Comparison of water-use characteristics of tropical tree saplings with implications for forest restoration.

Tropical forests are experiencing reduced productivity and will need restoration with suitable species. Knowledge of species-specific responses to changing environments during early stage can help identify the appropriate species for sustainable planting. Hence, we investigated the variability in whole-tree canopy conductance and transpiration (Gt and EL) in potted saplings of common urban species in Thailand, viz., Pterocarpus indicus, Lagerstroemia speciosa, and Swietenia macrophylla, across wet and dry seasons in 2017-2018. Using a Bayesian modeling framework, Gt and EL were estimated from sap flux density, informed by the soil, atmospheric and tree measurements. Subsequently, we evaluated their variations with changing vapor pressure deficit (VPD) and soil moisture across timescales and season. We found that Gt and EL were higher and highly variable in L. speciosa across seasons than S. macrophylla and P. indicus. Our results implied that water-use in these species was sensitive to seasonal VPD. L. speciosa may be suitable under future climate variability, given its higher Gt and EL across atmospheric and soil moisture conditions. With their lower Gt and EL, P. indicus and S. macrophylla may photosynthesize throughout the year, maintaining their stomatal opening even under high VPD. These findings benefit reforestation and reclamation programs of degraded lands.