Boreal Aspen Forest Research Articles

Re-assessment of the climatic controls on the carbon and water fluxes of a boreal aspen forest over 1996-2016: Changing sensitivity to long-term climatic conditions.

Recent evidence suggests that the relationships between climate and boreal tree growth are generally non-stationary; however, it remains uncertain whether the relationships between climate and carbon (C) fluxes of boreal forests are stationary or have changed over recent decades. In this study, we used continuous eddy-covariance and microclimate data over 21 years (1996-2016) from a 100-year-old trembling aspen stand in central Saskatchewan, Canada to assess the relationships between climate and ecosystem C and water fluxes. Over the study period, the most striking climatic event was a severe, 3-year drought (2001-2003). Gross ecosystem production (GEP) showed larger interannual variability than ecosystem respiration (Re ) over 1996-2016, but Re was the dominant component contributing to the interannual variation in net ecosystem production (NEP) during post-drought years. The interannual variations in evapotranspiration (ET) and C fluxes were primarily driven by temperature and secondarily by water availability. Two-factor linear models combining precipitation and temperature performed well in explaining the interannual variation in C and water fluxes (R2 > .5). The temperature sensitivities of all three C fluxes (NEP, GEP and Re ) declined over the study period (p < .05), and, as a result, the phenological controls on annual NEP weakened. The decreasing temperature sensitivity of the C fluxes may reflect changes in forest structure, related to the over-maturity of the aspen stand at 100 years of age, and exacerbated by high tree mortality following the severe 2001-2003 drought. These results may provide an early warning signal of driver shift or even an abrupt status shift of aspen forest dynamics. They may also imply a universal weakening in the relationship between temperature and GEP as forests become over-mature, associated with the structural and compositional changes that accompany forest ageing.

The incorporation of an organic soil layer in the Noah-MP land surface model and its evaluation over a boreal aspen forest

Abstract. A thick top layer of organic matter is a dominant feature in boreal forests and can impact land–atmosphere interactions. In this study, the multi-parameterization version of the Noah land surface model (Noah-MP) was used to investigate the impact of incorporating a forest-floor organic soil layer on the simulated surface energy and water cycle components at the BERMS Old Aspen site (OAS) field station in central Saskatchewan, Canada. Compared to a simulation without an organic soil parameterization (CTL), the Noah-MP simulation with an organic soil (OGN) improved Noah-MP-simulated soil temperature profiles and soil moisture at 40–100 cm, especially the phase and amplitude (Seasonal cycle) of soil temperature below 10 cm. OGN also enhanced the simulation of sensible and latent heat fluxes in spring, especially in wet years, which is mostly related to the timing of spring soil thaw and warming. Simulated top-layer soil moisture is better in OGN than that in CTL. The effects of including an organic soil layer on soil temperature are not uniform throughout the soil depth and are more prominent in summer. For drought years, the OGN simulation substantially modified the partitioning of water between direct soil evaporation and vegetation transpiration. For wet years, the OGN-simulated latent heat fluxes are similar to CTL except for the spring season when OGN produced less evaporation, which was closer to observations. Including organic soil produced more subsurface runoff and resulted in much higher runoff throughout the freezing periods in wet years.

Impact of harvesting intensity on wood-inhabiting fungi in boreal aspen forests of Eastern Canada

Environmental change, including human disturbance, can have a striking impact on the biodiversity of ecosystems. We used a molecular fingerprinting technique to determine how communities of saproxylic fungi on trembling aspen deadwood change under the influence of silvicultural treatments designed to emulate natural stand dynamics. We describe changes in richness, diversity, and species composition of fungal communities of trembling aspen logs and snags caused by these silvicultural practices. Our study was conducted in the SAFE Project, a series of silvicultural experiments that tests an ecosystem management model based on natural dynamics. We found that large trembling aspen logs and in advanced decay stages had approximately 9% higher fungal species richness and 10% higher fungal diversity than small and large logs at medium decay stages. The effect of log diameter was in turn strongly dependent on the silvicultural treatment. In burned stands, larger logs supported higher fungal richness and diversity, therefore potentially acting as fungal refuge. A negative relationship between the fungal diversity of logs and snags and the volume of fine woody debris was also related to silvicultural treatments, as fine woody debris increased with silvicultural intensity. Our results underline the negative effects of intense silvicultural practice on fungal diversity and species richness by modifying community composition, but they also highlight the benefits of partial harvest, which retain coarse woody debris volume.

Evaluation of Water Stress Impact on the Parameter Values in Stomatal Conductance Models Using Tower Flux Measurement of a Boreal Aspen Forest

Abstract The impact of water stress on plant stomatal conductance (g) has been widely studied but with little consensus as to the processes governing its responses. The photosynthesis-driven stomatal conductance models usually employ constant model parameters and attribute the decrease of g from water stress to the reduction of leaf photosynthesis. This has been challenged by studies showing that the model parameter values decrease when the plant is under water stress. In this study, the impact of plant water stress on the parameter values in stomatal conductance models is evaluated using the approach recently developed by S. Wang et al. and the tower flux measurements at a Canadian boreal aspen forest. Results show that the slope parameter (α) in the stomatal conductance models decreases substantially with the development of plant water stress. The magnitude of this reduction is dependent on how plant water stress is represented. Overall, the relative reduction of α from its maximum value is 28% when soil water content decreases from 0.38 to 0.18 m3 m−3, and is 38% when Bowen ratio increases from 0.25 to 3.5. Equations for α correction to account for water stress impacts are proposed. Further studies on different ecosystems are necessary to quantify the parameter variations with water stress among different climate regions and plant species.

Can a satellite-derived estimate of the fraction of PAR absorbed by chlorophyll (FAPAR chl) improve predictions of light-use efficiency and ecosystem photosynthesis for a boreal aspen forest?

We used daily MODerate resolution Imaging Spectroradiometer (MODIS) imagery obtained over a five-year period to analyze the seasonal and inter-annual variability of the fraction of absorbed photosynthetically active radiation (FAPAR) and photosynthetic light use efficiency (LUE) for the Southern Old Aspen (SOA) flux tower site located near the southern limit of the boreal forest in Saskatchewan, Canada. To obtain the spectral characteristics of a standardized land area to compare with tower measurements, we scaled up the nominal 500 m MODIS products to a 2.5 km × 2.5 km area (5 × 5 MODIS 500 m grid cells). We then used the scaled-up MODIS products in a coupled canopy-leaf radiative transfer model, PROSAIL-2, to estimate the fraction of absorbed photosynthetically active radiation (APAR) by the part of the canopy dominated by chlorophyll (FAPAR chl) versus that by the whole canopy (FAPAR canopy). Using the additional information provided by flux tower-based measurements of gross ecosystem production (GEP) and incident PAR, we determined 90-minute averages for APAR and LUE (slope of GEP:APAR) for both the physiologically active foliage (APAR chl, LUE chl) and for the entire canopy (APAR canopy, LUE canopy). The flux tower measurements of GEP were strongly related to the MODIS-derived estimates of APAR chl ( r 2 = 0.78) but only weakly related to APAR canopy ( r 2 = 0.33). Gross LUE between 2001 and 2005 for LUE chl was 0.0241 µmol C µmol − 1 PPFD whereas LUE canopy was 36% lower. Time series of the 5-year normalized difference vegetation index (NDVI) were used to estimate the average length of the core growing season as days of year 152–259. Inter-annual variability in the core growing season LUE chl (µmol C µmol − 1 PPFD) ranged from 0.0225 in 2003 to 0.0310 in 2004. The five-year time series of LUE chl corresponded well with both the seasonal phase and amplitude of LUE from the tower measurements but this was not the case for LUE canopy. We conclude that LUE chl derived from MODIS observations could provide a more physiologically realistic parameter than the more commonly used LUE canopy as an input to large-scale photosynthesis models.

Simulation of Evapotranspiration and Its Response to Plant Water and CO2 Transfer Dynamics

Abstract Evapotranspiration (ET) is controlled by atmospheric demand, plant and soil hydraulic constraints, and the plant physiological activities that determine canopy resistance. This paper introduces a new ET scheme developed for the Ecological Assimilation of Land and Climate Observations (EALCO) model that integrates these controls into one dynamic system. This scheme is based on solving the governing equation system that represents the coupled canopy energy–water–CO2 transfer dynamics, where the canopy temperature Tc, plant water potential ψc, and leaf intercellular CO2 concentrations Ci are simultaneously obtained and used in ET calculations. Modeled ET was compared with eddy correlation flux measurement at a boreal aspen forest. Results showed that the correlation coefficient (R) between modeled and measured daily ET was greater than 0.96. The average absolute error was approximately 0.3 mm day−1. Modeled ET was generally higher than measured ET by 10%. This is consistent with the energy balance closure analyses from observations that showed that turbulent energy flux was frequently less than 90% of the total available energy. The effects of the plant CO2 and water transfer dynamics on ET simulations were investigated by running the model in two additional settings. These were 1) static Ci—where the ratio of Ci to atmospheric CO2 concentration was set to a constant value, and 2) static ψc—where the ψc was linearly related to soil water potential. The dynamic CO2 transfer scheme and the static Ci scheme produced relatively small differences in ET that mainly occurred at a subdaily time scale. Differences in ET produced using the dynamic water transfer scheme and the static ψc scheme depended on ecosystem water conditions and were more significant when the plant was under water stress. Ignoring the dynamic water transfer process in the model decreased the correlation coefficient between modeled and measured ET more significantly in drier years. This implies that the dynamic water transfer scheme is of more importance for ET estimates in arid or semiarid ecosystems.

Impact of snow cover on soil temperature and its simulation in a boreal aspen forest

Abstract Snow cover has low thermal conductivity and high albedo, which have large impacts on the soil thermal regime and microclimate. Accurate simulations of snow and soil processes are critical to ecosystem and climate modelling. In this study, we developed a detailed snow and soil scheme in an ecosystem model EALCO to (1) improve the simulations of snow and soil thermal regimes and (2) investigate the effects of different model configurations and parameterizations including snow and soil layering resolutions, fractional snow cover parameterization, water table and total soil column depths, and the inclusion/exclusion of a surface organic layer and unfrozen water in frozen soil. The model includes dynamic layering of snow, user-adjustable layering of soil, snow cover compaction and destructive metamorphisms, implicit solutions of the soil heat and water transfer equations using a finite difference method, distinct parameterizations of the surface organic layer and unfrozen water in frozen soil. The model was tested using nine years (1994–2002) of winter field measurements obtained in a boreal aspen forest. The tested variables included snow depth, snow temperature, soil heat flux, and soil temperatures at various depths. The results showed that the model well reproduced the snow and soil thermal regimes and their interactions observed at the site. The results also indicated that the simulation of snow cover duration was affected by the snow layering resolution and fractional snow cover parameterization. The simulation of soil temperature was affected by the soil layering resolution, the depths of water table and soil column, and the inclusion of a soil organic layer. Inclusion of unfrozen water in frozen soil was found to have only a small effect on soil temperature simulation at this site.

Net ecosystem productivity of boreal aspen forests under drought and climate change: Mathematical modelling with Ecosys

The net ecosystem productivity (NEP) of boreal aspen is strongly affected by comparative rates of annual potential evapotranspiration ( E a) and precipitation ( P a). Changes in E a versus P a during future climate change will likely determine changes in aspen NEP and consequently the magnitude of the carbon sink/source of a significant part of the boreal forest. We hypothesize that the effects of E a versus P a on aspen NEP can be modelled with a soil–root–canopy hydraulic resistance scheme coupled to a canopy energy balance closure scheme that determines canopy water status and thereby CO 2 uptake. As part of the ecosystem model ecosys, these schemes were used to model diurnal declines in CO 2 and latent heat (LE) exchange during a 3-year drought (2001–2003) at the Fluxnet-Canada Research Network (FCRN) southern old aspen site (SOA). These declines were consistent with those measured by eddy covariance (EC) at SOA, except that ecosystem CO 2 effluxes modelled during most nights were larger that those measured by EC or gap-filled from other EC measurements. Soil CO 2 effluxes in the model were close to, but sometimes smaller than, those measured by automated surface chambers at SOA. Diurnal declines in CO 2 exchange during the drought caused declines in annual NEP in the model, and in gap-filled EC measurements (model versus EC in g C m −2: 275 versus 367 ± 110 in 2001, 82 versus 144 ± 43 in 2002 and 23 versus 104 ± 31 in 2003). Lower modelled NEP was attributed to the larger modelled CO 2 effluxes. Ecosys was then used to predict changes in aspen net biome productivity (NBP = NEP − C lost from disturbance) caused by 6-year versus 3-year recurring droughts during 100-year fire cycles under current climate versus climate change projected under the IPCC SRES A1B scenario. Although NBP was adversely affected during recurring 6-year droughts under current climate, it recovered quickly during non-drought years so that long-term NBP was maintained at 4 g C m −2 year −1. NBP rose by 10, 108 and 126 g C m −2 year −1 during the first, second and third centuries under climate change with recurring 3-year droughts, indicating a gradual rise in sink activity by boreal aspen. However recurring 6-year droughts during climate change caused recurring negative NBP (C losses), gradually depleting aspen C reserves and eventually causing dieback of the aspen overstory during the third century of climate change. This dieback was followed by a large decline in NBP. We conclude that NBP of boreal aspen will rise gradually under current projections of climate change, except under prolonged (e.g. 6 years) recurring droughts, which would eventually cause aspen to die back and substantial amounts of C to be lost.

Modelling multi-year coupled carbon and water fluxes in a boreal aspen forest

In order to test two hypotheses: (i) that carbon (C) and energy exchanges between terrestrial ecosystems and the atmosphere are closely constrained by soil water availability, and (ii) that vegetation is able to optimize soil water uptake from different soil layers; two model simulations were conducted. The Boreal Ecosystem Productivity Simulator (BEPS) model was run to simulate an aspen forest in Saskatchewan, Canada during the period 1997–2004. In Simulation 1, the effect of soil water availability in different soil layers on stomatal conductance was weighted only by root fraction. In Simulation 2, the influence of soil water availability in different soil layers on stomatal conductance was weighted according to both the root fraction and soil water availability, in order to allow easier access of roots to soil layers containing more water. Comparison against measured fluxes showed that Simulation 2 was an improvement over Simulation 1 in predicting C, water and energy fluxes at different time scales in dry years. In Simulation 1, the daytime C and water fluxes were underestimated during the transition from adequate to insufficient soil water content in the upper layers. In this run, the model captured 92, 79 and 91% of the daily variances in gross primary productivity (GPP), net ecosystem productivity (NEP), and ecosystem respiration ( R e) during 1997–2004. In Simulation 2, the daily variances of GPP, NEP, and R e explained by the model increased to 93, 82 and 92%, respectively. In Simulation 1, the annual NEP was considerably underestimated in the dry years and years with dry periods, with a root mean square error (RMSE) of 45 g C m −2 year −1 ( n = 8) from 1997 to 2004. In Simulation 2, the RMSE value of simulated annual NEP was reduced to 14 g C m −2 year −1, a relatively small value compared with the average NEP of 157 g C m −2 year −1 during 1997–2004. This suggested that the ability of plant roots to extract water from deep soil layers is critical for the forest to maintain growth when surface layers dried out. Our model results showed that NEP was very sensitive to water conditions at this site. In wet years, heterotrophic respiration was enhanced and NEP was reduced.

Impact of changing soil moisture distribution on net ecosystem productivity of a boreal aspen forest during and following drought

The interannual and seasonal variability of gross ecosystem photosynthesis (P), ecosystem respiration (R) and evapotranspiration (E), and their relationships to environmental factors were used to explain changes in net ecosystem productivity (F NEP) at the onset of, during, and following a 3-year-long (2001–2003) drought in a mature boreal aspen stand in central Saskatchewan, Canada. The forest was a moderate carbon (C) sink over its entire 11-year data record (F NEP = 153 � 99 g C m � 2 year � 1 ), including the peak drought years of 2002 and 2003. In 2001, the depletion of water near the soil surface likely reduced heterotrophic soil respiration while water remaining deep in the root zone maintained P above the pre-drought mean, resulting in above-average F NEP. In 2002 and 2003, the forest remained a C sink even though P was below average because R was also below average—a likely consequence of the influence of low soil water content in deep and shallow soil layers on both autotrophic and heterotrophic respiration. In 2004, the recharge of soil water in shallow soil layers allowed R to recover to its pre-drought values, whereas low spring temperatures, the slow recharge of soil water in deep soil layers in spring, late leaf emergence and diminished leaf area index combined to suppress P and produce the lowest annual F NEP of the 11-year record (4 g C m � 2 year � 1 ). The low F NEP and P were mirrored in the lowest stem growth and LAI values of the 11-year record. In 2005, a warm wet year, both the annual values and seasonal variations of F NEP, P and R returned to those of pre-drought years; the partial recovery of LAI to pre-drought values suggests that aspen P was able to adjust to this restriction on C assimilation. Growing season average dry surface conductance (gsv), the Priestley–Taylor coefficient (a) and light use efficiency (LUE) reached their lowest values in 2003 and became similar to predrought years in 2004–2005. Water use efficiency (WUE) was highest in 2003 and remained above average in 2004 and 2005. At the ecosystem scale, the above-average gains made in C sequestration in the first year of the drought were significantly offset by belowaverage stand F NEP in the final 2 years of the drought, and in the year following the drought. # 2006 Elsevier B.V. All rights reserved.

Climatic controls on the carbon and water balances of a boreal aspen forest, 1994–2003

AbstractThe carbon and water budgets of boreal and temperate broadleaf forests are sensitive to interannual climatic variability and are likely to respond to climate change. This study analyses 9 years of eddy‐covariance data from the Boreal Ecosystem Research and Monitoring Sites (BERMS) Southern Old Aspen site in central Saskatchewan, Canada and characterizes the primary climatic controls on evapotranspiration, net ecosystem production (FNEP), gross ecosystem photosynthesis (P) and ecosystem respiration (R). The study period was dominated by two climatic extremes: extreme warm and cool springs, which produced marked contrasts in the canopy duration, and a severe, 3‐year drought.Annual FNEP varied among years from 55 to 367 g C m−2 (mean 172, SD 94). Interannual variability in FNEP was controlled primarily by factors that affected the R/P ratio, which varied between 0.74 and 0.96 (mean 0.87, SD 0.06). Canopy duration enhanced P and FNEP with no apparent effect on R. The fraction of annual photosynthetically active radiation (PAR) that was absorbed by the canopy foliage varied from 38% in late leaf‐emergence years to 51% in early leaf‐emergence years. Photosynthetic light‐use efficiency (mean 0.0275, SD 0.026 mol C mol−1 photons) was relatively constant during nondrought years but declined with drought intensity to a minimum of 0.0228 mol C mol−1 photons during the most severe drought year. The impact of drought on FNEP varied with drought intensity. Years of mild‐to‐moderate drought suppressed R while having little effect on P, so that FNEP was enhanced. Years of severe drought suppressed both R and P, causing either little change or a subtle reduction in FNEP. The analysis produced new insights into the dominance of canopy duration as the most important biophysical control on FNEP.The results suggested a simple conceptual model for annual FNEP in boreal deciduous forests. When water is not limiting, annual P is controlled by canopy duration via its influence on absorbed PAR at constant light‐use efficiency. Water stress suppresses P, by reducing light‐use efficiency, and R, by limiting growth and/or suppressing microbial respiration. The high photosynthetic light‐use efficiency showed this site to be a highly productive boreal deciduous forest, with properties similar to many temperate deciduous forests.

Effects of tree harvesting, forest floor removal, and compaction on soil microbial biomass, microbial respiration, and N availability in a boreal aspen forest in British Columbia

The effects of timber harvesting and the resultant soil disturbances (compaction and forest floor removal) on relative soil water content, microbial biomass C and N contents (C mic and N mic), microbial biomass C:N ratio (C mic-to-N mic), microbial respiration, metabolic quotient ( qCO 2), and available N content in the forest floor and the uppermost mineral soil (0–3 cm) were assessed in a long-term soil productivity (LTSP) site and adjacent mature forest stands in northeastern British Columbia (Canada). A combination of principal component analysis and redundancy analysis was used to test the effects of stem-only harvest, whole tree harvest plus forest floor removal, and soil compaction on the studied variables. Those properties in the forest floor were not affected by timber harvesting or soil compaction. In the mineral soil, compaction increased soil total C and N contents, relative water content, and N mic by 45%, 40%, 34% and 72%, respectively, and decreased C mic-to-N mic ratio by 29%. However, these parameters were not affected by stem only harvesting or whole tree harvesting plus forest floor removal, contrasting the reduction of white spruce and aspen growth following forest floor removal and soil compaction reported in an earlier study. Those results suggest that at the study site the short-term effects of timber harvesting, forest floor removal, and soil compaction are rather complex and that microbial populations might not be affected by the perturbations in the same way as trees, at least not in the short term.

Response of forest vegetation and foliar δ 13C and δ 15N to soil compaction and forest floor removal in a boreal aspen forest

The effects of severe soil compaction and whole tree harvesting plus forest floor removal (referred to below as forest floor removal) on understory cover and species composition, and trembling aspen ( Populus tremuloides Michx.) and white spruce ( Picea glauca [Moench] Voss) growth and foliar δ 13C and δ 15N were investigated in a boreal aspen forest near Dawson Creek, BC, Canada. Soil compaction and forest floor removal shifted the understory community composition and understory total cover was highest in the compacted plus forest floor removed treatment, due to increased moss and shrub cover. Four years after the treatments were applied, soil compaction reduced aspen average height by 20% regardless of forest floor removal, while forest floor removal reduced aspen average height by 60% and unit leaf area by 18% regardless of soil compaction. Soil compaction reduced white spruce unit leaf area, unit leaf weight, and height increment by 29, 33, and 58%, respectively, when the forest floor was intact. In the non-compacted soil, forest floor removal reduced white spruce unit leaf area, unit leaf weight, and height increment by 29, 32, and 35%, respectively. Soil compaction or forest floor removal did not affect aspen or white spruce foliar δ 13C, which can be affected by water availability and reflects plant water use efficiency. Aspen foliar δ 15N was reduced by soil compaction when the forest floor was intact. Forest floor removal reduced aspen foliar N concentration but increased foliar δ 15N. Our results indicate that soil compaction and forest floor removal changed understory community structure but did not affect water availability for trees in the study year, while soil N dynamics or N acquisition by aspen was affected by soil compaction and forest floor removal.

Dynamics of surface albedo of a boreal forest and its simulation

Surface albedo determines the distribution of solar radiation between the earth's surface and the atmosphere. It affects the global climate directly by altering surface energy balance, and indirectly by controlling ecosystem processes and greenhouse gas exchange. In this study, a land surface albedo model was constructed based on the gap probability approach for ray tracing and the basic optical parameters of ecosystem elements. The model was applied to a boreal deciduous forest and results were compared with field measurements. Results show that seasonal and diurnal albedo dynamics were well simulated by the model. The standard deviation between the simulated and measured reflected radiation was 2.5–5.0 W m −2 in different seasons. The model also provided an insight into the relationships between surface albedo and radiation components (direct versus diffuse), solar zenith angle, and different wave bands. Model sensitivity analyses show that the surface albedo in winter is very sensitive to the forest wood area index for this boreal aspen forest, suggesting that accurate estimates of wood area index are necessary to improve the accuracy of surface albedo simulation in leafless seasons.

Seasonal variation and partitioning of ecosystem respiration in a southern boreal aspen forest

Continuous automatic chamber and eddy covariance (EC) measurements were made at an old aspen forest (SOA) located at southern boreal treeline in Saskatchewan, Canada to examine the temporal variability in soil ( R s), tree bole ( R b), and ecosystem respiration ( R E) during 2001. Climatic conditions were significantly warmer and drier than the 30-year climate normal, resulting in lower R E and an unprecedented increase in net ecosystem productivity (NEP). In the 7-year record (1994, and 1996–2001) of CO 2 exchange at SOA, the year 2001 showed the greatest carbon gain (300 g C m −2 year). Scaled chamber measurements (1315 g C m −2 per year) were 37% larger than the EC estimate of R E (961 g C m −2 per year). The difference between the scaled chambers and the EC estimate was reduced to 20% after correcting for lack of energy balance closure. Annual R E was approximately 170 g C m −2 per year lower than the average of the previous 6 years. Annual estimates of microbial-heterotrophic ( R h) (510 g C m −2 per year) and autotrophic respiration ( R a) (805 g C m −2 per year), based on chamber measurements, were used to help validate the EC estimate of R E. R a represented 61% of the total chamber respiration. This fraction was used to partition R E into R a and R h to calculate net primary production (NPP). The values of NEP (300 g C m −2 per year) and NPP (675 g C m −2 per year) were more characteristic of temperate forests. The NPP/ P g ratio of 0.54 was within the range of recently published values using biometric techniques and supports that the annual ecosystem respiration budget and its partitioning was well constrained. We recognize, however, that this ratio will vary interannually depending on climatic conditions. In 7 years of annual EC CO 2 flux measurements at SOA, this study provides the first evidence that drought can lead to a transient increase in CO 2 sequestration resulting from a reduction of R E.

The canopy conductance of a boreal aspen forest, Prince Albert National Park, Canada

AbstractAnnual fluxes of canopy‐level heat, water vapour and carbon dioxide were measured using eddy covariance both above the aspen overstory (Populus tremuloides Michx.) and hazelnut understory (Corylus cornuta Marsh.) of a boreal aspen forest (53·629 °N 106·200 °W). Partitioning of the fluxes between overstory and understory components allowed the calculation of canopy conductance to water vapour for both species. On a seasonal basis, the canopy conductance of the aspen accounted for 70% of the surface conductance, with the latter a strong function of the forest's leaf area index. On a half‐hour basis, the canopy conductance of both species decreased non‐linearly as the leaf‐surface saturation deficits increased, and was best parameterized and showed similar sensitivities to a modified form of the Ball–Berry–Woodrow index, where relative humidity was replaced with the reciprocal of the saturation deficit. The negative feedback between the forest evaporation and the saturation deficit in the convective boundary layer varied from weak when the forest was at full leaf to strong when the forest was developing or loosing leaves. The coupling between the air at the leaf surface and the convective boundary layer also varied seasonally, with coupling decreasing with increasing leaf area. Compared with coniferous boreal forests, the seasonal changes in leaf area had a unique impact on vegetation–atmosphere interactions. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Modelling carbon‐coupled energy and water dynamics of a boreal aspen forest in a general circulation model land surface scheme

AbstractThe ecosystem carbon (C) and nitrogen (N) processes and the C‐coupled energy and water dynamics were developed in the Canadian Land Surface Scheme (CLASS). The ecosystem C and N simulations include plant photosynthesis and respiration, plant tissue growth, senescence, root N uptake, soil heterotrophic respiration, and N mineralization and immobilization. These simulations are driven by variables (i.e. leaf temperature and water potential, and soil temperature and moisture) obtained from the C‐coupled energy and water balance simulations and feed back the model‐determined vegetation parameters (i.e. leaf area index, stomatal resistance, root length and distribution), which in turn control the land surface energy and water processes. In this paper, we introduce the C‐coupled energy and water balance scheme. The water flow process developed for the soil–plant–atmosphere system includes leaf and canopy stomatal resistance driven by leaf net CO2 fixation, plant water capacitance, and soil rhizosphere and plant root resistances. This water flow scheme is dynamically coupled with the canopy energy balance so that the full water balance and energy balance equations can be solved simultaneously. The model was run at a time step of 30 min and tested in a stand‐alone mode driven by meteorological observations obtained at the old aspen (Populus tremuloides) site in the southern study area of the Boreal Ecosystem–Atmosphere Study (BOREAS). Results show that the model reproduced the observed diurnal and seasonal patterns of energy fluxes fairly well. Canopy conductance in the mid‐growing season was simulated to reach above 0.5 mol m−2 s−1. Plant water capacitance was found to buffer the plant water flow process significantly and affect the canopy latent heat exchange under dry soil conditions. Comparisons of modelled and measured daily evapotranspiration in the two years of 1994 and 1996 gave the root‐mean‐square error of 0.71 mm day−1 and correlation coefficient of 0.75. Copyright © 2002 Royal Meteorological Society.

Comparison of two‐layer and single‐layer canopy models with Lagrangian and K‐theory approaches in modelling evaporation from forests

AbstractThe near‐field effect on flux calculation was examined using two approaches: (1) The performance of a Lagrangian two‐layer canopy model was compared with a K‐theory two‐layer canopy model and a K‐theory single‐layer canopy model; and (2) the near‐field resistance was placed in series with aerodynamic resistance in the Canadian Land Surface Scheme (CLASS). The first approach was tested using flux data measured from a boreal aspen forest because it had a thick understorey canopy. The second approach was tested using the aspen forest and a Douglas fir forest. Results from both approaches confirmed that the difference between simulations from K‐theory and the Lagrangian evaporation models was small due to the strong control by stomatal conductance. Furthermore, the two‐layer canopy models were inferior to the single canopy model in the calculation of the sensible and latent heat fluxes above the forest. Copyright © 2001 Royal Meteorological Society

Modelling plant carbon and nitrogen dynamics of a boreal aspen forest in CLASS — the Canadian Land Surface Scheme

The formulation of the physiological processes of plant carbon and nitrogen that have recently been implemented in CLASS — the Canadian Land Surface Scheme is presented, including plant photosynthesis of sunlit and shaded leaves, root nitrogen uptake, tissue respiration, growth, and senescence. Results from this formulation provide the vegetation parameters (e.g. stomatal resistance, leaf area index, and root length and distribution) for simulating energy and water exchange, and contribute to the carbon and nitrogen biogeochemical calculations and ecosystem CO 2 flux estimations in CLASS. The model was parameterized for deciduous trees and run at a time step of 30 min. Modelled results were tested with measurements of leaf CO 2 exchange, half-hourly and daily plant CO 2 exchange, and annual plant carbon budgets from the Old Aspen ( Populus tremuloides) site in the Southern Study Area (SSA-OA) of the Boreal Ecosystem-Atmosphere Study (BOREAS). Model tests at leaf level and half-hourly timescale improve confidence in the predictive capabilities of the model at canopy level and longer timescale. At half-hourly timescale, simulated plant CO 2 exchange explained 81% of the variance in the measured CO 2 flux derived from eddy correlation measurements and soil chamber measurements in the three comparing weeks in 1994. At a coarser timescale, simulated daily plant CO 2 exchange explained 86% of the variance in the measured values during 1994 and 1996. Annual gross primary production (GPP) accumulated from half-hourly values in the model in 1994 and 1996 averaged about 1053 g C m −2 for this aspen site, close to the estimations from field eddy correlation measurements. Of this carbon fixation, 59% was consumed by autotrophic respiration ( R a) for plant growth and maintenance in the model, similar to estimations from chamber measurements. Sensitivity analyses show that the modelled plant GPP, R a and net primary production NPP (=GPP− R a) were very sensitive to the changes in temperature, implying that this boreal aspen ecosystem is strongly constrained by temperature conditions.

The seasonal water and energy exchange above and within a boreal aspen forest

The seasonal water and energy exchange of a boreal aspen forest underlain by a hazelnut understory is described. Measurements of above-aspen latent and sensible heat, short-wave and net radiation, and photosynthetically active radiation are compared to those measured above the hazelnut understory. Understory radiation measurements were made using a tram system. Energy storage at each measurement height was determined, and measurements of the soil moisture, temperature, and heat flux were made using an array of probes.The mean annual air temperature and total precipitation during 1994 were 1.2°C and 488.4mm, respectively, above the 1951–1980 average −0.2°C and total 462.6mm. There was a pronounced seasonal development of leaves, with the maximum leaf area index of the hazelnut (3.3m2m−2) exceeding that of the aspen (2.3m2m−2). Beneath-aspen radiation decreased exponentially as the aspen leaf area increased, and the calculated effective extinction coefficients decreased as the plant area index increased. At full aspen leaf, 27, 23, and 20% of the above-aspen short-wave, net, and photosynthetically active radiation, respectively, reached the hazelnut. The diurnal energy balance at both heights showed pronounced seasonal trends. Sensible heat from the forest floor dominated during the leaf-free period, whereas latent heat from the overstory dominated during the leafed period. The fraction of the annual precipitation evaporated was 82–91%, with 67–68%, 26–28%, and 4–7% originating from the aspen, hazelnut, and soil, respectively. Over the leafed period, soil water was depleted from the root zone (0–60cm depth) and accumulated between the 61–123cm depth, overall resulting in a deficit of 34.7mm between 0–123cm depths. This soil water balance compared well with the daily integrated difference between precipitation and eddy-covariance determined measurements of evaporation.