Distal to proximal: a continuum of drivers shaping tree growth and carbon partitioning.

The relationship between tree carbon (C) assimilation and growth is central to understanding tree functioning and forecasting forest C sequestration, yet remains unresolved. The long-standing debate over C source vs sink limits to growth has yielded invaluable insight, but rests on a false dichotomy. Reframing this issue in terms of distal-to-proximal processes driving sink activity and placing it within a broader understanding of C partitioning offers new insights. Building on transport-resistance theory, I outline a framework where plant resource economies shape spatial gradients of resource availability along the leaf-to-root axis, thereby regulating local sink activity. This spatially explicit, trait-informed perspective aligns with optimality theory and provides a mechanistic link between C partitioning and the plant functional trait spectrum. By moving beyond binary limitations and emphasizing integrated physiological processes, this approach can improve understanding of tree function and biomass increment under climate change.