How do lianas and trees change their vascular strategy in seasonal versus rain forest?

Plants can alter wood anatomy to adjust water supply and mechanical stability demands. However, the different ways in which species can adjust rates of water supply through variation in size and number of vessels in the sapwood, and how this variation is related to the trade-off between hydraulic and mechanical functions remain unclear. We tested the hypothesis that plants with higher investment in mechanical support have relatively less margin to change their potential hydraulic conductivity in terms of vascular strategy: the total area of conducting tissue (vessel lumen fraction) and the combination of vessel sizes and numbers (vessel composition). We measured hydraulic and mechanical traits of xylem tissue and compared the relationship between those traits between trees and lianas co-occurring in a semi-deciduous seasonally dry forest (SDF) and an evergreen rainforest (RF). Along the axis of hydraulic-mechanical trait variation, SDF lianas showed a trait combination towards investment in hydraulic conductivity (higher vessel lumen area, percentage of xylem represented by vessels and potential hydraulic conductivity), whereas trees from both forests were characterized by investment in mechanical support (higher wood density, percentage of xylem represented by fibres and number of vessel per area) and RF lianas were intermediate between these spectrum. The main difference between trees and lianas was in vessel lumen fraction and vessel composition, indicating that not only the vessel size but the distribution between the size and number of vessels is important to explain the higher hydraulic conductivity of lianas. Between forests, trees did not differ in wood density (construction costs). Similarly, lianas did not differ in wood density among forests either; therefore, confirming that differences in potential hydraulic conductivity resulted from changes in the distribution between the size and number of vessels (vascular strategy). Our results suggest that vessel lumen fraction and the vessel composition are important dimensions driving variation in construction costs across woody plants. Thus, what makes lianas hydraulically distinctive from trees is the way vessel lumen fraction and vessel composition vary across environments rather than simply having wider vessels.