Leaf and Soil-plant Hydraulic Processes in the Transpiration of Tropical Forest

Abstract

In order to reveal the control of tree transpiration by the leaf ecophysiological traits and the hydraulic processes from the soil to leaves, transpiration rates of the tree species in tropical seasonal forests were monitored and modeled using independently measured leaf photosynthetic traits. Stand-level transpiration rate was modeled for rubber trees in a plantation and alien and native species in a community forest using a multilayer biophysical model that couples the energy balance and leaf ecophysiological processes. Model simulation was carried out on the assumption that leaf gas exchange was not limited by the hydraulic processes from the root to the leaves, while transpiration rates, which were independently monitored using sap flux measurements, were influenced both by the seasonal trends in leaf ecophysiological traits and the hydraulic processes. The modeled transpiration rate (Emodel) successfully captured the diurnal trend of the in situ measured one (Esap) in most rainy seasons in rubber plantation and in dry season in community forests, suggesting the absence of hydraulic limitation in soil-plant continuum. The decoupling between the Emodel and Esap was observed in mid dry season in rubber plantation and in a native species of the community forest. The daily-scale Emodel overestimated Esap by 20-40%, mainly due to the midday depression of Esap. On the other hand, in an alien eucalyptus species in community forest, overestimate of Emodel was observed in mid rainy season, suggesting the failure of water uptake by the roots under flooding conditions. The seasonal decreases in daily Esap matched the timing of the water transport limitation of soil-plant continuum. Under lowered Esap conditions, as high Emodel as other seasons was observed in each species but could not be met due to the water supply, suggesting the leaf ecophysiological traits oriented for high leaf water demand and their imbalance with the seasonally decreasing water supply capacity. In conclusion, seasonal trends in transpiration rate were strongly characterized by the limitation in the process of soil-plant water transport, rather than the seasonal trends in the leaf ecophysiological traits.