Tropical wet and dry forest tree species exhibit contrasting hydraulic architecture

Abstract

Forest tree species in wet and dry habitats are generally considered functionally divergent in leaf and stem functional traits such as leaf area, leaf mass per area, wood density and tree height. Yet, these traits have limited utility for characterizing plant water transport adaptations and strategies. We tested the hypothesis that wet and dry forest trees are functionally divergent in their water conducting apparatus. To assess trait differences and adaptations, we sampled branch wood from nine same-genus species-pairs, each species-pair occurring respectively in the wet (>1500 mm annual rainfall) and dry forest (<800 mm annual rainfall) in tropical Queensland, Australia. From branch wood sections, we measured anatomical traits involved in water conduction (stem vessel dimensions, fractions and their spatial distributions, theoretical water conductivities), storage (parenchyma), and providing hydraulic safety functions (fibres fractions, vulnerability index). Relative to wet forest species, we found on overall that dry forest trees had trait combinations showing adaptations to aridity such as more storage tissue and greater vessel connectivity which may provide alternative pathways for water transport should vessel embolism occur. Habitat is an environmental filter that influences trait behaviour across related species. However, depending on the genera, species in both dry and wet forest habitats also exhibit various tradeoffs in trait values, highlighting the existence of diverse hydraulic strategies within wet forest and dry forest trees.