Assisted range expansion and the potential for lodgepole pine to transform successional pathways after fire in Interior Alaska

Background/Question/Methods

Alaska's glacial history clearly shows that broad environmental changes inevitably bring shifts in species distributions. However, changes in tree distributions are constrained by slow processes of population spread and geographic barriers to migration. Once established in a location, the traits of individual tree species tend to support feedbacks that resist subsequent compositional changes, leading to lagged and often abrupt responses of forest communities to environmental changes. Here we examine a case study of potential range expansion of lodgepole pine (Pinus contorta var. latifolia) in Interior Alaska and address the following questions: a) to what degree does climate constrain the potential for pine to establish in natural post-fire communities, and b) what is the potential for pine to form new successional trajectories in Alaskan boreal forests? We used seed application experiments and planting of pine seedlings across 39 burned sites in interior Alaska to assess factors controlling initial establishment, growth, and survival over 10 years after fire. Experimental sites covered a range of environmental conditions in burned upland and treeline forests, providing the opportunity to assess environmental conditions that may support or restrict future range expansion of lodgepole pine in boreal Alaska.

Results/Conclusions

Seedlings of lodgepole pine were able to establish on organic layers left behind by low severity burns, but showed lower rates of growth and survivorship associated with the cold soils that persist under a thick ( >10 cm) organic layer. Site moisture initially supported pine seedling recruitment, but pine growth and survival was higher on more well-drained soils, where pine was able to outgrow black spruce, the former dominant species at the site. Pine growth was initially high at treeline sites, but severe winter conditions in 2012 caused substantial topkill and mortality, consistent with hypotheses that snow conditions may limit pine distributions at its northern range limits. Overall, high survival and growth at a majority of sites indicates strong potential for rapid pine expansion in Interior Alaska once bottlenecks of dispersal are overcome. The strong prevalence of pine as an alternative canopy dominant in both spruce and broadleaf forests in adjacent Canada suggests that assisted range expansion of pine into Alaska could substantially alter forest composition and successional dominance after fire.