Warming drives a front of white spruce establishment near western treeline, Alaska

• Published in: Global change biology Vol. 23; no. 12; pp. 5509 - 5522
• Main Authors: Miller, Amy E., Wilson, Tammy L., Sherriff, Rosemary L., Walton, James
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01-12-2017
• Subjects: Abundance; Alaska; boreal forest; Boreal forests; Climate; Climate Change; Climate models; Cores; Correlation analysis; Environmental changes; Environmental conditions; Environmental gradient; Environmental Monitoring; Evergreen trees; Forests; Generalized linear models; Global warming; Longitude; Null hypothesis; Picea - physiology; Picea glauca; range shift; Recruitment; Recruitment (fisheries); saplings; Seasons; Seedlings; Seedlings - physiology; Stand structure; Statistical models; Taiga; Temperature; Treeline; Trees; United States > US; Alaska; Picea glauca; recruitment; boreal forest; climate; range shift; seedlings; saplings
• DOI: 10.1111/gcb.13814
• EISSN: 1365-2486
• ISSN: 1354-1013

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.
We evaluated the effect of geospatial, climate, and plot covariates on the abundance of five life stages of white spruce from stands in southwest Alaska, and the relationship between growing season conditions and seedling establishment. We found a positive relationship between abundance and temperature that was consistent across all life stages, but a mixed response to longitude, with earlier life stages (seedlings, saplings) most abundant at the western end of the gradient, near the current range margin, and later life stages (trees) most abundant to the east. Likewise, white spruce establishment was positively associated with longer summers and/or greater heat accumulation, as inferred from growing degree days. Together, our results suggest a moving front of white spruce establishment, toward the species’ western range limit.