How do leaf phenology and microclimate influence carbon storage dynamics along the vertical canopy gradient of mature trees?

Leaves in tree crowns experience different microclimates depending on their vertical positions, particularly in light availability. This leads to different rates of carbon assimilation. Additionally, leaf phenology potentially varies along the same gradient. Therefore, it can be assumed that the size and seasonal dynamics of the non-structural carbohydrate (NSC) pool are different in sunlit and shaded twigs of individual trees.

Here we test how microclimatic gradients and leaf phenology influence the NSC dynamics of young twigs along the vertical gradient of individual tree crowns. Throughout the year 2020, we measured the NSC concentration in twigs from the top and bottom crown parts of mature trees from 9 species in a temperate mixed forest at the Swiss Canopy Crane II facility near Basel, Switzerland. We recorded the timing of budbreak along the canopy depth of those trees and continuously measured the light environment with loggers in various canopy positions in three consecutive seasons (2019-2021).

The timing of budbreak showed barely any difference between top and bottom crown parts in broadleaved species. However, in the conifers Abies alba and Picea abies, buds opened ca. 7 days earlier in the bottom crowns than the top. Light availability throughout the growing season in the lower crown parts was around 30 % of that at the top. In most species, the NSC pools were strikingly similar in sunlit and shaded twigs, both quantitatively and in terms of their seasonal dynamics. Only the two ring-porous species Quercus petraea and Fraxinus excelsior showed differences: in both, the lower twigs reached their minimum starch levels after budbreak about a week later than the top twigs, and took longer for the subsequent refilling. However, even in species that showed slight differences in the seasonal NSC dynamics between upper and lower canopy, the end of season NSC concentrations in late autumn were identical between top and bottom twigs.

The very similar NSC dynamics and pool sizes between twigs from upper and lower crown parts, despite stark differences in light availability, are surprising. Further analyses of carbon assimilation and the ratio of carbon source to sink tissues along these vertical canopy gradients will allow to better interpret those results and to get an improved understanding of how carbon storage is controlled in mature trees.