In tropical forests, abundance and biomass of lianas are positively related to the dry season length, and negatively to annual precipitation. In addition, they are increasing in some areas and causing negative effects on tree growth. It has been suggested that lianas have an advantage in the dry season growth because of their ability to maintain their photosynthetic rate and hydraulic condition compared to trees. We hypothesized that evergreen lianas extend their roots into deeper and wetter soil layers and have larger xylem vessels allowing them to extract and use more water. On the other hand, evergreen trees have more conservative leaf traits and higher water use efficiency than lianas to withstand seasonal droughts.
My study site was Ankarafantsika national park in northwestern Madagascar, which is a tropical dry forest with a dry season from April to October and annual precipitation of 1195mm-1615mm (2016-2019). I selected seven liana and eight tree species holding leaves in September 2024. In my study, predawn leaf water potential and δ18O of xylem water were measured as a proxy of soil wetness and depth of its water source. The radius and density of xylem vessels in branches were measured and used to estimate xylem hydraulic conductivity. Leaf-level δ13C, nitrogen content and leaf mass per area were measured as a proxy of water use efficiency, photosynthetic capacity, and sclerophylly, respectively.
I found that lianas tapped water in deeper and wetter soil layers than trees and had a higher xylem branch hydraulic conductivity. At the leaf level, water use efficiency and leaf nitrogen content did not show significant differences, but trees had more sclerophyllous leaves. Considering these results, I suggest that lianas invest more resources in their root extension to reach deeper and wetter water sources. They also transport water efficiently with larger and fewer xylem vessels which may be associated with a higher vulnerability to cavitation compared to trees. On the other hand, trees invest more resources in sclerophylly to withstand seasonal droughts. Despite higher sclerophylly, their leaf photosynthetic capacity and water use efficiency are similar to those of lianas.