Empirically linking fire history to seedbed quality, species establishment, and growth: Predictions for northern boreal forest succession under changing climate and disturbance regimes

Background/Question/Methods

Fire is a dominant process that initiates secondary succession in the boreal forest. Although boreal trees are well adapted for self replacement after fire, regeneration cycles may be interrupted when climate-disturbance interactions cause changes to the historic fire regime. Establishment and survival of trees is related to the post-fire characteristics of the forest floor substrate, or seedbed. To predict the effects of changes in fire regime on post-fire regeneration, we need a better understanding of how fire characteristics alter seedbed quality and thus, patterns of forest recovery. Our objective was to empirically link fire characteristics to seedbed quality and subsequent species establishment and growth. We applied experimental seeding treatments to recently burned black spruce stands in northern Yukon and Alaska where overlapping historic fires provide fire return intervals that are analogous to predicted increases in fire frequency. We applied seed and young seedlings of native tree species (black spruce, white spruce, trembling aspen, and paper birch) in 34 stands burned in 2004 (Alaska) and 2005 (Yukon). We measured seedling emergence, substrate cover, and growth of planted seedlings for three years. We linked seedling emergence, growth, and survival to substrate cover and site-level environmental characteristics using mixed-model Bayesian analyses and MANCOVA.

Results/Conclusions

Fire history exerts a major control on seedbed characteristics, species establishment, and seedling growth in these northern boreal forest stands. Microhabitat conditions affected species establishment and growth, but were less important than fire history effects. Regardless of species, seedling emergence in severely or frequently burned stands primarily occurred on post-fire substrates where a thin cover of Ceratadon-type moss had established. Local patterns of seedling recruitment across microhabitat types suggest that moisture and temperature interact to determine the optimal habitat for tree establishment. Planted seedlings of all species grew best in severely or frequently burned stands, likely due to warmer soils and higher nutrient availability. However, young seedlings are vulnerable to moisture stress and if these stands become too dry, recruitment will likely decline regardless of nutrient availability. These results lead us to predict a positive effect of warming, in the form of direct climate effects or indirect fire effects, on post-fire recruitment potential. However, this pattern could reverse if conditions become too dry. By linking fire characteristics to post-fire seedbeds and recruitment success, we can better predict future successional trajectories of northern boreal stands under changing climate and disturbance regimes, enabling us to better meet restoration and conservation goals.