Why is liana abundance low in semiarid climates?

Abstract

Lianas are abundant in seasonal tropical forests, where they avoid seasonal water stress presumably by accessing deep-soil water reserves. Although lianas are favoured in seasonal environments, their occurrence and abundance are low in semiarid environments. We hypothesized that lianas do not tolerate the great water shortage in the soil and air characteristic of semiarid environments, which would increase the risk of embolism. We compared the rooting depth of coarse roots, leaf dynamics, leaf water potential (ψleaf), embolism resistance (P50) and lethal levels of embolism (P88) between congeneric lianas that occur with different abundances in two semiarid sites differing in soil characteristics and vapour pressure deficit in the air (VPDair). Regardless of soil texture and depth, water availability was restricted to the rainy season. All liana species were drought deciduous and had superficial coarse roots (not deeper than 35 cm). P50 varied from 1.8 to 2.49MPa, and all species operated under narrow safety margins against catastrophic (P50) and irreversible hydraulic failure (P88), even during the rainy season. In short, lianas that occur in semiarid environments have lower resistance to cavitation and limit carbon fixation to the rainy season because of leaf fall in the early dry season. We suggest that leaf shedding and shallow roots impairing carbon gain and growth in the dry season may explain why liana abundance is lower in semiarid than in other seasonally dry environments