• Roles of neighboring plants and temperature on growth and survival of white spruce seedlings along elevational gradients in Alaska

      Okano, Kyoko; Bret-Harte, M. Syndonia; Mulder, Christa P. H.; Juday, Glenn P. (2018-05)
      Seedlings are the most vulnerable stage of a tree's life and their successful survival and growth are critical to support future forests. Recent rapid warming in Alaska has promoted the movement of treeline upward in elevation, while trees at low elevations have decreased their growth. Understanding the direct effects of warming and the indirect effects induced by warming, such as species interactions, on the dominant treeline species, white spruce (Picea glauca) is key to sustaining boreal forests, from low elevations to above current treeline. The objectives of my thesis were to assess the roles that warming, neighboring interaction, habitat type, elevation and season play in the survival and growth of white spruce in Denali National Park and Preserve and Fairbanks, Alaska, USA. I planted spruce seedlings where I manipulated summer temperature and neighbor plants at seven sites (forest or tundra) along an elevational gradient that crossed treeline. I measured survival after winter and summer seasons, and harvested the seedlings for biomass after the third growing season. I found that competition -- particularly light competition where seedlings were shaded -- was the most important factor for seedling growth, while along elevational gradients, temperature and season had inverse effects on their survival: more seedlings at high elevations survived in summer and under warming, but more seedlings at low elevations survived in winter and under ambient temperatures. More seedlings with neighbors survived in summer and in forests, suggesting facilitation through shading. I found some evidence for a trade-off between growth and survival. Seedlings with a high relative growth in height (RGR height) in 2012 had a lower survival rate than seedlings with a low RGR height in the following hot and dry summer of 2013. More seedlings planted with neighbors that had a small diameter in 2012 also survived in 2013, but not without neighbors. These results suggest that a trade-off between survival and growth occurred only when competition for water can be expected. No difference in survival was found after the second winter and third summer. Altogether, I concluded the most important factor affecting seedling growth in my experiment was light competition, while the most important factors for seedling survival were warming and water availability for the first two years in the subarctic montane and interior Alaska.
    • The snowshoe hare filter to spruce establishment in boreal Alaska

      Olnes, Justin; Kielland, Knut; Ruess, Roger; Juday, Glenn; Genet, Helene; Mann, Daniel (2018-05)
      Interior Alaska is a heterogeneous landscape within the circumpolar boreal forest and is largely composed of black and white spruce (Picea mariana and P. glauca). Improving our understanding of the factors affecting patterns in spruce regeneration is particularly important because these factors ultimately contribute to shaping the boreal forest vegetation mosaic. Herbivory by snowshoe hares (Lepus americanus) is one factor that likely drives patterns in spruce establishment. The interaction between spruce and snowshoe hares provides an opportunity to study how plant-herbivore interactions can affect succession, vegetation community composition, and consequently, how herbivory influences landscape heterogeneity. I explored how herbivory by snowshoe hares alters the survival and growth of spruce seedlings across Interior Alaska's boreal forest. I hypothesized that the survival and growth rate of regenerating spruce is significantly reduced by snowshoe hare herbivory and that snowshoe hare herbivory influences the pattern of spruce establishment across time and space. To address this hypothesis, I conducted research in three distinct vegetation communities across the region: productive lowland floodplains (Chapters 1 and 2), treeline (Chapters 3 and 4), and recently burned stands of black spruce (Chapter 5). Together these five chapters reveal that snowshoe hares affect spruce establishment across much of boreal Alaska. Where and when hares are abundant, spruce can be heavily browsed, resulting in suppressed seedling growth and increased seedling mortality. The results of these studies also reveal a consistent and predictable pattern in which this plant-herbivore interaction takes place. The snowshoe hare filter acts as a 'spatially aggregating force' to spruce establishment, where the potential for optimal regeneration is highest during periods of low hare abundance and where hares are absent from the landscape.