Thermal tolerance and recovery dynamics of urban tree species Acer campestre (field maple) under heat and drought stress derived from chlorophyll fluorescence

Rising temperatures present a significant challenge to urban tree species, affecting their physiological function and long-term survival. This study investigates the thermal tolerance and recovery capacity of Acer campestre under controlled heat stress conditions, using chlorophyll fluorescence (Fv/Fm) measurements to assess photosynthetic efficiency. Key thermal tolerance indicators, Tcrit and T50 and thermal damage index (TMI) were examined across four experimental trials, with the final trial incorporating severe drought stress to assess the combined effects of heat and water limitation. Results showed Tcrit decreasing from 40.47°C in the fourth trial to 28.81°C under drought stress. Despite this, post-recovery measurements indicated strong acclimation capacity, with Tcrit increasing significantly after 24 hours of hydration. T50 remained relatively stable across all trials, suggesting that Acer campestre maintains a fundamental resilience to extreme temperatures, despite variations in its initial thermal tolerance. The Thermal Damage Index (TDI) further substantiated these findings, increasing from 0.2-0.25 under normal conditions to over 0.38 during drought stress. These findings suggest that Acer campestre relies more on recovery mechanisms than sustained high thermal tolerance, which may support short-term survival but raises concerns in the face of prolonged heatwave conditions. As extreme temperature events become more frequent due to climate change, understanding the interactions between thermal stress, drought resilience, and recovery potential is essential for urban tree management. Future research should focus on the long-term effects of repeated heat exposure on tree physiology to better inform climate-adaptive urban forestry strategies aimed at enhancing the survival and health of urban trees in a warming world.