Small mammals, ectomycorrhizae, and conifer succession in beaver meadows

Beaver (Castor canadensis) build dams and create ponds on streams within forested areas. Upon abandonment, ponds often drain and the subsequent meadows persist for decades as graminoid patches that resist invasion of conifers despite their close proximity to seed sources. The lack of ectomycorrhizal fungi in soils of beaver meadows may limit the rate of conifer invasion. Small mammals such as the red backed vole (Clethrionomys gapperi) disperse spores of mycorrhizal fungi in their feces, and so the dispersal of spores by small mammals may be the rate-limiting step to conifer invasion of beaver meadows. We investigated the distribution and abundance of small mammals in beaver meadows and adjacent forests in northern Minnesota, USA during May and August, the abundance and species composition of fungal spores in their fecal pellets, and the growth of black spruce (Picea mariana) seedlings in beaver meadow soils inoculated with these spores. C. gapperi was present in the forest and at the forest edge but was absent from beaver meadows. Abundance of C. gapperi was almost three times higher in August than in May. Feces from C. gapperi in May and August contained fungal spores from a total of nine genera of fungi, and all but one are known to form ectomycorrhizal symbioses. Feces from August contained more fungal genera and more spores than feces from May. Seedlings grown in uninoculated beaver meadow soils did not form ectomycorrhizae during the length of this study, but beaver meadow soils which received feces formed ectomycorrhizae on 20% and 33% of plants using May and August feces, respectively. In contrast, seedlings grown in soils from forests adjacent to beaver meadows formed ectomycorrhizae on 100% of plants. Black spruce seedlings grown in beaver meadow soils containing August feces had significantly higher shoot, root, and total dry weights after 46 weeks of growth compared to uninoculated controls but plants grown in soils containing May feces were not significantly different from controls. In addition, seedlings grown in forest soils had significantly larger shoot, root, and total weights than plants grown in untreated beaver meadow soils at all sampling periods. Flooding of forest soils for eight weeks had no significant effect on seedling growth parameters or mycorrhizal development. Lack of ectomycorrhizal fungi is likely a factor that limits conifer invasion into beaver meadows. C. gapperi is a potential vector for ectomycorrhizal fungal spores info beaver meadows, but the expression of this potential is limited by habitat use patterns of this small mammal.