Interactions between climate, natural disturbances, and regeneration in boreal and hemi-boreal forests

Sammanfattning: Natural disturbance is an important driving force of community dynamics in many forest types around the globe. Understanding spatial and temporal properties of disturbance events in the present and in the past is important in formulating the nature conservation strategies as well as for the modeling of climate and human impacts on forest vegetation. In this thesis I studied wind and fire disturbances in natural spruce and pine dominated forests in European Russia. In the case of Norway spruce (Picea abies (L.) Karst.) forests, the goal was to quantitatively evaluate the role of naturally formed canopy gaps and their role in canopy dynamics. In the case of Scots pine (Pinus sylvestris L.) dominated forests, I analysed the link between climate variation, human presence in the landscape, and fire activity. Here, the primary goal was to establish a relationship between tree-ring variables and independently dated fire events to infer common climatic information contained in both datasets. By doing so, an attempt was made to improve the separation of climate- and human-related influences in site fire chronologies. Finally, I analysed the link between weather variation and fire activity at the regional scale, and evaluated the value of tree-ring data as a proxy for regional-scale fire activity. My studies in the spruce-dominated forests suggested that the impact of wind disturbance on canopy dynamics may be forest-type specific at the spatial scale of 103 ha and the temporal scale of a few decades. An agreement between the outcome of gap-associated tree regeneration and canopy composition was observed in boreal stands. In the hemi-boreal stands, a large proportion of stand area under canopy gaps and poor spruce regeneration in larger gaps led to an increase in the abundance of deciduous species implying a decrease in canopy spruce and an increase in deciduous species in the coming decades. In pine-dominated forests, it was possible to establish a relationship between tree-ring chronologies and independently dated fire events. Importantly, this relationship was dependent on the seasonal timing of fire events as recorded in the scars. Joint analysis of site fire histories and local tree-ring chronologies showed possibilities to quantitatively estimate the human impact in the reconstructed fire events. At regional scale, tree-ring-based reconstruction of fire activity appears to be a good predictor of high (annual) and low (decadal) frequency variability in the half-a-century long fire record of Komi republic (East European Russia, total area 415.9 thousand km2). My results supported the view of tree-ring chronologies as potentially realistic proxies for regional fire activity during longer time periods. Generally, considerable variability of disturbance events along temporal and spatial scales needs to be incorporated in any conceptual model of vegetation dynamics. This variability is intermingled with human activities in the past and present. To provide a sound practical advice in designing conservation strategies it is important to separate purely climatic forcing of disturbance regimes from the one mediated by land use patterns.