Decoupled phenology and PSII thermal plasticity in seasonally dry tropical forest trees

Abstract

We tested whether seasonal plasticity in photosystem II (PSII) heat tolerance aligns with leaf phenology in tropical trees, comparing evergreen and deciduous species across the wettopostwet transition in a seasonally dry tropical forest of the central Western Ghats, India. This transition, preceding droughtinduced senescence, represents the key window to assess true unstressed thermal plasticity. PSII thermal traits: damage onset (T5), damage midpoint (T50), and temperature between damage onset to full loss (T95-T5), decline width (DW) were quantified in 27 cooccurring species during the wet (27.5{square}{degrees}C) and postwet (31.6{square}{degrees}C) periods. Contrary to phenologybased predictions, PSII plasticity was not structured by leaf habit or successional status. Both T5 (+1.7{square}{degrees}C) and T50 (+0.9{square}{degrees}C) increased significantly across seasons, but responses were speciesspecific, with evergreen and deciduous trees acclimating similarly. The preventionversusforbearance tradeoff (T5 - DW relationship) remained conserved, though leaf habits diverged under postwet conditions. Thermal safety margins based on T50 were large, but T5 revealed vulnerable latesuccessional evergreens (Saraca asoca, Ficus spp.) and Careya arborea. These results show that PSII thermotolerance regulation operates largely independently of droughtavoidance phenology, indicating species identity and not leaf habit drives plasticity in PSII thermal response in seasonally dry tropical forests.