Understanding mechanisms shaping the distribution of species associations is a central theme of ecological research. This study uses an experimental approach to identify factors driving spatial variation in mycorrhizal associations. In alpine krummholz communities, arbuscular mycorrhizal associations (AMF) vary across small spatial scales; colonization is greater in plants growing in open meadows compared to plants in the willow understory. We tested whether the observed pattern in AMF colonization is due to 1) competition of host plants with willows for light, 2) competition of host plants with willows and/or their ectomycorrhizal partners (ECM) for belowground resources, or 3) leaf litter input by willows. To test the first hypothesis I covered open meadow plots with 80% shade cloth to simulate willow shading. To test the second hypothesis I transplanted mycorrhizal (M) and nonmycorrhizal (NM) willows into open meadow plots. I compared AMF colonization in these experimental treatments to AMF colonization in unmanipulated meadow (O), and understory (U) plots. To test the third hypothesis I added or removed leaf litter from open meadow and willow understory plots. I evaluated AMF and ECM colonization in all treatments for two years.  
Results/Conclusions

ECM abundance was high in the U treatment, moderate in the M treatment, and very low in the other three treatments. This indicates that the NM and M treatments successfully decoupled the effects of willows from the effects of ECM, and that colonization rates in the M treatment approached those observed in natural willow habitats. After two years, AMF colonization in the M and U plots was reduced compared to the other three treatments. Furthermore, AMF colonization was negatively correlated with ECM colonization regardless of treatment. Both trends suggest that competition with ECM negatively affects AMF associations in understory plants. The presence of leaf litter increased ECM colonization in the open meadow, but not in the willow understory. In contrast, leaf litter reduced AMF colonization in both habitats, particularly in the second year. Taken together, these results suggest that leaf litter indirectly affects AMF associations by promoting ECM colonization and, perhaps, by altering soil chemistry. Overall, this study demonstrates that indirect effects of willows via ECM partners and leaf litter drive the observed pattern in AMF colonization in these krummholz communities. The distribution of AMF associations is highly context dependent; as shown here, multiple disparate inputs into soil communities and soil chemistry are integrated into the overall pattern of mycorrhizal colonization.