A global analysis shows that for most tree species the largest trees are the fastest-growing trees, a finding that resolves conflicting assumptions about tree growth and that has implications for understanding forest carbon dynamics, resource allocation trade-offs within trees and plant senescence. Old forests and their leaves fix less carbon than do new forests, but does this apply at the individual tree level? This study uses a global analysis of more that 400 tree species to show that it does not. Rather, larger and older trees accumulate carbon more rapidly than do younger, smaller ones. This can be reconciled with the effects at other levels by taking into account increases in leaf number and reductions in forest density with age. The rapid growth of large trees means that, relative to their numbers, they could have a disproportionately important role in forest feedbacks to the terrestrial carbon cycle and global climate system. Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations1. Our ability to understand and predict changes in the forest carbon cycle—particularly net primary productivity and carbon storage—increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands2,3. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree4,5,6,7, in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level8,9,10 and stand-level10 productivity can be explained, respectively, by increases in a tree’s total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation11 and plant senescence12.

Rate of tree carbon accumulation increases continuously with tree size