Combining carbon isotopes with tree ring analysis – insights into water use efficiency and drought stress

The composition of heavy and light carbon isotopes in tree rings is influenced by the water supply during the time the tree ring was formed due to stomata opening and therefore the ability of the tree to favor lighter isotopes, since the absorbed carbon is used for tree ring formation. This allows calculating the Intrinsic Water Use Efficiency (iWUE) as an indicator for drought stress. With about 1400 samples (tree-rings) from 15 trees from Southwestern Germany, with three individual trees of European beech, Sessile oak, Norway spruce, Scots pine and Douglas Fir respectively, we created cross-dated time series Delta13C isotope ratio and annual growth (using ring width). Our sites included both good and bad site conditions. We linked our data with climate time series for the study area as independent variables. The variety of variables allowed us to determine, to what extent independent factors influence growth and iWUE as well as interactions between the individual factors and their cumulative effects. Statistical methods and time series analyses were used to quantify the complex relationships between water availability, competitive pressure, climatic conditions, and tree growth. This approach combines various fields of terrestrial ecosystem research on individual plant level.

First results show diverse reactions to drought depending on species and location. We found growth and iWUE to be highly dependent on the climate for all species, whereas correlations between these variables imply a tree´s strategy to cope with drought conditions. The correlations differ between individual trees of the same species meaning that location may play a greater role in its significance than previously assumed, conditional on the species. Further investigations could support hydrologically focused forest management with respect to understanding the impacts of climate change on forest ecosystems.

Therefore, an upscaling approach would allow for the depiction of iWUE on catchment level. By reconstructing past environmental conditions based on tree rings, valuable information can be obtained, contributing to the development of strategies for adapting forest stands to future climate conditions. For a deeper understanding and practical conclusions, the neighborhood relationships and competition should be quantified over the entire lifetime of an analyzed tree, since not all irregularities in isotope composition were captured.