The trade-off between stem biomechanics and hydraulics in Bauhinia lianas and trees

Mechanical support and water transport are the two main functions of plant stems. Stems need to be strong enough to hold their crown mass and to prevent breakage during strong wind. Stems also house the xylem, the vessels that transport water from the base of the plant to the leaves. However, there is often a trade-off between these two vital functions. The trade-off between mechanical safety and hydraulic efficiency has been proposed to be related to stem properties and wood anatomical traits, yet experimental studies testing the mechanical and hydraulic trade-off are relatively limited.

Bauhinia is one of the largest genera of Leguminosae, comprising about 300 species with life forms of trees, shrubs and lianas, pantropically distributed in the world. The Bauhinia genus therefore provides an excellent opportunity to test the differences in stem biomechanics and hydraulics between closely related lianas and trees. Lianas are non-self-supporting structural parasites. Their abundance and biomass in neotropical and Asian tropical seasonal forests have been increasing over the past decades. Comparing to self-supporting trees, lianas generally have lower wood density due to less carbon investment in mechanical support. They also tend to have higher hydraulic efficiency and faster photosynthetic rates than co-occurring trees in seasonal tropical forests.

In their new study published in AoBP, Xiao et al. measured stem mechanical and hydraulic traits for 10 Bauhinia lianas and 10 Bauhinia trees grown in a tropical common garden in Southwest China. Their results showed that a trade-off did indeed exist between stem biomechanics and hydraulics in both the trees and the lianas. Bauhinia lianas possessed lower stem mechanical strength, as indicated by both modulus of elasticity and modulus of rupture, and higher stem potential hydraulic conductivity than congeneric trees.

The results of Xiao et al. provide evidence for obvious differentiation in mechanical demand and hydraulic efficiency between congeneric lianas and trees. Their results provide a possible explanation for fast growth of lianas over congeneric trees. Findings from this study also have important implications for life-history strategies in non-self-supporting plants. The authors hope that moving forwards, further assessments of life-history strategies in non-self-supporting plants from different ecosystems will validate their results.