Stimulation of terrestrial productivity by rising CO~2~ concentration is projected to reduce the airborne fraction of anthropogenic CO~2~ emissions; coupled climate-carbon (C) cycle models, including those used in the IPCC Fourth Assessment Report (AR4), are sensitive to this negative feedback on atmospheric CO~2~^1^. The representation of the so-called CO~2~ fertilization effect in the 11 models used in AR4 and subsequent models^2,3^ was broadly consistent with experimental evidence from four free-air CO~2~ enrichment (FACE) experiments, which indicated that net primary productivity (NPP) of forests was increased by 23 +/- 2% in response to atmospheric CO~2~ enrichment to 550 ppm^4^. Substantial uncertainty remains, however, because of the expectation that feedbacks through the nitrogen (N) cycle will reduce the CO~2~ stimulation of NPP^5,6^; these feedbacks were not included in the AR4 models and heretofore have not been confirmed by experiments in forests^7^. Here, we provide new evidence from a FACE experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee, USA, that N limitation has significantly reduced the stimulation of NPP by elevated atmospheric CO~2~ concentration (eCO~2~). Isotopic evidence and N budget analysis support the premise that N availability in this forest ecosystem has been declining over time, and declining faster in eCO~2~. Model analyses and evidence from leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to eCO2. These results provide a strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.

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Summary Based on short‐term experiments, many plant growth models – including those used in global change research – assume that an increase in temperature stimulates plant respiration (R) more than photosynthesis (P), leading to an increase in the R/P ratio. Longer‐term experiments, however, have demonstrated that R/P is relatively insensitive to growth temperature. We show that both types of temperature response may be reconciled within a simple substrate‐based model of plant acclimation to temperature, in which respiration is effectively limited by the supply of carbohydrates fixed through photosynthesis. The short‐term, positive temperature response of R/P reflects the transient dynamics of the nonstructural carbohydrate and protein pools; the insensitivity of R/P to temperature on longer time‐scales reflects the steady‐state behaviour of these pools. Thus the substrate approach may provide a basis for predicting plant respiration responses to temperature that is more robust than the current modelling paradigm based on the extrapolation of results from short‐term experiments. The present model predicts that the acclimated R/P depends mainly on the internal allocation of carbohydrates to protein synthesis, a better understanding of which is therefore required to underpin the wider use of a constant R/P as an alternative modelling paradigm in global change research.

The observation that, for unstressed plants, light use efficiency of a plant canopy, defined as the ratio of net primary productivity (NPP) to absorbed photosynthetically active radiation (APAR), is approximately constant with respect to changes in APAR, implies that NPP can be modeled using a linear relationship with APAR. However, such a linear relationship is counter-intuitive because the relationship between leaf photosynthesis and absorbed light is strongly nonlinear. Three arguments have been advanced to explain the observed linear relationship between NPP and APAR. In this paper, a detailed, physiologically based model of canopy radiation absorption and photosynthesis (MAESTRO) was used to analyze these arguments. The first argument is that the canopy is structured so that radiation is distributed throughout the canopy such that most leaves are exposed to non-saturating quantum flux density, resulting in a linear response of canopy photosynthesis to APAR. Simulations of MAESTRO indicated that this explanation is inadequate, because daily values of canopy photosynthetic light use efficiency calculated with MAESTRO were highly variable regardless of canopy structure. The second argument is that variability in light use efficiency decreases with increasing time scale. The simulations showed that this is true to some extent, although simulated annual canopy photosynthetic light use efficiency still varies across sites with different LAI or light climate. The third argument is that changes in canopy nitrogen content act both to maximize net canopy photosynthesis and to keep light use efficiency constant. This argument could not be tested with the model, but the failure of the first two explanations suggests that this third explanation deserves closer attention.

This study was designed to estimate gross primary productivity (GPP) in the Duke Forest at both ambient and elevated CO 2 (ambient ϩ 200 L/L) concentrations using a physiologically based canopy model. The model stratified the canopy of loblolly pine (Pinus taeda L.) forest into six layers and estimated photosynthesis in each layer according to the Farquhar submodel coupled with the Ball-Berry stomatal conductance submodel. The model was parameterized with a suite of physiological measurements, including leaf area index (LAI), leaf nitrogen (N) concentration, photosynthesis-N relationships, and stomatal conductance. The model was validated against measured leaf photosynthesis and canopy carbon (C) fluxes estimated from eddy-covariance measurements (ECM). Application of this model to simulate canopy C fixation from 28 August 1996, the onset of CO 2 fumigation, to 31 December 1998 suggested that elevation of atmospheric [CO 2 ] to ambient ϩ 200 L/L resulted in increase of canopy C fixation by 35% in 1996, 39% in 1997, and 43% in 1998. The modeled GPP and its response to elevated [CO 2 ] were sensitive to parameter values of quantum yield of electron transport, leaf area index, and the vertical distribution of LAI within the canopy. Thus, further investigation on those parameters will help improve the precision of estimated ecosystem-scale C fluxes. Furthermore, comparison between the modeled and ECM-estimated canopy C fluxes suggested that soil moisture, in addition to air vapor pressure, controlled canopy photosynthesis during the drought period.