Measurements of gross and net ecosystem productivity and water vapour exchange of a Pinus ponderosa ecosystem,and an evaluation of two generalized models

Net ecosystem productivity (NEP), net primary productivity (NPP), and water vapour exchange of a mature Pinus ponderosa forest (44°30′ N, 121°37′ W) growing in a region subject to summer drought were investigated along with canopy assimilation and respiratory fluxes. This paper describes seasonal and annual variation in these factors, and the evaluation of two generalized models of carbon and water balance (PnET‐II and 3‐PG) with a combination of traditional measurements of NPP, respiration and water stress, and eddy covariance measurements of above‐and below‐canopy CO₂ and water vapour exchange. The objective was to evaluate the models using two years of traditional and eddy covariance measurements, and to use the models to help interpret the relative importance of processes controlling carbon and water vapour exchange in a water‐limited pine ecosystem throughout the year. PnET‐II is a monthly time‐step model that is driven by nitrogen availability through foliar N concentration, and 3‐PG is a monthly time‐step quantum‐efficiency model constrained by extreme temperatures, drought, and vapour pressure deficits. Both models require few parameters and have the potential to be applied at the watershed to regional scale. There was 2/3 less rainfall in 1997 than in 1996, providing a challenge to modelling the water balance, and consequently the carbon balance, when driving the models with the two years of climate data, sequentially. Soil fertility was not a key factor in modelling processes at this site because other environmental factors limited photosynthesis and restricted projected leaf area index to ～1.6. Seasonally, GEP and LE were overestimated in early summer and underestimated through the rest of the year. The model predictions of annual GEP, NEP and water vapour exchange were within 1–39% of flux measurements, with greater disparity in 1997 because soil water never fully recharged. The results suggest that generalized models can provide insights to constraints on productivity on an annual basis, using a minimum of site data.     

Measuring and modelling carbon dioxide and water vapour exchange over a temperate broad-leaved forest during the 1995 summer drought

Forests in the south-eastern United States experienced a prolonged dry spell and above-normal temperatures during the 1995 growing season. During this episode, nearly continuous, eddy covariance measurements of carbon dioxide and water vapour fluxes were acquired over a temperate, hardwood forest. These data are used to examine how environmental factors and accumulating soil moisture deficits affected the diurnal pattern and magnitude of canopy-scale carbon dioxide and water vapour fluxes. The field data are also used to test an integrative leaf-to-canopy scaling model (CANOAK), which uses micrometeorological and physiological theory, to calculate mass and energy fluxes. When soil moisture was ample in the spring, peak rates of net ecosystem CO₂ exchange (N_F) occurred around midday and exceeded 20 μmol m^?2 s^?1. Rates of N_K were near optimal when air temperature ranged between 22 and 25°C. The accumulation of soil moisture deficits and a co-occurrence of high temperatures caused peak rates of daytime carbon dioxide uptake to occur earlier in the morning. High air temperatures and soil moisture deficits were also correlated with a dramatic reduction in the magnitude of N_E. On average, the magnitude of N_E decreased from 20 to 7 μmol m^?2 s^?1 as air temperature increased from 24 to 30°C and the soil dried. The CANAOK model yielded accurate estimates of canopy-scale carbon dioxide and water vapour fluxes when the forest had an ample supply of soil moisture. During the drought and heat spell, a cumulative drought index was needed to adjust the proportionality constant of the stomatal conductance model to yield accurate estimates of canopy CO₂ exchange. The adoption of the drought index also enabled the CANOAK model to give improved estimates of evaporation until midday. On the other hand, the scheme failed to yield accurate estimates of evaporation during the afternoon.     

Integration of Process-based Soil Respiration Models with Whole-Ecosystem CO2 Measurements

We integrated soil models with an established ecosystem process model (SIPNET, simplified photosynthesis and evapotranspiration model) to investigate the influence of soil processes on modelled values of soil CO₂ fluxes (R _Soil). Model parameters were determined from literature values and a data assimilation routine that used a 7-year record of the net ecosystem exchange of CO₂ and environmental variables collected at a high-elevation subalpine forest (the Niwot Ridge AmeriFlux site). These soil models were subsequently evaluated in how they estimated the seasonal contribution of R _Soil to total ecosystem respiration (TER) and the seasonal contribution of root respiration (R _Root) to R _Soil. Additionally, these soil models were compared to data assimilation output of linear models of soil heterotrophic respiration. Explicit modelling of root dynamics led to better agreement with literature values of the contribution of R _Soil to TER. Estimates of R _Soil/TER when root dynamics were considered ranged from 0.3 to 0.6; without modelling root biomass dynamics these values were 0.1–0.3. Hence, we conclude that modelling of root biomass dynamics is critically important to model the R _Soil/TER ratio correctly. When soil heterotrophic respiration was dependent on linear functions of temperature and moisture independent of soil carbon pool size, worse model-data fits were produced. Adding additional complexity to the soil pool marginally improved the model-data fit from the base model, but issues remained. The soil models were not successful in modelling R _Root/R _Soil. This is partially attributable to estimated turnover parameters of soil carbon pools not agreeing with expected values from literature and being poorly constrained by the parameter estimation routine. We conclude that net ecosystem exchange of CO₂ alone cannot constrain specific rhizospheric and microbial components of soil respiration. Reasons for this include inability of the data assimilation routine to constrain soil parameters using ecosystem CO₂ flux measurements and not considering the effect of other resource limitations (for example, nitrogen) on the microbe biomass. Future data assimilation studies with these models should include ecosystem-scale measurements of R _Soil in the parameter estimation routine and experimentally determine soil model parameters not constrained by the parameter estimation routine.     

Scaling carbon dioxide and water vapour exchange from leaf to canopy in a deciduous forest. II. Model testing and application

The scaling of CO₂ and water vapour transfer from leaf to canopy dimensions was achieved by integrating mechanistic models for physiological (photosynthesis, stomatal conductance and soil/root and bole respiration) and micrometeorological (radiative transfer, turbulent transfer and surface energy exchanges) processes. The main objectives of this paper are to describe a canopy photosynthesis and evaporation model for a temperate broadleaf forest and to test it against field measurements. The other goal of this paper is to use the validated model to address some contemporary ecological and physiological questions concerning the transfer of carbon and water between forest canopies and the atmosphere. In particular, we examine the role of simple versus complex radiative transfer models and the effect of environmental (solar radiation and CO₂) and ecophysiological (photo-synthetic capacity) variables on canopy-scale carbon and water vapour fluxes.     

Modelling the discrimination of 13CO2 above and within a temperate broad-leaved forest canopy on hourly to seasonal time scales

Fluxes and concentrations of carbon dioxide and ¹³CO₂ provide information about ecosystem physiological processes and their response to environmental variation. The biophysical model, CANOAK, was adapted to compute concentration profiles and fluxes of ¹³CO₂ within and above a temperate deciduous forest (Walker Branch Watershed, Tennessee, USA). Modifications to the model are described and the ability of the new model (CANISOTOPE) to simulate concentration profiles of ¹³CO₂, its flux density across the canopy–atmosphere interface and leaf‐level photosynthetic discrimination against ¹³CO₂ is demonstrated by comparison with field measurements. The model was used to investigate several aspects of carbon isotope exchange between a forest ecosystem and the atmosphere. During the 1998 growing season, the mean photosynthetic discrimination against ¹³CO₂, by the deciduous forest canopy (Δ_canopy), was computed to be 22·4‰, but it varied between 18 and 27‰. On a diurnal basis, the greatest discrimination occurred during the early morning and late afternoon. On a seasonal time scale, the greatest diurnal range in Δ_canopy occurred early and late in the growing season. Diurnal and seasonal variations in Δ_canopy resulted from a strong dependence of Δ_canopy on photosynthetically active radiation and vapour pressure deficit of air. Model calculations also revealed that the relationship between canopy‐scale water use efficiency (CO₂ assimilation/transpiration) and Δ_canopy was positive due to complex feedbacks among fluxes, leaf temperature and vapour pressure deficit, a finding that is counter to what is predicted for leaves exposed to well‐mixed environments.     

Carbon,water and nutrient relations of two mistletoes and their hosts: A hypothesis*

Abstract. The carbon, water and nutrient relations of the xylem parasites Loranthus europaeus and Viscum laxum and their respective hosts. Quercus robur and Pinus sylvestris, were followed throughout clear days in July in order to study water and nutrient interactions in a simple system in which the plant growth depends on the host for its water and nutrients. At similar quantum flux densities, temperatures and vapour pressure deficits, the mistletoes had higher rates of transpiration and lower leaf water potentials than their hosts, but similar rates of CO₂ assimilation. Based on measurements of the nutrient content of the xylem and on seasonal measurements of the biomass and the tissue nutrient content, the present study suggests that the high rates of transpiration may be necessary for the parasites to take up sufficient nitrogen from the xylem of the host for production of biomass (leaves, fruits and stems).     

Seasonal variations of CO2 and water vapour exchange rates over a temperate deciduous forest

Long-term and direct measurements of CO₂ and water vapour exchange are needed over forested ecosystems to determine their net annual fluxes of carbon dioxide and water. Such measurements are also needed to parameterize and test biogeochemical, ecological and hydrological assessment models. Responding to this need, eddy covariance measurements of CO₂ and water vapour were made ever a deciduous forest growing near Oak Ridge, TN, between April 1993 and April 1994. Periodic measurements were made of leaf area index, stomatal resistance, soil moisture and pre-dawn leaf water potential to characterize the gas exchange capacity of the canopy. Four factors had a disproportionate influence on the seasonal variation of CO₂ flux densities. These factors were photon flux densities (during the growing season), temperature (during the dormant season), leaf area index and the occurrence of drought The drought period occurred during the peak of the growing season and caused a significant decline in daily and hourly CO₂ flux densities, relative to observations over the stand when soil moisture was plentiful. The annual net uptake of carbon was calculated by integrating flux measurements and filling missing and spurious data with the relations obtained between measured CO₂ fluxes and environmental forcing variables. The net flux of carbon for the period between April 1993 and April 1994 was -525 g C m^?2 y^?1. This value represents a net flux of carbon from the atmosphere and into the forest. The net annual carbon exchange of this southern temperate broadleaved forest exceeded values measured over a northern temperate forest (which experiences a shorter growing season and has less leaf area) by 200 g C m^?2 y^?1 (cf. Wofsy et al 1993). The seasonal variation of canopy evaporation (latent heat flux) was controlled mostly by changes in leaf area and net radiation. A strong depression in evaporation rates was not observed during the drought Over a broadleaved forest large vapour pressure deficits promote evaporation and trees in a mixed stand are able to tap a variety of deep and shallow water sources.     

Exposure to an enriched CO2 atmosphere alters carbon assimilation and allocation in a pine forest ecosystem

We linked a leaf-level CO₂ assimilation model with a model that accounts for light attenuation in the canopy and measurements of sap-flux-based canopy conductance into a new canopy conductance-constrained carbon assimilation (4C-A) model. We estimated canopy CO₂ uptake (A_nC) at the Duke Forest free-air CO₂ enrichment (FACE) study. Rates of A_nC estimated from the 4C-A model agreed well with leaf gas exchange measurements (A_net) in both CO₂ treatments. Under ambient conditions, monthly sums of net CO₂ uptake by the canopy (A_nC) were 13% higher than estimates based on eddy-covariance and chamber measurements. Annual estimates of A_nC were only 3% higher than carbon (C) accumulations and losses estimated from ground-based measurements for the entire stand. The C budget for the Pinus taeda component was well constrained (within 1% of ground-based measurements). Although the closure of the C budget for the broadleaf species was poorer (within 20%), these species are a minor component of the forest. Under elevated CO₂, the C used annually for growth, turnover, and respiration balanced only 80% of the A_nC. Of the extra 700 g C m⁻² a⁻¹ (1999 and 2000 average), 86% is attributable to surface soil CO₂ efflux. This suggests that the production and turnover of fine roots was underestimated or that mycorrhizae and rhizodeposition became an increasingly important component of the C balance. Under elevated CO₂, net ecosystem production increased by 272 g C m⁻² a⁻¹: 44% greater than under ambient CO₂. The majority (87%) of this C was sequestered in a moderately long-term C pool in wood, with the remainder in the forest floor–soil subsystem.     

18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

In this paper we describe measurements and modeling of ¹⁸O in CO₂ and H₂O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ¹⁸O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ¹⁸O intercepts for net soil‐surface CO₂ and ecosystem CO₂ and H₂O fluxes during three periods of the 2000 growing season. Daytime discrimination against C¹⁸OO, as calculated from measured above‐canopy CO₂ and δ¹⁸O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H₂¹⁸O and C¹⁸OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ¹⁸O value of net ecosystem CO₂ and H₂O fluxes. Simulations indicate that incomplete equilibration between CO₂ and H₂O within C₄ plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ¹⁸O value of above‐canopy vapor had a small impact on the predicted δ¹⁸O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ¹⁸O value of above‐canopy CO₂ substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem ¹⁸O‐isoflux in CO₂ during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C¹⁸OO fluxes requires accurate predictions of both soil and leaf ¹⁸O‐isofluxes.     

Drought controls over conductance and assimilation of a Mediterranean evergreen ecosystem: scaling from leaf to canopy

Drought control over conductance and assimilation was assessed using eddy flux and meteorological data monitored during four summer periods from 1998 to 2001 above a closed canopy of the Mediterranean evergreen oak tree Quercus ilex. Additional discrete measurements of soil water content and predawn leaf water potential were used to characterize the severity of the drought. Canopy conductance was estimated through the big‐leaf approach of Penman–Monteith by inverting latent heat fluxes. The gross primary production ( GPP ) was estimated by adding ecosystem respiration to net ecosystem exchange. Ecosystem respiration was deduced from night flux when friction velocity ( u _*) was greater than 0.35 m s^?1. Empirical equations were identified that related maximal canopy conductance and daily ecosystem GPP to relative soil water content ( RWC) , the ratio of current soil water content to the field capacity, and to the predawn leaf water potential. Both variables showed a strong decline with soil RWC for values lower than 0.7. The sharpest decline was observed for GPP . The curves reached zero for RWC =0.41 and 0.45 for conductance and GPP , respectively. When the predawn leaf water potential was used as a surrogate for soil water potential, both variables showed a hyperbolic decline with decreasing water potential. These results were compared with already published literature values obtained at leaf level from the same tree species. Scaling up from the leaf to ecosystem highlighted the limitation of two big‐leaf representations: Penman–Monteith and Sellers' Π factor. Neither held completely for comparing leaf and canopy fluxes. Tower measurements integrate fluxes from foliage elements clumped at several levels of organization: branch, tree, and ecosystem. The Q. ilex canopy exhibited non‐random distribution of foliage, emphasizing the need to take into account a clumping index, the factor necessary to apply the Lambert–Beer law to natural forests. Our results showed that drought is an important determinant in water losses and CO₂ fluxes in water‐limited ecosystems. In spite of the limitations inherent to the big‐leaf representation of the canopy, the equations are useful for predicting the influence of environmental factors in Mediterranean woodlands and for interpreting ecosystem exchange measurements.     

Rising sea level,temperature, and precipitation impact plant and ecosystem responses to elevated CO2 on a Chesapeake Bay wetland: review of a 28‐year study

An ongoing field study of the effects of elevated atmospheric CO₂ on a brackish wetland on Chesapeake Bay, started in 1987, is unique as the longest continually running investigation of the effects of elevated CO₂ on an ecosystem. Since the beginning of the study, atmospheric CO₂ increased 18%, sea level rose 20 cm, and growing season temperature varied with approximately the same range as predicted for global warming in the 21st century. This review looks back at this study for clues about how the effects of rising sea level, temperature, and precipitation interact with high atmospheric CO₂ to alter the physiology of C3 and C4 photosynthetic species, carbon assimilation, evapotranspiration, plant and ecosystem nitrogen, and distribution of plant communities in this brackish wetland. Rising sea level caused a shift to higher elevations in the Scirpus olneyi C3 populations on the wetland, displacing the Spartina patens C4 populations. Elevated CO₂ stimulated carbon assimilation in the Scirpus C3 species measured by increased shoot and root density and biomass, net ecosystem production, dissolved organic and inorganic carbon, and methane production. But elevated CO₂ also decreased biomass of the grass, S. patens C4. The elevated CO₂ treatment reduced tissue nitrogen concentration in shoots, roots, and total canopy nitrogen, which was associated with reduced ecosystem respiration. Net ecosystem production was mediated by precipitation through soil salinity: high salinity reduced the CO₂ effect on net ecosystem production, which was zero in years of severe drought. The elevated CO₂ stimulation of shoot density in the Scirpus C3 species was sustained throughout the 28 years of the study. Results from this study suggest that rising CO₂ can add substantial amounts of carbon to ecosystems through stimulation of carbon assimilation, increased root exudates to supply nitrogen fixation, reduced dark respiration, and improved water and nitrogen use efficiency.     

Seasonal patterns and environmental control of carbon dioxide and water vapour exchange in an ecotonal boreal forest

Carbon dioxide, water vapour, and sensible heat fluxes were measured above and within a spruce dominated forest near the southern ecotone of the boreal forest in Maine, USA. Summer, mid-day carbon dioxide uptake was higher than at other boreal coniferous forests, averaging about – 13 μmol CO₂ m^–2 s^–1. Nocturnal summer ecosystem respiration averaged ≈ 6 μmol CO₂ m^–2 s^–1 at a mean temperature of ≈ 15 °C. Significant ecosystem C uptake began with the thawing of the soil in early April and was abruptly reduced by the first autumn frost in early October. Half-hourly forest CO₂ exchange was regulated mostly by the incident photosynthetically active photon flux density (PPFD). In addition to the threshold effects of freezing temperatures, there were seasonal effects on the inferred photosynthetic parameters of the forest canopy. The functional response of this forest to environmental variation was similar to that of other spruce forests. In contrast to reports of carbon loss from northerly boreal forest sites, in 1996 the Howland forest was a strong carbon sink, storing about 2.1 t C ha^–1.     

Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites: revision of current hypotheses?

Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top‐down approaches in conjunction with a mechanistic ecosystem gas‐exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO₂/H₂O exchange. The three sites include two nearly monospecific Quercus ilex L. forests – one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) – and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO₂ exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO₂ fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman‐Monteith equation. In a top‐down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity ( Q ₁₀) in response to drought. Soil temperature and soil water content explained 70–80% of the seasonal variability of ecosystem respiration. During the drought, light‐saturated ecosystem gross carbon uptake and day‐time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water‐use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water‐use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO₂, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.     

Carbon dioxide exchange in a high-arctic fen estimated by eddy covariance measurements and modelling

The high-arctic environment is an environment where the consequences of global warming may be significant. In this paper we report on findings on carbon dioxide and water vapour fluxes above a sedge-dominated fen at Zackenberg (74°28′N, 20°34′ W) in The National Park of North and East Greenland. Eddy covariance measurements were initiated at the start of the growing season and terminated shortly before its end lasting 45 days. The net CO₂ flux during daytime reaches a high of 10 μmol m^–2s^–1, and around the summer solstice, net CO₂ assimilation occurred at midnight, resulting in net carbon gain during the night. The measured carbon dioxide fluxes compare well to estimates based on the photosynthesis model by Collatz et al. (1991 ). The total growing-season net ecosystem CO₂ exchange was estimated to be 96 g C m^–2 based on the carbon dioxide model and micrometeorological data. Finally, the combined CO₂ assimilation and soil respiration models are used for examining the dependence of the carbon dioxide budget on temperature. The ecosystem is found to function optimally given the present temperature conditions whereas either an increase or a decrease in temperature would reduce the ecosystem CO₂ accumulation. An increase in temperature by 5 °C would turn the ecosystem into a carbon dioxide source.     

Canopy gradients in leaf intercellular CO2 mole fractions revisited: interactions between leaf irradiance and water stress need consideration

Intercellular CO₂ mole fractions (C_i) are lower in the upper canopy relative to the lower canopy leaves. This canopy gradient in C_i has been associated with enhanced rates of carbon assimilation at high light, and concomitant greater draw‐downs in C_i. However, increases in irradiance in the canopy are generally also associated with decreases in leaf water availability. Thus, stress effects on photosynthesis rates (A) and stomatal conductance (G), may provide a further explanation for the observed C_i gradients. To test the hypotheses of the sources of canopy variation in C_i, and quantitatively assess the influence of within‐canopy differences in stomatal regulation on A, the seasonal and diurnal variation in G was studied in relation to seasonal average daily integrated quantum flux density (Q_int) in tall shade‐intolerant Populus tremula L. trees. Daily time‐courses of A were simulated using the photosynthesis model of Farquhar et al. (Planta 149, 78–90, 1980). Stable carbon isotope composition of a leaf carbon fraction with rapid turnover rate was used to estimate canopy gradient in C_i during the simulations. Daily maximum G (G_max) consistently increased with increasing Q_int. However, canopy differences in G_max decreased as soil water availability became limiting during the season. In water‐stressed leaves, there were strong mid‐day decreases in G that were poorly associated with vapour pressure deficits between the leaf and atmosphere, and the magnitude of the mid‐day decreases in G occasionally interacted with long‐term leaf light environment. Simulations indicated that the percentage of carbon lost due to mid‐day stomatal closure was of the order of 5–10%, and seasonal water stress increased this percentage up to 20%. The percentage of carbon lost due to stomatal closure increased with increasing Q_int. Canopy differences in light environment resulted in a gradient of daily average C_i of approximately 20 µmol mol^?1. The canopy variation in seasonal and diurnal reductions in G led to a C_i gradient of approximately 100 µmol mol^?1, and the actual canopy C_i gradient was of the same magnitude according to leaf carbon isotope composition. This study demonstrates that stress effects influence C_i more strongly than within‐canopy light gradients, and also that leaves acclimated to different irradiance and water stress conditions may regulate water use largely independent of foliar photosynthetic potentials.     

Cultivation of a perennial grass for bioenergy on a boreal organic soil – carbon sink or source?

The area under the cultivation of perennial bioenergy crops on organic soils in the northern countries is fast increasing. To understand the impact of reed canary grass (RCG, Phalaris arundinaceae L.) cultivation on the carbon dioxide (CO₂) balance of an organic soil, net ecosystem CO₂ exchange (NEE) was measured for four years in a RCG cultivated cutover peatland in eastern Finland using the eddy covariance technique. There were striking differences among the years in the annual precipitation. The annual precipitation was higher during 2004 and 2007 and lower during 2005 and 2006 than the 1971–2000 regional mean. During wet growing seasons, moderate temperatures, high surface soil moisture and low evaporative demand favoured high CO₂ uptake. During dry seasons, owing to soil moisture and atmospheric stress, photosynthetic activity was severely restricted. The CO₂ uptake [gross primary productivity (GPP)] was positively correlated with soil moisture, air temperature and inversely with vapour pressure deficit. Total ecosystem respiration (TER) increased with increasing soil temperature but decreased with increasing soil moisture. The relative responses of GPP and TER to moisture stress were different. While changes in TER for a given change in soil moisture were moderate, variations in GPP were drastic. Also, the seasonal variations in TER were not as conspicuous as those in GPP implying that GPP is the primary regulator of the interannual variability in NEE in this ecosystem. The ecosystem accumulated a total of 398 g C m^?2 from the beginning of 2004 until the end of 2007. It retained some carbon during a wet year such as 2004 even after accounting for the loss of carbon in the form of harvested biomass. Based on this CO₂ balance analysis, RCG cultivation is found to be a promising after‐use option on an organic soil.     

Growth of Eastern Cottonwoods (Populus deltoides) in elevated [CO2] stimulates stand-level respiration and rhizodeposition of carbohydrates, accelerates soil nutrient depletion, yet stimulates above- and belowground biomass production

We took advantage of the distinctive system‐level measurement capabilities of the Biosphere 2 Laboratory (B2L) to examine the effects of prolonged exposure to elevated [CO₂] on carbon flux dynamics, above‐ and belowground biomass changes, and soil carbon and nutrient capital in plantation forest stands over 4 years. Annually coppiced stands of eastern cottonwoods (Populus deltoides) were grown under ambient (400 ppm) and two levels of elevated (800 and 1200 ppm) atmospheric [CO₂] in carbon and N‐replete soils of the Intensive Forestry Mesocosm in the B2L. The large semiclosed space of B2L uniquely enabled precise CO₂ exchange measurements at the near ecosystem scale. Highly controllable climatic conditions within B2L also allowed for reproducible examination of CO₂ exchange under different scales in space and time. Elevated [CO₂] significantly stimulated whole‐system maximum net CO₂ influx by an average of 21% and 83% in years 3 and 4 of the experiment. Over the 4‐year experiment, cumulative belowground, foliar, and total aboveground biomass increased in both elevated [CO₂] treatments. After 2 years of growth at elevated [CO₂], early season stand respiration was decoupled from CO₂ influx aboveground, presumably because of accelerated fine root production from stored carbohydrates in the coppiced system prior to canopy development and to the increased soil carbohydrate status under elevated [CO₂] treatments. Soil respiration was stimulated by elevated [CO₂] whether measured at the system level in the undisturbed soil block, by soil collars in situ, or by substrate‐induced respiration in vitro. Elevated [CO₂] accelerated depletion of soil nutrients, phosphorus, calcium and potassium, after 3 years of growth, litter removal, and coppicing, especially in the upper soil profile, although total N showed no change. Enhancement of above‐ and belowground biomass production by elevated [CO₂] accelerated carbon cycling through the coppiced system and did not sequester additional carbon in the soil.     

Seasonal and within‐canopy variation in shoot‐scale resource‐use efficiency trade‐offs in a Norway spruce stand

Previous leaf‐scale studies of carbon assimilation describe short‐term resource‐use efficiency (RUE) trade‐offs where high use efficiency of one resource requires low RUE of another. However, varying resource availabilities may cause long‐term RUE trade‐offs to differ from the short‐term patterns. This may have important implications for understanding canopy‐scale resource use and allocation. We used continuous gas exchange measurements collected at five levels within a Norway spruce, Picea abies (L.) karst., canopy over 3 years to assess seasonal differences in the interactions between shoot‐scale resource availability (light, water and nitrogen), net photosynthesis (A_n) and the use efficiencies of light (LUE), water (WUE) and nitrogen (NUE) for carbon assimilation. The continuous data set was used to develop and evaluate multiple regression models for predicting monthly shoot‐scale A_n. These models showed that shoot‐scale A_n was strongly dependent on light availability and was generally well described with simple one‐ or two‐parameter models. WUE peaked in spring, NUE in summer and LUE in autumn. However, the relative importance of LUE for carbon assimilation increased with canopy depth at all times. Our results suggest that accounting for seasonal and within‐canopy trade‐offs may be important for RUE‐based modelling of canopy carbon uptake.     

Functional diversity of photosynthesis during drought in a model tropical rainforest – the contributions of leaf area, photosynthetic electron transport and stomatal conductance to reduction in net ecosystem carbon exchange

The tropical rainforest mesocosm within the Biosphere 2 Laboratory, a model system of some 110 species developed over 12 years under controlled environmental conditions, has been subjected to a series of comparable drought experiments during 2000–2002. In each study, the mesocosm was subjected to a 4–6 week drought, with well‐defined rainfall events before and after the treatment. Ecosystem CO₂ uptake rate (A_eco) declined 32% in response to the drought, with changes occurring within days and being reversible within weeks, even though the deeper soil layers did not become significantly drier and leaf‐level water status of most large trees was not greatly affected. The reduced A_eco during the drought reflected both morphological and physiological responses. It is estimated that the drought‐induced 32% reduction of A_eco has three principal components: (1) leaf fall increased two‐fold whereas leaf expansion growth of some canopy dominants declined to 60%, leading to a 10% decrease in foliage coverage of the canopy. This might be the main reason for the persistent reduction of A_eco after rewatering. (2) The maximum photosynthetic electron transport rate at high light intensities in remaining leaves was reduced to 71% for three of the four species measured, even though no chronic photo‐inhibition occurred. (3) Stomata closed, leading to a reduced ecosystem water conductance to water vapour (33% of pre‐drought values), which not only reduced ecosystem carbon uptake rate, but may also have implications for water and energy budgets of tropical ecosystems. Additionally, individual rainforest trees responded differently, expressing different levels of stress and stress avoiding mechanisms. This functional diversity renders the individual response heterogeneous and has fundamental implications to scale leaf level responses to ecosystem dynamics.     

FLUXNET and modelling the global carbon cycle

Measurements of the net CO₂ flux between terrestrial ecosystems and the atmosphere using the eddy covariance technique have the potential to underpin our interpretation of regional CO₂ source–sink patterns, CO₂ flux responses to forcings, and predictions of the future terrestrial C balance. Information contained in FLUXNET eddy covariance data has multiple uses for the development and application of global carbon models, including evaluation/validation, calibration, process parameterization, and data assimilation. This paper reviews examples of these uses, compares global estimates of the dynamics of the global carbon cycle, and suggests ways of improving the utility of such data for global carbon modelling. Net ecosystem exchange of CO₂ (NEE) predicted by different terrestrial biosphere models compares favourably with FLUXNET observations at diurnal and seasonal timescales. However, complete model validation, particularly over the full annual cycle, requires information on the balance between assimilation and decomposition processes, information not readily available for most FLUXNET sites. Site history, when known, can greatly help constrain the model‐data comparison. Flux measurements made over four vegetation types were used to calibrate the land‐surface scheme of the Goddard Institute for Space Studies global climate model, significantly improving simulated climate and demonstrating the utility of diurnal FLUXNET data for climate modelling. Land‐surface temperatures in many regions cool due to higher canopy conductances and latent heat fluxes, and the spatial distribution of CO₂ uptake provides a significant additional constraint on the realism of simulated surface fluxes. FLUXNET data are used to calibrate a global production efficiency model (PEM). This model is forced by satellite‐measured absorbed radiation and suggests that global net primary production (NPP) increased 6.2% over 1982–1999. Good agreement is found between global trends in NPP estimated by the PEM and a dynamic global vegetation model (DGVM), and between the DGVM and estimates of global NEE derived from a global inversion of atmospheric CO₂ measurements. Combining the PEM, DGVM, and inversion results suggests that CO₂ fertilization is playing a major role in current increases in NPP, with lesser impacts from increasing N deposition and growing season length. Both the PEM and the inversion identify the Amazon basin as a key region for the current net terrestrial CO₂ uptake (i.e. 33% of global NEE), as well as its interannual variability. The inversion's global NEE estimate of −1.2 Pg [C] yr⁻¹ for 1982–1995 is compatible with the PEM‐ and DGVM‐predicted trends in NPP. There is, thus, a convergence in understanding derived from process‐based models, remote‐sensing‐based observations, and inversion of atmospheric data. Future advances in field measurement techniques, including eddy covariance (particularly concerning the problem of night‐time fluxes in dense canopies and of advection or flow distortion over complex terrain), will result in improved constraints on land‐atmosphere CO₂ fluxes and the rigorous attribution of mechanisms to the current terrestrial net CO₂ uptake and its spatial and temporal heterogeneity. Global ecosystem models play a fundamental role in linking information derived from FLUXNET measurements to atmospheric CO₂ variability. A number of recommendations concerning FLUXNET data are made, including a request for more comprehensive site data (particularly historical information), more measurements in undisturbed ecosystems, and the systematic provision of error estimates. The greatest value of current FLUXNET data for global carbon cycle modelling is in evaluating process representations, rather than in providing an unbiased estimate of net CO₂ exchange.