Relative carbon costs for growth and starch formation of young branches are similar along the vertical microclimatic gradient of mature forest canopies

The canopies of mature trees establish a vertical microclimatic gradient, especially in light availability. The decreasing irradiance from top to bottom of canopies lead to significant differences in the seasonal C assimilation between the uppermost and lowest branches of mature tree crowns. But whether this translates also to differences in the seasonal net C-balance of sun- vs. shade branches remains unclear. Here, we present in-situ measurements of upper and lower branches from mature canopies of three conifer species and 6 broadleaved tree species at the mixed temperate forest of the Swiss Canopy Crane II facility. We combined a light-driven model of the seasonal photosynthesis with branch functional growth analyses to test whether the relative C investment in structural biomass and C reserves (starch) of one-year-old branches differ between the uppermost, sun-exposed and lowest, most shaded branches.

We found that amortization times for the C costs of one-year-old branches varied widely among species, but only in a few species also between sun and shade branches. Interestingly however, expressed as a percentage of the total branch C uptake, the structural C-costs were surprisingly similar across species and crown positions between 15 and 25 % of the total seasonal C assimilation per branch. Key shade acclimations included SLA, dark respiration rates and photosynthetic low-light efficiency. We further found that a similar proportion of the total C assimilation is required for the seasonal starch build-up in sun and shade branches. Our results thus show that the balance of assimilation and both structural and non-structural C costs at the branch-level is finely tuned along the vertical light gradient, suggesting a high degree of C autonomy even in the most shaded branches of our investigated trees.