Impacts of natural disturbances on the development of European forest resources: application of model approaches from tree and stand levels to large-scale scenarios

Abstract

Natural disturbances can significantly affect the sustainable production of forest services. Until now there has been no concise overview of the damage such disturbances have caused to European forests, and their role in projection models has often been ignored. This dissertation aims to contribute in filling those gaps. A literature review in Paper I revealed that from 1950 to 2000 the annual average timber volume damaged by disturbances was 35 million m3: 53% by storms, 16% by fire, 8% by bark beetles and 8% by other biotic factors. A natural disturbance module was added to a large-scale scenario model, which was then applied to Switzerland and Austria. For Switzerland, it was found that the inclusion of natural disturbances significantly affected the development of growing stock, both under current and changing climatic conditions (paper II). In Austria, climate change doubled the expected damage by bark beetles by the end of the century (paper III). Adaptation through replanting with different tree species after clear-felling had only a small mitigating effect, since older forests are the most vulnerable. To study how silvicultural regimes affect the wind damage risk, a wind damage module was added to an individual-based forest simulator. The explicit inclusion of shelter and support from neighbouring trees enabled both individual tree and whole stand stability to be simulated in detail (papers IV-V). Silvicultural regimes leading to relatively low tree height to stem diameter (h/d) ratios experienced the least damage. Low h/d-ratios could be obtained by maintaining low stand densities in even-aged stands or by favouring trees with a low h/d ratio when thinning in uneven-aged stands. It is concluded that the inclusion of disturbances in projection models of different scales offers great possibilities for exploring alternative scenarios and associated risks, for example for adapting to expected future climate change.