Mycorrhizal responses to host plant competition, facilitation and disturbance in pinyon-juniper woodlands

Pinyon-juniper woodlands cover approximately 24 million hectares of the western United States and provide food and refuge for several species. The work presented here is an attempt to understand how competition with the co-dominant juniper and facilitation by a conspecific nurse plant affect the performance of pinyon pine (Pinus edulis) and their ectomycorrhizal (EM) fungal mutualists in light of predicted climate changes. Furthermore, pinyon juniper woodlands are now more intensively managed, but it is unclear how management techniques that involve significant disturbance will affect the plants and their associated mycorrhizal fungi. The research described here thus also examines the effect of fuel wood harvest and slash pile burning on the pinyon-juniper understory plant community and the arbuscular mycorrhizae that associate with these plants.; Six major findings resulted from these studies. First, mature pinyons respond to reduced belowground competition from juniper by increasing fine root biomass and the number of short roots available for EM colonization. Second, pinyon seedlings re-establishing in juniper-dominated zones are challenged by reduced abundance and diversity of EM inoculum. Third, nurse plants are important for pinyon seedling survival, particularly during drought. In moderate years, an established seedling performs better when its nurse dies. However, during severe drought, seedlings nursed by dead conspecifics are likely to die. Fourth, although a few EM fungal species are shared by nurses and nurslings, the EM fungal communities of nurslings and nurses differ, suggesting that seedlings are not entirely dependent on nurses for EM inoculum. Fifth, the soil beneath a dead conspecific nurse is inhospitable to the establishment of pinyon seedlings. Sixth, fuel wood harvest followed by slash-burning altered the understory plant community by increasing exotic species, an effect that has persisted for five years, but that is unrelated to the arbuscular mycorrhizal fungal inoculum potential of the soil.; Taken together, these findings suggest that during extreme times pinyons could be lost from the landscape. Because pinyons are the only host for EM fungi in many pinyon juniper woodlands, loss of pinyons also means the loss of EM fungi. These changes are likely to be highly persistent and may result in altered ecosystem processes.