Mycorrhizal symbioses and Arctic shrubification

A unifying feature of the deciduous and evergreen shrubs contributing to the greening of the Arctic landmass-the process known as shrubification-is that their roots consistently form ectomycorrhizal or ericoid mycorrhizal symbioses with soil fungi. Here, we review the potential roles of these symbioses in shrubification, focusing on their effects on shrub nitrogen (N) acquisition from N-limited Arctic tundra soils. Enhanced shrub growth in response to warming or elevated atmospheric CO₂ is attributable to several mycorrhizal-mediated processes associated with N acquisition, including increased frequencies of fungal symbionts on, in, or around shrub roots, elevated activities of extracellular enzymes secreted by mycorrhizal hyphae, or enhanced hyphal access to N at the permafrost thaw front. An analysis of the responses to experimental warming of 11 frequent ectomycorrhizal fungal genera shows no effects on their abundances but a 20% reduction in taxonomic richness in warmed soil, suggesting that elevated temperature selects for specific fungal taxa. The effects of warming on ectomycorrhizas are most pronounced in mesic soils. We conclude that ectomycorrhizal and ericoid mycorrhizal symbioses most probably influence shrubification, but that further research is needed to determine the processes by which they do so.