Stem Xylem Differences in Congeneric Lianas Between Forests Are Unrelated to Hydraulic Safety but Partly Explain Efficiency

Lianas are known for their distinctive vascular anatomy and remarkable hydraulic efficiency. Yet they exhibit considerable variation in hydraulic safety across and within forest types. This observation suggests different structure-functional strategies among lianas growing under contrasting levels of drought. Here, we compared xylem features at the cellular and intervessel pit levels and investigated their relationships with hydraulic safety and efficiency in five pairs of congeneric Bignonieae lianas from a seasonally dry forest (SDF) and a wet rainforest (RF). We hypothesize that rainforest lianas have xylem traits that maximize conductivity, while lianas from seasonally dry forests show greater woodiness and investment in storage tissues, and that xylem features at different levels drive the hydraulic safety and efficiency. The SDF liana species had a higher vessel density and grouping, and thinner fibers than rainforest lianas, but none of the features measured were related to hydraulic safety. Our results do not support that vessel or pit quantitative properties predict hydraulic safety in lianas. However, a higher hydraulic vessel diameter, total pit membrane area, and lower intervessel wall-lumen ratio were associated with high hydraulic efficiency, regardless of the forest type. These findings highlight the complexity of hydraulic structure-function relationships in lianas. While we found distinct xylem anatomical differences between species from contrasting forest types, only some traits were associated with hydraulic efficiency, and none predicted hydraulic safety, suggesting that other factors may be at play.