Low sensitivity of gross primary production to elevated CO.sub.2 in a mature eucalypt woodland

The response of mature forest ecosystems to a rising atmospheric carbon dioxide concentration (C.sub.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 C.sub.a (eC.sub.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 CO.sub.2 enrichment (the EucFACE experiment). In this experiment, light-saturated leaf photosynthesis increased by 19 % in response to a 38 % increase in C.sub.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.sub.a . We assessed the direct impact of eC.sub.a, as well as the indirect effect of photosynthetic acclimation to eC.sub.a and variability among treatment plots using different model scenarios.