10% in terms of CO2 equivalents. Considering exported biomass, both sites were carbon sources in terms of Net Biome Production (NBP).
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| 12. |
- Jocher, Georg, et al.
(author)
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Apparent Winter CO2 uptake by a boreal forest due to decoupling
- 2017
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 232, s. 23-34
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Journal article (peer-reviewed)abstract
- Net uptake of carbon dioxide (CO2) was observed during the winter when using the eddy covariance (EC) technique above a 90-year-old Scots pine (Pinus sylvestris L.) stand in northern Sweden. This uptake occurred despite photosynthetic dormancy. This discrepancy led us to investigate the potential impact of decoupling of below- and above-canopy air mass flow and accompanying below-canopy horizontal advection on these measurements. We used the correlation of above- and below-canopy standard deviation of vertical wind speed (sigma(w)), derived from EC measurements above and below the canopy, as the main mixing criterion. We identified 0.33 m s(-1) and 0.06 m s(-1) as site-specific o thresholds for above and below canopy, respectively, to reach the fully coupled state. Decoupling was observed in 45% of all cases during the measurement period (5.11.2014-25.2.2015). After filtering out decoupled periods the above-canopy mean winter NEE shifted from -0.52 mu mol m(-2) s(-1) to a more reasonable positive value of 0.31 mu mol m(-2) s(-1). None of the above-canopy data filtering criteria we tested (i.e., friction velocity threshold; horizontal wind speed threshold; single-level sigma(w) threshold) ensured sufficient mixing. All missed critical periods that were detected only by the two-level filtering approach. Tower-surrounding topography induced a predominant below-canopy wind direction and consequent wind shear between above- and below-canopy air masses. These processes may foster decoupling and below-canopy removal of CO2 rich air. To determine how broadly such a topographical influence might apply, we compared the topography surrounding our tower to that surrounding other forest flux sites worldwide. Medians of maximum elevation differences within 300m and 1000 m around 110 FLUXNET forest EC towers were 24 m and 66 m, respectively, compared to 24 m and 114 m, respectively, at our site. Consequently, below canopy flow may influence above-canopy NEE detections at many forested EC sites. Based on our findings we suggest below-canopy measurements as standard procedure at sites evaluating forest CO2 budgets.
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| 13. |
- Lindfors, Lauri
(author)
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A mechanistic model of winter stem diameter dynamics reveals the time constant of diameter changes and the elastic modulus across tissues and species
- 2019
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 272, s. 20-29
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Journal article (peer-reviewed)abstract
- When freezing occurs in trees in the autumn living cells are able to withstand the rapid dehydration that they experience due to the chemical properties of ice that fills apoplastic spaces. Elasticity and hydraulic permeability of living tissue are important properties influencing the frost tolerance of trees, because of their effects on how rapidly the dehydration occurs. Stem diameter change measurements have been used to quantify these tissue properties during the summer, but no such attempt has yet been made during winter when the apoplast is frozen. Here the dynamics of xylem, phloem and whole stem diameter changes for three tree species were simulated by formulating a mechanistic model using temperature of ice as a driver for water exchange between symplast and apoplast in accordance with theory of extracellular freezing. Hence in our model formulation, a decrease in the temperature of the apoplastic ice leads to a decrease in the apoplastic water potential drawing water from the living cells, which could be also observed in diameter change measurements in the field. The model was successful in explaining diameter changes in the case of all tree species when modelled diameter changes were fitted against measured diameter changes. Estimates for the elastic modulus, the time constant of diameter changes and the hydraulic permeability at tissue level were obtained by the fitting of three model parameters. The elastic modulus of the living tissue was found to exhibit a negative temperature dependency, while the time constant of diameter changes was found to exhibit a positive temperature dependency. Overall, this modelling approach offers an easy and non-destructive way of gaining valuable information about physiological properties of trees and their tissues in the winter. However, a complete understanding of the parameters estimated by the model requires further investigation into the physical processes that result in winter diameter changes.
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| 14. |
- Marshall, John
(author)
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Forest productivity varies with soil moisture more than temperature in a small montane watershed
- 2018
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 259, s. 211-221
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Journal article (peer-reviewed)abstract
- Mountainous terrain creates variability in microclimate, including nocturnal cold air drainage and resultant temperature inversions. Driven by the elevational temperature gradient, vapor pressure deficit (VPD) also varies with elevation. Soil depth and moisture availability often increase from ridgetop to valley bottom. These variations complicate predictions of forest productivity and other biological responses. We analyzed spatiotemporal air temperature (T) and VPD variations in a forested, 27-km(2) catchment that varied from 1000 to 1650 m in elevation. Temperature inversions occurred on 76% of mornings in the growing season. The inversion had a clear upper boundary at midslope (similar to 1370 m a.s.l.). Vapor pressure was relatively constant across elevations, therefore VPD was mainly controlled by T in the watershed. We assessed the impact of microclimate and soil moisture on tree height, forest productivity, and carbon stable isotopes (delta C-13) using a physiological forest growth model (3-PG). Simulated productivity and tree height were tested against observations derived from lidar data. The effects on photosynthetic gas-exchange of dramatic elevational variations in T and VPD largely cancelled as higher temperature (increasing productivity) accompanies higher VPD (reducing productivity). Although it was not measured, the simulations suggested that realistic elevational variations in soil moisture predicted the observed decline in productivity with elevation. Therefore, in this watershed, the model parameterization should have emphasized soil moisture rather than precise descriptions of temperature inversions.
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| 15. |
- Matthews, B., et al.
(author)
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Transpiration deficits increase host susceptibility to bark beetle attack : Experimental observations and practical outcomes for Ips typographus hazard assessment
- 2018
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In: Agricultural and Forest Meteorology. - : Elsevier B.V.. - 0168-1923 .- 1873-2240. ; 263, s. 69-89
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Journal article (peer-reviewed)abstract
- The projected increase in the frequency and severity with which bark beetle disturbances occur is forecasted to be partially driven by increases in drought episodes. Drought is widely considered to predispose host conifer trees to bark beetle attack; however, experimental data supporting this hypothesis are scarce. This study revisits the Rosalia Roof Project, the first throughfall manipulation experiment to investigate how attack by the Eurasian spruce bark beetle (Ips typographus) on mature Norway spruce (Picea abies) trees is affected by drought stress. Using the in situ “attack box” method, this study explores whether increased host acceptance by I. typographus and/or reduced host defense against attack coincide with increased tree transpiration deficits (i.e. the reduction from a potential transpiration caused by soil water limitation). To estimate transpiration deficits of the respective control and drought stress-induced (full-cover) trees, sap flow measurements were combined with simulations from a simple forest water balance routine. The model, which was calibrated against in situ hydrological measurements, has been developed for a hazard rating tool (PHENIPS-TDEF) which simulates both potential I. typographus phenology and tree drought stress in Norway spruce stands. While host acceptance appeared unaffected by tree transpiration deficits, acute and chronic transpiration deficits did lead to reduced host defense. Full cover trees for instance, which experienced an estimated 93 mm transpiration deficit in the previous May-Sep, could only defend against <10% of the total individual attack attempts between spring and midsummer compared to the control trees which experienced a corresponding deficit of 9 mm and defended >70% of attacks. However, similar defended attack percentages on the full-cover and control trees during late summer demonstrate the difficulty in deriving simple stress proxy-infestation risk relationships. The experiment therefore highlights the utility and limitations of transpiration deficits within I. typographus disturbance models and hazard assessment tools, such as PHENIPS-TDEF.
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| 16. |
- Oliva Palau, Jonàs
(author)
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Past logging, drought and pathogens interact and contribute to forest dieback
- 2015
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 208, s. 85-94
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Journal article (peer-reviewed)abstract
- Forest dieback is one of the most widespread responses to global-change drivers, such as climate warming-related drought stress and the spread of pathogens. Although both climatic and biotic stressors have been studied separately, much less is known on how drought and pathogens interact and induce dieback, particularly in formerly used forests. We determine the roles played by each of those drivers as factors causing recent dieback in three Pyrenean silver fir stands: a managed site subjected to past logging and two unmanaged sites not logged for the past 50 years. The age, size, recent competition, and basal-area increment (BAI) trends of non-declining and declining trees, and the presence of fungal pathogens were investigated. Growth patterns at yearly to decadal time scales were compared to distinguish the roles and interactions played by the different stressors. In the managed site, declining trees displayed low growth already before logging (1950-1970s). In both unmanaged sites, declining and non-declining trees displayed divergent growth patterns after extreme droughts, indicating that dieback was triggered by severe water deficit. We did not find indications that fungal pathogens are the primary drivers of dieback, since a low proportion of declining trees were infested by primary pathogens (10%). However, trees with the primary fungal pathogen Heterobasidion showed lower BAI than non-declining trees. On the other hand, the secondary fungal pathogen Amylostereum was isolated from a higher number of trees than expected by chance. These findings highlight the importance of legacies, such as the past use in driving recent forest dieback. Past forest use could predispose to dieback by selecting slow-growing trees and thus, making some them more vulnerable to drought and fungal pathogens. (C) 2015 Elsevier B.V. All rights reserved.
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| 17. |
- Olsson, Cecilia, et al.
(author)
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Budburst model performance: The effect of the spatial resolution of temperature data sets
- 2015
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 200, s. 302-312
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Journal article (peer-reviewed)abstract
- Phenological models have mainly been developed to capture the seasonal development of individual trees and local populations, using data from meteorological stations. Ecosystem models that incorporate phenology are however commonly driven by gridded climate data. Using two phenological models to simulate budburst of birch in Germany, we assessed how combining phenological point observations with gridded climate data in model calibration and evaluation influence model accuracy. The models were driven by observed temperature from a nearby meteorological station, gridded temperature, and observed and gridded temperature adjusted to the location of the tree. Our results indicate that the spatial resolution of temperature can influence the models performance at individual sites, but with no temperature data set generating significantly more accurate simulations than the other temperatures. Irrespective of temperature data, the model simulations represented the average of several trees better than any individual tree. When evaluating the models performance using point observations, the error became smaller when driving the model with adjusted temperature data, and then calculating grid-cell averages based on several observations. (C) 2014 Elsevier B.V. All rights reserved.
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| 18. |
- Oren, Ram
(author)
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The way the wind blows matters to ecosystem water use efficiency
- 2016
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 217, s. 1-9
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Journal article (peer-reviewed)abstract
- In many regions, atmospheric conditions change frequently with shifts of wind direction, extending maritime influences far inland or continental influences to coastal ecosystems. Climate models predict changes in both wind direction and velocity; these changes could potentially impact ecosystems mass and energy exchanges with the atmosphere. Using data on climate (wind speed and direction, air temperature and humidity) and ecosystem-scale fluxes (eddy-covariance evapotranspiration and CO2 flux) from the Mediterranean island of Sardinia, we evaluated whether the frequency of certain wind characteristics, potentially affecting ecosystem CO2 uptake, have changed over five decades, and whether these characteristics are indeed linked to ecosystem gas-exchange responses of the studied ecosystem. The analyses show that days dominated by summer Mistral winds decreased on average 3% per decade, and that wind direction affects biosphere-atmosphere exchange of carbon but not water. High velocity cool Mistral winds from continental Europe undergo limited thermal equilibration with the land surface after embarking on the land, trebling vapor pressure deficit (D) as they cross the island. In contrast, arriving with a similar D, lower velocity, warmer Saharan Sirocco winds heat up, thus increasing D 5-fold only 50 km inland. Over a mixed pasture-woodland (grass-wild olive), while soil moisture was low and constant, daytime net carbon exchange (NEEd) averaged 2.3-fold higher (p < 0.001) in Mistral than Sirocco days, largely reflecting the theoretically expected response of canopy conductance (g(c)) to variation of D. Because the product of g(c) and D encodes the key ecosystem compensatory mechanism, the reciprocal g(c)-D response maintained similar ecosystem evapotranspiration (E-e). Thus, summertime ecosystem water-use efficiency (W-e = NEEd/E-e) was similar to 66% higher during Mistral than other days. The historical decrease of Mistral frequency reduced the estimated summertime NEEd > 30%. The analyses demonstrate that alteration of dominance of air masses predicted with future climate will amplify or negate the positive effect of increased atmospheric [CO2] on We, and should be considered when assessing climate change impact on NEE. (C) 2015 Elsevier B.V. All rights reserved.
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| 19. |
- Palmroth, Sari, et al.
(author)
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Nitrogen supply and other controls of carbon uptake of understory vegetation in a boreal Picea abies forest
- 2019
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 276
-
Journal article (peer-reviewed)abstract
- In boreal forests, carbon (C) uptake by understory may be too large to be ignored and too variable in space to be assumed a constant fraction of the ecosystem gross primary production. To improve estimates of understory production in these ecosystems, we need to better account for its main controls. In this study, we estimated C uptake of field-layer vegetation, dominated by Vaccinium myrtillus, V. vitis-idaea, and Deschampsia flexuosa, in a boreal Picea abies stand in northern Sweden. Nitrogen (N) availability in the stand has been manipulated through annual N additions since 1996 at the rates of 0, 12.5, and 50 kg N ha(-1) yr(-1). To assess the relative importance of N supply, and interannual fluctuations in leaf biomass and weather, in controlling field-layer photosynthetic production, we calculated C uptake over eight growing seasons using a canopy photosynthesis model. Without N additions, tree leaf area index (L) was already high (8.5) and field-layer C uptake was small, 27 g Cm-2 (or similar to 3% of stand C uptake). An increase in tree L with N additions further reduced light availability for the understory, yet the concurrent increase in the relative abundance of the more physiologically active D. flexuosa sustained the contribution of the field-layer to stand photosynthetic production. Based on a literature survey, in which site quality or stand age generated a wide range in L, understory contribution to ecosystem C uptake increases linearly with the fraction of available light reaching the forest floor across high latitude forests. Understory contributes only similar to 5% to ecosystem C uptake where trees intercept similar to 80% of incoming light, increasing to 100% after clearcut tree harvest. While the availability of solar energy, both spatially and temporally, is the primary driver of understory production, our analyses suggest that the predicted increases in drought severity and frequency at high latitudes may affect understory communities more than trees. Future empirical and modeling studies should focus on functional and ecological responses to drought of not only trees but also understory species, which contribute to biodiversity and convert their photosynthates to important non-timber products.
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| 20. |
- Peichl, Matthias
(author)
-
Land surface phenology derived from normalized difference vegetation index (NDVI) at global FLUXNET sites
- 2017
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 233, s. 171-182
-
Journal article (peer-reviewed)abstract
- Phenology is an important indicator of annual plant growth and is also widely incorporated in ecosystem models to simulate interannual variability of ecosystem productivity under climate change. A comprehensive understanding of the potentials of current algorithms to detect the start and end for growing season (SOS and EOS) from remote sensing is still lacking. This is particularly true when considering the diverse interactions between phenology and climate change among plant functional types as well as potential influences from different sensors. Using data from 60 flux tower sites (376 site-years in total) from the global FLUXNET database, we applied four algorithms to extract plant phenology from time series of normalized difference vegetation index (NDVI) from both MODIS and SPOT-VGT sensors. Results showed that NDVI-simulated phenology had overall low correlation (R-2 <0.30) with flux-derived SOS/EOS observations, but this predictive strength substantially varied by fitting algorithm, sensor and plant functional type. Different fitting algorithms can produce significantly different phenological estimates, but this difference can also be influenced by sensor type. SPOT-VGT simulated better EOS but no difference in the accuracy of SOS was found with different sensors. It may be due to increased frequency of data sampling.(1 0 days for SPOT-VGT vs. 16 days for MODIS) during spring season when rapid plant growth does not help SPOT-VGT more sensitive to growth. In contrast, more frequent data acquisition favors better modeling of plant growth in autumn when a gradual decrease in photosynthesis occurs. Our study results highlight that none of these algorithm can provide consistent good accuracy in modeling SOS and EOS with respect to both plant functional types and sensors. More importantly, a rigorous validation of phenology modeling against ground data is necessary before applying these algorithms at regional or global scales and consequently previous conclusions on regional SOS/EOS trends should be viewed with caution if independent validation is lacking. (C) 2016 Elsevier B.V. All rights reserved.
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| 21. |
- Peltola, O., et al.
(author)
-
Studying the spatial variability of methane flux with five eddy covariance towers of varying height
- 2015
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 214, s. 456-472
-
Journal article (peer-reviewed)abstract
- In this study, the spatial representativeness of eddy covariance (EC) methane (CH4) measurements was examined by comparing parallel CH4 fluxes from three short (6 m) towers separated by a few kilometres and from two higher levels (20 m and 60 m) at one location. The measurement campaign was held on an intensively managed grassland on peat soil in the Netherlands. The land use and land cover types are to a large degree homogeneous in the area. The CH4 fluxes exhibited significant variability between the sites on 30-min scale. The spatial coefficient of variation (CVspa) between the three short towers was 56% and it was of similar magnitude as the temporal variability, unlike for the other fluxes (friction velocity, sensible heat flux) for which the temporal variability was considerably larger than the spatial variability. The CVspa decreased with temporal averaging, although less than what could be expected for a purely random process (1/root N), and it was 14% for 26-day means of CH4 flux. This reflects the underlying heterogeneity of CH4 flux in the studied landscape at spatial scales ranging from 1 ha (flux footprint) to 10 km(2) (area bounded by the short towers). This heterogeneity should be taken into account when interpreting and comparing EC measurements. On an annual scale, the flux spatial variability contributed up to 50% of the uncertainty in CH4 emissions. It was further tested whether EC flux measurements at higher levels could be used to acquire a more accurate estimate of the spatially integrated CH4 emissions. Contrarily to what was expected, flux intensity was found to both increase and decrease depending on measurement height. Using footprint modelling, 56% of the variation between 6 m and 60 m CH4 fluxes was attributed to emissions from local anthropogenic hotspots (farms). Furthermore, morning hours proved to be demanding for the tall tower EC where fluxes at 60 m were up to four-fold those at lower heights. These differences were connected with the onset of convective mixing during the morning period. (C) 2015 The Authors. Published by Elsevier B.V.
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| 22. |
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| 23. |
- Rodríguez, A., et al.
(author)
-
Implications of crop model ensemble size and composition for estimates of adaptation effects and agreement of recommendations
- 2019
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 264, s. 351-362
-
Journal article (peer-reviewed)abstract
- Climate change is expected to severely affect cropping systems and food production in many parts of the world unless local adaptation can ameliorate these impacts. Ensembles of crop simulation models can be useful tools for assessing if proposed adaptation options are capable of achieving target yields, whilst also quantifying the share of uncertainty in the simulated crop impact resulting from the crop models themselves. Although some studies have analysed the influence of ensemble size on model outcomes, the effect of ensemble composition has not yet been properly appraised. Moreover, results and derived recommendations typically rely on averaged ensemble simulation results without accounting sufficiently for the spread of model outcomes. Therefore, we developed an Ensemble Outcome Agreement (EOA) index, which analyses the effect of changes in composition and size of a multi-model ensemble (MME) to evaluate the level of agreement between MME outcomes with respect to a given hypothesis (e.g. that adaptation measures result in positive crop responses). We analysed the recommendations of a previous study performed with an ensemble of 17 crop models and testing 54 adaptation options for rainfed winter wheat (Triticum aestivum L.) at Lleida (NE Spain) under perturbed conditions of temperature, precipitation and atmospheric CO2 concentration. Our results confirmed that most adaptations recommended in the previous study have a positive effect. However, we also showed that some options did not remain recommendable in specific conditions if different ensembles were considered. Using EOA, we were able to identify the adaptation options for which there is high confidence in their effectiveness at enhancing yields, even under severe climate perturbations. These include substituting spring wheat for winter wheat combined with earlier sowing dates and standard or longer duration cultivars, or introducing supplementary irrigation, the latter increasing EOA values in all cases. There is low confidence in recovering yields to baseline levels, although this target could be attained for some adaptation options under moderate climate perturbations. Recommendations derived from such robust results may provide crucial information for stakeholders seeking to implement adaptation measures.
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| 24. |
- Sundqvist, Elin, et al.
(author)
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Upscaling of methane exchange in a boreal forest using soil chamber measurements and high-resolution LiDAR elevation data
- 2015
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 214, s. 393-401
-
Journal article (peer-reviewed)abstract
- Forest soils are generally considered to be net sinks of methane (CH4), but CH4 fluxes vary spatially depending on soil conditions. Measuring CH4 exchange with chambers, which are commonly used for this purpose, might not result in representative fluxes at site scale. Appropriate methods for upscaling CH4 fluxes from point measurements to site scale are therefore needed. At the boreal forest research site, Norunda, chamber measurements of soils and vegetation indicate that the site is a net sink of CH4, while tower gradient measurements indicate that the site is a net source of CH4. We investigated the discrepancy between chamber and tower gradient measurements by upscaling soil CH4 exchange to a 100 ha area based on an empirical model derived from chamber measurements of CH4 exchange and measurements of soil moisture, soil temperature and water table depth. A digital elevation model (DEM) derived from high-resolution airborne Light Detection and Ranging (LiDAR) data was used to generate gridded water table depth and soil moisture data of the study area as input data for the upscaling. Despite the simplistic approach, modeled fluxes were significantly correlated to four out of five chambers with R>0.68. The upscaling resulted in a net soil sink of CH4 of -10 mu mol m(-2) h(-1), averaged over the entire study area and time period June-September, 2010). Our findings suggest that additional contributions from CH4 soil sources outside the upscaling study area and possibly CH4 emissions from vegetation could explain the net emissions measured by tower gradient measurements. (C) 2015 Elsevier B.V. All rights reserved.
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| 25. |
- Svystun, Tetiana, et al.
(author)
-
Modelling Populus autumn phenology : The importance of temperature and photoperiod
- 2019
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 271, s. 346-354
-
Journal article (peer-reviewed)abstract
- In late summer-early autumn, trees undergo growth cessation culminating in bud set. For a wide range of plants, including Populus, photoperiod is considered as the primary environmental cue determining timing of growth cessation and shoot to bud transition. However, studies on Populus have revealed temperature influence on seasonal growth cessation and bud set. In this study we examine the role of temperature in regulating the transition between phenological stages of bud development, from a growing apex to a closed bud. We test different model structures incorporating cues from both temperature and photoperiod (M1 and M2) as compared to the previously established model based on photoperiod only (M0, null hypothesis). All models simulate the date of bud set forPopulustrees from 12 latitudinal populations grown in the common garden of Sävar. For two of the three years (2005 and 2007) M1 and M2 models outperformed the null model in predicting bud set date. Poor predictions for 2006 were related to stressful weather conditions. This indicates that under non-stressful conditions temperature can be a factor modifying the photoperiod response, while other regulatory mechanisms can be at play during stressful conditions. A phenological model of bud development has to account for both types of responses. Thus, our study highlights the importance of temperature as a factor that should be considered in addition to the well-established photoperiodic signal in studies on autumnal bud set under natural conditions.
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| 26. |
- Tor-Ngern, Pantana, et al.
(author)
-
Water balance of pine forests: Synthesis of new and published results
- 2018
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 259, s. 107-117
-
Journal article (peer-reviewed)abstract
- The forest hydrologic cycle is expected to have important feedback responses to climate change, impacting processes ranging from local water supply and primary productivity to global water and energy cycles. Here, we analyzed water budgets of pine forests worldwide. We first estimated local water balance of forests dominated by two wide-ranging species: Pinus taeda (36 degrees N) and Pinus sylvestris (64 degrees N). In these stands, growing season evapotranspiration (ET) was largely insensitive to inter-annual variation of precipitation (P), consistent with the insensitivity of canopy transpiration to P. Extending the analyses to include published data from 117 studies on 27 pine species, we found that pine forests annually use -66% +/- 17% (SD) of P as ET, regardless of climatic regime, leaving a third of P as runoff to downstream aquatic ecosystems and users. However, during the growing season, pine forests used more water as ET than P in regions where P 5 326 39 (SE) mm. Forests in regions of low growing season P exist in their current state only where the rooting depth is sufficient to supply trees with water from soil storage in addition to P, and these forests are likely to support only ephemeral streams that dry down during the growing season. Thus, globally, water use by pine forests is adapted to mean annual P, but shows a limited capacity to respond to inter-annual variability in P. Forests with a small buffer of growing season water availability (P + soil water storage - E-T), are likely to be most sensitive to variation in P regimes, changing canopy leaf area, tree density, and species composition depending on the degree, direction and persistence of the change in P.
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| 27. |
- Wu, Mousong, et al.
(author)
-
Global parameters sensitivity analysis of modeling water, energy and carbon exchange of an arid agricultural ecosystem
- 2019
-
In: Agricultural and Forest Meteorology. - : Elsevier B.V.. - 0168-1923 .- 1873-2240. ; 271, s. 295-306
-
Journal article (peer-reviewed)abstract
- Agricultural ecosystems are important for regulating terrestrial hydrological and carbon cycles. Hydrological and carbon processes in agricultural ecosystem models are complex due to interactions between parameters. It is therefore crucial to identify parameter sensitivity before a process-based model is applied for simulations and predictions of water, energy and carbon fluxes in agricultural ecosystems. In this study, we investigated the sensitivity and equifinality of the CoupModel parameters in modeling an arid agricultural ecosystem in northwestern China. In total, 27 model parameters were analyzed using a global parameters sensitivity analysis approach and a combination of multiple in situ and remotely sensed data sets. Among the five major model processes, we found that the energy balance process account for much of the importance in the model, followed by soil hydrology, plant growth, soil heat, and soil carbon processes. Meanwhile, parameters from the plant growth process exhibited higher equifinalities than other processes. We found that net ecosystem exchange (NEE) is controlled by soil heat, soil hydrology and energy balance processes, which is mainly due to a high equifinality (0.91) between the parameters g max (maximal stomatal conductance) and V cmax (maximal carboxylation rate). The equifinalities between different parameters result in a trade-off in model performance metrics (i.e. determination coefficient R 2 and mean error ME) in the water, energy and carbon balance simulations. We revealed that daytime and yearly accumulated eddy fluxes (sensible heat H s , latent heat LE and NEE) can constrain the model parameters better. Remotely sensed data were also promising as additional constraints on soil water contents and energy fluxes. This study introduced a systematic global parameter sensitivity analysis approach together with the equifinality identification in an ecosystem model. The approach proposed here is applicable to other studies and the equifinalities detected in this study can be important implications for modelling arid agricultural ecosystems. Additional exploration on remotely sensed data in constraining the model from different aspects are highly recommended in modeling agricultural ecosystems.
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| 28. |
- Zhang, Quan, et al.
(author)
-
Changes in photosynthesis and soil moisture drive the seasonal soil respiration-temperature hysteresis relationship
- 2018
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 259, s. 184-195
-
Journal article (peer-reviewed)abstract
- In nearly all large-scale terrestrial ecosystem models, soil respiration is represented as a function of soil temperature. However, the relationship between soil respiration and soil temperature is highly variable across sites and there is often a pronounced hysteresis in the soil respiration-temperature relationship over the course of the growing season. This phenomenon indicates the importance of biophysical factors beyond just temperature in controlling soil respiration. To identify the potential mechanisms of the seasonal soil respiration-temperature hysteresis, we developed a set of numerical models to demonstrate how photosynthesis, soil moisture, and soil temperature, alone and in combination, affect the hysteresis relationship. Then, we used a variant of the model informed by observations of soil respiration, soil temperature, photosynthesis, and soil moisture from multiple mesic and semi-arid ecosystems to quantify the frequency of hysteresis and identify its potential controls. We show that the hysteresis can result from the seasonal cycle of photosynthesis (which supplies carbon to rhizosphere respiration), and soil moisture (which limits heterotrophic respiration when too low or too high). Using field observations of soil respiration, we found evidence of seasonal hysteresis in 9 out of 15 site-years across 8 diverse biomes. Specifically, clockwise hysteresis occurred when photosynthesis preceded seasonal soil temperature and counterclockwise hysteresis occurred when photosynthesis lagged soil temperature. We found that across all sites, much of the respiration-temperature lag was explained by the decoupling of photosynthesis and temperature, highlighting the importance of recently assimilated carbon to soil respiration. An analysis of observations from 129 FLUXNET sites revealed that time lags between gross primary productivity (a proxy for canopy photosynthesis) and soil temperature were common phenomena, which would tend to drive counterclockwise hysteresis at low-latitude sites and clockwise hysteresis at high-latitude sites. Collectively, our results show that incorporating photosynthesis and soil moisture in the standard exponential soil respiration-temperature model (i.e., Q(10) model) improves the explanatory power of models at local scales.
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| 29. |
- Zhang, Wenxin, et al.
(author)
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Model-data fusion to assess year-round CO2 fluxes for an arctic heath ecosystem in West Greenland (69°N)
- 2019
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 272-273, s. 176-186
-
Journal article (peer-reviewed)abstract
- Quantifying net CO2 exchange (NEE) of arctic terrestrial ecosystems in response to changes in climatic and environmental conditions is central to understanding ecosystem functioning and assessing potential feedbacks of the carbon cycle to future climate changes. However, annual CO2 budgets for arctic tundra are rare due to the difficulties of performing measurements during non-growing seasons. It is still unclear to what extent arctic tundra ecosystems currently act as a CO2 source, sink or are in balance. This study presents year-round eddy-covariance (EC) measurements of CO2 fluxes for an arctic heath ecosystem on Disko Island, West Greenland (69 °N) over five years. Based on a fusion of year-round EC-derived CO2 fluxes, soil temperature and moisture, the process-oriented model (CoupModel) has been constrained to quantify an annual budget and characterize seasonal patterns of CO2 fluxes. The results show that total photosynthesis corresponds to -202 ± 20 g C m−2 yr-1 with ecosystem respiration of 167 ± 28 g C m-2 yr-1, resulting in NEE of -35 ± 15 g C m-2 yr-1. The respiration loss is mainly described as decomposition of near-surface litter. A year with an anomalously deep snowpack shows a threefold increase in the rate of ecosystem respiration compared to other years. Due to the high CO2 emissions during that winter, the annual budget results in a marked reduction in the CO2 sink. The seasonal patterns of photosynthesis and soil respiration were described using response functions of the forcing atmosphere and soil conditions. Snow depth, topography-related soil moisture, and growing season warmth are identified as important environmental characteristics which most influence seasonal rates of gas exchange.
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| 30. |
- Björklund, Jesper, et al.
(author)
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Disentangling the multi-faceted growth patterns of primary Picea abies forests in the Carpathian arc
- 2019
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 271, s. 214-224
-
Journal article (peer-reviewed)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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| 31. |
- Jönsson, Anna Maria, et al.
(author)
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Potential use of seasonal forecasts for operational planning of north European forest management
- 2017
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 244-245, s. 122-135
-
Journal article (peer-reviewed)abstract
- Weather and climate conditions can have large impacts on the outcome of forest management operations: Suboptimal conditions can increase the amount of driving damage to forest ground caused by the heavy machines used for harvesting, forwarding and soil scarification. Planting of tree seedlings is commonly practised after clear cutting, and drought in summer or soil frost uplifting in autumn reduces the likelihood of successful plant establishment. Weather and climate also influence the risk of forest fires and the occurrence and development of pest and pathogens, and thereby the timing suitable for surveillance and countermeasures. In this study, the potential use of seasonal forecasts to support the operational planning of forest management in northern Europe was assessed. The analysis was based on temperature and precipitation data from WFDEI System 4 with 15 ensemble members representing seasonal hindcasts (retrospective predictions) for the period of 1981–2010. The data was used directly and as input to a soil model from which monthly indices of frozen soil and plant water stress were calculated. Relatively low skills were found for most months, and in particular for longer lead times. Highest skill was found for bias corrected temperature of January to March, with one month lead time. The skill was higher for the soil model indices, in particular those related to soil frost, as they are influenced by cumulative processes and the initial model conditions contribute to the skill. Probabilistic forecasts on frozen soil can thus be valuable for planning of which areas to harvest, taking the risk of driving damage to forest soils and forest roads into account.
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| 32. |
- Lindroth, Anders, et al.
(author)
-
Effects of low thinning on carbon dioxide fluxes in a mixed hemiboreal forest
- 2018
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 262, s. 59-70
-
Journal article (peer-reviewed)abstract
- We used eddy-covariance (EC) measurements of net ecosystem exchange (NEE) above canopy to assess the effects of thinning on CO2 fluxes at the ICOS Sweden site Norunda in central Sweden. This forest site consists of mixed pine and spruce stands approx. 100 years old. The thinning during late autumn 2008, performed in a semi-circle from the mast extending 200 m outwards harvested about 25% of the volume. Measurements were conducted from 2007 to 2016 and thus, above canopy fluxes were recorded two years before and eight years after the thinning. We also measured the net flux from the forest floor with automatic chambers in three locations and with below-canopy EC during shorter periods before and after thinning. The chamber measurements during the first part of the growing season after thinning showed strongly enhanced effluxes in the order of 150–250% of the pre-thinning values. These chamber measurements were made on drier places within the thinned area because waterlogging made it impossible to use chambers at all available locations. The below-canopy EC measurements, which had a larger footprint as compared to the chambers, showed less enhanced fluxes (in the order of 35%). This footprint included also wetter areas. The above canopy EC measurements showed a reduction of daytime net flux by approx. 30% during the first summer after thinning. The median growing season fluxes then slowly increased but were not restored to the pre-thinning levels eight years after thinning. There was also a small decrease in growing season ecosystem respiration during the first summer after thinning and with a continued decreasing trend over time. It was concluded that this decrease in respiration was caused by successively decreasing decomposition of coarse organic substrates resulting from the thinning. This respiration decrease over time persisted even under gradual biomass increase, which otherwise would indicate increasing autotrophic respiration. The light-response and respiration models fitted to all data did not show any trends in daytime or nighttime fluxes so the conclusion was that the trends were caused by the thinning and not because of trends in meteorological drivers. The annual values contrasted with the summertime results since only a minor effect was observed on the annual NEE. Both ecosystem respiration and gross primary productivity were reduced as an effect of thinning. We explained the different summertime versus annual effects to be caused by the decrease in ecosystem respiration since respiration is dominating the NEE during non-growing season periods when photosynthesis is very low or even zero. Our results are a strong indication that the NEE of a forest could be maintained over time with harvesting practices that avoids clear-cutting and thereby enhance the total carbon uptake of forests.
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| 33. |
- Nicolini, Giacomo, et al.
(author)
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Impact of CO2 storage flux sampling uncertainty on net ecosystem exchange measured by eddy covariance
- 2018
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 248, s. 228-239
-
Journal article (peer-reviewed)abstract
- Complying with several assumption and simplifications, most of the carbon budget studies based on eddy covariance (EC) measurements quantify the net ecosystem exchange (NEE) by summing the flux obtained by EC (FC) and the storage flux (SC). SC is the rate of change of a scalar, CO2 molar fraction in this case, within the control volume underneath the EC measurement level. It is given by the difference in the quasi-instantaneous profiles of concentration at the beginning and end of the EC averaging period, divided by the averaging period. The approaches used to estimate SC largely vary, from measurements based on a single sampling point usually located at the EC measurement height, to measurements based on profile sampling. Generally a single profile is used, although multiple profiles can be positioned within the control volume. Measurement accuracy reasonably increases with the spatial sampling intensity, however limited resources often prevent more elaborated measurement systems. In this study we use the experimental dataset collected during the ADVEX campaign in which turbulent and non-turbulent fluxes were measured in three forest sites by the simultaneous use of five towers/profiles. Our main objectives are to evaluate both the uncertainty of SC that derives from an insufficient sampling of CO2 variability, and its impact on concurrent NEE estimates.Results show that different measurement methods may produce substantially different SC flux estimates which in some cases involve a significant underestimation of the actual SC at a half-hourly time scales. A proper measuring system, that uses a single vertical profile of which the CO2 sampled at 3 points (the two closest to the ground and the one at the lower fringe of the canopy layer) is averaged with CO2 sampled at a certain distance and at the same height, improves the horizontal representativeness and reduces this (proportional) bias to 2–10% in such ecosystems. While the effect of this error is minor on long term NEE estimates, it can produce significant uncertainty on half-hourly NEE fluxes.
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| 34. |
- Tagesson, Torbern, et al.
(author)
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Spatiotemporal variability in carbon exchange fluxes across the Sahel
- 2016
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 226-227, s. 108-118
-
Journal article (peer-reviewed)abstract
- Semi-arid regions play an increasingly important role as a sink within the global carbon (C) cycle and is the main biome driving inter-annual variability in carbon dioxide (CO2) uptake by terrestrial ecosystems. This indicates the need for detailed studies of spatiotemporal variability in C cycling for semi-arid ecosystems. We have synthesized data on the land-atmosphere exchange of CO2 measured with the eddy covariance technique from the six existing sites across the Sahel, one of the largest semi-arid regions in the world. The overall aim of the study is to analyse and quantify the spatiotemporal variability in these fluxes and to analyse to which degree spatiotemporal variation can be explained by hydrological, climatic, edaphic and vegetation variables. All ecosystems were C sinks (average ± total error -162 ± 48 g C m-2 y-1), but were smaller when strongly impacted by anthropogenic influences. Spatial and inter-annual variability in the C flux processes indicated a strong resilience to dry conditions, and were correlated with phenological metrics. Gross primary productivity (GPP) was the most important flux process affecting the sink strength, and diurnal variability in GPP was regulated by incoming radiation, whereas seasonal dynamics was closely coupled with phenology, and soil water content. Diurnal variability in ecosystem respiration was regulated by GPP, whereas seasonal variability was strongly coupled to phenology and GPP. A budget for the entire Sahel indicated a strong C sink mitigating the global anthropogenic C emissions. Global circulation models project an increase in temperature, whereas rainfall is projected to decrease for western Sahel and increase for the eastern part, indicating that the C sink will possibly decrease and increase for the western and eastern Sahel, respectively.
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| 35. |
- Wang, L., et al.
(author)
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Validation of the global land data assimilation system based on measurements of soil temperature profiles
- 2016
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 218-219, s. 288-297
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Journal article (peer-reviewed)abstract
- Soil temperature is a key parameter in the soil–vegetation–atmosphere system. It plays an important role in the land surface water and energy cycles, and has a major influence on vegetation growth and other hydrological aspects. We evaluated the accuracy of the soil temperature profiles from the Global Land Data Assimilation System (GLDAS) using nine observational networks across the world and aimed to find a reliable global soil temperature profile dataset for future hydrological and ecological studies. In general, the soil temperature profile data generated by the Noah model driven by the GLDAS forcing data (GLDAS_Noah10 and GLDAS_Noah10_v2) were found to have high skills in terms of daily, monthly, and mean seasonal variations, indicated by smaller bias and root-mean-square-error (RMSE) (both <3 °C) and correlation coefficients larger than 0.90. Conversely, the Community Land Model (CLM) results (GLDAS_CLM10) generally showed larger bias and RMSE (both >4 °C). Further analysis showed that the overestimation by GLDAS_CLM10 was mainly caused by overestimation of the ground heat flux, determined by the thermal conductivity parameterization scheme, whereas the underestimation by GLDAS_Noah10 was due to underestimation of downward longwave radiation from the forcing data. Thus, more accurate forcing data should be required for the Noah model and an improved thermal parameterization scheme should be developed for the CLM. These approaches will improve the accuracy of simulated soil temperatures. To our knowledge, it is the first study to evaluate the GLDAS soil temperatures with comprehensive in situ observations across the world, and has a potential to facilitate an overall improvement of the GLDAS products (not only soil temperatures but also the related energy and water fluxes) as well as a refinement of the land surface parameterization used in GLDAS.
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| 36. |
- Yao, Yunjun, et al.
(author)
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Assessment and simulation of global terrestrial latent heat flux by synthesis of CMIP5 climate models and surface eddy covariance observations
- 2016
-
In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 223, s. 151-167
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Journal article (peer-reviewed)abstract
- The latent heat flux (LE) between the terrestrial biosphere and atmosphere is a major driver of the global hydrological cycle. In this study, we evaluated LE simulations by 45 general circulation models (GCMs) in the Coupled Model Intercomparison Project Phase 5 (CMIP5) by a comparison with eddy covariance (EC) observations from 240 globally distributed sites from 2000 to 2009. In addition, we improved global terrestrial LE estimates for different land cover types by synthesis of seven best CMIP5 models and EC observations based on a Bayesian model averaging (BMA) method. The comparison results showed substantial differences in monthly LE among all GCMs. The model CESM1-CAM5 has the best performance with the highest predictive skill and a Taylor skill score (S) from 0.51-0.75 for different land cover types. The cross-validation results illustrate that the BMA method has improved the accuracy of the CMIP5 GCM's LE simulation with a decrease in the averaged root-mean-square error (RMSE) by more than 3 W/m2 when compared to the simple model averaging (SMA) method and individual GCMs. We found an increasing trend in the BMA-based global terrestrial LE (slope of 0.018 W/m2 yr-1, p <0.05) during the period 1970-2005. This variation may be attributed directly to the inter-annual variations in air temperature (Ta), surface incident solar radiation (Rs) and precipitation (P). However, our study highlights a large difference from previous studies in a continuous increasing trend after 1998, which may be caused by the combined effects of the variations of Rs, Ta, and P on LE for different models on these time scales. This study provides corrected-modeling evidence for an accelerated global water cycle with climate change.
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| 37. |
- Zhang, Gangfeng, et al.
(author)
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Impact of near-surface wind speed variability on wind erosion in the eastern agro-pastoral transitional zone of Northern China, 1982–2016
- 2019
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 271, s. 102-115
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Journal article (peer-reviewed)abstract
- © 2019 Elsevier B.V. Wind erosion in arid and semi-arid areas is an important global environmental issue, and changes in wind speed trends over time play a key role in wind erosion dynamics. In a warming climate, scientists have recently observed a widespread decline in wind speed, termed “stilling”. Here, we apply the Revised Wind Erosion Equation Model (RWEQ) to simulate the variability of wind erosion and quantify the impact of wind speed changes on soil degradation dynamics over the eastern agro-pastoral transitional zone of Northern China from 1982 to 2016. Our results show that a significant (i.e., p < 0.05) decrease (-0.007 m s −1 year −1 ) of near-surface wind speed was observed annually, with significant declining trends in spring (-0.010 m s −1 year −1 )and autumn (-0.009 m s −1 year −1 ). At the same time, wind erosion simulations reveal a negative trend for the annual soil loss from wind erosion (-6.20 t hectare -2 year -1 , p < 0.05; affecting 99.8% of the study region), with significant declining trends in all seasons, particularly in spring (-3.49 t hectare -2 year −1 ) and autumn (-1.26 ha -2 year −1 ). Further, we isolate the effects of wind variability on wind erosion from 1982 to 2016 by the model variable control method. This shows that wind speed variability strongly weakens wind erosion at -8.14 t hectare -2 year -1 (p < 0.05) annually, with the strongest stilling recorded in spring leading to major decreases of wind erosion in spring (-4.77 t hectare -2 year −1 , p < 0.05). Meanwhile, the weakest stilling in summer had the opposite influence on wind erosion (+0.40 t hectare -2 year -1 , p < 0.10). To summarize, our findings have shown a significant impact of wind stilling on the decline of soil erosion rates in Northern China.
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