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

Abstract

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.

Factors shaping alternate successional trajectories in burned black spruce forests of Alaska

“…Our sampling design explicitly included a broad range of site moisture conditions and fire severity across the black spruce forests within each burn scar, and the extensive burning in 2004 allowed us to sample a large elevational range from valley bottoms to high elevation sites at local treelines. The variation in post‐fire regeneration across this broad sample very clearly picked out a strong effect of elevation that most likely represents topographic effects on microclimate that constrain the distribution of forests across elevational gradients in interior Alaska (Calef et al 2005, Miller et al 2017). Interestingly, although black spruce is more common than deciduous trees at these elevational treelines, our models suggest a similar magnitude of negative effects of elevation on recruitment of both spruce and deciduous trees.…”

“…The fate of open stands where seedling recruitment was <1 stem/m 2 remains uncertain. Persistence of open canopies is likely to depend on environmental constraints as well as biotic interactions including herbivory and plant interactions that affect potential infilling of spruce seedlings (Miller et al 2017, Olnes et al 2017, Pacé et al 2018).…”

Divergent responses to climate change and disturbance drive recruitment patterns underlying latitudinal shifts of tree species

“…As disturbances can provide opportunities for recruitment separately or in interaction with climate change (Leithead et al, ; Zhang, Bergeron, Zhao, & Drobyshev, ), we then estimate the probability of observing a recruitment event in response to changes in climate, disturbance and their interaction, using a multimodel selection approach. We generally expected the probability of sapling recruitment to increase with warming, especially at these high latitudes, but that relationship can change in space (Cavanaugh et al, ; Fisichelli, Frelich, & Reich, ; Miller, Wilson, Sherriff, & Walton, ). We also expected the disturbance signal to be more important than the climate signal given the prevalence of disturbances in the study area.…”

Nest use dynamics of an undisturbed population of bald eagles

“…Although this is the case, the observed relationship between temperature and productivity underscores the importance of continuing to monitor the species in Alaska where warming is expected to be most intense (IPCC 2013). In addition to temperature and precipitation effects, climate change and ocean acidification could change the base of the marine food web (Edwards & Richardson, 2004; Kroeker, Kordas, Crim, & Singh, 2010; Lam, Cheung, & Sumaila, 2016), the timing of salmon runs (Quinn, Hodgson, Flynn, Hilborn, & Rogers, 2007), and distribution of nesting substrate (Miller, Wilson, Sherriff, & Walton, 2017). These factors may influence bald eagle nesting populations in the future.…”