Uniform carbon reserve dynamics along the vertical light gradient in mature tree crowns

Understanding the within-tree variability of non-structural carbohydrates (NSC) is crucial for interpreting point measurements and calculating whole-tree carbon balances. Yet, little is known about how the vertical light gradient within tree crowns influences branch NSC concentrations and dynamics. We measured NSC concentrations, irradiance and key leaf traits in uppermost, sun-exposed and lowest, shaded branches in the crowns of mature, temperate trees from nine species with high temporal resolution throughout one growing season. Measurements from two additional years allowed us to test the generality of our findings amongst climatically contrasting years. Despite the vertical light gradient, we found very similar seasonal NSC dynamics and concentrations between sun and shade branches in most species. This can at least partially be explained by acclimations in specific leaf area and photosynthetic leaf traits compensating the different light availability between the top and bottom canopy. Only in the ring-porous species Quercus petraea x robur and Fraxinus  excelsior was starch refilling after budbreak slower in lower branches. End-of-season NSC concentrations were similar between canopy positions and amongst observation years. Only Fagus sylvatica had 40 and 29% lower starch concentrations by the end of the extremely dry year 2020, relative to the other 2 years. We show that NSC measured anywhere in a tree crown is often representative of the whole crown. Overall, our results suggest that carbon reserve dynamics in trees are largely insensitive to both microclimatic gradients and inter-annual climatic variation, and only deviate under severe carbon deficits, as was presumably the case with Fagus in our study.