Differential effects of elevation on leaf size of overstorey and understorey species in a temperate rainforest

Abstract

ABSTRACTConspicuous geographic variation in the average leaf size of plant communities suggests the physical environment must exert strong selection pressures on this trait. Leaf energy balance theory has recently been applied to explain global patterns in leaf size, through its strong influence on vulnerability to (a) overheating in dry sunny conditions and (b) chilling on cold clear nights. The same theory should also predict leaf size patterns on local elevational gradients, but there are fewer data available to examine this prediction. We collected recently-fallen leaf litter at 50 m elevational intervals in temperate rainforest on Mt. Te Aroha, New Zealand, hypothesising that average leaf size of angiosperm tree and shrub species should decrease with increasing elevation. Data from overstorey species (≥ 10 m tall) strongly supported this hypothesis (R2 = 0.36), average leaf size declining from 24.5 cm2 at 150 m to 4.3 cm2 at 850 m. Among understorey trees and shrubs (< 10 m tall) the relationship with elevation was weaker (R2 = 0.18) and the effect size smaller, average leaf size falling from 29.8 cm2 at 150 m to 9.8 cm2 at 850 m. The data are therefore consistent with limitation of leaf size of overstorey species by an elevational decrease in minimum temperatures. Leaf size of shorter tree and shrub species appears to be less constrained by elevational declines in temperature, a pattern that may be partly explained by protection from radiative cooling by overtopping taller trees. However, the occasional occurrence of large-leaved (notophyll) small trees and shrubs on open, recently-disturbed sites at high elevation requires clarification.