The role of fire in facilitating treeline expansion: Getting to the root of the matter

Background/Question/Methods

Fire is the primary landscape-scale disturbance in the boreal forest, and in the last half-century fires have increased in severity and extent in the boreal forest and tundra. In the boreal forest, climate-induced changes in the fire regime may be a more critical driver of landscape processes and species migrations than the direct effects of warming.  Across Alaska, changes in vegetation with warming over the last century include shrub cover expansion and shifts in treeline. Invasion of tundra by trees may be further facilitated by wildfire disturbance, which exposes seedbeds, increases nutrient availability, and creates opportunities for establishment in an ecosystem where recruitment is otherwise rare.

Post-fire vegetation recovery can include shifts in current vegetation. One of the most critical factors that may influence tree seedling establishment at and beyond treeline is the availability of ectomycorrhizae inoculum. Comprehensive studies of treeline position have noted that ectomycorrhizal fungi (EMF) may be an important and often overlooked factor delineating the boundary between forest and tundra. However, these critical tree-fungal symbioses are sensitive to wildfires. The goal of this research is to understand the importance of EMF relative to other biophysical factors to boreal tree seedling performance across the treeline ecotone after fire.

Results/Conclusions

At treeline and tundra sites we investigated the interactions between seedling establishment, fire severity, and EMF dynamics. We found that: 1) Tundra fire reduced ECM availability for establishing boreal tree seedlings and shrubs. Analysis of EMF DNA sequences showed post-fire soils provided no EMF inoculum. Seedlings inoculated with burned tundra soils had reduced biomass compared with seedling that received control inoculum (p=0.03338, F*=4.5774). 2) Post-fire resprouting shrubs act as refugia for EMF that may be critical to seedling establishment. Shrubs in unburned tundra have higher proportion of EMF colonization than in burned sites (p=0.0249, F*=4.0560), but DNA sequence analysis suggests that even in severely burned sites resprouting shrubs maintain late successional EMF that may associate with tree seedlings. 3) At post-fire treeline sites establishing boreal tree seedlings share 26-52% of their EMF with adjacent resprouting shrubs, and high severity fires reduced the amount of overlapping EMF (p=0.0699, F*=3.3921). Our studies indicate that fire reduces EMF inoculum potential in previously unforested sites.  However, interactions between resprouting tundra shrubs and newly established tree seedlings after fire could either maintain boreal community dynamics at the limit of tree establishment or provide a mechanism for treeline expansion under future scenarios of warming and fire.