Competition, precipitation and temperature shape deviations from scaling laws in the crown allometries of miombo woodlands

Abstract

Scaling relationships between different axes of tree size, such as height, crown radius, crown depth and stem diameter, play a direct role in shaping forest structure and function. Theoretical models such as metabolic scaling theory postulate that they are optimized for biomechanical stability and hydraulic sap distribution. However, empirical data often show that such models are only good enough as first order approximations because they do not account for differences in species traits and environmental conditions where trees grow. Nevertheless, the vast majority of research has focused on temperate systems or tropical rainforests, so we continue to lack a full understanding of what factors shape allometries of trees in tropical dry forests. Here, we compile data on tree height, diameter, crown radius and depth from miombo woodlands across Zambia and use Bayesian hierarchical modelling framework to explore how allometric scaling relationships are shaped by climate and competition. Similar to previous studies, our results revealed that allometric scaling relationships deviate substantially from theoretical expectations. We found that competition, precipitation and temperature all affect crown allometric scaling relationships, with trees becoming more slender where neighbourhood competition was greater, while crowns were wider and deeper in warmer and wetter climates. Our study highlights how the structure and function of miombo woodlands is shaped by more than just water availability. Moreover, by developing improved crown allometric models for miombo woodlands, we provide new tools to aid the estimation of aboveground biomass and calibration of remote sensing products in these critically important dry forest ecosystems.