Mature tree crowns experience strong microclimatic gradients, particularly in solar radiation. These gradients can lead to different carbon assimilation rates within the canopy. While certain adaptations to different light regimes, such as the SLA, are well studied, little is known about how the actual C balance, C storage dynamics and C amortization time of branches differ along these light gradients. We utilized the crane at the Swiss Canopy Crane II site in Basel, Switzerland, to assess the vertical microclimatic gradients in a mature temperate forest. We further measured non-structural carbohydrate (NSC) storage in sun- and shade twigs throughout the season and assessed the net C balance of new shoots, all in trees from six broadleaved and three conifer species.
Results/Conclusions
Light availability was reduced on average by 70% in the lower canopy. Yet the NSC concentrations in sun- and shade twigs was strikingly similar, both quantitatively and in their seasonal dynamics. A C-assimilation model, based on in situ light response curves of photosynthesis and continuous light measurements, confirmed the higher net C assimilation in sun leaves: C gains per leaf area were ca 50% lower in shade leaves. Due to differences in biomass allocation depending on the light environment, i.e., higher SLA and more leaf area per shoot mass in the shade, the estimated C-amortization times (the time a one-year-old shoot needs to assimilate the equivalent amount of C structurally invested in it) were not much longer in shade twigs. Sun- and shade shoots of broadleaved trees took on average 27 and 43 days, to amortize. This pattern was less consistent in the conifer species, but the amortization times were longer than in the broadleaved trees, around 300 days.
Our results highlight how changes in branch morphology can compensate for different per area C assimilation in the upper and lower canopy of mature trees and help to explain the similar NSC dynamics we observed in sun- and shade twigs.
