Drivers of regional and local boreal forest dynamics during the Holocene

Palaeoecological information provides means for understanding the processes behind past changes in forest composition, and offers valuable information on the potential impacts of predicted changes in climate on boreal vegetation. To fully understand the processes behind the long-term boreal forest dynamics both local and regional factors need to be considered. In this work, the Holocene history of the western taiga forests, a continental variant of boreal forest characterized by the presence of Siberian larch (Larix Sibirica), in northern Europe is investigated using fossil pollen and stomata records from small forest hollow sites. The importance of the potential drivers of long-term boreal forest composition is quantitatively assessed using novel approaches in a palaeoecological context. The statistical method variation partitioning is employed to assess relative importance of climate, forest fires, local moisture conditions and human population size on longterm boreal forest dynamics at both regional (lake records) and local scales (small hollow records). Furthermore, wavelet coherence analysis is applied to examine the significance of individual forest fires on boreal forest composition. The results demonstrate that Siberian larch and Norway spruce have been present in the region since the early Holocene. The expansion of spruce at 8000 – 7000 cal yr BP caused a notable change in forest structure towards dense spruce dominated forests, and appears to mark the onset of the migration of spruce into Fennoscandia. The mid-Holocene dominance of spruce and constant presence of Siberian larch suggests that taiga forest persisted throughout the Holocene at the study sites in eastern Russian Karelia. Climate is the main driver of long-term vegetation changes at the regional scale. However, at the local scale the role of climate is smaller and the influence of local factors increases, suggesting that intrinsic site-specific factors have an important role in stand-scale dynamics in the boreal forest. When the whole 9000 year period is considered, forest fires explain relatively little of the variation in stand-scale boreal forest composition. However, this may be attributable to the variation partitioning method. Forest fires have a significant role in stand-scale forest dynamics when observed in shorter time intervals and the results from wavelet coherence analysis suggests that fires can have a significant effect on shortterm changes in individual tree taxa as well as a longer profound effect on forest structure. The relative importance of human population size on variation in long-term boreal vegetation was statistically assessed for the first time using this type of human population size data and the results showing unexpectedly low importance of human population size as a driver of vegetation change may be biased because of the difference in spatial representativeness between the human population size data and the pollen-derived forest composition data. Although the results strongly suggest that climate is the main driver of long-term boreal forest dynamics, the local disturbances, such as fires, species interactions and local site specific characteristics can dictate the importance of climate on stand-scale boreal forest dynamics.