| Summary: | Despite the expectation that treeline will advance as climate continues to warm, it has only done so in approximately half of all sites studied. One of the explanations for the nonresponse of treelines is based on reproductive limitations including seed production, germination, establishment and growth of young seedlings. To that end, I studied the white spruce (Picea glauca) alpine treeline in the Kluane region of southwest Yukon to explore two important questions related to treeline ecology: (1) Is there sufficient viable seed dispersed at and beyond treeline for successful advance? (2) What are the most important variables influencing germinant presence, seedling growth and seedling survival in the alpine treeline ecotone? Four specific investigations were used to answer these questions, each one addressing a key aspect of seed production, germination, and seedling growth: (1) Seed production, dispersal, and predation were quantified across the treeline ecotone. Results demonstrated that over 88% of seeds were damaged or not viable prior to dispersal. Less than 2% of all seeds were dispersed into alpine tundra. Additionally, germinant losses were also one third higher when seedlings were not protected from predators. (2) Germination success was highest in partially scarified quadrats relative to unscarified and completely scarified quadrats. This was likely due to higher soil moisture and decreased temperature range. In non-experimental areas, naturalized seedlings were found at a higher frequency in areas with evidence of fine-scale soil disturbance. (3) The most important variable that explained the presence of seedlings was the presence of shrubs: the higher the shrub cover, the greater the likelihood of finding tree seedlings up to a certain threshold, after which the probability declined. (4) Tree seedlings grew at a slower pace in the presence of shrubs, but had significantly less tissue damage and higher survivorship than in plots where shrubs had been removed. Collectively, my results suggest that seedling success is contingent on many factors, most notably the amount of erect shrubs in the immediate vicinity. Understanding the variables that influence seedling dynamics is critical, as treeline advance will only occur with the successful emergence, survival and growth of new individuals. PhD
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