| 1. |
- Boudreault, Louis-Etienne, et al.
(författare)
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A LiDAR method of canopy structure retrieval for wind modeling of heterogeneous forests
- 2015
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 201, s. 86-97
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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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| 2. |
- Dai, L. L., et al.
(författare)
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Reduced photosynthetic thermal acclimation capacity under elevated ozone in poplar (Populus tremula) saplings
- 2021
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Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 27:10, s. 2159-2173
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Tidskriftsartikel (refereegranskat)abstract
- The sensitivity of photosynthesis to temperature has been identified as a key uncertainty for projecting the magnitude of the terrestrial carbon cycle response to future climate change. Although thermal acclimation of photosynthesis under rising temperature has been reported in many tree species, whether tropospheric ozone (O-3) affects the acclimation capacity remains unknown. In this study, temperature responses of photosynthesis (light-saturated rate of photosynthesis (A(sat)), maximum rates of RuBP carboxylation (V-cmax), and electron transport (J(max)) and dark respiration (R-dark) of Populus tremula exposed to ambient O-3 (AO(3), maximum of 30 ppb) or elevated O-3 (EO3, maximum of 110 ppb) and ambient or elevated temperature (ambient +5 degrees C) were investigated in solardomes. We found that the optimum temperature of A(sat) (T-optA) significantly increased in response to warming. However, the thermal acclimation capacity was reduced by O-3 exposure, as indicated by decreased T-optA, and temperature optima of V-cmax (T-optV) and J(max) (T-optJ) under EO3. Changes in both stomatal conductance (g(s)) and photosynthetic capacity (V-cmax and J(max)) contributed to the shift of T-optA by warming and EO3. Neither R-dark measured at 25 degrees C (Rdark25) nor the temperature response of R-dark was affected by warming, EO3, or their combination. The responses of A(sat), V-cmax, and J(max) to warming and EO3 were closely correlated with changes in leaf nitrogen (N) content and N use efficiency. Overall, warming stimulated growth (leaf biomass and tree height), whereas EO3 reduced growth (leaf and woody biomass). The findings indicate that thermal acclimation of A(sat) may be overestimated if the impact of O-3 pollution is not taken into account.
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| 3. |
- De Kauwe, M. G., et al.
(författare)
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A test of the ‘one-point method’ for estimating maximum carboxylation capacity from field-measured, light-saturated photosynthesis
- 2016
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Ingår i: New Phytologist. - : Wiley. - 0028-646X .- 1469-8137. ; 210:3, s. 1130-1144
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Tidskriftsartikel (refereegranskat)abstract
- Simulations of photosynthesis by terrestrial biosphere models typically need a specification of the maximum carboxylation rate (Vcmax). Estimating this parameter using A–Ci curves (net photosynthesis, A, vs intercellular CO2 concentration, Ci) is laborious, which limits availability of Vcmax data. However, many multispecies field datasets include net photosynthetic rate at saturating irradiance and at ambient atmospheric CO2 concentration (Asat) measurements, from which Vcmax can be extracted using a ‘one-point method’. We used a global dataset of A–Ci curves (564 species from 46 field sites, covering a range of plant functional types) to test the validity of an alternative approach to estimate Vcmax from Asat via this ‘one-point method’. If leaf respiration during the day (Rday) is known exactly, Vcmax can be estimated with an r2 value of 0.98 and a root-mean-squared error (RMSE) of 8.19 μmol m−2 s−1. However, Rday typically must be estimated. Estimating Rday as 1.5% of Vcmax, we found that Vcmax could be estimated with an r2 of 0.95 and an RMSE of 17.1 μmol m−2 s−1. The one-point method provides a robust means to expand current databases of field-measured Vcmax, giving new potential to improve vegetation models and quantify the environmental drivers of Vcmax variation.
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| 4. |
- Dewar, R. C., et al.
(författare)
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Why does leaf nitrogen decline within tree canopies less rapidly than light? An explanation from optimization subject to a lower bound on leaf mass per area
- 2012
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Ingår i: Tree Physiology. - : Oxford University Press (OUP). - 0829-318X .- 1758-4469. ; 32:5, s. 520-534
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Tidskriftsartikel (refereegranskat)abstract
- A long-established theoretical result states that, for a given total canopy nitrogen (N) content, canopy photosynthesis is maximized when the within-canopy gradient in leaf N per unit area (N-a) is equal to the light gradient. However, it is widely observed that N-a declines less rapidly than light in real plant canopies. Here we show that this general observation can be explained by optimal leaf acclimation to light subject to a lower-bound constraint on the leaf mass per area (m(a)). Using a simple model of the carbon-nitrogen (C-N) balance of trees with a steady-state canopy, we implement this constraint within the framework of the MAXX optimization hypothesis that maximizes net canopy C export. Virtually all canopy traits predicted by MAXX (leaf N gradient, leaf N concentration, leaf photosynthetic capacity, canopy N content, leaf-area index) are in close agreement with the values observed in a mature stand of Norway spruce trees (Picea abies L. Karst.). An alternative upper-bound constraint on leaf photosynthetic capacity (A(sat)) does not reproduce the canopy traits of this stand. MAXX subject to a lower bound on m(a) is also qualitatively consistent with co-variations in leaf N gradient, m(a) and A(sat) observed across a range of temperate and tropical tree species. Our study highlights the key role of constraints in optimization models of plant function.
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| 5. |
- He, Hongxing, 1987, et al.
(författare)
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Forests on drained agricultural peatland are potentially large sources of greenhouse gases – insights from a full rotation period simulation
- 2015
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Ingår i: Biogeosciences Discussions. - : Copernicus GmbH. - 1810-6277. ; 12, s. 19673-19710
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Tidskriftsartikel (refereegranskat)abstract
- The CoupModel was used to simulate a Norway Spruce forest on fertile drained peat over 60 years, from planting in 1951 until 2011, describing abiotic, biotic and greenhouse gas (GHG) emissions (CO2 and N2O). By calibrating the model against tree ring data we obtained a "reference" model by which we were able to describe the fluxes and controlling factors over the 60 years. We discuss some conceptual issues relevant to improving the model in order to better understand peat soil simulations. However, the present model was able to describe the most important ecosystem dynamics such as the plant biomass development and GHG emissions. The GHG fluxes are composed of two important quantities, the forest carbon (C) uptake, 405 g C m−2 yr−1 and the decomposition of peat soil, 396 g C m−2 yr−1. N2O emissions contribute to the GHG emissions by 0.5 g N m−2 yr−1, corresponding to 56.8 g C m−2 yr−1. The 60-year-old Spruce forest has an accumulated biomass of 164 Mg C ha−1. However, over this period 208 Mg C ha−1 GHG has been added to the atmosphere, which means a net addition of GHG emissions. The main losses are from the peat soil and, indirectly, from forest thinning products, which we assume have a short lifetime. We conclude that after harvest at an age of 80 years, most of the stored biomass carbon is liable to be released, the system having captured C only temporarily and with a cost of disappeared peat, adding CO2 to the atmosphere.
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| 6. |
- He, Hongxing, 1987, et al.
(författare)
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Forests on drained agricultural peatland are potentially large sources of greenhouse gases – insights from a full rotation period simulation
- 2016
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 13, s. 2305-2318
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Tidskriftsartikel (refereegranskat)abstract
- The CoupModel was used to simulate a Norway spruce forest on fertile drained peat over 60 years, from planting in 1951 until 2011, describing abiotic, biotic and greenhouse gas (GHG) emissions (CO2 and N2O). By calibrating the model against tree ring data a “vegetation fitted” model was obtained by which we were able to describe the fluxes and controlling factors over the 60 years. We discuss some conceptual issues relevant to improving the model in order to better understand peat soil simulations. However, the present model was able to describe the most important ecosystem dynamics such as the plant biomass development and GHG emissions. The GHG fluxes are composed of two important quantities, the spruce forest carbon (C) uptake, 413 g C m-2 yr-1 and the decomposition of peat soil, 399 gCm-2 yr-1. N2O emissions contribute to the GHG emissions by up to 0.7 gNm-2 yr-1, corresponding to 76 g Cm-2 yr-1. The 60-year old spruce forest has an accumulated biomass of 16.0 kg Cm-2 (corresponding to 60 kgCO2 m-2). However, over this period, 26.4 kg m-2 (97 kgCO2eqm-2) has been added to the atmosphere, as both CO2 and N2O originating from the peat soil and, indirectly, from forest thinning products, which we assume have a short lifetime. We conclude that after harvest at an age of 80 years, most of the stored biomass carbon is liable to be released, the system having captured C only temporarily and with a cost of disappeared peat, adding CO2 to the atmosphere.
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| 7. |
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| 8. |
- Henriksson, Nils, et al.
(författare)
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Stem compression reversibly reduces phloem transport in Pinus sylvestris trees
- 2015
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Ingår i: Tree Physiology. - : Oxford University Press (OUP). - 0829-318X .- 1758-4469. ; 35, s. 1075-1085
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Tidskriftsartikel (refereegranskat)abstract
- Manipulating tree belowground carbon (C) transport enables investigation of the ecological and physiological roles of tree roots and their associated mycorrhizal fungi, as well as a range of other soil organisms and processes. Girdling remains the most reliable method for manipulating this flux and it has been used in numerous studies. However, girdling is destructive and irreversible. Belowground C transport is mediated by phloem tissue, pressurized through the high osmotic potential resulting from its high content of soluble sugars. We speculated that phloem transport may be reversibly blocked through the application of an external pressure on tree stems. Thus, we here introduce a technique based on compression of the phloem, which interrupts belowground flow of assimilates, but allows trees to recover when the external pressure is removed. Metal clamps were wrapped around the stems and tightened to achieve a pressure theoretically sufficient to collapse the phloem tissue, thereby aiming to block transport. The compression's performance was tested in two field experiments: a C-13 canopy labelling study conducted on small Scots pine (Pinus sylvestris L.) trees [2-3 m tall, 3-7 cm diameter at breast height (DBH)] and a larger study involving mature pines (similar to 15 m tall, 15-25 cm DBH) where stem respiration, phloem and root carbohydrate contents, and soil CO2 efflux were measured. The compression's effectiveness was demonstrated by the successful blockage of C-13 transport. Stem compression doubled stem respiration above treatment, reduced soil CO2 efflux by 34% and reduced phloem sucrose content by 50% compared with control trees. Stem respiration and soil CO2 efflux returned to normal within 3 weeks after pressure release, and C-13 labelling revealed recovery of phloem function the following year. Thus, we show that belowground phloem C transport can be reduced by compression, and we also demonstrate that trees recover after treatment, resuming C transport in the phloem.
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| 9. |
- Henriksson, Nils, et al.
(författare)
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The mycorrhizal tragedy of the commons
- 2021
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Ingår i: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 24:6, s. 1215-1224
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Tidskriftsartikel (refereegranskat)abstract
- Trees receive growth-limiting nitrogen from their ectomycorrhizal symbionts, but supplying the fungi with carbon can also cause nitrogen immobilization, which hampers tree growth. We present results from field and greenhouse experiments combined with mathematical modelling, showing that these are not conflicting outcomes. Mycorrhizal networks connect multiple trees, and we modulated C provision by strangling subsets of Pinus sylvestris trees, assuming that carbon supply to fungi was reduced proportionally to the strangled fraction. We conclude that trees gain additional nitrogen at the expense of their neighbours by supplying more carbon to the fungi. But this additional carbon supply aggravates nitrogen limitation via immobilization of the shared fungal biomass. We illustrate the evolutionary underpinnings of this situation by drawing on the analogous tragedy of the commons, where the shared mycorrhizal network is the commons, and explain how rising atmospheric CO2 may lead to greater nitrogen immobilization in the future.
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| 10. |
- Jocher, Georg, et al.
(författare)
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Apparent Winter CO2 uptake by a boreal forest due to decoupling
- 2017
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 232, s. 23-34
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Tidskriftsartikel (refereegranskat)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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