Changes in subalpine spruce forests under the impact of climate change

A number of climate scenarios for the Balkan Peninsula forecast a dramatic decline in summer precipitation together with higher temperatures. For this reason the forests there are an especially appropriate point of reference for examining the effects of climate change. But they are suitable from another perspective as well. Unlike in our region, expansive forests grow there that have not been cultivated for more than 150 years – providing ideal conditions for a study of natural forest dynamics.

The WSL Institute for Snow and Avalanche Research SLF is participating in a research project that is investigating uncultivated subalpine Norway spruce-dominated forests in Bulgaria. The focus is on natural disturbances, such as windthrow, bark beetle infestation, or forest fire, and the frequency with which these types of damage occur in such forests. How large is the disturbance and what role could it play, together with cultivation and the species composition, in future loss events? The researchers are also examining how the trees have responded thus far to extreme climate events, and how they could meet similar challenges in future.

Initial results show that the greatest damage occurred in forests which were between 120 and 160 years old and lacked significant structural diversity – forests in which the treetrunks were all similarly thick. The extent of the disturbance varied between 60 ha (windthrow) and 200 ha (bark beetle). Older forests and those with a more diverse structure suffered damage over a smaller area, but possessed numerous small gaps – caused mostly by windthrow.

Annual ring analyses of almost 650 living and dead trees further indicate that climatic extremes – primarily dry summers – influence the fatality rate of spruces to a greater extent than was previously assumed. It was generally supposed that trees in dense spruce forests die during the self-differentiation phase predominantly as a consequence of competition for water, nutrients or light. Although the effect of competition between trees was very important in the examined forests in Bulgaria as well, it was substantially amplified by climatic extremes. In forests such as these, the fatality rate is likely to increase accordingly under the influence of climate change.

The results of the study in Bulgaria also delivered evidence of another phenomenon. Treetrunks that thickened only slowly at the start of their life cycle died after 40 to 50 years as a consequence of extreme drought more frequently than specimens that had a better start to life. In other words, factors that control the initial growth of trees influence the development of forests over the long term as well.