Lianas exhibit lower leaf drought resistance than trees in both tropical dry and wet forests in Thailand

The abundance and diversity of tropical lianas have been increasing, particularly in forests experiencing seasonal drought. Leaf drought resistance is a critical leaf function associated with plant survival, growth, and reproduction. However, it remains debated whether leaf drought resistance between coexisting trees and lianas differs along gradients of water availability. In this study, we compared leaf pressure volume curves and leaf morphological traits of 78 co-occurring liana and tree species from two tropical seasonal forests with differing water availability in Thailand. We found that lianas showed a less negative (lower drought resistance) water potential at turgor loss (Ψ_tlp) than co-occurring trees in both tropical dry and wet forests. Lianas and trees from the tropical dry forest exhibited a higher leaf drought resistance than those from the tropical wet forest. Overall, growth-form and site explained more variation in leaf drought resistance than phylogeny, indicating that leaf drought resistance is labile under contrasting water availability. When considering the effects of phylogeny, there is a correlated evolution of leaf drought resistance and morphological traits (e.g. leaf thickness, leaf mass per area, leaf density, and leaf dry mass content) across species. We conclude that lianas have lower leaf drought resistance compared to co-occurring trees in both tropical dry and wet forests. Further studies are necessary to examine how these trait differences between co-occurring lianas and trees change along a broader aridity gradient.