Exploring the ‘upper hidden half’ of trees: Influence of microclimatic gradients on biological processes within mature temperate tree crowns

Tree crowns are characterised by strong, vertical gradients in light availability, temperature and humidity. These microclimatic gradients drive many physiological processes and may therefore be important when modelling biological processes, including the carbon (C) balance, at the whole tree or forest level. So far, much of the research on vertical canopy gradients has focused on how leaves acclimate morphologically and physiologically in order to optimise light interception and C uptake of the whole tree. However, because of the relative inaccessibility of mature tree crowns, key knowledge gaps remain on how microclimatic gradients influence leaf phenology, carbon storage dynamics and the overall branch C balance within mature tree crowns.
Over three years, we assessed these factors at high temporal resolution in upper, sun exposed and lower, shaded branches of nine tree species growing in a mature, temperate forest at the Swiss Canopy Crane Site II near Basel, Switzerland. The studied species covered a wide range of common European species, including three evergreen conifers and six deciduous broadleaved species.
We found that microclimatic gradients were pronounced during the growing season, but mostly negligible from late autumn to early spring. Spring leaf phenology occurred simultaneously in upper and lower branches of most species, but several angiosperms retained lower leaves longer in autumn, likely because of the lower environmental stress in the shade crown. Surprisingly, the seasonal dynamics and tissue concentrations of non-structural carbohydrates (NSC) were very similar in upper and lower branches of most species, despite the strong light gradient. Particularly at the end of the season, NSC concentrations were largely identical across the canopy gradient and among climatically contrasting years. Finally, by combining a simple C assimilation model with functional growth analyses of current-year shoots, we found that adjustments in certain key traits (specific leaf area, leaf area per twig mass) largely compensate the light gradient, resulting in very similar relative C costs (i.e. the percentage of total shoot C uptake invested in current-year shoot tissues) both along the vertical light gradient and across the diverse range of species. This suggests that branch and leaf costs are finely tuned to their specific light availability.
With this thesis, I highlight the extraordinary capacity of trees to acclimate to within-crown light and other microclimatic gradients, overall resulting in surprisingly uniform relative carbon balance and carbon storage dynamics of branches across the whole crown. These findings have important implications for modelling of the studied processes: I confirm with in situ measurements on a wide range of temperate tree species, that when studying at the forest scale, the simple assumption of a uniform canopy, ignoring vertical microclimatic gradients, will in fact reflect the whole-tree leaf phenology, C storage dynamics and seasonal branch C balance quite accurately. However, I also found species- or functional group specific exceptions in certain aspects, highlighting the necessity to still consider within-crown variation when studying individual trees or comparing species.