Switching belowground partners: biogeochemical consequences of mycorrhizal type in aspen forests

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Microbes are paramount in the health of humans, their food and the ecosystems with which they interact. Of the microbes that live in soil, fungi are key players in cycling of nutrients and carbon. Fungi decompose organic matter, and through mycorrhizas, a symbiosis between roots and mycorrhizal fungi, they affect the nutrition and growth of plants. The proposed research addresses how soil fungi may mediate changes to soils associated with current range shifts of a widespread tree species. This research is important because it will evaluate how carbon cycling in terrestrial systems is controlled by interactions between trees and fungi, a critical link often overlooked when assessing the impacts of climate change. In western Canada, shifts in forest composition have been predicted and already observed to be intense at the southern limits of boreal forests. Trembling aspen is a widespread tree in this region of range tension, extending north into the boreal forest and south into grasslands. Aspen is also one of the few tree species worldwide that forms symbioses with arbuscular and ectomycorrhizal fungi, the two major groups of mycorrhizas. All mycorrhizal fungi rely on living hosts for carbon, but arbuscular mycorrhizal fungi and ectomycorrhizal fungi differ in their physiology, carbon demands and the extent and types of enzymes excreted into soils. In consequence, the dominance of ectomycorrhizas versus arbuscular mycorrhizas in forests may have pronounced effects on soils. In particular, predicting soil carbon fluxes is imperative as even small changes in fluxes can have substantial impacts on carbon cycling because the soil carbon pool is so large. Little is known on the ecological factors controlling which mycorrhizal type dominates on dually colonized hosts. Over the next five years, my research program will identify causes and consequences of shifts between ectomycorrhizas and arbuscular mycorrhizas in aspen forests occurring in regions of range tension. The anticipated significance of this research is threefold. 1) Historically, these groups of fungi have been studied in isolation. The proposed research considers their interaction as a response to forest dynamics, an innovative perspective ignored to date. 2) Shifts in arbuscular and ectomycorrhizal dominance have been tracked across landscapes, but not within a single, widespread species. Novel approaches described in this proposed research test whether the expression of a changing climate through a single tree species can alter soil fungi to the extent they change nutrient and carbon cycling in soils. 3) The proposed research builds on emerging perspectives on how mycorrhizas mediate carbon fluxes between soils and plants. This perspective is a turning point in recognizing the complex role mycorrhizas play in forests aside from plant nutrition.