Factors influencing the establishment and growth of tree seedlings at Subarctic alpine treelines

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.