Characterising Thermotolerance Traits and Their Acclimation Potential in Two Important Forestry Species of Eucalyptus

As climate warming and more frequent heatwave events cause increasing damage to native and managed forest trees, there is an urgent need to characterise species thermotolerance thresholds and define their capacity to acclimate to warmer growth conditions. This study characterised a range of thermotolerance traits related to thresholds of photosystem II (PSII) function under increasing temperature (T_crit) and heat load in saplings and adults of two commercially and ecologically important eucalypt species, Eucalyptus globulus and E. nitens. We also measured photosynthetic and leaf economic traits and examined the acclimation response of leaf thermotolerance in saplings grown under different temperatures and exposed to heatwave conditions. Saplings and adults of each species recorded similar optimum temperatures (T_opt) for maximum photosynthesis (E. globulus = 26.8°C and E. nitens = 25.9°C) and similar leaf thermotolerance thresholds (T_crit~42°C). When grown under different day/night temperatures (cool, 20/10°C; mild, 25/15°C; and warm, 30/20°C), we found evidence of plasticity in T_crit in E. globulus saplings grown under cool (T_crit = 42°C) versus warm (T_crit = 45°C) conditions, while T_crit in E. nitens saplings did not significantly vary. During heatwave conditions, PSII dysfunction increased in saplings of both species across all growth conditions with increasing heatwave intensity, with substantial loss of function at 45°C. We establish for the first time heat intensity-duration relationships and define thermal acclimation potential for E. globulus and E. nitens saplings. These results provide forest managers with insights into canopy vulnerability when optimising species selection for heat-prone sites and developing proactive climate adaptation strategies for plantations in a warming environment.