Abstract
Abstract. The response of mature forest ecosystems to a rising atmospheric carbon dioxide concentration (Ca) 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 (eCa), 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 Ca. 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 eCa. We assessed the direct impact of eCa, as well as the indirect effect of photosynthetic acclimation to eCa and variability among treatment plots using different model scenarios. At the canopy scale, MAESPA estimated a GPP of 1574 g C m−2 yr−1 under ambient conditions across 4 years and a direct increase in the GPP of +11 % in response to eCa. 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 eCa of 6 % with a 95 % confidence interval (−2 %, 14 %). These findings highlight that the GPP response of mature forests to eCa is likely to be considerably lower than the response of light-saturated leaf photosynthesis. Our results provide an important context for interpreting the eCa responses of other components of the ecosystem carbon cycle.
Highlights
Forests represent the largest long-term terrestrial carbon storage (Bonan, 2008; Pan et al, 2011)
We have shown how a large response of leaf-level photosynthesis to elevated Ca (eCa) diminishes when integrated to the canopy scale, according to the synthesis of 4 years of leaf measurements at Eucalyptus FACE experiment (EucFACE) with a stand-scale model, MAESPA
Our version of MAESPA was evaluated against leaf photosynthesis and whole-tree sap flow measurements in EucFACE (R2 of 0.77 and 0.8 respectively; Yang et al, 2019)
Summary
Forests represent the largest long-term terrestrial carbon storage (Bonan, 2008; Pan et al, 2011). Ca is projected to continue to increase by 1–5 μmol mol−1 yr−1 into the future (IPCC, 2014), but the rate of this rise depends on the magnitude of the forest feedback on Ca. At the leaf scale, the direct physiological effects of rising Ca are well understood: elevated Ca (eCa) stimulates plant photosynthesis (Kimball et al, 1993; Ellsworth et al, 2012) and reduces stomatal conductance (Morison, 1985; Saxe et al, 1998), which together increase leaf water-use efficiency (De Kauwe et al, 2014). The direct physiological effects of rising Ca are well understood: elevated Ca (eCa) stimulates plant photosynthesis (Kimball et al, 1993; Ellsworth et al, 2012) and reduces stomatal conductance (Morison, 1985; Saxe et al, 1998), which together increase leaf water-use efficiency (De Kauwe et al, 2014) Projecting the response of the terrestrial carbon sink to future increases in Ca is a major uncertainty in models (Friedlingstein et al, 2014), highlighting an urgent need to make greater use of data from manipulative experiments at the leaf scale to inform terrestrial biosphere models (Medlyn et al, 2015)
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