Mycorrhizal colonisation of mountain birch (Betula pubescens ssp. czerepanovii) along three environmental gradients: does life in harsh environments alter plant-fungal relationships?

Environmental stress affects ectomycorrhizal communities (ECM), but it is not known how general the detected ECM responses are. We investigated ECM fungi on roots of mountain birch, Betula pubescens subsp. czerepanovii (Orlova) Hämet-Ahti, along three environmental gradients, two natural (altitude, seashore) and one human-induced (pollution), within the Kola Peninsula, NW Russia. Chlorophyll fluorescence of birch leaves indicated no environmental stress even in the conditions that were presumed most stressful in terms of abiotic environment, where the biomass and population density of birches were strongly reduced. Although neither overall ECM colonisation nor root fungal biomass showed stress-related patterns, colonisation by Cenococcum geophilum tended to decrease with abiotic stress. ECM morphotype diversity declined with abiotic stress, and along altitudinal gradient this decline was related to an increase in proportion of morphotypes with high fungal biomass. Polycormic birches had higher ECM colonisation than monocormic birches at high stress sites only. ECM morphotype diversity increased with foliar nitrogen concentration at low stress sites, but not at high stress sites. Birches with higher chlorophyll fluorescence had lower chitin concentration in their roots (indicating lower proportion of fungal structures) at high stress sites only. Our results suggest that at high stress sites (1) mechanical shelter created by polycormic trees may favour ECM fungi and (2) mountain birches maintain lower ECM diversity than at low stress sites.