Getting to the root of the matter: Fire effects on mycorrhizal seedling establishment and tree migration in Alaska

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. Fire, an indirect effect of climate warming, may overshadow the direct effects of climate change on species distribution and migration. Soil microbes are key drivers of ecosystem processes, yet their role in regulating landscape-scale vegetation change is not known. Comprehensive studies of treeline position have noted that ectomycorrhizal fungi (EMF) may be an important factor delineating the boundary between forest and tundra. Yet, these critical plant-fungal symbioses are sensitive to wildfires. We know that fire affects seedling establishment success and fungal community structure; however, the influence of fire on mycorrhizal seedling establishment and the importance of these symbioses to seedling performance post-fire at and beyond current treeline are not well understood. At treeline and tundra sites we investigated the interactions between seedling establishment, fire severity, and EMF dynamics. We found that 1) Tundra fire reduced EMF availability for establishing boreal tree seedlings and shrubs and seedling biomass was related to fungal community structure of inoculum from burned soils (F_{(114, 1)}=4.2921, P_value=0.0003). 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 basidiomycete colonization than in burned sites (F_{(43, 1)} =6.9409, P_value=0.0117), 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 (F(70,1)=3.3921, Pvalue=0.0699,). 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. This research is a cross-site project connecting post-fire boreal and tundra seedling establishment to treeline dynamics.