Low Sensitivity Of Gross Primary Production To Elevated Co2 In A Mature Eucalypt Woodland

The response of mature forest ecosystems to a rising atmospheric carbon dioxide concentration (C-a) is a major uncertainty in projecting the future trajectory of the Earth's climate. Although leaf-level net photosynthesis is typically stimulated by exposure to elevated Ca (eC(a)), it is unclear how this stimulation translates into carbon cycle responses at the ecosystem scale. Here we estimate a key component of the carbon cycle, the gross primary productivity (GPP), of a mature native eucalypt forest exposed to free-air CO2 enrichment (the EucFACE experiment). In this experiment, light-saturated leaf photosynthesis increased by 19% in response to a 38% increase in C-a. We used the process-based forest canopy model, MAESPA, to upscale these leaf-level measurements of photosynthesis with canopy structure to estimate the GPP and its response to eC(a). We assessed the direct impact of eC(a), as well as the indirect effect of photosynthetic acclimation to eC(a) and variability among treatment plots using different model scenarios.At the canopy scale, MAESPA estimated a GPP of 1574 gCm(-2) yr(-1) under ambient conditions across 4 years and a direct increase in the GPP of C11% in response to eC(a). The smaller canopy-scale response simulated by the model, as compared with the leaf-level response, could be attributed to the prevalence of RuBP regeneration limitation of leaf photosynthesis within the canopy. Photosynthetic acclimation reduced this estimated response to 10 %. After taking the baseline variability in the leaf area index across plots in account, we estimated a field GPP response to eC(a) of 6% with a 95% confidence interval (-2 %, 14 %). These findings highlight that the GPP response of mature forests to eC(a) is likely to be considerably lower than the response of light-saturated leaf photosynthesis. Our results provide an important context for interpreting the eC(a) responses of other components of the ecosystem carbon cycle.