Similar Relative Carbon Costs for Construction and Storage of Sun and Shade Branches in Mature Temperate Trees

Irradiance strongly affects the morphology, carbon (C) uptake and construction costs of leaves and branches. Within tree crowns, light decreases from the top downwards, but whether this translates to differences in the C balance of sun and shade branches remains unclear. Here, we combined a light-driven photosynthesis model, parameterised with empirical data, with functional growth analyses to estimate the C costs and amortisation times of upper, sun exposed and lower, shaded branches in the crowns of mature trees from nine European species in a diverse and relatively open mixed forest. Amortisation times for the C costs of 1-year-old branches varied among species, but not between sun and shade branches except for two species. Expressed as percentage of the branch C uptake, branch costs were similar between crown positions in most species. Finally, a similar proportion of C assimilation is used for the seasonal build-up of starch in upper and lower branches. Our results show that, at least in forests with relatively open canopies as the one studied here, the balance of assimilation and structural and non-structural C costs at the branch-level is finely tuned along the light gradient, suggesting a high degree of C autonomy even in shaded branches.