Significant treeline advance can only occur with successful establishment, growth, and survival of new seedlings. Several studies have examined microsite factors at single locations to explain the presence/absence of seedlings at treeline. We conducted a much larger observational study and included multiple factors to determine (1) which variable(s) was/were most important, (2) whether their importance differed between aspects, and (3) whether the same variables explained variation in seedling growth and damage. We analyzed five biophysical and six shrub variables along four forest–tundra ecotones in southwest Yukon at 640 points. The model that best explained seedling occurrence was similar between north‐ and south‐facing slopes. Of all variables, seedling occurrence was best explained by the proximity, height, and upslope orientation of shrubs (relative to the seedling). The data indicate an optimal range of shrub cover, which differed with aspect. On north‐facing slopes, seedlings occurred most often when shrub cover exceeded 13%, while on south‐facing slopes seedlings occurred most when shrub cover was between 9% and 72%. We also found that with the exception of shrub‐related factors, very few biophysical variables explained size and growth characteristics and damage of tree seedlings, suggesting that the relative strength and importance of variables change depending on the life stage and size of the tree seedling. Collectively, our results demonstrate a non‐random distribution of seedlings in the forest–tundra ecotone, suggesting that as shrub distributions change with climate change, colonization sites for seedlings will also be influenced.

The recent widespread expansion of deciduous shrubs across much of the Arctic has been largely attributed to climate warming. This study investigated decadal growth rates of dwarf birch (Betula glandulosa) across a low Arctic landscape in the continental interior of Canada. Detailed birch cover (100 m²replicate plots) and individual shrub stature measurement datasets for five representative habitat-types were compared between 2006 and 2016, and evaluated in relation to environmental characteristics. Furthermore, dendrochronologically-based annual growth rates were assessed in relation to the 20-year climate record. Birch height, lateral dimensions, and patch groundcover all increased 20–25% relative to 2006 values, but these increases were similar among the habitat-types. Together, the limited evidence of recent warming at this site, the absence of significant habitat-type growth rate differences, and the lack of correlation between annual climate and stem secondary growth strongly suggest that climate change was not the principal cause. Instead, we propose that release from caribou impacts following the recent severe herd decline may explain the net shrub growth. Individual shrub growth rates were correlated with soil nutrient availability, but the latter was highly variable, suggesting that growth rates are primarily determined by fine-scale rather than habitat-scale spatial heterogeneity in nutrient supply. Together, our results demonstrate that birch growth has been enhanced across a variety of habitat-types in the Daring Lake landscape over the decade since 2006, and suggest that the recent severe caribou herd declines may be at least as significant as climate warming in driving birch shrub expansion in the Canadian central low Arctic.