Drivers of regional and local boreal forest dynamics during the Holocene

• Main Author: Kuosmanen, Niina
• Other Authors: Ohlson, Mikael, University of Helsinki, Faculty of Science, Department of Geosciences and Geography, Biogeotieteen osasto, Helsingin yliopisto, matemaattis-luonnontieteellinen tiedekunta, geotieteiden ja maantieteen laitos, Helsingfors universitet, matematisk-naturvetenskapliga fakulteten, institutionen för geovetenskaper och geografi, Seppä, Heikki
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Helsingin yliopisto 2015
• Subjects: Geologia / Paleoekologia; Norway; Fennoscandia; taiga
• Online Access: http://hdl.handle.net/10138/159208

In this work, the Holocene history of the western taiga forests, at the modern western range limit of Siberian larch (Larix Sibirica) in northern Europe, is investigated using fossil pollen and stomata records from small forest hollow sites. The relative 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 long-term 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 (Picea abies) 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 changes in boreal vegetation at the regional scale. However, at the local scale the role 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 study period is considered, forest fires explain relatively little of the variation in stand-scale boreal forest composition. However, forest fires have a significant role in stand-scale forest dynamics when observed in shorter time intervals and the results suggests that fires can have a significant effect on short-term changes in individual tree taxa as well as a longer profound effect on forest ...