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

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.

Does growing atmospheric CO2 explain increasing carbon sink in a boreal coniferous forest?

“…The model results suggest ecosystem response is weaker than that observed at the leaf level, similar to what was recently reported for EucFACE (Yang et al, 2020). For the ca.…”

Estimating intraseasonal intrinsic water‐use efficiency from high‐resolution tree‐ring δ13C data in boreal Scots pine forests

“…At leaf level, rising c a tightly regulates iWUE gas (Table 1 ) by enhancing assimilation rate (Streit et al ., 2014 ) and reducing g s (Brodribb et al ., 2009 ). However, the c a effect on whole‐tree level iWUE iso was reduced (Table 1 ), because the prevalent photosynthesis at lower canopy is limited by RuBP‐regeneration and less sensitive to rising c a compared with Rubisco‐limited photosynthesis at top canopy (Yang et al ., 2020 ). At ecosystem level, the c a effect was further dampened (iWUE EC in Table 1 ) due to species‐specific responses of iWUE to c a (Marchand et al ., 2020 ) and changes in leaf area and soil water savings (Lavergne et al ., 2019 ).…”

Evaluating a land surface model at a water-limited site: implications for land surface contributions to droughts and heatwaves

“…These results again highlight uncertainties in the translation of soil texture informa-tion to hydraulic parameters for water retention and hydraulic conductivity via the empirical functions, which are derived from limited observations (Van Looy et al, 2017). The development in pedotransfer functions via machine learning or multi-model ensemble provides new avenues to reduce errors from parameters (Zhang and Schaap, 2017;Dai et al, 2019). High-resolution global soil datasets (e.g.…”