Birch effects on root fungal colonisation of crowberry are uniform along different environmental gradients

Changes in plant–fungal interactions were often suggested as one of possible mechanisms behind facilitative plant–plant effects in harsh environments. We asked how the mycorrhizal and dark septate endophyte (DSE) colonisations of understorey crowberry ( Empetrum nigrum ssp. hermaphroditum ) are affected by proximity to mature mountain birch trees ( Betula pubescens ssp. czerepanovii ) along three abiotic stress gradients (pollution, elevation, seashore) in the Kola peninsula, NW Russia. Stress level affected shoot growth and reproduction in crowberry, but had no effect on root fungal colonisation. In contrast, proximity to a mountain birch tree had no effect on either growth or reproduction of crowberry, but changed all characteristics of root colonisation. The mycorrhizal coil colonisation of crowberry was on average 21% higher near a birch tree, whereas other parameters were higher outside of canopy area (hyaline hyphae: 12%; DSE hyphae: 16%; DSE sclerotia: 42%). Effects of birch tree on root fungal colonisation in crowberry did not depend on the level of abiotic stress. Although we detected a weak positive association between growth of crowberry and its mycorrhizal coil colonisation, we conclude that mycorrhizal and DSE colonisations of crowberry are primarily affected by the abiotic environment. None of the detected patterns was consistent with the patterns expected from the theories concerning stress effects on plant–plant interactions.