Antecedent growth temperatures and soil water effects on heat tolerance during a heatwave and post-recovery in Pinus massoniana from two provenances

Rising frequency and intensity of heatwaves pose a major threat to forest carbon sequestration worldwide. However, intraspecific differences in heat tolerance and the plasticity of these responses under combined warming, drought, and heatwaves remain poorly understood. We grew one-year-old seedlings of Pinus massoniana from two provenances (cooler-drier and warmer-wetter) under three growth temperature regimes (22 °C, 26 °C, 30 °C) and two water conditions (well-watered and drought), followed by a controlled heatwave (40 °C for seven days) and 14-day recovery phase. Heat tolerance (T_crit,  T₅₀), gas exchange, and chlorophyll fluorescence were assessed, integrating with a synthesis of available global literatures. We show four key findings: (1) Warmer-wetter seedlings showed higher heat tolerance, but these tolerance metrics showed a poor correlation with actual photosynthesis during heatwaves; (2) antecedent warming increased heat tolerance (T_crit,  T₅₀) and accelerated post-heatwave recovery; (3) Drought increased heat tolerance but reduced photosynthetic rates during the heatwave; (4) Combined antecedent warming and drought synergistically enhanced photosynthetic performance under heatwaves. Our study demonstrates that heat tolerance is shaped by multidimensional stress (growth temperature, soil water availability, and climates of origin), rather than isolated factors. These findings highlight the importance of considering provenance differences and stress history in forest silviculture, particularly for seed sourcing, nursery propagation, and reforestation under future subtropical climate extremes.