Search

Models represent our primary method for integration of small-scale, process-level phenomena into a comprehensive description of forest-stand or ecosystem function. They also represent a key method for testing hypotheses about the response of forest ecosystems to multiple changing environmental conditions. This paper describes the evaluation of 13 stand-level models varying in their spatial, mechanistic, and temporal complexity for their ability to capture intra- and interannual components of the water and carbon cycle for an upland, oak-dominated forest of eastern Tennessee. Comparisons between model simulations and observations were conducted for hourly, daily, and annual time steps. Data for the comparisons were obtained from a wide range of methods including: eddy covariance, sapflow, chamber-based soil respiration, biometric estimates of stand-level net primary production and growth, and soil water content by time or frequency domain reflectometry. Response surfaces of carbon and water flux as a function of environmental drivers, and a variety of goodness-of-fit statistics (bias, absolute bias, and model efficiency) were used to judge model performance.A single model did not consistently perform the best at all time steps or for all variables considered. Intermodel comparisons showed good agreement for water cycle fluxes, but considerable disagreement among models for predicted carbon fluxes. The mean of all model outputs, however, was nearly always the best fit to the observations. Not surprisingly, models missing key forest components or processes, such as roots or modeled soil water content, were unable to provide accurate predictions of ecosystem responses to short-term drought phenomenon. Nevertheless, an inability to correctly capture short-term physiological processes under drought was not necessarily an indicator of poor annual water and carbon budget simulations. This is possible because droughts in the subject ecosystem were of short duration and therefore had a small cumulative impact. Models using hourly time steps and detailed mechanistic processes, and having a realistic spatial representation of the forest ecosystem provided the best predictions of observed data. Predictive ability of all models deteriorated under drought conditions, suggesting that further work is needed to evaluate and improve ecosystem model performance under unusual conditions, such as drought, that are a common focus of environmental change discussions.Read More: http://www.esajournals.org/doi/abs/10.1890/03-4049

Differences in the seasonal pattern of assimilatory and respiratory processes are responsible for divergences in seasonal net carbon exchange among ecosystems. Using FLUXNET data ( http://www.eosdis.ornl.gov/FLUXNET ) we have analyzed seasonal patterns of gross primary productivity (FGPP), and ecosystem respiration (FRE) of boreal and temperate, deciduous and coniferous forests, Mediterranean evergreen systems, a rainforest, temperate grasslands, and C3 and C4 crops. Based on generalized seasonal patterns classifications of ecosystems into vegetation functional types can be evaluated for use in global productivity and climate change models. The results of this study contribute to our understanding of respiratory costs of assimilated carbon in various ecosystems. Seasonal variability of FGPP and FRE of the investigated sites increased in the order tropical Mediterranean temperate coniferous temperate deciduous boreal forests . Together with the boreal forest sites, the managed grasslands and crops show the largest seasonal variability. In the temperate coniferous forests, seasonal patterns of FGPP and FRE are in phase, in the temperate deciduous and boreal coniferous forests FRE was delayed compared to FGPP, resulting in the greatest imbalance between respiratory and assimilatory fluxes early in the growing season. FGPP adjusted for the length of the carbon uptake period decreased at the sampling sites across functional types in the order C4 crops, temperate and boreal deciduous forests (7.5–8.3 g C m −2 per day )> temperate conifers, C3 grassland and crops (5.7–6.9 g C m −2 per day )> boreal conifers (4.6 g C m−2 per day). Annual FGPP and net ecosystem productivity (FNEP) decreased across climate zones in the order tropical>temperate>boreal. However, the decrease in FNEP with latitude was greater than the decrease in FGPP, indicating a larger contribution of respiratory (especially heterotrophic) processes in boreal systems.

Quantifying net carbon (C) storage by forests is a necessary step in the validation of carbon sequestration estimates and in assessing the possible role of these ecosystems in offsetting fossil fuel emissions. In eastern North America, five sites were established in deciduous forests to provide measurements of net ecosystem CO2 exchange (NEE) using micro-meteorological methods, and measures of major carbon pools and fluxes, using a combination of forest mensurational, eco-physiological, and other biometric methods. The five study sites, part of the AmeriFlux network, ranged across 10 ◦ of latitude and 18 ◦ of longitude, but were all of similar age, canopy height, and stand basal area. Here we present a cross-site synthesis of annual carbon storage estimates, comparing meteorological and biometric approaches, and also comparing biometric estimates based on analyses of autotrophic carbon pools and heterotrophic carbon fluxes (net ecosystem production, NEP) versus those based on measurements of change in two major carbon pools (� C). Annual above-ground net primary production (ANPP) varied nearly two-fold among sites and was strongly correlated with average annual temperature and with annual soil nitrogen mineralization (Nmin). Estimates of NEP ranged from 0.7 Mg C per hectare per year in northern lower Michigan to 3.5 Mg C per hectare per year in central Indiana, and were also well correlated with Nmin. There was less variation among sites in estimates

○ The hydrological response of forests to rising CO 2 is a critical biotic feedback in the study of global climate change. Few studies, however, have investigated this highly dynamic response at relevant temporal and spatial scales. ○ A combination of leaf and whole-tree measurements and stand-level extrapolations were used to assess how stomatal conductance, canopy transpiration and conductance, and evapotranspiration might be affected by future, higher CO 2 concentrations. ○ Midday measurements of stomatal conductance for leaves sampled in a 12-yr-old sweetgum (Liquidambar styraciflua) stand exposed to free-air CO 2 enrichment were up to 44% lower at elevated than at ambient CO 2 concentrations, whereas canopy conductance, averaged over the growing season, was only 14% lower in stands exposed to CO 2 enrichment. The magnitude of this response was dependent on vapor pressure deficit and soil water potential. Annual estimates of evapotranspiration showed relatively small reductions due to atmospheric CO 2 enrichment. ○ These data illustrate that the hydrological response of a closed-canopy plantation to elevated CO 2 depends on the temporal and spatial scale of observation. They emphasize the importance of interacting variables and confirm that integration of measurements over space and time reduce what, at the leaf level, might otherwise appear to be a large and significant response.

A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with a mean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO2 fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated

At a deciduous forest in the southeast United States (Walker Branch Watershed, Oak Ridge, Tennessee), as at other sites with tall vegetation and/or complex terrain, it is difficult to temporally integrate eddy covariance data to obtain long-term estimates of net ecosystem exchange of carbon dioxide (NEE), primarily because of suspected systematic nocturnal errors. Therefore, although eddy covariance data can be invaluable, additional tools such as empirical gap-filling methods, independent measurements of CO2 flux using chambers, and simulations using canopy process models are often necessary to obtain reliable annual carbon uptake estimates. Two independent approaches for estimating annual NEE using these tools at the Walker Branch site are discussed. One approach is to cumulatively sum the full set of eddy covariance measurements over time. The second approach is to sum the output of NEE from a biophysical canopy exchange model (CANOAK). CANOAK incorporates independent chamber measurements on leaves, soil, and stems and is driven using the observed canopy architecture, meteorology, soil water content, and soil temperatures to predict NEE. Both methods estimate similar trends and magnitudes of daytime (daylight hours) soil respiration and NEE over 5 years. Both methods also suggest similar differences among years (interannual variability). These two estimates of NEE are used to address possible measurement bias errors at this site and to provide plausible estimates of annual NEE. The estimated mean annual NEE at this site is −574 g C m−2 yr−1 between 1995 and 1999, ranging from −470 g C m−2 y−1 (1995) to −629 g C m−2 yr−1 (1999) (negative NEE indicates uptake by forest).

Fluxes of carbon dioxide, water and energy between a temperate deciduous forest and the atmosphere were quantified across time scales of hours, days, seasons, years and decades. This exercise was performed using stand-level eddy covariance flux measurements and a biophysical model, CANOAK. The CANOAK model was tested with measurements of carbon dioxide, water vapor and energy flux densities we have been collecting since October 1994. Model calculations reproduced 80% of CO2 and water vapor flux variance that are contained in a year-long time series, when the model was forced with hourly weather data and seasonal information on plant structure and physiological capacity. Spectral analysis of measured and computed time series revealed that peak time scales of flux variance have periods of a day, half-week, season and year. We examined questions relating to inter-annual variability of mass and energy exchange by forcing the validated model with a decade-long meteorological record. Theoretical estimates of year-to-year variability of net ecosystem CO2 exchange were on the order of ±200 gC m−2 year. We also deduced that significant variance of water vapor and CO2 exchange occurs on the time scale of 5–6 years, the time scale associated with El Nino phenomena. Sensitivity tests performed with the model examined issues associated with model complex and parameterization issues. Of particular importance were the effects of leaf clumping and length of the growing season on canopy photosynthesis and net ecosystem CO2 exchange. Ignoring the effects of leaf clumping caused an error as large as 50% in the estimation of annual biosphere–atmosphere net carbon exchange. Each incremental day change in the length of the growing season altered the net ecosystem CO2 exchange by 5.9 gC m−2.

Temporal trends in photosynthetic capacity are a critical factorin determining the seasonality and magnitude of ecosystem carbonfluxes. At a mixed deciduous forest in the south-eastern United States (Walker Branch Watershed, Oak Ridge, TN, USA), we independently measured seasonal trends in photosynthetic capacity (using single-leaf gas exchange techniques) and the whole-canopycarbon flux (using the eddy covariance method). Soil respiration was also measured using chambers and an eddy covariance system beneath the canopy. These independent chamber and eddy covariance measurements, along with a biophysical model (CANOAK), areused to examine how leaf age affects the seasonal pattern of carbon uptake during the growing season. When the measured seasonality in photosynthetic capacity is representedin the CANOAK simulations, there is good agreement with the eddy covariance data on the seasonal trends in carbon uptake. Removing the temporal trends in the simulations by using the early season maximum value of photosynthetic capacity over the entire growing season over estimates the annual carbon uptake by about 300 g C m−2 year−1– halfthe total estimated annual net ecosystem exchange. Alternatively, use of the mean value of photosynthetic capacity incorrectly simulates the seasonality in carbon uptake by the forest. In addition to changes related to leaf development and senescence, photosynthetic capacitydecreased in the middle and late summer, even when leaf nitrogenwas essentially constant. When only these middle and late summer reductions were neglected in the model simulations, CANOAK still overestimated the carbon uptake by an amount comparable to 25% ofthe total annual net ecosystem exchange.

At present a network of over 100 field sites are measuring carbon dioxide, water vapor and sensible heat fluxes between the biosphere and atmosphere, on a nearly continuous basis. Gaps in the long term measurements of evaporation and sensible heat flux must be filled before these data can be used for hydrological and meteorological applications. We adapted methods of gap filling for NEE (net ecosystem exchange of carbon) to energy fluxes and applied them to data sets available from the EUROFLUX and AmeriFlux eddy covariance databases. The average data coverage for the sites selected was 69% and 75% for latent heat (λE) and sensible heat (H). The methods were based on mean diurnal variations (half-hourly binned means of fluxes based on previous and subsequent days, MDV) and look-up tables for fluxes during assorted meteorological conditions (LookUp), and the impact of different gap filling methods on the annual sum of λE and H is investigated. The difference between annual λE filled by MDV and λE filled by LookUp ranged from - 120 to 210 MJ m-2 per year, i.e. -48 to +86 mm per year, or -13 to +39% of the annual sum. For annual sums of H differences between -140 and + 140 MJ m-2 per year or -12 to +19% of the annual sum were found.

Fluxes of carbon dioxide, water and sensible heat were measured using three different eddy covariance systems above the forest floor of a closed deciduous forest (leaf area index 6). The primary objective was to examine the representativeness of a single eddy covariance system in estimating soil respiration for time scales ranging from one-half hour to more than one week. Exper- iments were conducted in which the eddy covariance sensors were in one of three configurations: i) collocated, ii) separated horizontally or iii) separated vertically. A measure of the variation between the three systems (CV 0 , related to the coefficient of variation) for half-hour carbon dioxide fluxes was 0.14 (collocated systems), 0.34 (vertically separated systems at 1, 2 and 4 m above the surface), and 0.57 (systems horizontally separated by 30 m). A similar variation was found for other scalar fluxes (sensible and latent heat). Variability between systems decreased as the number of half-hour sampling periods used to obtain mean fluxes was increased. After forty-eight hours (means from ninety-six half-hour samples), CV 0 values for carbon dioxide fluxes were 0.07, 0.09 and 0.16 in the collocated, vertically separated and horizontally separated experiments, respectively. The time de- pendence of variability has implications on the appropriateness of using short-term measurements in modelling validation studies. There are also implications concerning the appropriate number of half- hour samples necessary to obtain reliable causal relationships between flux data and environmental parameters. Based on the longer-term measurements, we also discuss the representativeness of a single eddy covariance system in long-term monitoring of soil respiration and evaporation beneath forest canopies using the eddy covariance method.

The advent of long-term studies on CO2 and water vapor exchange provides us with new information on how the atmosphere and biosphere interact. Conventional time series analysis suggests that temporal fluctuations of weather variables and mass and energy flux densities occur on numerous time scales. The time scales of variance that are associated with annual time series of meteorological variables, scalar flux densities and their covariance with one another, however, remain unquantified. We applied Fourier analysis to time series (4 years in duration) of photon flux density, air temperature, wind speed, pressure and the flux densities of CO 2 and water vapor. At the daily time scale, strong spectral peaks occurred in the meteorological and flux density records at periods of 12 and 24 h, due to the daily rising and setting of the sun. At the synoptic time scale (3‐7 days) the periodic passage of weather fronts alter available sunlight and temperature. In turn, variations in these state variables affect carbon assimilation, respiration and transpiration. At the seasonal and semi-annual time scales, a broad spectral peak occurs due to seasonal changes in weather and plant functionality and phenology. In general, 21% of the variance of CO2 exchange is associated with the annual cycle, 43% of the variance is associated with the diurnal cycle and 9% is associated with the semi-annual time scale. A pronounced spectral gap was associated with periods 3‐4 weeks long. Interactions between CO2 flux density ( Fc) and sunlight, air temperature and latent heat flux density were quantified using co-spectral, coherence and phase angle analyzes. Coherent and in-phase spectral peaks occur between CO2 exchange rates and water vapor exchange on annual, seasonal and daily time scales. A 180 phase shift occurs between Fc and photon flux density (Qp) on seasonal and daily time scales because the temporal course of sunlight corresponds with the withdrawal of CO2 from the atmosphere, a flux that possesses a negative sign. Covariations between Fc and Tair experience a 180 phase shift with one another at the seasonal time scale because rising temperatures are associated with more carbon uptake. At daily time scales the phase angle between Fc and Tair is on the order of 130. This phase lag can be explained by the strong dependence of canopy photosynthesis on available light and the 2‐3 h lags, which occur between the daily course of sunlight and air temperature.

Heightened awareness of global change issues within both science and political communities has increased interest in using the global network of eddy covariance flux towers to more fully understand the impacts of natural and anthropogenic phenomena on the global carbon balance. Comparisons of net ecosystem exchange ( F NEE ) responses are being made among biome types, phenology patterns, and stress conditions. The comparisons are usually performed on annual sums of F NEE ; however, the average data coverage during a year is only 65%. Therefore, robust and consistent gap filling methods are required. We review several methods of gap filling and apply them to data sets available from the EUROFLUX and AmeriFlux databases. The methods are based on mean diurnal variation (MDV), look-up tables (LookUp), and nonlinear regressions (Regr.), and the impact of different gap filling methods on the annual sum of F NEE is investigated. The difference between annual F NEE filled by MDV compared to F NEE filled by Regr. ranged from −45 to +200 g C m −2 per year (MDV−Regr.). Comparing LookUp and Regr. methods resulted in a difference (LookUp−Regr.) ranging from −30 to +150 g C m −2 per year. We also investigated the impact of replacing measurements at night, when turbulent mixing is insufficient. The nighttime correction for low friction velocities ( u ∗ ) shifted annual F NEE on average by +77 g C m −2 per year, but in certain cases as much as +185 g C m −2 per year. Our results emphasize the need to standardize gap filling-methods for improving the comparability of flux data products from regional and global flux networks.

A multi-year, multi-technique study was conducted to measure evapotranspiration and its components within an uneven-aged mixed deciduous forest in the Southeastern United States. Four different measurement techniques were used, including soil water budget (1 year), sap flow (2 years), eddy covariance (5 years), and catchment water budget (31 years). Annual estimates of evapotranspiration were similar for the eddy covariance and catchment water balance techniques, averaging 571±16 mm (eddy covariance) and 582±28 mm (catchment water balance) per year over a 5-year period. There were qualitative similarities between sap flow and eddy covariance estimates on a daily basis, and sap flow estimates of transpiration were about 50% of annual evapotranspiration estimated from eddy covariance and catchment studies. Soil evaporation was estimated using a second eddy covariance system below the canopy, and these measurements suggest that soil evaporation explains only a small portion of the difference between sap flow estimates of transpiration and eddy covariance and catchment water budget estimates of evapotranspiration. Convergence of the catchment water balance and eddy covariance methods and moderately good energy balance closure suggests that the sap flow estimates could be low, unless evaporation of canopy-intercepted water was especially large. The large species diversity and presence of ring-porous trees at our site may explain the difficulty in extrapolating sap flow measurements to the spatial scales representative of the eddy covariance and catchment water balance methods. Soil water budget estimates were positively correlated with eddy covariance and sap flow measurements, but the data were highly variable and in error under conditions of severe surface dryness and after rainfall events.

Equations are presented to correct eddy-covariance measurements for both fluctuations in density and non-zero mean advection, induced by convergence or divergence of flow, and spatial source/sink inhomogeneity, under steady-state and transient conditions. This correction collapses to the Webb-Pearman-Leuning expression if the mean vertical velocity is zero, and formally adds the Webb-Pearman-Leuning expression to the corrections suggested by Lee for conditions of non-zero vertical velocity and source/sink and mean scalar horizontal homogeneity. The equation requires measurement of the mean vertical gradients of the scalar concentration of interest (air temperature, humidity, CO2) as well as an accurate estimation of the mean vertical velocity, in addition to the vertical eddy covariance of the scalar. Simple methods for the approximation of sensor tilt and complex terrain flow angle are presented, to allow estimation of non-zero mean vertical velocities. The equations are applied to data from a maize crop and a forest to give examples of when the correction is significant. In addition, a term for the thermodynamic expansion energy associated with water vapour flux is derived, which implies that the sonic temperature derived sensible heat flux will accurately include this contribution.

There is a strong need to extend whole-tree measurements of sap flow into broad-leaved forests where characteristics of stand structure, surface roughness, leaf dimension, and aerodynamic and canopy conductance may interact to partially decouple the canopy from the atmosphere. The implications of this partial decoupling to understanding the environmental control of canopy transpiration and to the modeling of forest water use are many. Therefore, thermal dissipation probes were used over a three-month period (June through August, 1997) to quantify day-to-day and tree-to-tree variation in whole-tree sap flow (Q) for 12 red maple (Acerrubrum L.) trees growing in an upland oak forest of eastern Tennessee. Whole-tree Q was calculated as the product of measured sap velocity, sapwood area and the fraction of sapwood functional in water transport. Daily canopy transpiration (Ec) was calculated from whole-tree Q and projected crown area, whereas average daily conductance (gc) was derived by inverting the Penman–Monteith equation. Maximum Q averaged 73 kg per tree per day and varied between 45 and 160 kg per day for trees that ranged in stem diameter (DBH) from 17 to 35 cm, and from 19 to 26 m in height. Canopy transpiration peaked at 3.0 mm per day in early July and averaged 1.5 mm per day over the 3-month measurement period. Tree-to-tree variability for Ec was high. Maximum rates of Ec varied from 1.9 mm per day for the tree with the smallest projected crown area to 5.7 mm per day for one of the largest trees. Day-to-day variation in Ec was a function of daily differences in net radiation (Rn) and atmospheric humidity deficit (δe). Increases in daily Rn and δe led to linear increases in canopy transpiration and there was no indication that a plateau-style relationship existed between Ec and average daily δe. Mean daily gc ranged from 1.4 to 6.7 mm s−1, and averaged 3.4 mm s−1 across the 12 study trees. Some of the tree-to-tree variation observed for Ec and gc was related to the fact that not all trees occupied the same vertical position within the stand. Variation in estimates of the daily decoupling coefficient (0≤Ω≤1) was also considerable and for individual trees the seasonally-averaged Ω varied from 0.12 to 0.37, and averaged 0.23 for the 12 study trees. An Ω of this magnitude indicates that red maple canopies are partially decoupled from the atmosphere and suggests that significant vertical gradients of air temperature and δe from the canopy surface to the bulk air several meters above the canopy are possible. Model analysis of hourly data indicated that simulated surface temperatures in mid-July were 3.6–5.8°C higher than above-canopy reference temperatures, and δe at the canopy surface was 0.3 kPa higher than that of the bulk atmosphere. These calculations were partially supported by leaf-level measurements taken on one of the trees from a 20-m canopy-access tower. The implications of this partial decoupling to understanding and modeling the environmental control of canopy transpiration are discussed.

The energy balance components were measured above the ground surface of a temperate deciduous forest over an annual cycle using the eddy covariance technique. Over a year, the net radiation at the forest floor was 21.5% of that above the canopy, but this proportion was not constant, primarily because of the distinct phenological stages separated by the emergence and senescence of leaves. The dominant response to seasonal changes in net radiation was through corresponding changes in the sensible heat flux, and both net radiation and sensible heat flux peaked just before leaf emergence. Evaporation at the forest floor was typically less than 0.5 mm per day, and unlike sensible heat flux, was not closely coupled to seasonal changes in net radiation. Instead, evaporation at the forest floor responded primarily to rapid changes in litter water content. Forest floor evaporation was limited by the water-holding capacity of litter, and when the atmospheric demand was large, the litter layer dried on the time scale of several hours. After this rapid period of drying, net radiation and sensible heat flux dominated the energy budget. When leaves were present during the growing season, the sensible and latent energy fluxes at the forest floor were less than 10% of the total canopy fluxes, and the mean Bowen ratio was similar to that above the canopy. However, during the dormant season, the controls of the energy budget at the forest floor largely determine the whole canopy fluxes. On an annual basis, the fluxes from the forest floor are roughly 15–22% of those above the canopy and the evaporation was 86 mm.

The components of the surface energy balance were measured for 3 years over a broadleaved deciduous forest using the eddy covariance technique. Within years, the magnitude and distribution of fluxes was controlled by seasonal changes in solar radiation, drought, as well as leaf emergence and senescence. Evapotranspiration increased by a factor greater than five (from about 0.5 to 3 mm day 1 ) after leaves emerged in spring. Large decreases in sensible heat flux were observed over the same period (6 to 2 MJ day 1 ) despite increases in solar radiation. The most influential effect on annual fluxes was the occurrence and extent of drought, with lesser control exerted by differences in the timing of leaf expansion and leaf senescence. Average annual evapotranspiration over the period was 567 mm and ranged from 537 to 611 mm. The year with the lowest precipitation, soil moisture content and surface conductance also had the lowest evapotranspiration. Although evapotranspiration was quite sensitive to surface conductance and surface conductance was reduced substantially by drought, the correlation of low surface conductance and high humidity deficit reduced the effects of drought on evapotranspiration. Differences in net radiation among years were only a minor source of variability in evapotranspiration. In addition to surface conductance, other bulk parameters are calculated to describe the general exchange characteristics of this forest. ©2000 Elsevier Science B.V. All rights reserved.

Summary The direct effect of elevated carbon dioxide on evapotranspiration over a growing season was investigated by scaling up single-leaf gas exchange measurements on soybean and corn plants grown and measured at three carbon dioxide concentrations. Stomatal conductance decreased markedly with increasing carbon dioxide in these species under most conditions. Coupled soil–vegetation–atmosphere models were used to scale up these single-leaf level measurements to simulate evapotranspiration at the regional scale from planting to harvest. The coupled modelling system introduced feedbacks over the season that are not present at the measurement level, which decreased the effect of carbon dioxide on evapotranspiration. Four sets of simulations were performed to evaluate specifically the magnitude of four feedbacks; two resulting from scale, surface layer and mixed layer feedback, one resulting from soil evaporation and one resulting from the interactions of stomatal conductance and the simulated canopy microclimate (physiological feedback). The feedbacks occurring from scale were consistent with previous analytical work indicating that transpiration becomes less dependent on stomatal conductance at larger scales. Evaporation from the soil has been generally neglected in past studies on carbon dioxide effects, but was especially important in decreasing the effects of carbon dioxide on evapotranspiration and showed a seasonal dynamic. The feedback resulting from physiological responses has also received less attention than the feedbacks from scale, but was only moderately important in these simulations. We also investigated the seasonal dynamics of how the observed increase in leaf area at elevated carbon dioxide affects evapotranspiration. Considering all the feedbacks and the observed increase in leaf area at elevated carbon dioxide, the simulated decrease in evapotranspiration was not negligible but was much less than the decrease in stomatal conductance. At the regional scale and maximum complexity in our model, the simulated decrease in seasonal evapotranspiration at doubled carbon dioxide (700 μmol mol–1) was 5.4% for soybeans and 8.6% for corn.

[1] Future climate change is expected to affect ecosystematmosphere CO2 exchange, particularly through the influence of temperature. To date, however, few studies have shown that differences in the response of net ecosystem CO2 exchange (NEE) to temperature among ecosystems can be explained by differences in the photosynthetic and respiratory processes that compose NEE. Using a new nonparametric statistical model, we analyzed data from four forest ecosystems. We observed that differences among forests in their ability to assimilate CO2 as a function of temperature were attributable to consistent differences in the temperature dependence of photosynthesis and respiration. This observation provides empirical validation of efforts to develop models of NEE from the first-principle relationships between photosynthetic and respiratory processes and climate. Our results also showed that models of seasonal dynamics in NEE that lack specific consideration of the temperature dependence of respiration and photosynthesis are likely to carry significant uncertainties. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1615 Global Change: Biogeochemical processes (4805); 3307 Meteorology and Atmospheric Dynamics: Boundary layer processes; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 4806 Oceanography: Biological and Chemical: Carbon cycling. Citation: Yi, C., et al. (2004), A nonparametric method for separating photosynthesis and respiration components in CO2 flux measurements, Geophys. Res. Lett., 31, L17107, doi:10.1029/2004GL020490.

We tested the hypothesis that the date of the onset of net carbon uptake by temperate deciduous forest canopies corresponds with the time when the mean daily soil temperature equals the mean annual air temperature. The hypothesis was tested using over 30 site-years of data from 12 field sites where CO(2) exchange is being measured continuously with the eddy covariance method. The sites spanned the geographic range of Europe, North America and Asia and spanned a climate space of 16 degrees C in mean annual temperature. The tested phenology rule was robust and worked well over a 75 day range of the initiation of carbon uptake, starting as early as day 88 near Ione, California to as late as day 147 near Takayama, Japan. Overall, we observed that 64% of variance in the timing when net carbon uptake started was explained by the date when soil temperature matched the mean annual air temperature. We also observed a strong correlation between mean annual air temperature and the day that a deciduous forest starts to be a carbon sink. Consequently we are able to provide a simple phenological rule that can be implemented in regional carbon balance models and be assessed with soil and temperature outputs produced by climate and weather models.

[1] We present the first direct, multisite observations in support of the hypothesis that atmospheric aerosols affect the regional terrestrial carbon cycle. The daytime growing season (summer) CO2 flux observations from six sites (forest, grasslands, and croplands) with collocated aerosol and surface radiation measurements were analyzed for high and low diffuse radiation; effect of cloud cover; and effect of high and low aerosol optical depths (AOD). Results indicate that, aerosols exert a significant impact on net CO2 exchange, and their effect may be even more significant than that due to clouds. The response appears to be a general feature irrespective of the landscape and photosynthetic pathway. The CO2 sink increased with aerosol loading for forest and crop lands, and decreased for grassland. The cause for the difference in response between vegetation types is hypothesized to be canopy architecture.

[1] Data from a network of eddy covariance stations in Europe and North America (FLUXNET) were analyzed to examine the diurnal patterns of surface energy and carbon fluxes during the summer period across a range of ecosystems and climates. Diurnal trends were quantified by assessing the time of day surface fluxes and meteorological variable reached peak values, using the “diurnal centroid” method; the diurnal centroid enabled us to discern whether the peak activity of the variable of interest is weighted more toward the morning or afternoon. In this paper, diurnal centroid estimates were used to diagnose which atmospheric and physiological processes controlled carbon dioxide, water vapor, and sensible heat fluxes across different ecosystems and climates. Sensitivity tests suggested that the diurnal centroids for latent (LE) and sensible (H) heat flux depend on atmospheric resistance, static stability in the free atmosphere, stomatal response to vapor pressure deficit, and advection. With respect to diurnal trends of surface energy fluxes at FLUXNET sites, maximum LE occurred later in the day relative to H at most tall forests with continental climates. The lag between LE and H was reduced or reversed at sites that were influenced by advection or by afternoon stomatal closure. The time of peak carbon uptake of temperate forests occurred earlier relative to the temporal peak of photosynthetically active radiation, as compared to boreal forests. The timing of this peak occurred earlier during periods with low soil water content, as it did during the summer in Mediterranean climates. In this case, the diurnal centroid for the CO2 flux was influenced by the response of respiration and photosynthesis to increasing afternoon temperature and by afternoon stomatal closure.

Gas exchange techniques were used to investigate light-saturated carbon assimilation and its stomatal and non-stomatal limitations over two seasons in mature trees of five species in a closed deciduous forest. Stomatal and non-stomatal contributions to decreases in assimilation resulting from leaf age and drought were quantified relative to the maximum rates obtained early in the season at optimal soil water contents. Although carbon assimilation, stomatal conductance and photosynthetic capacity (V(cmax)) decreased with leaf age, decreases in V(cmax) accounted for about 75% of the leaf-age related reduction in light-saturated assimilation rates, with a secondary role for stomatal conductance (around 25%). However, when considered independently from leaf age, the drought response was dominated by stomatal limitations, accounting for about 75% of the total limitation. Some of the analytical difficulties associated with computing limitation partitioning are discussed, including path dependence, patchy stomatal closure and diffusion in the mesophyll. Although these considerations may introduce errors in our estimates, our analysis establishes some reasonable boundaries on relative limitations and shows differences between drought and non-drought years. Estimating seasonal limitations under natural conditions, as shown in this study, provides a useful basis for comparing limitation processes between years and species.

We used gas exchange techniques to estimate maximum rate of carboxylation (V cmax ), a measure of photosynthetic capacity, in the understory and upper crown of a closed deciduous forest over two seasons. There was extensive variability in photosynthetic capacity as a result of vertical canopy position, species type, leaf age and drought. Photosynthetic capacity was greater in oaks than in maples and greater in the overstory than in the understory. Parameter V cmax was maximal early in the season but declined slowly throughout most of the summer, and then more rapidly during senescence. There was also an apparent decline during drought in some trees. Variability in V cmax as a result of species or vertical canopy gradients was described well by changes in leaf nitrogen per unit area (N a ). However, temporal changes in V cmax were often poorly correlated with leaf nitrogen, especially in spring and summer and during drought. This poor correlation may be the result of a seasonally dependent fractional allocation of leaf nitrogen to Rubisco; however, we could not discount Rubisco inactivation, patchy stomatal closure or changes in mesophyll resistance. Consequently, when a single annual regression equation of V cmax versus N a was used for this site, there were substantial errors in the temporal patterns in V cmax that will inevitably result in modeling errors.

Changes in regional precipitation patterns will directly impact soil and foliage water status, resulting in physiological modifications in trees that can affect carbon assimilation rates (Briggs et al. 1986; Teskey et al. 1986; Ni and Pallardy 1992). Decreasing water potentials in the root and/or foliage can affect the carbon assimilation by adjusting stomatal conductance (Hinckley et al. 1978a; Epron and Dreyer 1993; Lowenstein and Pallardy 1998) or possibly by directly impacting the biochemical potential for carbon assimilation in the leaf (Lawlor 1995; Escalona et al. 1999). Tree species differ in their morphology and use of physiologic adaptations to avoid the negative impacts of drought on carbon assimilation (Hinckley et al. 1978b; Bahari et al. 1985; Briggs et al. 1986; Ni and Pallardy 1992; Abrams and Mostoller 1995; Lowenstein and Pallardy 1998; Tschaplinski et al. 1998). Species-specific differences in gas-exchange response to drought depend on characteristics such as rooting depth, stomatal sensitivity, osmotic adjustment, cavitation avoidance, and increased tolerance of desiccation (Abrams 1990; Lowenstein and Pallardy 1998; Tshaplinski et al. 1998). For example, because of their deep rooting habit, Quercus species are expected to outcompete the more mesic Acer and Cornus species in dry climates (Hinckley et al. 1979; Bahari et al. 1985; Abrams 1990). Species also may differ in their ability to recover predrought photosynthetic rates following a precipitation event (Ni and Pallardy 1992). Because of shallower rooting depths and limited water-storage capacity, photosynthesis of understory species and saplings is expected to be more sensitive to drying when compared to large overstory trees (Donovan and Ehleringer 1991; Flanagan et al. 1992).

This paper focuses on how canopy structure, its physiological functioning and the environment interact to control and drive the exchange of carbon dioxide (CO2) and water vapor between a temperate forest stand and the atmosphere. First, we present an overview of how temporal and spatial variations in canopy structure (e.g., leaf area index, species, leaf inclination angles, leaf clumping) and physiological functioning (e.g., maximal stomatal conductance, photosynthetic capacity) modulate CO2 and water vapor fluxes. Then, with the biophysical model CANOAK, we quantify the effects of leaf dimension and thickness, vertical variations in leaf area and photosynthetic capacity, leaf clumping, leaf inclination angles, photosynthetic capacity, stomatal conductance and weather on the annual sums of CO2, water vapor and sensible heat exchange. Finally, we discuss how much detail is needed in a model to predict fluxes of CO2 and water vapor with acceptable fidelity.

The exchanges of solar energy, carbon dioxide, water vapor and trace gases between a forest and the atmosphere are among the most fundamental processes to be quantified when studying the physiological and ecological functioning of a forest and the chemistry and climate of its overlying atmosphere. A forest must attain energy to sustain the work that is needed to assimilate carbon dioxide, for biosynthesis, to evaporate water, and to transport nutrients from the soil to the plant. Concurrently, these activities require flows of substrate material, which are obtained from the atmosphere and soil.

International audience; As length and timing of the growing season are major factors explaining differences in carbon exchange of ecosystems. we analyzed seasonal patterns of net ecosystem carbon exchange (F-NEE) using eddy covariance data of the FLUXNET data base (http://www-eosdis.ornl.gov/FLUXNET). The study included boreal and temperate. deciduous and coniferous forests, Mediterranean evergreen systems, rainforest, native and managed temperate grasslands, tundra, and C-3 and C-4 crops. Generalization of seasonal patterns are useful for identifying functional vegetation types for global dynamic vegetation models, as well as for global inversion studies, and can help improve phenological modules in SVAT or biogeochemical models. The results of this study have important validation potential for global carbon cycle modeling. The phasing of respiratory and assimilatory capacity differed within forest types: for temperate coniferous forests seasonal uptake and release capacities are in phase, for temperate deciduous and boreal coniferous forests, release was delayed compared to uptake. According to seasonal pattern of maximum nighttime release (evaluated over 15-day periods. F-max) the study sites can be grouped in four classes: (1) boreal and high altitude conifers and grasslands: (2) temperate deciduous and temperate conifers; (3) tundra and crops; (4) evergreen Mediterranean and tropical forest,,, Similar results are found for maximum daytime uptake (F-min) and the integral net carbon flux, but temperate deciduous forests fall into class 1. For forests, seasonal amplitudes of F-max and F-min increased in the order tropical < Mediterranean and temperate coniferous < temperate deciduous and boreal forests, and the pattern seems relatively stable for these groups. The seasonal amplitudes of F-max and F-min are largest for managed grasslands and crops. Largest observed values of F-min varied between -48 and -2 mumol m(-2) s(-1), decreasing in the order C-4-crops > C-3-crops > temperate deciduous forests > temperate conifers > boreal conifers > tundra ecosystems. Due to data restrictions, our analysis centered mainly on Northern Hemisphere temperate and boreal forest ecosystems. Grasslands, crops, Mediterranean ecosystems, and rainforests are under-represented. as are savanna systems, wooded grassland, shrubland, or year-round measurements in tundra systems. For regional or global estimates of carbon sequestration potentials, future investigations of eddy covariance should expand in these systems.

The warm season (mid-June through late August) partitioning between sensible (H) and latent (LE) heat flux, or the Bowen ratio (beta=H/LE), was investigated at 27 sites over 66 site years within the international network of eddy covariance sites (FLUXNET). Variability in beta across ecosystems and climates was analyzed by quantifying general climatic and surface characteristics that control flux partitioning. The climatic control on beta was quantified using the climatological resistance (R-i), which is proportional to the ratio of vapor pressure deficit (difference between saturation vapor pressure and atmospheric vapor pressure) to net radiation (large values of R-i decrease beta). The control of flux partitioning by the vegetation and underlying surface was quantified by computing the surface resistance to water vapor transport (R-c, with large values tending to increase beta). There was a considerable range in flux partitioning characteristics (R-c, R-i and beta) among sites, but it was possible to define some general differences between vegetation types and climates. Deciduous forest sites and the agricultural site had the lowest values of R-c and beta (0.25-0.50). Coniferous forests typically had a larger R-c and higher beta (typically between 0.50 and 1.00 but also much larger). However, there was notable variability in R-c and R-i between coniferous site years, most notably differences between oceanic and continental climates and sites with a distinct dry summer season (Mediterranean climate). Sites with Mediterranean climates generally had the highest net radiation, R-c, R-i, and beta. There was considerable variability in beta between grassland site years, primarily the result of interannual differences in soil water content and R-c