Climate and hydraulic traits interact to set thresholds for liana viability

Trees and lianas, or woody vines, dominate the canopy of tropical forests and comprise the majority of tropical aboveground carbon storage. These growth forms respond differently to contemporary variation in climate and resource availability, but their responses to future climate change are poorly understood because there are almost no predictive ecosystem models that represent lianas. We compiled an extensive database of liana functional traits (846 species) and compared liana and tree trait distributions. We then parameterized a mechanistic model with observed allometric and hydraulic traits to simulate liana-tree competition under different tropical hydroclimate scenarios. We found that the difference between liana and tree hydraulic conductivity represents a critical source of inter-growth form variation and can be used to develop viability thresholds. Specifically, on average, liana hydraulic conductivity was over three times greater than tree hydraulic conductivity. Using our model, we found that both liana and tree hydraulic conductivities are more sensitive to changes in vapor pressure deficit than soil water potential. Furthermore, we estimated that liana hydraulic conductivity is approximately 21 times more sensitive to drying atmospheric conditions than tree hydraulic conductivity. We conclude by showing that lianas, as a growth form, are more susceptible to reaching a hydraulic threshold for viability in the tropics by 2100 than trees. Liana hydraulic conductivity should be recognized as a key trait affecting liana-tree competition, tropical forest carbon cycling, ecosystem services, and forest management. Liana functional traits, such as hydraulic conductivity, should be incorporated into liana-enabled dynamic global vegetation models to improve global carbon cycle predictions.