White spruce woodlands and open forest are an important landscape component in Lake Clark and Katmai. Warming since the 1980s has led to increased spruce beetle-related mortality in older, denser forest stands, but also to increases in spruce growth and seedling establishment in more open sites, particularly near the forest-tundra margin. Two new studies using monitoring and tree-ring data collected by the Southwest Alaska Network describe recent changes in forest dynamics near western treeline, represented by Boundary Creek in southern Katmai. The first uses tree-ring data to examine the effects of warmer summer temperatures on growth at sites extending from northern Lake Clark to southern Katmai. This study found that trees in all but a handful of forested sites have been growing more rapidly in recent decades and that late-summer precipitation has become increasingly important in maintaining enhanced growth (Sherriff et al. 2017). The second study found that spruce establishment since the mid-twentieth century has been greatest in warm years, but that the majority of establishment has occurred at sites near western treeline where growing season conditions are generally cooler and wetter than at sites farther north (Miller et al. 2017). This wave of new spruce establishment across southern and western Katmai could result in the creation of new forested areas over the course of decades, offsetting to some extent the dieback currently occurring on other areas of the landscape.
Warming drives a front of white spruce establishment near western treeline, Alaska
Regional warming has led to increased productivity near the boreal forest margin in Alaska. To date, the effects of warming on seedling recruitment have received little attention, in spite of forecasted forest expansion. Here, we used stand structure and environmental data from 95 white spruce (Picea glauca) plots sampled across a longitudinal gradient in southwest Alaska to explore factors influencing spruce establishment and recruitment near western treeline. We used total counts of live seedlings, saplings, and trees, representing five life stages, to evaluate whether geospatial, climate, and measured plot covariates predicted abundance, using current abundance distributions as a surrogate for climate conditions in the past. We used generalized linear models to test the null hypothesis that conditions favorable for recruitment were similar along the environmental gradient represented by longitude, by exploring relationships between per‐plot counts of each life stage and the covariates hypothesized to affect abundance. We also examined the relationship between growing degree days (GDD) and seedling establishment over a period of three decades using tree‐ring chronologies obtained from cores taken at a subset of our sites (n = 30). Our results indicated that seedling, sapling, and tree abundance were positively correlated with temperature across the study area. The response to longitude was mixed, with earlier life stages (seedlings, saplings) most abundant at the western end of the gradient, and later life stages (trees) most abundant to the east. The differential relationship between longitude and life‐stage abundance suggests a moving front of white spruce establishment through time, driven by changes in environmental conditions near the species’ western range limit. Likewise, we found a positive relationship between periods of seedling establishment and GDD, suggesting that longer summers and/or greater heat accumulation might enhance establishment, consistent with the positive relationship we found between life‐stage abundance and temperature.
Miller, A. E., T. L. Wilson, R. L. Sherriff, and J. Walton. 2017. Warming drives a front of white spruce establishment near western treeline, Alaska. Global Change Biology 23:5509-5522.
Spruce growth responses to warming vary by ecoregion and ecosystem type near the forest‐tundra boundary in southwest Alaska
A critical concern for boreal ecosystems centres on broad‐scale responses to warming, i.e. declining growth and mortality, or enhanced growth and greater productivity. However, few studies have synthesized tree growth along biogeographic gradients to address this issue. This study develops a broader understanding of how growth of a dominant conifer has responded to recent warming near the western forest margin of Alaska.
Sherriff, R. L., A. E. Miller, K. Muth, M. Shriver, and R. Batzel. 2017. Spruce growth responses to warming vary by ecoregion and ecosystem type near the forest-tundra boundary in southwest Alaska. Journal of Biogeography 44:1457-1468.