Getting to the root of the matter: The role of mycorrhizal fungi in post-fire seedling establishment at treeline

Background/Question/Methods

Ectomycorrhizal fungi (EMF) are critical symbionts of all boreal tree species.  These mycobionts are potentially limiting to seedling establishment beyond current treeline in tundra. In addition, fire frequency and extent has increased in the last half century in the boreal forest and tundra in response to warmer weather and lower precipitation. These indirect effects of climate change have been hypothesized to increase vegetation transitions from tundra to forest; yet, fire can reduce fungal inoculum along with host plant abundance. We investigated the relative importance of EMF and other factors to growth of naturally established seedlings at treeline and outplanted seedlings at Arctic treeline and tundra sites after fire. We quantified EMF colonization of seedling roots and used molecular tools to characterize fungal communities. We related these fungal variables to aboveground biomass and measurements of foliar C:N, d¹³C, and d¹⁵N. We used Bray-Curtis ordinations and Canonical Correspondence Analysis to evaluate correlations between fungal composition and seedling biomass and foliar measurements. We employed Random Forest regression trees to evaluate the importance of fungal variables to seedling growth in relation to fire and environment.

Results/Conclusions

Bray-Curtis ordinations of fungal composition resulted in a 3-D solution that explained 91.3% of the variance in fungal community structure. EMF composition was most strongly correlated with vegetation composition (r²=0.29) and fire severity (the depth of the organic horizon; r²=0.22). EMF composition on tree seedlings was also significantly related to the EMF composition of the closest resprouting shrub (r= 0.130, p=0.001), and distance to these shrubs was the main factor determining similarity in fungal communities. There were also significant relationships between EMF composition and seedling C:N ratio (0.608, p=0.050) and biomass (0.444, p=0.050). Random Forest analysis showed that fungal composition was the fourth most important variable predicting seedling biomass, after seedling age, site, and fire severity (organic depth). Stable isotope analysis on outplanted seedlings demonstrated the nutritional consequences of fungal colonization for different species in treeline and tundra sites. These results suggest that EMF mycobionts provided by resprouting post-fire vegetation are important to nutrient acquisition and biomass accrual of naturally establishing tree seedlings at treeline and in tundra.