Beyond recruitment limitation: post fire growth of seedling transplants across a boreal fire severity gradient

Background/Question/Methods

: Plant community assembly after disturbance is governed by processes that affect demographic stages of seedling germination, establishment, survival, and growth. Here we use experimental outplantings of four boreal tree seedlings across a range of fire severities in Alaskan post-fire black spruce forests to examine environmental and biotic filters on survival and growth in the absence of recruitment limitations. We asked: 1) How do environmental drivers of seedling growth differ between boreal tree species; 2) How does the relative importance of those drivers change between early life stages; and 3) What are the relative growth responses of species through time? We expected that the same environmental drivers (soil moisture, residual organic layer depth, summer temperature, pH) would affect seedling performance and seedling establishment, but that their relative importance would change between life stages. We addressed these questions using i) structural equation modeling (SEM) and ii) general linear mixed effects modeling (GLMM) for each of four species of seedlings: two evergreens (black and white spruce) and two deciduous species (Alaska paper birch and trembling aspen).

Results/Conclusions

: Growth of spruce species was negatively associated with soil moisture (GLMM, SEM), with no direct effects of residual soil organic layer depth. Conversely, deciduous seedling growth was negatively associated with organic layer depth, with no direct effects of soil moisture. Our GLMMs also revealed that the constraining effects of residual organic layers on deciduous seedling emergence persists across life stages, reducing growth. Finally, our GLMM analysis showed that the residual organic layer depth effects persisted through time in deciduous species, but was unimportant in the two spruces. Aspen and birch seedling growth responses to residual organic layers likely reflects their shallow rooting depth compared to spruce, as well as the deciduous species’ growth response to warmer soils resulting from the deeper active layers and increased nutrient access from the mineral soil. We conclude that, in the absence of seed and establishment limitations, increasingly frequent and severe fires in the Alaskan boreal forest will likely result in a mix of stand dominance across the landscape, where deciduous species may dominate in more severely burned sites. We suggest that similar experimental and analytical approaches can help us understand where, and under what conditions, forest community assembly may diverge from pre-fire stand characteristics.