Effects of nurse shrubs on symbioses between soil fungi and associated plants along a tropical alpine elevation gradient

Studies in alpine environments indicate that nurse plants can facilitate other species mainly through direct mechanisms (i.e., improvements in local abiotic conditions). However, far fewer studies consider indirect facilitation, including the effect on plant–plant interactions of symbiosis with soil fungi. We asked whether the nurse shrub Hypericum laricifolium affected the colonization and activity of fungal symbionts of plants showing contrasting patterns of local spatial association with this nurse in four sites between 4100 and 4400 m in the tropical Andes. We selected three abundant herb species (Senecio wedglacialis, Castilleja fissifolia, and Agrostis tolucensis) which showed positive spatial associations with the shrub, and two herbs (Agrostis breviculmis and the exotic Rumex acetosella), which showed predominantly negative associations. We collected roots of these species from individuals under the shrub’s crown and outside, and measured colonization and activity of arbuscular mycorrhizal fungi (AMF) and colonization of dark septate fungi (DSE), as well as glomalin content in soil samples from both study situations. We found no consistent effect of the nurse across species or elevations on the degree of AMF and DSE colonization, but there was a consistent increase in AMF phosphatase activity in plants positively associated with the shrub, as well as an increase in the content of easily extractable glomalin in soils under its influence across elevations. Thus, our results suggest that an increased AMF metabolic activity and soil stabilization mediated by the increase in extractable glomalin could be linked with an indirect facilitation effect of this nurse shrub on its beneficiary plants.