Small-scale disturbance and vegetation dynamics in trembling aspen dominated forests in northeastern Ontario
Hill, Steven Burton
Master of Science
SubjectForest dynamics (Ontario, Northeastern)
Forest canopy ecology
Boreal forest stand development
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The scientific understanding of boreal forest stand development and vegetation dynamics has been well documented with respect to stand-initiating disturbances; however, the role of subtler disturbance events has received less attention. Two scenarios that warrant the study of small-scale disturbance in the boreal forest of Ontario include i) forest management practices and changing climatic factors that range in spatial scale from local to global are predicted to skew age class distribution of forests towards mature, oldgrowth stages of development, and ii) during these stages of forest development, small-scale disturbance is hypothesized to be the major disturbance mechanism to influence vegetation dynamics. Many principles and methods of the research that has been conducted on vegetation dynamics and small-scale disturbance in deciduous temperate and tropical forests can be applied to the boreal forest as a study system. This thesis attempts to understand vegetation dynamics during intermediate stages of boreal forest development, with an emphasis on the role of small-scale disturbance. The thesis is presented in four chapters that, i) develop the rationale for studying small-scale disturbances in boreal forests, ii) characterize the extent of small-scale disturbance in trembling aspen forests, and ii) quantify the effect of small-scale disturbance on understorey plant communities. Analyses varied considerably among chapters. Multivariate ordination techniques and average basal area change measurements were used to assess stand and species dynamics of 217 forests stands sampled across northeastern Ontario. Relationships between small-scale disturbance with time since stand initiation (60 - 120 years) of ten trembling aspen were assessed using linear models. Juvenile tree recruitment patterns and community changes (woody and herbaceous plant species) in gaps and understorey environments of five trembling aspen stands were assessed using general linear models and various multivariate methods. Consistent patterns in boreal forest stand development include early dominance of shade intolerant species such as Populus tremuloides and Pinus banksiana, followed by increasing basal area of shade tolerant conifer species such as balsam fir, black spruce, and white cedar. Important declines, between 80 and 100 years since stand initiation, in the basal area of species that dominate the initial post-disturbance cohort point towards the occurrence of small-scale disturbance (i.e. mortality of mature canopy trees). The occurrence of such disturbance warrants the investigation of i) how important small-scale disturbance actually is, and ii) the effect that it has on the vegetation dynamics of understorey plant communities. Results from chapter two clearly show that small-scale disturbances are an important and conspicuous element of forest stand development when trembling aspen are the dominant tree species. The most important factors that lead to tree mortality include infection by wood decay pathogens and wind. The increasing number of gap makers per gap, and number of gap maker decay classes per gap with time suggests that gap expansion, resulting from punctuated disturbance events is an important mechanism of gap formation. Snapped gap makers, followed by standing dead and uprooted gap makers were the most important structural attributes of gaps, and their relative importance remained similar with time. Despite the importance of small-scale disturbance during 95-120 years since stand initiation, juvenile tree recruitment patterns and plant community response to gap environments of different sizes and ages was negligible. There was no detectable difference in stem density and growth of all important juvenile tree species (including Abies balsamea, P. tremuloides, Picea mariana, Picea glauca, and Betula papyrifera) when compared among gap environments and the closed canopy understorey condition. Similarly, understorey plant communities showed no difference when compared among different gap types. High across-stand variability in community composition limited the ability to detect consistent compositional trends with respect to gap size and age. However, regardless of gap type, slight increases in species richness and abundance were detected for understorey woody species of heights between 2 and 5 m. The effect of small-scale canopy tree mortality on understorey plant communities in trembling aspen dominated forests is minimal. This is in contrast to predictions that were made based on expectations from the role of small-scale disturbance dynamics in temperate and tropical forests. Based on the results of this thesis, future studies of boreal forest development and understorey vegetation dynamics should consider i) the establishment mechanisms of shade tolerant conifer species such as A. balsamea during early stages of forest development, ii) minimal gap sizes that facilitate growth of shade intolerant tree species that are taller than two metres, and iii) appropriately replicate different understorey community types.