Combined effects of elevated CO2 and warmer temperature on limitations to photosynthesis and carbon sequestration in yellow birch

Abstract

Abstract Elevated CO2 and warmer temperature occur simultaneously under the current climate change. However, their combined effects on the photosynthetic traits in boreal trees are not well understood. This study investigated the morphological and photosynthetic responses of yellow birch (Betula alleghaniensis Britt.) to a combined treatment of CO2 and temperature (ambient, ACT (400 μmol mol−1 CO2 and current temperature) vs elevated, ECT (750 μmol mol−1 CO2 and current +4 °C temperature)). It was found that ECT significantly reduced leaf-area based photosynthetic rate (An), maximum Rubisco carboxylation rate (Vcmax), photosynthetic electron transport rate (Jmax), leaf nitrogen concentration, respiration and mesophyll conductance. There were two interesting findings: first, the primary mechanism of photosynthetic limitation shifted from Ribulose-1,5-bisphosphate (RuBP) carboxylation (related to Vcmax) to RuBP regeneration (related to Jmax) in response to ECT, leading to decreased transition point (Ci-t and An-t) from RuBP carboxylation to regeneration; second, the increase in total leaf area in response to ECT more than compensated for the downregulation of leaf-area based photosynthesis, leading to greater biomass in ECT than in ACT. We proposed a new protocol for evaluating photosynthetic limitations by comparing the relative relationship between the transition point (Ci-t and An-t) and the photosynthetic rate at growth CO2 (Ci-g and An-g). Furthermore, we found that Jmax (RuBP regeneration) was the primary limitation to An under ECT.