The Relationship Between Leaf Temperature, Leaf Traits, and Sap Flow Rate in a Panamanian Tropical Moist Forest

Rising temperatures and increased droughts caused by climate change are threatening tropical forests through leaf thermal damage and subsequent thermal mortality. As temperatures are predicted to continue rising, understanding what mechanisms tropical tree species have to cool their leaves is important. Therefore, this study examines whether the rate of stem sap flow is significantly driven by changes in leaf temperature, other climate variables, and leaf size. Thermal videos of five different tropical tree species were collected at San Lorenzo National Park (Panama), alongside sap flow, weather, and leaf trait data. These data sets were used to estimate average leaf temperatures, rates of sap flow, leaf level vapor pressure deficit (VPD), and average leaf area for each tree species. In an initial analysis, average leaf temperatures and leaf level VPD were compared to rates of sap flow using nonlinear least squares regression. The greatest rate of change in the increase of the rate of sap flow as leaf temperature increased, (kTleaf), was compared to the average leaf areas in a second analysis using linear regression. For the first analysis, there was a positive correlation between the rate of sap flow and average leaf temperature, which implied that leaf temperature did partially drive changes in the rate of sap flow. The positive correlation between rates of sap flow and leaf level VPD demonstrated that VPD affected sap flow, but only up to certain values of VPD. The plateau of sap flow rates also suggested that individual root and vascular systems limited the volume of water that could be transported at once. For the second analysis, there was no correlation between leaf area and changes in rates of sap flow. These results imply that tropical tree species with the largest maximum rates of sap flow will be able to evaporatively cool in hotter climates. Furthermore, the lack of relationship between increased average leaf area and kTleaf for the analyzed species suggests that different measurements should be used to study the relationship boundary layers and the rate of sap flow in future, or that there potentially was an unidentified variable that influenced this relationship.