Do post-fire plant-fungal interactions shape biome shifts?

Background/Question/Methods

At high latitudes, uncertainties about the drivers of vegetation migration patterns weaken predictions about future ecosystem responses to environmental change.  Across Alaska, changes in vegetation with warming over the last century include shrub cover expansion and shifts in treeline. Mechanisms controlling vegetation migration include ecological factors that influence seedling establishment. Evidence suggests that these factors are in fact the most critical determinants of treeline and shrubline advances globally. Soil biota, both mutualists and pathogens, influence vegetation establishment, thereby impacting this critical step in vegetation migration and expansion. The composition and net outcome of mutualist and pathogenic effects of soil biota on migrating plant species, however, are likely sensitive to wildfire. Climate-sensitive changes in the tundra fire regime are expected to accelerate the rate of vegetation change associated with warming in the Arctic. Wildfire disturbance exposes high-quality establishment microsites creating opportunities for establishment in an ecosystem where recruitment is otherwise rare.  However, these changes in the fire regime may alter the composition of soil biota critical to seedling establishment. The goal of this research is to understand the relative contribution of soil biota on the migration of trees and expansion of shrubs into tundra after fire.

Results/Conclusions

At treeline and tundra sites we investigated the interactions between fire severity, soil biota, and seedling establishment. We found that 1) There was a positive trend between the proportion of pathogens and fire severity, which reduced black spruce and alder seedling growth. 2) Resprouting shrubs, regardless of fire severity, maintained the presence of dominant taxa, including late-stage ectomycorrhizal fungi (EMF), after fire beyond current treeline; 3) At post-fire treeline sites boreal tree seedlings (aspen, birch, black and white spruce) shared 26-52% of their EMF with adjacent resprouting shrubs (birch, willow, and bearberry), and high severity fires reduced the amount of overlapping EMF; 4) Survivorship of seedlings beyond current treeline was high regardless of inoculation with boreal forest fungi. These results suggest a hierarchy of ecological filters on seedling establishment where both abiotic and biotic factors can determine successful establishment. Under scenarios of extreme fire severity the absence of EMF and high proportion of pathogens may decrease seedling establishment and dampen migration and expansion.  However, after moderate burns, which will likely be more common in tundra, mycorrhizal associations with tundra shrubs could provide critical inoculum that enables the northward expansion of trees into tundra.