Traits and gradients influence the canopy position of small-statured rain forest trees

Plant height determines a species’ position in the canopy and regulates access to light. Shifts in trait values for assemblages (plots) arrayed along abiotic gradients can reflect changes in species composition, and shifts in species trait values. Multivariate analysis was used to quantify the relationship of assemblage-level floristic composition to environmental gradients. Species trait values for maximum height, leaf area, seed size and wood density were quantified for woody species in the assemblage samples, and partitioned into within- and among-assemblage components to enable trait correlations to be identified, including in relation to abiotic gradients. Assemblages in upslope topographic positions had lower height, smaller leaves and higher wood density. Across the assemblages, shifts in species composition, decreasing canopy height and the position of smaller trees in the canopy were all linked to decreasing soil depth in upslope topographic positions. Regardless of stand height, the canopy position of most main canopy dominants remained largely unchanged in response to shifts in environmental gradients. In contrast, shorter-stature tree species retained height along the gradient and subsequently shifted from the subcanopy to the canopy as soil depth and site (plot) canopy height decreased. Within a community, height and position in the canopy can shift under differing environmental conditions.