Cool-Dry Season Depression in Gas Exchange of Canopy Leaves and Water Flux of Tropical Trees at the Northern Limit of Asian Tropics

Trees on the northern boundary of Asian tropics experience hot-humid and cool-dry seasons, but little is known about their seasonal dynamics in canopy physiology. We used a canopy crane to reach the canopy of nine tropical tree species and measured canopy leaf gas exchange, water status, and trunk sap flux during the hot-humid and cool-dry seasons in Xishuangbanna, China. We found that most tree species exhibited significant reductions in maximum photosynthetic rate (Amax), stomatal conductance (gsmax), predawn and midday leaf water potentials, and maximum sap flux density in the cool-dry season. Compared to the hot-humid season, Amax declined by 19–60%, and maximum water flux declined by −14% (an increase) to 42%. The cool-dry season decline in Amax of four species can be partly explained by an increased stomatal limitation (decreased gsmax and intercellular CO2 concentrations). Therefore, a predicted increase in drought in this region may decrease the carbon sequestration and productivity of these forests. We did not find a tradeoff between performance (Amax in the hot-humid season) and persistence through the cool-dry season; species with higher Amax in the hot-humid season did not show higher percent seasonal declines in the cool-dry season. Amax was significantly and positively associated with the trunk sap flux for both seasons, but the association was weaker in the cool-dry season. Thus, our results suggest that some tradeoffs and trait associations are environment dependent. Our results are important for understanding carbon and water fluxes of seasonal tropical forests and their responses to environmental changes.