Tree species diversity and stability in disturbance-driven boreal forest

Abstract

Disturbance is a potent driver of forests. Components of disturbance such as frequency (i.e., time since last stand-replacing disturbance) and intensity (e.g., stand-replacing vs. non-stand-replacing disturbance) plays a major role in influencing plant species diversity. However, majority of studies often consider the effect of disturbance frequency alone on plant species diversity, few studies consider interactive effects between disturbance frequency and intensity of disturbance on plant species diversity. In this dissertation, my first goal was to conduct a meta-analyses to examine the relationship between overstory tree species diversity and disturbance frequency and intensity of disturbance. Across tropical and temperate biomes, tree species richness was greatest at intermediate disturbance frequency with intermediate intensity of disturbance (i.e., non-stand replacing disturbance). Furthermore, research on diversity-disturbance relationships (DDRs) often exclude other critical factors such as climate and local site conditions and thus limit understanding on DDR. Using observational data from a natural forest in Canada, I examine DDR, under the influences of climate and local site conditions. I found that the most important factor regulating tree species diversity was disturbance frequency and local site conditions, indicating that they are important factors in maintaining biodiversity in the boreal forest landscape. Empirical evidence often show that wildfire frequency is strongly influence by local site factors, but broad-scale driver of fires such as climate is rarely considered. I evaluated effects of local site factors and climate on fire regimes across a large natural boreal forest (about 892,000 ha) with no commercial forest harvesting activity; as such, human influences on wildfire is relatively little. My results indicated that mean annual temperature and precipitation were the two most crucial factors driving fire regime in the natural boreal forest studied. Species diversity has often been linked to temporal stability of ecosystem functions; however, forest stand development, species composition and soil resource availability may affect community stability, but these predictors are often overlooked. I investigated whether community stability (measured as stand level basal area) relates to tree species diversity, forest stand development, tree species composition and soil resource availability. Temporal stability was measured as coefficient of variation, which is a principal component of ecosystem stability. I used repeated measurement plot data from a central boreal forest, which is often associated with non-stand-replacing disturbances such as spruce budworm and forest tent caterpillar outbreaks, which selectively kill trees. I highlighted that temporal stability of stand basal area is influenced by forest stand development and species composition depending on the type of insect outbreak.