High‐resolution temperature responses of leaf respiration in snow gum (Eucalyptus pauciflora) reveal high‐temperature limits to respiratory function

The response of leaf energy metabolism to temperature is central to predicting the impact of environmental gradients and future climate regimes on carbon exchange in forests. Here, we tested whether Eucalyptus pauciflora (an evergreen, broadleaved tree) growing in thermally contrasting environments (including winter‐acclimated trees encased in ice at high altitudes) exhibit generalizable temperature response functions of leaf respiration and fluorescence. We found that leaf energy metabolism was surprisingly heat tolerant (with maximal rates of respiration occurring at 51–57°C), with temperature responses varying seasonally. Collectively, our results: (1) highlight high‐temperature limits of energy metabolism in E. pauciflora; and, (2) provide a framework for improving representation of T‐responses of leaf respiration in predictive models. We tested whether snow gum (Eucalyptus pauciflora) trees growing in thermally contrasting environments exhibit generalizable temperature (T) response functions of leaf respiration (R) and fluorescence (Fo). Measurements were made on pot‐grown saplings and field‐grown trees (growing between 1380 and 2110 m a.s.l.). Using a continuous, high‐resolution protocol, we quantified T response curves of R and Fo – these data were used to identify an algorithm for modelling R–T curves at subcritical T's and establish variations in heat tolerance. For the latter, we quantified Tmax [T where R is maximal] and Tcrit [T where Fo rises rapidly]. Tmax ranged from 51 to 57 °C, varying with season (e.g. winter > summer). Tcrit ranged from 41 to 49 °C in summer and from 58 to 63 °C in winter. Thus, surprisingly, leaf energy metabolism was more heat‐tolerant in trees experiencing ice‐encasement in winter than warmer conditions in summer. A polynomial model fitted to log‐transformed R data provided the best description of the T‐sensitivity of R (between 10 and 45 °C); using these model fits, we found that the negative slope of the Q10–T relationship was greater in winter than in summer. Collectively, our results (1) highlight high‐T limits of energy metabolism in E. pauciflora and (2) provide a framework for improving representation of T‐responses of leaf R in predictive models.