Implications of leaf-scale physiology for whole tree transpiration under seasonal flooding and drought in central Cambodia

In central Cambodia, rapidly growing non-endemic species are often planted for timber production. The introduction of non-endemic species into stands with native species has been found to alter the forest transpiration characteristics in this region with distinct dry and rainy seasons defined by the different rainfall patterns. However, less is understood about the underlying processes, especially the ecophysiological characteristics of native species that are adapted to the highly seasonal environments and the non-endemic species that acclimate to those environments. Leaf ecophysiological traits were measured for two native and two representative non-endemic tree species in central Cambodia to examine whether species-specific seasonal trends in transpiration could be explained by leaf ecophysiological traits. Photosynthetic capacity as represented by the maximum carboxylation rate (Vcmax25) of each species remained constant in the rainy and dry seasons. Native species had high stomatal conductance (gsw) and the parameter for its response to the environments (m) in the rainy season and lower levels in the dry season. Additionally, the seasonal change in gsw for non-endemic species was less clear despite the seasonality in the tree-level transpiration. A multi-layer model that incorporated the measured data related to the leaf ecophysiological traits and their seasonal changes successfully reproduced the daily canopy-scale transpiration rate (Ecanopy). The modeled Ecanopy successfully reproduced most of the seasonal and day-to-day changes in measured Ecanopy using sap flux measurements and showed the strong control of leaf ecophysiological traits over Ecanopy and the species-specific seasonal trends. Two reduction events in measured Ecanopy were, however, not explained by the simulation, possibly as a result of species-specific factors that were not considered in this study, i.e., the reduction in water uptake by a native species caused by drought and the hypoxic effects of flooding on the roots of a non-endemic species.