| Summary: | The tundra biome has undergone dramatic vegetation shifts in recent decades, which have been partly attributed to climate warming. Shrub species in particular are expanding widely throughout the Pan-Arctic region, and are involved in complex vegetation-atmosphere interactions that have important implications for the global energy balance and carbon budget. However, projections of vegetation change and associated feedbacks are complicated by the high variability in the sensitivity of shrub growth to temperature among sites and species. A mechanistic understanding of the individual-to-regional controls of climate sensitivity is therefore needed to accurately predict future vegetation change at the biome scale. This thesis quantifies the influence of environmental and ecological factors, and especially of plant-plant interactions, on the growth response of Arctic shrub communities to climate change. Climate change in the Arctic has resulted in warmer, but also longer growing seasons in many locations due to earlier snowmelt. These two factors are often treated as one single control of plant growth, but with scarce records of green-up and senescence dates for the Arctic, few studies have measured the sensitivity of shrub growth to changes in growing season length. Using radial growth time series from over 300 shrubs collected at four sites of contrasting climatic regimes and greening trajectories in Northern Canada, I measured the sensitivity of shrub growth to summer temperature and satellite-derived growing season length. I found that growing season length and summer temperature were decoupled within sites and had inconsistent effects on growth across the four sites. My findings indicate that longer and warmer growing seasons do not necessarily act as combined drivers of vegetation change across the biome. My research also demonstrated that growth at the root collar of shrubs is more climate sensitive than stem growth, possibly indicating differential internal resource allocation strategies, and highlighting the ...
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