Interactions between an above-ground plant parasite and below-ground ectomycorrhizal fungal communities on pinyon pine

1 Recent research has demonstrated important linkages between above- and below-ground components of terrestrial ecosystems, but the relationships between aerial parasitic plants, such as dwarf mistletoes, and below-ground organisms, such as mycorrhizal fungi, have not been examined in detail. 2 We examined the relationship between dwarf mistletoe infection, host vigour and the ectomycorrhizal colonization and fungal community composition of pinyon pine (Pinus edulis) using a combination of field observations and glasshouse studies. 3 High levels of dwarf mistletoe infection were not associated with increased mortality or needle loss of infected pinyons, but infected trees had lower shoot growth. 4 Ectomycorrhizal colonization was positively associated with dwarf mistletoe infection severity at two sites in two years. In addition, the ectomycorrhizal fungal community structure of trees with low, intermediate and high levels of dwarf mistletoe were significantly different, primarily due to a shift in the dominance of ascomycete fungi. 5 These higher levels of ectomycorrhizal colonization were associated with increased fungal inoculum under the crowns of pinyons heavily infected with dwarf mistletoe. In addition, 33% more pinyon seedlings were found in the understories of mistletoe-infected trees than uninfected trees. 6 These findings point to complex multi-trophic interactions between the above-ground and below-ground communities of pinyon pine, and suggest that a detailed understanding of host-parasite relationships may require study of other symbionts associated with the host.