UBC Theses and Dissertations
Interspecific boreal shrub growth response to climate, fertilization and herbivory. Grabowski, Meagan M.
Substantial evidence indicates a circumpolar ‘greening’ trend largely attributed to an increase in tall shrub abundance. Shrub expansion creates positive feedbacks to the global climate, involving the release of stored carbon due to shifts in permafrost stability and nutrient cycling. Understanding drivers of shrub growth is therefore essential to predicting how northern ecosystems will respond to climate change. After a decade of research into circumpolar shrub expansion, there remain large gaps in our understanding of shrub expansion below treeline in the boreal forest biome. In the Kluane region of southwest Yukon, there has been an increase in the aboveground standing biomass of boreal shrubs by one and a half to two times over the past 27 years. I used dendroecological methods to assess the relative impacts of climate, fertilization, and herbivory on three boreal understory shrub species (Betula glandulosa, Salix glauca, and Shepherdia canadensis) and found interspecific variation in growth response. I used a mixed effects model selection analysis to determine which factors were strongest in determining annual ring width. B. glandulosa had increased ring widths with added nutrients, S. glauca was not climate or nutrient sensitive, and S. canadensis responded to precipitation and drought index climate variables. When the same growth analysis was performed on canopy trees in the same area, Picea glauca ring widths were found to be more climate and nutrient sensitive than the shrubs growing beneath them. However, the response of shrub growth to the aforementioned factors was not strongly associated with canopy cover by trees. This indicates there is a difference between trees and shrubs in their growth response other than the influence of canopy species on understory light availability. If shrubs continue to increase, and boreal trees decline in health (drought, insects, fire) as predicted, there could be a shift from a more tree-dominated to a more shrub-dominated system. This would influence local habitat for key wildlife species and global climate feedbacks via carbon storage or release. The boreal forest is one of the largest forests in the world, and understanding vegetation change in this biome is essential for adapting to climate change.
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