Do stem and leaf functional traits explain differences in water use between tropical evergreen trees and lianas during the dry season?

Liana abundance increases with the seasonality of rainfall in tropical forests, and evergreen lianas and trees may compete for water during the dry season. Several studies suggest functional divergence of water use strategies between lianas and trees, with lianas experiencing less water stress and exhibiting growth advantage over trees during the dry season.

Our aim was to test the hypothesis that evergreen lianas are less drought tolerant and more water spending than evergreen trees. If so, we expected that, compared to trees, lianas should have (i) less sclerophyllous leaves, (ii) larger but fewer xylem vessels in their branches, (iii) higher predawn and midday water potentials, (iv) lower water use efficiency, (v) access to deeper water sources.

We tested our hypothesis in a seasonally dry tropical forest on a white-sand hill in Ankarafantsika National Park, north-western Madagascar (MAP, 1600 mm; dry season, April to November). We studied three liana and three tree species in June 2023, and seven liana and eight trees species in September 2024. We measured leaf mass per area, nitrogen content and δ¹³C, leaf predawn and midday water potentials, diameter and density of xylem vessels of branches, and δ¹⁸O of xylem water.

We found that lianas had lower leaf mass per area, higher leaf predawn water potentials, larger and fewer xylem vessels, and lower δ¹⁸O of xylem water than trees. Overall, our results confirm that evergreen trees are more drought tolerant than lianas while evergreen lianas are more water spending than trees but not consistently across families.