Tissue-specific variation of δC in mature canopy trees in a temperate forest in central Europe

Stable carbon isotope analysis has become a key tool in functional ecology, yet considerable natural variability often limits the interpretations. In this study we document the spatial, taxonomic, temporal and tissue-specific δ¹³C variability in 10 tree species of a temperate European forest. The Swiss Canopy Crane provided access to the three dimensional space within 55 trees 30–35 m high representing the genera Acer, Carpinus, Fagus, Prunus, Quercus, Tilia, Abies, Larix, Picea and Pinus. The results from six broad-leaved and four conifer species (seven deciduous, three evergreen) documented that the species effect was not significant in contrast to tissue-specific and spatial differences in the canopy. Year-to-year differences were not large but still significant.

Our analysis confirmed a significant difference between δ¹³C of foliage collected in the upper and lower canopy, but revealed no systematic differences with respect to azimuthal directions in tree crowns of the broad-leaved trees, as opposed to the conifers, which show clear differences between the sun-exposed and the shaded side. Tissue-specific differences were significant, despite surprisingly similar mean values for most tissue types. Such tissue effect was largely due to young branch xylem, which exhibited a systematic less negative deviation from the other tissue types. These findings were consistent across the species tested and provided some guidelines towards a representative sampling strategy for ¹³C analysis.