Thermal tolerance of Acer campestre (field maple) under heat and drought stress derived from chlorophyll fluorescence

Climate change is intensifying extreme heat events, making the thermal tolerance of street trees increasingly crucial for sustainable urban forestry. In this study, we quantified the physiological responses of Acer campestre (field maple) to heat stress under varying hydration conditions by measuring chlorophyll fluorescence parameters. We assessed critical temperature thresholds (T _crit ) and temperature at 50% reduction in photosystem II efficiency (T ₅₀ ) through four experimental trials, with the final trial incorporating drought stress. Our results demonstrated significant hydration-dependent plasticity in thermal tolerance thresholds. Well-watered specimens exhibited T _crit values of 37.0–44.0°C, which declined sharply to 29.0°C under drought conditions. Following 24-hour rehydration, T _crit significantly increased across all trials (t = 7.63, df = 31, p < 0.01), with the most pronounced recovery in drought-stressed specimens from 29.0°C to 42.0°C with a mean difference of 8.81°C (95% CI: 6.46–11.17°C). In contrast, T ₅₀ values remained relatively stable (47.0–50.0°C) with only modest post-hydration of mean difference 1.19°C (t = 2.83, df = 31, p < 0.01, 95% CI: 0.33–2.04°C). Principal component analysis revealed that hydration status explained 80.41% of variation in thermal responses, with K-means clustering (silhouette score = 0.45) separating samples into distinct hydrated and non-hydrated physiological groups. PERMANOVA confirmed significant treatment effects on multivariate thermal profiles (F = 5.47, R² = 0.41, p = 0.001). The significant correlation between T _crit and T ₅₀ (r = 0.61, p < 0.01, 95% CI: 0.43–0.74°C) suggested coordinated protection across different levels of thermal stress. These findings demonstrate that immediate hydration status, rather than prior drought conditioning, predominantly determines thermal resilience in Acer campestre , with critical implications for urban tree management under increasingly frequent heat extremes.