We investigated how abundance and diversity of soil fungi and invertebrates responded to a reduction in fresh belowground photosynthate supply in treeline birch and willow, achieved using stem girdling. We hypothesised that birch forest would support greater abundance of ectomycorrhizal fungal species and fauna than willow shrubs, and that girdling would result in a rapid switch from ectomycorrhizal fungi to saprotrophs as canopy-supply of C was cut, with a concomitant decline in soil fauna.
Birch forest had greater fungal and faunal abundance with a large contribution of root-associated ascomycetes (ericoid mycorrhizal fungi and root endophytes) compared to willow shrub plots, which had a higher proportion of saprotrophs and, contrary to our expectations, ectomycorrhizal fungi. Broad-scale soil fungal and faunal functional group composition was not significantly changed by girdling, even in the third year of treatment. Within the ectomycorrhizal community, there were some changes, with genera that are believed to be particularly C-demanding declining in girdled plots. However, it was notable how most ectomycorrhizal fungi remained present after three years’ isolation of the belowground compartment from contemporary photosynthate supply.
Synthesis: In a treeline/tundra ecosystem, distinct soil communities existed in contrasting vegetation patches within the landscape, but the structure of these communities was resistant to canopy disturbance and concomitant reduction of autotrophic C inputs.