New insights into carbon allocation by trees from the hypothesis that annual wood production is maximized

Summary Allocation of carbon ( C ) between tree components (leaves, fine roots and woody structures) is an important determinant of terrestrial C sequestration. Yet, because the mechanisms underlying C allocation are poorly understood, it is a weak link in current earth‐system models. We obtain new theoretical insights into C allocation from the hypothesis ( MaxW ) that annual wood production is maximized. MaxW is implemented using a model of tree C and nitrogen ( N ) balance with a vertically resolved canopy and root system for stands of Norway spruce ( Picea abies ). MaxW predicts optimal vertical profiles of leaf N and root biomass, optimal canopy leaf area index and rooting depth, and the associated optimal pattern of C allocation. Key insights include a predicted optimal C–N functional balance between leaves at the base of the canopy and the deepest roots, according to which the net C export from basal leaves is just sufficient to grow the basal roots required to meet their N requirement. MaxW links the traits of basal leaves and roots to whole‐tree C and N uptake, and unifies two previous optimization hypotheses (maximum gross primary production, maximum N uptake) that have been applied independently to canopies and root systems.