Application of stable isotope methods (13C,18O) to link climate change-induced drought stress and bark beetle susceptibility in Austrian forests

The impacts of climate change in forests are often cascading, affecting plant growth, plant performance but often also rendering trees susceptible to insect pathogen attack. Potentially these insect infestations could have a greater impact on the forest’s carbon sequestration potential, than the direct climate effects on plant growth and forestry production. Regional and context-specific responses to a changing climate are expected: In regions where forest productivity is constrained by low temperatures, such as high altitudes,  growth is likely to increase as temperatures rise, whilst in regions with limited water availability, a decline in growth is predicted as a result of more frequent and prolonged droughts and/or changing precipitation patterns.

Norway spruce is high-yielding, easy to manage and has a number of economic advantages over broadleaf production.  Spruce is economically the most important and common tree species in Austria, making up over 50% the Austrian forestry area. Area-wide planting throughout Austria was supported for decades, which often led to planting in regions where the production risk for spruce was and remains high and where active management does not necessarily cover costs. This legacy of this planting policy means that stands are often still planted on sites, where the trees are close to the limit of suitability. Spruce has shallow roots and less access to deeper soil moisture, so it is particularly susceptible to drought stress. Trees that suffer from changing site conditions are likely to be more susceptible to disease and insect infestation than trees that are not exposed to additional site stresses. In an effort to identify sites that have suffered from drought in the past and that are vulnerable to possible pest infestations, a method to predict bark beetle susceptibility is being developed, based on stable isotope signatures in tree rings. Specifically, we will test whether the stable isotope data (¹³C,¹⁸O) in the tree rings can provide an accurate archive of information about past climate variability and physiological responses to environmental and geomorphological conditions: These data can provide historical insights into water status, in particular soil water availability and use, temperature and water consumption of individual trees. They can be correlated with detailed archived weather and precipitation data as well as easily measured parameters such as canopy temperature or spectral data. We will present our approach and the first results, which are based on samples from the Austrian forest inventory and complemented with samples from additional bark-beetle infested areas.