Responses of Sap Flux Densities of Different Plant Functional Types to Environmental Variables Are Similar in Both Dry and Wet Seasons in a Subtropical Mixed Forest

Subtropical mixed forest ecosystems are experiencing dramatic changes in precipitation and different plant functional types growing here are expected to respond differently. This study aims to unravel the water use patterns of different plant functional types and their responses to environmental changes in a typical subtropical mixed forest in southern China. Diurnal and seasonal sap flux densities of evergreen broad-leaved trees (EBL), deciduous broad-leaved trees (DBL), and conifers (CON), as well as environmental variables, were recorded simultaneously from May 2016 to March 2019. The results showed that the sap flux density of EBL was significantly higher than those of CON and DBL in all seasons, irrespective of dry or wet seasons. Path analysis revealed that seasonal differences in sap flux density were mainly due to variations in photosynthetic photon flux density (PPFD). At saturating PPFD, changes in sap flux density during the day were in response to vapor pressure deficit (VPD). Regression analyses showed that sap flux density increased logarithmically with PPFD, irrespective of functional type. The hysteresis loops of sap flux density and VPD were different among different plant functional types in wet and dry seasons. Our results demonstrated converging response patterns to environmental variables among the three plant functional types considered in this study. Our findings contribute to a better understanding of the water use strategies of different plant functional types in subtropical mixed forests