Tropical tree species vary in photosynthetic temperature sensitivity, with species from warmer habitats or those acclimated to higher temperatures typically displaying higher thermal optima for net CO2 assimilation (Topt, Anet). Sustaining photosynthesis at elevated temperatures likely requires increased allocation of resources (ATP, NADPH, nitrogen, carbon) toward heat stress management, particularly PSII repair. However, under extreme heat, repair demands may exceed available resources, potentially limiting acclimation. It is unclear whether higher Topt, Anet reflects inherently greater PSII heat stability.We studied 11 tropical tree species across a topographic (hilltop, slope, valley) and thermal gradient (summer peaks: 46.1, 40.1, 31.8 °C, respectively) in India’s Central Western Ghats forest, measuring photosynthetic temperature responses and PSII thermal tolerance (T5, the temperature causing 5% PSII efficiency decline) at peak summer. We found an inverse correlation between T5 and Topt, Anet (p = 0.005): lower Topt, Anet was associated with higher PSII heat stability (higher T5), and vice versa. This could suggest a trade-off between investing resources to achieve higher Topt, Anet and maintaining PSII heat stability. ·      Species may struggle to simultaneously acclimate to elevated temperatures and remain resilient to extreme heat events. These findings have implications for understanding tropical forest tree responses to climate warming.

Microclimate differences in water availability can diversify seasonal water use and photosynthetic strategies among co-occurring tropical tree species, especially in forests with strongly seasonal climates. We studied a tropical forest site in the Western Ghats, India, and characterised seasonal differences in photosynthetic CO 2 assimilation rates ( A net) among three tree species groups spanning leaf habit and topographic affinity: deciduous species in dry hilltops, dry-affinity evergreens on slopes, and wet-affinity evergreens in valleys. As expected, deciduous species on dry hilltops showed higher P opt ( A net at optimal temperature, T opt) during the wet period, while evergreen species showed no overall seasonal differences. Interestingly, dry-affinity slope evergreens showed higher P opt during the dry period compared to the wet period despite lower soil moisture, suggesting sufficient water availability and warmer thermal niche preference. Across species, stomatal conductance ( g s) at T opt was generally higher during the wet period, except for one evergreen species. Surface soil moisture was lowest in hilltops, intermediate on slopes, and highest in valleys, with higher levels during the wet period compared to the dry period. Our findings highlight the diverse seasonal photosynthetic strategies among tropical tree species with different leaf habits and water affinities.

Tropical forests are experiencing unprecedented high temperature conditions due to climate change that could limit their photosynthetic functions. We studied the high temperature sensitivity of photosynthesis in a rainforest site in southern Amazonia, where some of the highest temperatures and most rapid warming in the Tropics have been recorded. The quantum yield (Fv/Fm) of photosystem II was measured in seven dominant tree species using leaf discs exposed to varying levels of heat stress. T50 was calculated as the temperature at which Fv/Fm) was half the maximum value. T5 is defined as the breakpoint temperature, at which Fv/Fm) decline was initiated. Leaf thermotolerance in the rapidly warming Southern Amazonia was the highest recorded for forest tree species globally. T50 and T5 varied between species, with one mid storey species, Amaioua guianensis, exhibiting particularly high T50 and T5 values. While the T50 values of the species sampled were several degrees above the maximum air temperatures experienced in southern Amazonia, the T5 values of several species are now exceeded under present-day maximum air temperatures.