Contrasting seasonal plasticity of photosynthesis in evergreen and deciduous tree species from a tropical forest

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.