Introduction


Fine-scale topographic-edaphic gradients are common in tropical forests and drive spatial turnover in species composition and marked changes in forest structure and function. We evaluate how hydraulic traits of dipterocarp species relate to vertical and horizontal spatial niche specialization along such a gradient.


Objective(s)/Hypothesis(es)


We investigated how hydraulic traits relate to habitat, tree height and their interaction on a topographic-edaphic gradient with uniform climate in Borneo. We tested 1) if forests with higher soil water availability have selected for dipterocarp species with higher water transport efficiency, and lower hydraulic safety than forests on well drained sandier soils; 2) if dipterocarp species adjust their hydraulic traits to become safer and more efficient with increasing size and if this adjustment is different across forest types with different soil conditions; and 3) if topographic-edaphic gradients not only select for specific hydraulic traits, but also change the magnitude of hydraulic trait-trait coordination or trade-offs.


Methods


We measured key hydraulic traits in 156 individuals of differing heights in 13 dipterocarp species: embolism resistance, maximum hydraulic conductivity, leaf specific conductivity, leaf-to-sapwood area and wood density.


Results


We found that embolism resistance increases from forests on more clay to more sandy soil, but did not vary with tree height. In contrast, water transport showed strong increases with tree height. Habitat and height only interact for hydraulic efficiency, with height slope changing from positive to negative from the clayey to the sandier soil. Habitat type influenced trait-trait relationships for all traits except wood density.


Implications/Conclusions


Our data reveal that variation in the hydraulic traits of dipterocarp trees is driven by a combination of topographic-edaphic conditions, tree height and taxonomic identity. Our work indicates that hydraulic traits play a significant role in shaping the topographic-edaphic and vertical structure and niche specialization of dipterocarp species.