Disentangling interspecific and intraspecific variability in height–diameter allometry of dominant tree species in the Rocky Mountains across broad spatial scales

Abstract Height–diameter (H-D) allometric relationships provide us with insights into tree growth strategies and resource utilization. Yet, we have a poor understanding about the underlying drivers that limit or alter tree H-D allometries at large scales. Based on 569 502 pairs of height versus diameter from 53 808 plots of 20 species in the Rocky Mountains, USA, we used a mixed-effects approach to model the variability in H-D allometry across species (interspecific variation) and within-species (intraspecific variation) along gradients of site condition, climate, and competition. The allometric theories derived from physics or biological laws provide a fundamental framework for tree growth and performance, but our findings also show that physiological and environmental factors further shape the interspecific and intraspecific patterns in tree allometry with the intraspecific variation being smaller than the interspecific variation along all the gradients we considered. In general, trees growing in favorable site conditions and highly competitive environments tend to comparatively invest more in height growth, while trees may preferentially invest in diameter growth when constrained by water stress or other unfavorable climatic condition. Trees will choose the most appropriate growth strategies with corresponding allometric patterns to adapt to environmental changes. Our study is expected to provide applied value for estimating biomass and carbon stocks at broad environmental scales.