Convergent acclimation of leaf photosynthesis and respiration to prevailing ambient temperatures under current and warmer climates in Eucalyptus tereticornis

Understanding physiological acclimation of photosynthesis and respiration is important in elucidating the metabolic performance of trees in a changing climate. Does physiological acclimation to climate warming mirror acclimation to seasonal temperature changes? We grew Eucalyptus tereticornis trees in the field for 14 months inside 9-m tall whole-tree chambers tracking ambient air temperature (Tair) or ambient Tair + 3°C (i.e. 'warmed'). We measured light- and CO2-saturated net photosynthesis (Amax) and night-time dark respiration (R) each month at 25°C to quantify acclimation. Tree growth was measured, and leaf nitrogen (N) and total non-structural carbohydrate (TNC) concentrations were determined to investigate mechanisms of acclimation. Warming reduced Amax and R measured at 25°C compared to ambient-grown trees. Both traits also declined as mean daily Tair increased, and did so in a similar way across temperature treatments. Amax and R (at 25°C) both increased as TNC concentrations increased seasonally; these relationships appeared to arise from source-sink imbalances, suggesting potential substrate regulation of thermal acclimation. We found that photosynthesis and respiration each acclimated equivalently to experimental warming and seasonal temperature change of a similar magnitude, reflecting a common, nearly homeostatic constraint on leaf carbon exchange that will be important in governing tree responses to climate warming.