Functional responses to increasing tree height within and between trees and their potential drivers

Trees adjust their traits along their vertical crown profile in response to hydraulic limitations imposed by height and/or prevailing environmental conditions. To date, it remains unclear whether these changes are general or specific to particular species or ecosystems, and what the primary factors driving these adjustments are. In this meta-analysis, we examined whether general patterns exist in the variation of key functional traits with tree height, considering 101 species and 119 sites, and whether these patterns reflect trait adjustments in response to environmental factors associated with tree height.

We focused on height-related phenotypic adjustments in a set of hydraulic and carbon-economy traits at two scales: comparing conspecific tall- and short-statured trees and comparing lower and upper crowns within trees. We assessed whether trait variation along tree height could be related to changes in environmental factors by comparing multiple global datasets of trait responses along radiation, nutrient content, water availability and temperature gradients.

Results indicated similar responses in upper canopies and taller trees, with reduced midday water potentials, specific leaf area, leaf mass-based assimilation rates and stomatal conductance. Most traits responded similarly to tree height and multiple environmental factors. However, specific leaf area, net assimilation rate per leaf mass, and stomatal conductance exhibited between-tree responses similar to those observed under drought. Conversely, within-tree variation in specific leaf area mirrored that to increased radiation.

Overall, leaf economics trait variation in taller trees mostly reflected water limitation, variation in leaf morphology in upper canopies was mainly associated with radiation, and temperature did not exert an apparent effect. Future drier conditions may exacerbate declines in leaf gas exchange and leaf water potential in taller trees. Yet their capacity to offset these declines across different species and ecosystems is unresolved.