Variation of leaf-level gas exchange rates and leaf functional traits of dominant trees across three successional stages in a Southeast Asian tropical forest
Deforestation has created heterogeneous patches of old-growth and secondary forests throughout Southeast Asia, posing challenges for understanding the hydrological and carbon cycles. In addition to changes in species composition, environmental conditions differ across successional stages which in turn can influence forest water use and productivity. Here, we investigated leaf-level area-based photosynthesis (Aarea) and stomatal conductance (gs) of 11 tree species dominating an old-growth (OF; >200 years), an intermediate (IF; ~44 years), and a young forest (YF; ~4 years) in Thailand during both the wet and dry season. Specifically, we compared Aarea and gs and assessed the sensitivity of gs to vapor pressure deficit (VPD). We also examined relationships between gas exchange parameters and key functional leaf traits, including leaf mass per area (LMA), nitrogen (N), phosphorus (P), and chlorophyll concentration. All three forests showed comparable Aarea and gs in the wet season, whereas significantly lower values were observed in IF during the dry season. All forest stages displayed similar sensitivity of gs to VPD. Among the leaf functional traits considered, LMA, N and P were significantly higher in YF compared to the other two successional stages. Our results suggested that forest succession may not influence gas exchange, rather, canopy development associated with forest stage produced the main effect. Furthermore, the young forest was the most active in resource acquisition with its high LMA and leaf nutrient concentrations, which could result in high photosynthetic rates. However, low soil water availability in YF possibly limit the gas exchange rates thereby making them similar to those in the old-growth forest. These findings highlight the potential effects of canopy characteristics inherent in successional forests on water and carbon exchanges between trees and the atmosphere and their sensitivity to atmospheric drought. These results call for the need for further studies to identify the main factors influencing forest productivity during secondary succession in the tropics, particularly in the Southeast Asian region where such information is lacking.