Simulating post glacial vegatation dynamics of forest ecosystems in the Eastern Alps

Scenarios of a drastic climate change caused by the release of green house gases into the atmosphere focused much attention to the development of simulation models which are capable to project the possible consequences of climate change on ecosystems vulnerable to such changes. Forest ecosystems are particularly relevant for climate change impact studies because they cover large portions of the land surface, are an important sink for carbon and thus linked to the climate system, provide timber supply for industry and contribute to the maintainance of biodiversity on earth. PICUS is a forest dynamics model which was developed at the Institute of Silvivulture at the University of Agricultural Sciences, Vienna. Aim of this project was, to evaluate how good the forest model is capable to reproduce postglacial vegetation development of a region in the Eastern Alps in Austria. This constitutes an experiment relevant for climate change impact analyses due to the climatic fluctuations during the Holocene. This simulation experiment covered a 7000-year period from 11000 BP to 4000 BP and an area of about 90 km 2 . As a prerequisite for the simulation exercises climate and soil information from the Holocene had to be collected. This is not an easy task due to large uncertainties in our knowledge on both, Holocene climate and soils. To recognise this uncertainty several scenarios were put together to test for effects on the simulated vegetation. The simulated vegetation development was then compared with the vegetation as indicated by a pollen profile from lake Leopoldstein, Eisenerz region. Prior to comparison the pollen proportions need to be modified by means of R- values. This procedure allows to compare simulated biomass proportions from the model output with pollen proportions from the pollen profile. In general the simulation results compared well to the pollen record. However, particularly during the early pahse of the simulated 7000-year period major discrepancies between modeloutput and pollen record appeared. This was attributed to the immigration and expansion history of individual tree species. Vice versa it could be clearly demonstrated that vegetation composition during the Holocene was not just determined by climate but to a large extent by species immigration.