Wildfires did not ignite boreal forest range expansion into tundra ecosystems in subarctic Yukon, Canada

The resilience of northern ecosystems is being tested as the temporal and spatial distribution of wildfires change. Wildfires at treeline are predicted to facilitate a northward advance of boreal tree species, as the tundra is in close proximity to fire-adapted species climatically primed for range expansion. To study the effects of wildfire at treeline, we asked (1) how does wildfire change the environmental characteristics of treeline microsites, and (2) in the absence of viable seed limitation, do burned treelines increase the likelihood of tree seedling emergence? We measured numerous biotic and abiotic factors at three treelines in Yukon, Canada, each consisting of a burned and unburned treatment. Our observational study was paired with a manipulative seeding experiment of black spruce (Picea mariana (Mill) B.S.P.) and white spruce (Picea glauca (Moench) Voss) to determine what environmental characteristics facilitated seedling emergence potential across site treatments. When comparing site treatments, results indicated that within the first two decades after wildfire, soil temperatures (GDD) were more extreme at burned treelines during the summer and winter. An examination of plot data in ordination space showed clear differences in organic layer depth and vegetation cover between treatments. While some of these changes would seem to benefit germination potential, spruce seedling emergence was extremely limited over the study period. Model results indicated little association between seedling emergence and recent fire; however, seedling emergence was positively associated with moss substrate found across treatments and reduced cover of tall multi-stemmed shrubs. We conclude that wildfire can create seedbed conditions that benefit seedling emergence, but low-severity wildfires create a variety of microsite conditions that limit overall availability of these seedbeds. Furthermore, emerging seedlings may experience new barriers to establishment, as burned treelines exhibited changes in soil temperature extremes and snow depth.