Tree biomass allocation is governed by allometry but modulated by optimization

Understanding the patterns and drivers of biomass allocation among organs at a broad scale is crucial for predicting the responses of plant growth and carbon sequestration to environmental change. However, the extent to which the general rules govern these patterns and the key factors affecting biomass allocation remain poorly understood. Using a global dataset of 239 tree species, we tested the two prevailing theories (i.e., the allometric partitioning theory (APT) and the optimal partitioning theory (OPT)) by investigating the scaling relationships between plant organs and how environmental factors and phylogeny shape the patterns of biomass allocation. Our results generally support APT at the global scale, with variations in biomass allocation patterns explained by OPT. As plant size increased, a significant shift in biomass allocation from leaves to roots and stems, as well as from roots to stems, was observed. Specific environmental factors (including temperature, precipitation variables, and soil properties) significantly influenced biomass allocation with distinct patterns in the angiosperms and gymnosperms, even when the allometric effects were taken into account. We conclude that tree biomass allocation among organs (i.e., the ratios of leaf to stem, leaf to root, stem to root, and aboveground to belowground) is governed by allometry but modulated by optimization at the global scale. Our findings highlight the importance of considering both the ontogenetic and environmental effects in predicting the responses of biomass sequestration to phylogenetic and environmental factors.