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Northern peatland stores a large amount of organic soil carbon and is considered to be one of the most significant CH4 sources among wetlands. The default wetland CH4 emission scheme in JULES (land surface model of the UK Earth System model) only takes into account the CH4 emissions from inundated areas in a simple way. However, it is known that the processes for peatland CH4 emission are complex. In this work, we coupled the process-based peatland CH4 emission model HIMMELI (HelsinkI Model of MEthane buiLd-up and emIssion for peatlands) with JULES (JULES-HIMMELI) by taking the HIMMELI input data from JULES simulations. Firstly, the soil temperature, water table depth (WTD) and soil carbon simulated by JULES, as well as the prescribed maximum leaf area index (LAI) in JULES were evaluated against available datasets at the studied northern wetland sites. Then, the simulated CH4 emissions from JULES and JULES-HIMMELI simulations were compared against the observed CH4 emissions at these sites. Moreover, sensitivities of CH4 emissions to the rate of anoxic soil respiration (anoxic Rs), surface soil temperature and WTD were investigated. Results show that JULES can well represent the magnitude and seasonality of surface (5–10 cm) and relatively deep (34–50 cm) soil temperatures, whereas the simulated WTD and soil carbon density profiles show large deviations from the site observations. The prescribed maximum LAI in JULES was within one standard deviation of the maximum LAIs derived from the Sentinel-2 satellite data for Siikaneva, Kopytkowo and Degerö sites, but lower for the other three sites. The simulated CH4 emissions by JULES have much smaller inter-annual variability than the observations. However, no specific simulation setup of the coupled model can lead to consistent improvements in the simulated CH4 emissions for all the sites. When using observed WTD or modified soil decomposition rate, there were only improvements in simulated CH4 fluxes at certain sites or years. Both simulated and observed CH4 emissions at sites strongly depend on the rate of anoxic Rs, which is the basis of CH4 emission estimates in HIMMELI. By excluding the effect from the rate of anoxic Rs on CH4 emissions, it is found that the Rs-log-normalized CH4 emissions (log normalization of the ratio of CH4 emission to anoxic Rs rate) show similar increasing trends with increased surface soil temperature from both observations and simulations, but different trends with raised WTD which may due to the uncertainty in simulated O2 concentration in HIMMELI. In general, we consider the JULES-HIMMELI model is more appropriate in simulating the wetland CH4 emissions than the default wetland CH4 emission scheme in JULES. Nevertheless, in order to improve the accuracy of simulated wetland CH4 emissions with the JULES-HIMMELI model, it is still necessary to better represent the peat soil carbon and hydrologic processes in JULES and the CH4 production and transportation processes in HIMMELI, such as plant transportation of gases, seasonality of parameters controlling oxidation and production, and adding microbial activities.

Methane emission from trees may partially or completely offset the methane sink in upland soils, the only process that has been regularly included in methane budgets for forest ecosystems. Our objective was to analyze multiple biogeochemical processes that influence the production, oxidation and transport of methane in a riparian cottonwood ecosystem and its adjacent river. We combined chamber flux measurements on tree stems, forest soil and the river surface with eddy covariance measurements of methane net ecosystem exchange. In addition, we tested whether methanogens were present in cottonwood stems, shallow soil layers and alluvial groundwater. Average midday peak in net methane emission measured by eddy covariance was c. 12 nmol m-2 s-1 . The average uptake of methane by soils (0.87 nmol m-2 s-1 ) was largely offset by tree stem methane emission (0.75 nmol m-2 s-1 ). There was evidence of methanogens in tree stems but not in shallow soil. Growing season (May-September) cumulative net methane emission (17.4 mmol CH4 m-2 ) included methane produced in cottonwood stems and methane input to the nocturnal boundary layer from the forest and the adjacent river. The multiple processes contributing to methane emission illustrated the linked nature of these adjacent terrestrial and aquatic ecosystems.

We compiled hydrogen and oxygen stable isotope compositions (δ

In the global methane budget, the largest natural source is attributed to wetlands, which encompass all ecosystems composed of waterlogged or inundated ground, capable of methane production. Among them, northern peatlands that store large amounts of soil organic carbon have been functioning, since the end of the last glaciation period, as long-term sources of methane (CH4) and are one of the most significant methane sources among wetlands. To reduce uncertainty of quantifying methane flux in the global methane budget, it is of significance to understand the underlying processes for methane production and fluxes in northern peatlands. A methane model that features methane production and transport by plants, ebullition process and diffusion in soil, oxidation to CO2, and CH4 fluxes to the atmosphere has been embedded in the ORCHIDEE-PEAT land surface model that includes an explicit representation of northern peatlands. ORCHIDEE-PCH4 was calibrated and evaluated on 14 peatland sites distributed on both the Eurasian and American continents in the northern boreal and temperate regions. Data assimilation approaches were employed to optimized parameters at each site and at all sites simultaneously. Results show that methanogenesis is sensitive to temperature and substrate availability over the top 75 cm of soil depth. Methane emissions estimated using single site optimization (SSO) of model parameters are underestimated by 9 g CH4 m−2 yr−1 on average (i.e., 50 % higher than the site average of yearly methane emissions). While using the multi-site optimization (MSO), methane emissions are overestimated by 5 g CH4 m−2 yr−1 on average across all investigated sites (i.e., 37 % lower than the site average of yearly methane emissions).

Riparian cottonwood forests in dry regions of western North America do not typically receive sufficient growing season precipitation to completely support their relatively high transpiration requirements. Water used in transpiration by riparian ecosystems must include alluvial groundwater or water stored in the potentially large reservoir of the unsaturated soil zone. We used the stable oxygen and hydrogen isotope composition of stem xylem water to evaluate water sources used by the dominant riparian cottonwood (Populus spp.) trees and shrubs (Shepherdia argentea and Symphoricarpos occidentalis) in Lethbridge, Alberta, during 3 years of contrasting environmental conditions. Cottonwoods did not exclusively take up alluvial groundwater but made extensive use of water sourced from the unsaturated soil zone. The oxygen and hydrogen isotope compositions of cottonwood stem water did not strongly overlap with those of alluvial groundwater, which were closely associated with the local meteoric water line. Instead, cottonwood stem water δ¹⁸O and δ²H values were located below the local meteoric water line, forming a line with a low slope that was indicative of water exposed to evaporative enrichment of heavy isotopes. In addition, cottonwood xylem water isotope compositions had negative values of deuterium excess (d‐excess) and line‐conditioned (deuterium) excess (lc‐excess), both of which provided evidence that water taken up by the cottonwoods had been exposed to fractionation during evaporation. The shrub species had lower values of d‐excess and lc‐excess than had the cottonwood trees due to shallower rooting depths, and the d‐excess values declined during the growing season, as shallow soil water that was taken up by the plants was exposed to increasing, cumulative evaporative enrichment. The apparent differences in functional rooting pattern between cottonwoods and the shrub species, strongly influenced the ratio of net photosynthesis to stomatal conductance (intrinsic water‐use efficiency), as shown by variation among species in the δ¹³C values of leaf tissue.

Within the grassland-dominated landscape of the North American Great Plains, riparian forest ecosystems exist along river floodplains. We compared cumulative evapotranspiration (ET) and ecosystem water-use efficiency (WUE) between a cottonwood forest and a nearby native grassland ecosystem in southern Alberta, using eddy covariance measurements during May-September (growing season) of three study years. Our objective was to test predictions about mechanistic controls on ecosystem water-use, and to provide insights into the amount of alluvial groundwater and stored soil water required to support a healthy riparian forest within the Great Plains biome. Grassland ET was dependent on precipitation inputs during the growing season. Cumulative growing season ET at the cottonwood site (375–451 mm) exceeded grassland ET (111–213 mm) by 2.1- to 3.4-fold depending on study year, despite slightly higher WUE in the cottonwood ecosystem. The difference in cumulative ET between ecosystems ranged from 238 to 264 mm in different years, in a region that normally receives 258 mm of cumulative precipitation during May-September. The large ET at the cottonwood site was caused by two-fold higher LAI, and associated greater canopy conductance than was apparent at the grassland site. The additional soil water required for the higher cottonwood ET was supplied by access to alluvial groundwater, which is recharged by river water, and was also supported by a larger soil volume to store water from precipitation and river flooding inputs. These factors resulted in a relatively long interval for cottonwood photosynthetic gas exchange that was consistent among years despite widely different environmental conditions, while the grassland had shorter growing season lengths that were constrained further as precipitation and soil moisture declined among years. Our analyses contribute to understanding the water requirements of these contrasting ecosystems and will help to improve management procedures for regulating river flow rates in order to sustain healthy riparian cottonwood ecosystems.

Peatlands and forests cover large areas of the boreal biome and are critical for global climate regulation. They also regulate regional climate through heat and water vapour exchange with the atmosphere. Understanding how land-atmosphere interactions in peatlands differ from forests may therefore be crucial for modelling boreal climate system dynamics and for assessing climate benefits of peatland conservation and restoration. To assess the biophysical impacts of peatlands and forests on peak growing season air temperature and humidity, we analysed surface energy fluxes and albedo from 35 peatlands and 37 evergreen needleleaf forests-the dominant boreal forest type-and simulated air temperature and vapour pressure deficit (VPD) over hypothetical homogeneous peatland and forest landscapes. We ran an evapotranspiration model using land surface parameters derived from energy flux observations and coupled an analytical solution for the surface energy balance to an atmospheric boundary layer (ABL) model. We found that peatlands, compared to forests, are characterized by higher growing season albedo, lower aerodynamic conductance, and higher surface conductance for an equivalent VPD. This combination of peatland surface properties results in a similar to 20% decrease in afternoon ABL height, a cooling (from 1.7 to 2.5 degrees C) in afternoon air temperatures, and a decrease in afternoon VPD (from 0.4 to 0.7 kPa) for peatland landscapes compared to forest landscapes. These biophysical climate impacts of peatlands are most pronounced at lower latitudes (similar to 45 degrees N) and decrease toward the northern limit of the boreal biome (similar to 70 degrees N). Thus, boreal peatlands have the potential to mitigate the effect of regional climate warming during the growing season. The biophysical climate mitigation potential of peatlands needs to be accounted for when projecting the future climate of the boreal biome, when assessing the climate benefits of conserving pristine boreal peatlands, and when restoring peatlands that have experienced peatland drainage and mining.

Detailed knowledge of the interactions between the atmosphere, biosphere and pedosphere is essential for an understanding of global carbon dynamics in terrestrial ecosystems. This chapter addresses ecosystem level variation of Δe, possible impacts of seasonality, of characteristic features of the dominant plant species within an ecosystem (life form, canopy structure, age) as well as the influence of climate and soil nutrient availability. It focuses on where to obtain tro-pospheric baseline data, and then discuss how to determine 13carbon of respired CO2. Canopy air should be collected dry, and stored in inert flasks that do not leak or exchange CO2, with seals that do not exchange with CO2. Natural landscapes comprise mosaics of evergreen and deciduous ecosystems, most pronounced in temperate and boreal regions. Information about the influence of different life forms on ecophysiological processes is therefore critical for understanding what controls carbon dynamics in evergreen and deciduous forest ecosystems, and how to integrate over diverse regions.

Terrestrial ecosystems play a major role in the global carbon cycle. There is a great need, therefore, to understand how global changes, such as increasing atmospheric CO2 and enhanced levels of nitrogen deposition, will influence physiological responses of terrestrial vegetation, and how the vegetation can feedback to influence the global carbon cycle and the earth’s climate system. Despite significant progress in using micrometerological and remote sensing techniques, attempts to extend physiological measurements to large areas are often difficult for a variety of technical reasons. The source of water taken up by plants is precipitation and/or ground water, and since no fractionation occurs during water uptake by most plants, the isotope ratio of water in plant roots is the same as that of the water available in the soil.

Episodic precipitation pulses are important for driving biological activity in desert ecosystems. The pattern of precipitation, including the size of rain events and the duration of time between events, can influence ecosystem net CO2 exchange (NEE) by shifting the balance between ecosystem photosynthesis and respiration. Our objective was to measure the response of NEE and its major components, to seasonal variation in precipitation and other environmental conditions. The study was conducted at a site, where 40–60% of annual precipitation comes from the North American Monsoon that typically brings rain in July–September, a time period when temperatures are near the seasonal peak. The results were compared to a model of the expected responses of NEE to seasonal changes in precipitation and temperature. We measured NEE using the eddy covariance technique during September 2015–August 2016. The ecosystem showed large (fivefold) seasonal variation in maximum photosynthesis and ecosystem respiration rate at 10 °C that corresponded to seasonal variation in precipitation and temperature. Ecosystem respiratory activity exceeded photosynthetic activity, so the ecosystem was a net source of CO2 to the atmosphere during June–October, a period that included monsoon rain inputs. Only during the winter months (November–March) did photosynthesis exceed respiration, resulting in net ecosystem carbon sequestration. The ecosystem recorded a net loss of 10 g C m−2 year−1, which was likely caused by below normal annual precipitation during the study. Our results illustrated the important interaction between seasonal variation in precipitation and temperature in controlling the ecosystem carbon budget.

Forest mortality has been widely observed across the globe during recent episodes of drought and extreme heat events. But the future of forest mortality remains poorly understood. While the direct effects of future climate and elevated CO2 on forest mortality risk have been studied, the role of lateral subsurface water flow has rarely been considered. Here we demonstrated the fingerprint of lateral flow on the forest mortality risk of a riparian ecosystem using a coupled plant hydraulics-hydrology model prescribed with multiple Earth System Model projections of future hydroclimate. We showed that the anticipated water-saving and drought ameliorating effects of elevated CO2 on mortality risk were largely compromised when lateral hydrological processes were considered. Further, we found lateral flow reduce ecosystem sensitivity to climate variations, by removing soil water excess during wet periods and providing additional water from groundwater storage during dry periods. These findings challenge the prevailing expectation of elevated CO2 to reduce mortality risk and highlight the need to assess the effects of lateral flow exchange more explicitly moving forward with forest mortality projections.

Water table depth (WTD) effects on net ecosystem CO2 exchange of boreal peatlands are largely mediated by hydrological effects on peat biogeochemistry and the ecophysiology of peatland vegetation. The lack of representation of these effects in carbon models currently limits our predictive capacity for changes in boreal peatland carbon deposits under potential future drier and warmer climates. We examined whether a process-level coupling of a prognostic WTD with (1) oxygen transport, which controls energy yields from microbial and root oxidation–reduction reactions, and (2) vascular and nonvascular plant water relations could explain mechanisms that control variations in net CO2 exchange of a boreal fen under contrasting WTD conditions, i.e., shallow vs. deep WTD. Such coupling of eco-hydrology and biogeochemistry algorithms in a process-based ecosystem model, ecosys, was tested against net ecosystem CO2 exchange measurements in a western Canadian boreal fen peatland over a period of drier-weather-driven gradual WTD drawdown. A May–October WTD drawdown of ∼ 0.25 m from 2004 to 2009 hastened oxygen transport to microbial and root surfaces, enabling greater microbial and root energy yields and peat and litter decomposition, which raised modeled ecosystem respiration (Re) by 0.26 µmol CO2 m−2 s−1 per 0.1 m of WTD drawdown. It also augmented nutrient mineralization, and hence root nutrient availability and uptake, which resulted in improved leaf nutrient (nitrogen) status that facilitated carboxylation and raised modeled vascular gross primary productivity (GPP) and plant growth. The increase in modeled vascular GPP exceeded declines in modeled nonvascular (moss) GPP due to greater shading from increased vascular plant growth and moss drying from near-surface peat desiccation, thereby causing a net increase in modeled growing season GPP by 0.39 µmol CO2 m−2 s−1 per 0.1 m of WTD drawdown. Similar increases in GPP and Re caused no significant WTD effects on modeled seasonal and interannual variations in net ecosystem productivity (NEP). These modeled trends were corroborated well by eddy covariance measured hourly net CO2 fluxes (modeled vs. measured: R2 ∼ 0.8, slopes ∼ 1 ± 0.1, intercepts ∼ 0.05 µmol m−2 s−1), hourly measured automated chamber net CO2 fluxes (modeled vs. measured: R2 ∼ 0.7, slopes ∼ 1 ± 0.1, intercepts ∼ 0.4 µmol m−2 s−1), and other biometric and laboratory measurements. Modeled drainage as an analog for WTD drawdown induced by climate-change-driven drying showed that this boreal peatland would switch from a large carbon sink (NEP ∼ 160 g C m−2 yr−1) to carbon neutrality (NEP ∼ 10 g C m−2 yr−1) should the water table deepen by a further ∼ 0.5 m. This decline in projected NEP indicated that a further WTD drawdown at this fen would eventually lead to a decline in GPP due to water limitation. Therefore, representing the effects of interactions among hydrology, biogeochemistry and plant physiological ecology on ecosystem carbon, water, and nutrient cycling in global carbon models would improve our predictive capacity for changes in boreal peatland carbon sequestration under changing climates.

Data from the hyperspectral imager for coastal ocean (HICO), mounted on the International Space Station (ISS), were used to develop and test algorithms for remotely retrieving ecosystem productivity. Twenty-six HICO images were used from four study sites representing different vegetation types: grasslands, shrubland, and forest. Gross ecosystem production (GEP) data from eddy covariance were matched with HICO-derived spectra. Multiple algorithms were successful relating spectral reflectance with GEP, including: spectral vegetation indices (SVI), SVI in a light-use efficiency model framework, spectral shape characteristics through spectral derivatives and absorption feature analysis, and statistical models leading to multiband hyperspectral indices from stepwise regressions and partial least squares regression. Successful algorithms were able to achieve r 2 better than 0.7 for both GEP at the overpass time and daily GEP. These algorithms were successful using a diverse set of observations combining data from multiple years, multiple times during growing season, different times of day, with different view angles, and different vegetation types. The demonstrated robustness of the algorithms presented in this study over these conditions provides some confidence in mapping spatial patterns of GEP, describing variability within fields, as well as the regional patterns. The ISS orbit provides periods with multiple observations collected at different times of the day within a period of a few days. Diurnal GEP patterns were estimated comparing the half-hourly average GEP from the flux tower against HICO estimates of GEP ( r 2 = 0.87) if morning, midday, and afternoon observations were available.

The survival of riparian forests depends on water input from their adjacent rivers. There are multiple, additional demands on river water that threatens to reduce the water supply, prompting the question, how much water must be left flowing within a river to sustain the native riparian ecosystems? To address this, we made eddy covariance measurements of riparian cottonwood forest evapotranspiration (ET) in a representative site and used remote sensing and the Penman-Monteith equation to up-scale ET along the 171 km river corridor from the Oldman River Dam to Lethbridge, Alberta. Our study was conducted in two growing seasons with contrasting weather conditions, including one with over-bank flooding (2014). Measured cumulative forest ET from May-September (2014: 451 ± 90 mm; 2015: 411 ± 18 mm) was very similar in the two years despite contrasting May-September cumulative precipitation input (2014: 362 mm; 2015: 181 mm). Integrated over the 56 km 2 area of riparian forest along the Oldman River corridor, the cumulative forest ET during May-September was 19.1 million m 3 (339 mm/season), about 0.9% of the average cumulative Oldman River discharge during 2008–2013. Cottonwood forest ET during May-June was less than 1% of average river flow rates during 2008–2013, but the ratio of ET to average river flow rate increased markedly to peak values of 4–5% in late July and early August. Our analysis indicated that the water-use rates of riparian cottonwood forests were high, even for a broad-leaf deciduous forest functional type, particularly given the modest leaf area index (1.4 m 2 m −2 ) we measured for riparian forests along the Oldman River corridor. The high ET rates were possible because of tree access to alluvial groundwater to support transpiration. Our ET measurements and calculations provided perspective on riparian forest water use in relation to precipitation inputs and total river discharge, knowledge that is important for successfully managing the multiple demands on river water use.

Improved predictive capacity of hydrology and surface energy exchange is critical for conserving boreal peatland carbon sequestration under drier and warmer climates. We represented basic processes for water and O2 transport and their effects on ecosystem water, energy, carbon, and nutrient cycling in a process-based model ecosys to simulate effects of seasonal and interannual variations in hydrology on peat water content, water table depth (WTD), and surface energy exchange of a Western Canadian fen peatland. Substituting a van Genuchten model (VGM) for a modified Campbell model (MCM) in ecosys enabled a significantly better simulation of peat moisture retention as indicated by higher modeled versus measured R2 and Willmot's index (d) with VGM (R2~0.7, d~0.8) than with MCM (R2~0.25, d~0.35) for daily peat water contents from a wetter year 2004 to a drier year 2009. With the improved peat moisture simulation, ecosys modeled hourly WTD and energy fluxes reasonably well (modeled versus measured R2: WTD ~0.6, net radiation ~0.99, sensible heat >0.8, and latent heat >0.85). Gradually declining ratios of precipitation to evapotranspiration and of lateral recharge to discharge enabled simulation of a gradual drawdown of growing season WTD and a consequent peat drying from 2004 to 2009. When WTD fell below a threshold of ~0.35 m below the hollow surface, intense drying of mosses in ecosys caused a simulated reduction in evapotranspiration and an increase in Bowen ratio during late growing season that were consistent with measurements. Hence, using appropriate water desorption curve coupled with vertical-lateral hydraulic schemes is vital to accurately simulate peatland hydrology and energy balance.

The interacting effects of temperature and precipitation changes on grasslands remain hard to quantify. Research now indicates widespread and consistent increases in North American grassland productivity under climate change despite greater aridity.

In the Great Plains, grassland carbon dynamics differ across broad gradients of precipitation and temperature, yet finer-scale variation in these variables may also affect grassland processes. Despite the importance of grasslands, there is little information on how fine-scale relationships compare between them regionally. We compared grassland C exchanges, energy partitioning and precipitation variability in eight sites in the eastern and western Great Plains using eddy covariance and meteorological data. During our study, both eastern and western grasslands varied between an average net carbon sink and a net source. Eastern grasslands had a moderate vapor pressure deficit (VPD = 0.95 kPa) and high growing season gross primary productivity (GPP = 1010 ± 218 g C m−2 yr−1). Western grasslands had a growing season with higher VPD (1.43 kPa) and lower GPP (360 ± 127 g C m−2 yr−1). Western grasslands were sensitive to precipitation at daily timescales, whereas eastern grasslands were sensitive at monthly and seasonal timescales. Our results support the expectation that C exchanges in these grasslands differ as a result of varying precipitation regimes. Because eastern grasslands are less influenced by short-term variability in rainfall than western grasslands, the effects of precipitation change are likely to be more predictable in eastern grasslands because the timescales of variability that must be resolved are relatively longer. We postulate increasing regional heterogeneity in grassland C exchanges in the Great Plains in coming decades.

The light-use efficiency (LUE) model of photosynthesis is widely used to estimate ecosystem photosynthesis and net primary production from remote sensing measurements. The fraction of absorbed photosynthetically active radiation ( f APAR ) is a dominant term in this model, and it is fundamentally important for model calculations of ecosystem productivity across large areas. The LUE term is sometimes considered a constant, but may be best represented as a variable scalar under stress conditions. The main objective of this study was to better understand factors influencing f APAR , its relationship with seasonal variation in canopy greenness (Normalized Difference Vegetation Index (NDVI)), and the consequences of potential seasonal changes in the NDVI- f APAR relationship for LUE model calculations of ecosystem photosynthesis in a semi-arid grassland. We used two approaches to determine f APAR , (i) direct incoming and outgoing radiation measurements above and below the canopy, and (ii) an inversion approach based on incident photosynthetically active radiation and the light response curve of net ecosystem productivity measured by eddy covariance at low light levels. The two approaches resulted in f APAR values that were very strongly correlated during the initial development of the canopy until peak leaf area index (LAI) was reached. During this time, a strong linear relationship also occurred between f APAR and NDVI calculated from spectral reflectance measurements of the grassland canopy. After peak LAI, there was hysteresis in the NDVI– f APAR relationship, and the two f APAR estimates diverged. Light-use efficiency model calculations of ecosystem photosynthesis made using f APAR values were strongly correlated with chamber CO 2 exchange measurements during the initial development of the canopy leaf area. After peak LAI, a stress function, based on either soil moisture or vapour pressure deficit (VPD) measurements, was necessary to reduce quantum yield and model calculations of ecosystem photosynthesis during periods of relatively low soil moisture and higher VPD later in the growing season. Both stress functions were similarly effective in improving the correlation between modeled and measured ecosystem photosynthesis values, and indicated reduced LUE under late season conditions. Modulating LUE based on the Photochemical Reflectance Index or the Water Band Index (both proposed as possible indicators of LUE) was not effective to improve the correlation between modeled and measured ecosystem photosynthesis values in this ecosystem.

Peatlands store substantial amount of carbon, are vulnerable to climate change. To predict the fate of carbon stored in peatlands, the complex interactions between water, peat and vegetations need more attention. This study describes a modified version of the ORCHIDEE land surface model for simulating the hydrology, surface energy and CO2 fluxes of peatlands on daily to annual time scales. The model, referred to as ORCHIDEE-PEAT, includes a separate soil tile in each 0.5° grid-cell, defined from a global peatland map and identified with peat-specific soil hydraulic properties. Runoff from non-peat vegetation with a grid-cell containing a fraction of peat is routed to this peat soil tile, which maintains shallow water tables. The water table position separates oxic from anoxic decomposition. The model is evaluated against eddy-covariance (EC) observations from 30 northern peatland sites, with the maximum rate of carboxylation (Vcmax) being optimized at each site to match the peak of growing season gross primary productivity (GPP), derived from direct EC measurements. Regarding short-term variations from day to day, the model performance was good for the variations in GPP (r2 = 0.76, Nash-Sutcliff modeling efficiency, MEF = 0.76), with lesser accuracy for latent heat fluxes (LE, r2 = 0.42, MEF = 0.14) and Net ecosystem CO2 exchange (NEE, r2 = 0.38, MEF = 0.26). Seasonal variations in GPP, NEE and energy fluxes on monthly scales showed moderate to high r2 values ranging from 0.57 to 0.86. For spatial across-sites gradients of annual mean GPP, NEE and LE, r2 of 0.93, 0.27, and 0.71, respectively, were achieved. The water table variations are not well predicted (r2 < 0.1), likely due to the uncertain water input to the peat from surrounding areas. However, when using the observed water table in the carbon module to define the fraction of oxic and anoxic decomposition instead of the modeled water table, ORCHIDEE-PEAT shows a small improvement in reproducing NEE. Moreover, we found a significant relationship between optimized Vcmax and the latitude (temperature), which can better reflect the spatial gradients of annual NEE than using an average Vcmax value. In a future version of ORCHIDEE-PEAT, the influences of water table on photosynthesis and depth-dependent influences of soil temperature on respiration may be included.

Water table depth (WTD) effects on net ecosystem CO2 exchange of boreal peatlands are largely mediated by hydrological effects on peat biogeochemistry, and eco-physiology of peatland vegetation. Lack of representation of these effects in carbon models currently limits our predictive capacity for changes in boreal peatland carbon deposits under future drier and warmer climates. We therefore tested whether the effects of WTD variation on net ecosystem CO2 exchange of a Western Canadian boreal fen peatland could be modelled through a process-level coupling of a prognostic WTD dynamic, which arises from equilibrium between vertical and lateral water fluxes, with oxygen transport, which controls energy yields from microbial and root oxidation–reduction reactions, and vascular and non-vascular plant water relations in an ecosystem model ecosys. Ecosys successfully simulated a May–October WTD drawdown by ~ 0.25 m measured in the fen from 2004 to 2008, which was attributed to reduced precipitation relative to evapotranspiration, and reduced lateral recharge relative to discharge. This WTD drawdown hastened oxygen transport to microbial and root surfaces, enabling greater microbial and root energy yields, and peat and litter decomposition, which raised modelled ecosystem respiration (Re) by ~ 0.26 μmol CO2 m−2 s−1 per 0.1 m of WTD drawdown. It also augmented nutrient mineralization, and hence root nutrient availability and uptake, which resulted in improved leaf nutrient (nitrogen) status that facilitated carboxylation, and raised modelled vascular gross primary productivity (GPP) and plant growth. The increase in modelled vascular GPP exceeded declines in modelled non-vascular (moss) GPP due to greater shading from increased vascular plant growth, and moss drying from near surface peat desiccation, thereby causing a net increase in modelled growing season GPP by ~ 0.39 μmol CO2 m−2 s−1 per 0.1 m of WTD drawdown. Similar increases in GPP and Re left no significant WTD effects on modelled seasonal and interannual variations in net ecosystem productivity (NEP). These modelled trends were corroborated against eddy covariance hourly net CO2 fluxes (modelled vs. measured: R2 ~ 0.8, slopes ~ 1 ± 0.1, intercepts ~ 0.05 μmol m−2 s−1), and against other automated chamber, biometric, and laboratory measurements. Modelled drainage as an analog for climate change showed that this boreal peatland would switch from a large carbon sink (NEP ~ 160 g C m−2 yr−1) to carbon neutrality (NEP ~ 10 g C m−2 yr−1) should water table deepened by a further ~ 0.5 m. Therefore, representing the effects of interactions among hydrology, biogeochemistry and plant physiological ecology on ecosystem carbon, water, and nutrient cycling in global carbon models would improve our predictive capacity for changes in boreal peatland carbon sequestration under changing climates.

Leaf carbon isotope ratios (δ13C) and photosynthetic gas exchange were measured on plants growing in hanging garden communities in southern Utah, USA. Hanging gardens are unusual, mesic cliff communities occurring where water seeps from the sandstone bedrock in an otherwise extremely arid region; there is very limited overlap in species distributions inside and outside these gardens. Solar exposure in hanging gardens varied with orientation and one of the gardens (Ribbon Garden) was shaded throughout the day. The leaf δ13C values of plants in hanging gardens were significantly more negative than for plants from either nearby ephemeral wash or riparian communities. In Ribbon Garden, the observed δ13C values were as low as -34.8‰, placing them among the most negative values reported for any terrestrial plant species growing in a natural environment. Hanging garden plants were exposed to normal atmospheric CO2 with an average δ13C value of -7.9‰ and so the low leaf δ13C values could not be attributed to exposure to a CO2 source with low 13C content. There was a seasonal change toward more negative leaf δ13C values at the end of the growing season. The observed leaf δ13C values were consistent with photosynthetic gas exchange measurements that indicated unusually high leaf intercellular CO2 concentrations associated with the relatively low light levels in hanging gardens. Thus, extremely negative leaf δ13C values would be expected if significant amounts of the seasonal carbon gain occur at light levels low enough to be near the light compensation point. Maximum observed photosynthetic rates varied with light levels at each of the gardens, with maximum rates averaging 20.3, 14.6, and 3.1 μmol m-2 s-1 at Double Garden, Lost Garden, and Ribbon Garden, respectively. Leaf nitrogen contents averaged 18.5 mg g-1 in species from the more shaded hanging gardens (Lost and Ribbon). When expressed on a leaf area basis, nitrogen contents averaged 117 mmol N m-2 at Lost Garden and 65 mmol N m-2 at Ribbon Garden (shadiest of the two gardens). Leaf nitrogen isotope ratios averaged -2.3‰ (range of -0.7 to -6.1‰), suggesting that most of the nitrogen was derived from a biological fixation source which is most likely the Nostoc growing on the sandstone walls at the seep. These values contrast with leaf nitrogen isotope ratios of 5-9‰ which have been previously reported for arid zone plants in nearby ecosystems.

Ecosystem phenology plays an important role in carbon exchange processes and can be derived from continuous records of carbon dioxide (CO2) exchange data. In this study we examined the potential use of phenological indices for characterizing cumulative annual CO2 exchange in four contrasting northern peatland ecosystems. We used the approach of Jonsson and Eklundh (2004) to derive a set of phenological indices based on the daily time series of gross primary production (GPP), ecosystem respiration (R-e), and net ecosystem production (NEP) measured in the four peatland sites. The main objectives of this study were (a) to examine the variation in phenological indices across sites and (b) to determine the relationships among phenological indices, environmental conditions, and cumulative annual CO2 exchange. The phenological index used to define the "start of the growing season" showed good potential for differentiation among sites based on their average annual site GPP. Sites with earlier growing seasons had the highest average annual site GPP. The "peak CO2 exchange rate" phenological index performed best in reflecting variations among sites and for estimating annual values of GPP, R-e, and NEP (Pearson correlation coefficients ranged between 0.77 and 0.99, p

Measurements of the carbon (δ(13) Cm ) and oxygen (δ(18) Om ) isotope composition of C3 plant tissue provide important insights into controls on water-use efficiency. We investigated the causes of seasonal and inter-annual variability in water-use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf-scale) and eddy covariance measurements (ecosystem-scale). The positive relationship between δ(13) Cm and δ(18) Om values for samples collected during 1998-2001 indicated that variation in stomatal conductance and water stress-induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ(13) Cm and biomass production during this time. By comparison, the lack of a significant relationship between δ(13) Cm and δ(18) Om values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water-use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf ci /ca during the drier conditions of 2000. Calculated values of leaf-scale water-use efficiency were 2-3 times higher than ecosystem-scale water-use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.

The interacting effects of altered temperature and precipitation are expected to have significant consequences for ecosystem net carbon storage. Here we report the results of an experiment that evaluated the effects of elevated temperature and altered precipitation, alone and in combination, on plant biomass production and soil respiration rates in a northern Great Plains grassland, near Lethbridge, Alberta Canada. Open-top chambers and rain shelters were used to establish an experiment in 2011 with two temperature treatments (warmed and control), each combined with three precipitation treatments (minus 50%, ambient (no manipulation), and plus 50%). A smaller experiment with only the two temperature treatments was conducted in 2012, a year with less rain than 2011. Our objectives were to determine the sensitivity of plant biomass production and soil respiration to temperature and moisture manipulations, and to test for direct and indirect effects of the environmental changes on soil respiration rates. The experimental manipulations resulted primarily in a significant increase in air temperature in the warmed treatment. There were no significant temperature or precipitation treatment effects on soil moisture content. Aboveground biomass was not significantly affected by the experimental manipulations, but the warmed plots of the ambient precipitation treatment showed an increase in root biomass relative to the control plots in 2011. The warmed treatment increased the cumulative loss of carbon in soil respiration (July–September) compared to the control by 497 g C m−2 during 2011, and by 185 g C m−2 during 2012. This higher soil respiration rate in both years was not directly caused by significant differences among treatments in soil temperature or soil moisture, but was likely an indirect result of increased carbon substrate availability in the warmed relative to the control treatment.

article i nfo We describe a pragmatic approach for evaluating the spatial representativeness of flux tower measurements based on footprint climatology modeling analyses of land cover and remotely sensed vegetation indices. The approach was applied to the twelve flux sites of the Canadian Carbon Program (CCP) that include grassland, wetland, and temperate and boreal forests across an east-west continental gradient. The spatial variation within the footprint area was evaluated by examining the spatial structure of Normalized Difference Vegeta- tion Index (NDVI) and land cover using geostatistical analyses of frequency distribution, variogram and win- dow size. The results show that at most sites (i) the percentages of the target vegetation functional type (dominant land cover) observed by the CCP towers were higher than 60%; (ii) to some extent, most of the CCP sites presented anisotropically distributed patterns of NDVI in the 90% annual footprint climatology area; and (iii) the land surface heterogeneity within the flux footprint area differed among sites. Overall, the forest sites had larger fine-scale spatial variation than the grassland and wetland sites. The coniferous bo- real forest sites had greater spatial variability than the two wetland sites and a coniferous temperate forest site. We conclude that the combination of footprint modeling, semivariogram and window size techniques, together with moderate spatial resolution remotely-sensed image data, is a pragmatic approach for assessing the spatial representativeness of flux tower measurements.

Photosynthetic capacity in boreal coniferous forests varies on a seasonal basis in response to the strong fluctuations in environmental conditions, and this contributes significantly to temporal changes in the concentration and stable isotope composition of atmospheric carbon dioxide. Our objectives in this study were to compare measurements of seasonal variation in ecosystem-scale photosynthesis and the carbon isotope composition of ecosystem-respired CO 2 ( δ R ) with calculations done using a model that included seasonal changes in the temperature acclimation of photosynthesis. Our measurements and model calculations were conducted in three boreal coniferous forests during the main growing season months (May–September) in three different years (2004–2006) as part of the Fluxnet-Canada Research Network. We observed good agreement between measured and modeled ecosystem photosynthesis, with measured ecosystem photosynthesis based on eddy covariance measurements of net ecosystem CO 2 exchange. In addition, good agreement was observed between measured and modeled δ R values, which helped to provide validation of our calculations of ecosystem-scale carbon isotope discrimination. There were important seasonal changes in both ecosystem photosynthesis rate and carbon isotope discrimination that affect the concentration and stable isotope composition of atmospheric CO 2 . The seasonal patterns of change in ecosystem photosynthesis and carbon isotope discrimination determined in this study closely matched the timing of measured changes in the concentration and isotope composition of atmospheric CO 2 recorded at Cold Bay, Alaska.

The effects of climatic factors and vegetation type on evapotranspiration (E) and water use efficiency (WUE) were analyzed using tower-based eddy-covariance (EC) data for nine mature forest sites, two peatland sites and one grassland site across an east–west continental-scale transect in Canada during the period 2003–2006. The seasonal pattern of E was closely linked to growing-season length and rainfall distribution. Although annual precipitation (P) during the observation period was highly variable among sites (250−1450 mm), minimum annual E was not less than 200 mm and was limited to 400−500 mm where annual P exceeded 700 mm. Site-specific interannual variability in E could be explained by either changes in total P or variations in solar irradiance. A highly positive linear correlation was found between monthly mean values of E and net radiation (Rn) at the grassland site (AB-GRL), the two peatland sites (AB-WPL and ON-EPL), and only one of the forest sites (coastal Douglas-fir, BC-DF49) whereas a hysteretic relationship at the other forest sites indicated that E lagged behind the typical seasonal progression of Rn. Results of a cross-correlation analysis between daily (24-h) E and Rn revealed that site-specific lag times were between 10 and 40 days depending on the lag of vapour pressure deficit (D) behind Rn and the decoupling coefficient, Ω. There was significant seasonal variation in daytime mean dry-foliage Priestley–Taylor α with maxima occurring in the growing season at all sites except BC-DF49 where it was relatively constant (∼0.55) throughout all years. Annual means of daytime dry-foliage α mostly ranging between 0.5 and 0.7 implied stomatal limitation to transpiration. Increasing D significantly decreased canopy conductance (gc) at the forest sites but had little effect at the peatland and grassland sites, while variation in soil water content caused only minor changes in gc. At all sites, a strong linear correlation between monthly mean values of gross primary production (GPP) and E resulted in water use efficiency being relatively constant. While at most sites, WUE was in the range of 2.6–3.6 g C kg−1 H2O, the BC-DF49 site had the highest WUE of the twelve sites with values near 6.0 g C kg−1 H2O. Of the two peatland sites, AB-WPL, a western treed fen, had a significantly higher WUE (∼3.0 g C kg−1 H2O) than ON-EPL, an eastern ombrotrophic bog (∼1.8 g C kg−1 H2O), which was related to peatland productivity and plant functional type.

Droughts are among the world's most costly natural disasters and collectively affect more people than any other form of natural disaster. The Canadian Prairies are very susceptible to drought and have experienced this phenomenon many times. However, the recent 1999–2005 Prairie drought was one of the worst meteorological, agricultural and hydrologic droughts over the instrumental record. It also had major socio-economic consequences, adding up to losses in the billions of dollars. This recent drought was the focus of the Drought Research Initiative (DRI), the first integrated network focusing on drought in Canada. This article addresses some of the key objectives of DRI by providing a collective summary, understanding and synthesis of the 1999–2005 drought. Bringing together the many datasets used in this study was in itself a major accomplishment. This drought exhibited many important, and sometimes surprising, features. This includes, for example, (1) a non-steady large-scale atmospheric circulation (an...

Peatland ecosystems have been consistent carbon (C) sinks for millennia, but it has been predicted that exposure to warmer temperatures and drier conditions associated with climate change will shift the balance between ecosystem photosynthesis and respiration providing a positive feedback to atmospheric CO 2 concentration. Our main objective was to determine the sensitivity of ecosystem photosynthesis, respiration and net ecosystem production (NEP) measured by eddy covariance, to variation in temperature and water table depth associated with interannual shifts in weather during 2004-2009. Our study was conducted in a moderately rich treed fen, the most abundant peatland type in western Canada, in a region (northern Alberta) where peatland ecosystems are a significant landscape component. During the study, the average growing season (May-October) water depth declined approximately 38 cm, and temperature [expressed as cumulative growing degree days (GDD, March-October)] varied approximately 370 GDD. Contrary to previous predictions, both ecosystem photosynthesis and respiration showed similar increases in response to warmer and drier conditions. The ecosystem remained a strong net sink for CO 2 with an average NEP ( ± SD) of 189 ± 47 g C m ―2 yr ―1 . The current net CO 2 uptake rates were much higher than C accumulation in peat determined from analyses of the relationship between peat age and cumulative C stock. The balance between C addition to, and total loss from, the top 0-30 cm depth (peat age range 0-70 years) of shallow peat cores averaged 43 ± 12 g C m ―2 yr ―1 . The apparent long-term average rate of net C accumulation in basal peat samples was 19-24 g C m ―2 yr ―1 . The difference between current rates of net C uptake and historical rates of peat accumulation is likely a result of vegetation succession and recent increases in tree establishment and productivity.

We describe an approach for evaluating the representativeness of eddy covariance flux measurements and assessing sensor location bias (SLB) based on footprint modelling and remote sensing This approach was applied to the 12 main sites of the Fluxnet-Canada Research Network (FCRN)/Canadian Carbon Program (CCP) located along an east-west continental-scale transect covering grassland forest and wetland biomes For each site monthly and annual footprint climatologies e monthly or annual cumulative footprints) were calculated using the Simple Analytical Footprint model on Eulerian coordinates (SAFE) The resulting footprint climatologies were then overlaid on to Images of the Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) derived from LANDSAT Thematic Mapper (TM) imagery which were used as surrogates of land surface fluxes to estimate SLB Results indicate that (i) the sizes of annual footprint climatology Increased exponentially with increasing cumulative footprint percentages and for a given percentage of footprint climatology the footprint areas were significantly different among the sites Typically the 90% annual footprint climatology areas varied from 1 1 km(2) to 5 0 km(2) (ii) using either NDVI or EVI as the flux surrogate the SLB was less than 5% for most sites with respect to the reference area of interest (A(r)) at 90% annual footprint climatology (scenario A) and a circular area with radius of 1 km centred at the individual tower (scenario B) with several exceptions (iii) the SLB decreased with increasing size of footprint climatology for all sites for both scenarios A and B (iv) out of 12 eight flux towers represented most of the ecosystem surrounding the towers for an area of 0 3 km(2) up to 10 km(2) with a satisfactorily low bias of

Climate models predict rising temperatures and more frequent and prolonged droughts, particularly in the northern hemisphere and in the Canadian Prairies. However, few studies have examined the interannual variation in evapotranspiration (E) of northern terrestrial ecosystems in relation to drought. This study analyses multi-year (1998-2006), eddy-covariance measurements to quantify the seasonal and interannual variability in E from grassland and mature aspen, black spruce, and jack pine forest ecosystems in the dry interior plains of western Canada. It also investigates the response of E to the historic 2001-2003 drought in this region. Leaf area index (LAI) was a primary factor controlling the difference in E among different ecosystems in the same ecozone. Annual E was higher and more variable for the aspen forest (405 ± 84 mm, mean ± s.d.) and grassland (395 ± 90 mm) than the black spruce (374 ± 34 mm) and jack pine (300 ± 20 mm) forests. Interannual variation of E was controlled by early spring soil temperature at all four sites, with warm springs enhancing annual E. Only the aspen forest and grassland showed a significant suppression of E by summer drought, related to reduced surface conductance at the aspen site, and to reduced surface conductance and early leaf senescence at the grassland. We conclude that the potential for drought impacts on annual E of northern ecosystems is greatest for grasslands, moderate for deciduous broadleaf aspen forests, and smallest for coniferous black spruce and jack pine forests. The forests of the Boreal Plains, adjacent to the prairie region, may ameliorate the onset of drought through the recycling of moisture to the atmosphere, whereas the prairie grasslands have only limited capacity to counteract drought through moisture recycling.

Continuous half-hourly net CO(2) exchange measurements were made using nine automatic chambers in a treed fen in northern Alberta, Canada from June-October in 2005 and from May-October in 2006. The 2006 growing season was warmer and drier than in 2005. The average chamber respiration rates normalized to 10 degrees C were much higher in 2006 than in 2005, while calculations of the temperature sensitivity (Q(10)) values were similar in the two years. Daytime average respiration values were lower than the corresponding, temperature-corrected respiration rates calculated from night-time chamber measurements. From June to September, the season-integrated estimates of chamber photosynthesis and respiration were 384 and 590 g C m(-2), respectively in 2006, an increase of 100 and 203 g C m(-2) over the corresponding values in 2005. The season-integrated photosynthesis and respiration rates obtained using the eddy covariance technique, which included trees and a tall shrub not present in the chambers, were 720 and 513 g C m(-2), respectively, in 2006, an increase of 50 and 125 g C m(-2) over the corresponding values in 2005. While both photosynthesis and respiration rates were higher in the warmer and drier conditions of 2006, the increase in respiration was more than twice the increase in photosynthesis.

Remote sensing studies are affected by the inherently anisotropic nature of reflectance from natural surfaces. The objective of this study was to investigate the effect of anisotropic reflectance on the 970 nm water band index (WBI) for Pleurozium schreberi moss from the Fluxnet Canada Western Peatland site in northern Alberta. A series of hyperspectral bidirectional reflectance measurements from the University of Lethbridge Goniometer System (ULGS-I) were assessed for their effect on a variety of WBI and NDVI spectral indices. These indices are often used to estimate ecophysiological parameters such as plant water content, pigment content, and leaf area and subsequently have the potential to contribute to estimates of ecosystem CO2 flux across large regions. Estimates of the bidirectional reflectance distribution function (BRDF) from laboratory ULGS-I measurements of P. schreberi moss were made under controlled illumination conditions from which WBI and NDVI were computed to evaluate the variation in ind...

Spectral Network (SpecNet) began as a Working Group in 2003 with the goals of integrating remote sensing with biosphere-atmosphere carbon flux measurements and standardizing field optical sampling methods. SpecNet has evolved into an international network of collaborating sites and investigators, with a particular focus on matching optical sampling tools to the temporal and spatial scale of flux measurements and ecological sampling. Current emphasis within the SpecNet community is on greater automation of field optical sampling using simple cost-effective technologies, improving the light-use-efficiency (LUE) model of carbon dioxide flux, consideration of view and illumination angle to improve physiological retrievals, and incorporation of informatics and cyberinfrastructure solutions that address the increasing data dimensionality of cross-site and multiscale sampling. In this review, we summarize recent findings and current directions within the SpecNet community and provide recommendations for the larg...

Many current models of ecosystem carbon exchange based on remote sensing, such as the MODIS product termed MOD17, still require considerable input from ground based meteorological measurements and look up tables based on vegetation type. Since these data are often not available at the same spatial scale as the remote sensing imagery, they can introduce substantial errors into the carbon exchange estimates. Here we present further development of a gross primary production (GPP) model based entirely on remote sensing data. In contrast to an earlier model based only on the enhanced vegetation index (EVI), this model, termed the Temperature and Greenness (TG) model, also includes the land surface temperature (LST) product from MODIS. In addition to its obvious relationship to vegetation temperature, LST was correlated with vapor pressure deficit and photosynthetically active radiation. Combination of EVI and LST in the model substantially improved the correlation between predicted and measured GPP at 11 eddy correlation flux towers in a wide range of vegetation types across North America. In many cases, the TG model provided substantially better predictions of GPP than did the MODIS GPP product. However, both models resulted in poor predictions for sparse shrub habitats where solar angle effects on remote sensing indices were large. Although it may be possible to improve the MODIS GPP product through improved parameterization, our results suggest that simpler models based entirely on remote sensing can provide equally good predictions of GPP.

We developed and applied an ecosystem-scale model that calculated leaf CO2 assimilation, stomatal conductance, chloroplast CO2 concentration and the carbon isotope composition of carbohydrate formed during photosynthesis separately for sunlit and shaded leaves within multiple canopy layers. The ecosystem photosynthesis model was validated by comparison to leaf-level gas exchange measurements and estimates of ecosystem-scale photosynthesis from eddy covariance measurements made in a coastal Douglas-fir forest on Vancouver Island. A good agreement was also observed between modelled and measured delta13C values of ecosystem-respired CO2 (deltaR). The modelled deltaR values showed strong responses to variation in photosynthetic photon flux density (PPFD), air temperature, vapour pressure deficit (VPD) and available soil moisture in a manner consistent with leaf-level studies of photosynthetic 13C discrimination. Sensitivity tests were conducted to evaluate the effect of (1) changes in the lag between the time of CO2 fixation and the conversion of organic matter back to CO2; (2) shifts in the proportion of autotrophic and heterotrophic respiration; (3) isotope fractionation during respiration; and (4) environmentally induced changes in mesophyll conductance, on modelled delta(R) values. Our results indicated that deltaR is a good proxy for canopy-level C(c)/C(a) and 13C discrimination during photosynthetic gas exchange, and therefore has several applications in ecosystem physiology.