Abstract: Tropical forests play a critical role in global carbon and water cycles, driven by interactions and feedbacks among diverse species across forest successional stages. However, species-specific carbon–water strategies and their environmental drivers remain poorly understood. We investigated variations of leaf-level gas exchange and water-use efficiency in the dry- and wet seasons in old-growth (> 200 yrs) and young secondary (∼10 yrs) tropical forests in Thailand. Stomatal conductance, net photosynthesis, transpiration, and water-use efficiency (WUE) generally declined with rising vapor pressure deficit (VPD), but the responses varied markedly among species and between forest successional stages. During the wet season, trees in the young forest exhibited fast-growing strategies characterized by a lower WUE, higher gas exchanges, and greater sensitivity to increasing VPD, whereas trees in the old-growth forest exhibited more conservative water-use strategies. During the dry season, despite VPD being twice higher in the young forest, similar WUE was recorded between the two successional stages. Variations of specific leaf area (SLA) and chlorophyll contents across forests and seasons failed to explain variations of the gas exchanges or WUE. Instead, these variations were better explained by the interactions and feedback between microclimate conditions and forest structure. Our findings suggest that use of average leaf-traits for regional or stand level modeling is unadvisable in the diverse tropical ecosystems with varying successional stages.

Key words: leaf gas exchange, vapor pressure deficit, forest succession, carbon–water trade-off, leaf functional traits