Separating the role of genetics and environment in determining the mycorrhizal community composition of hybridizing cottonwoods

Over 5000 species of fungi form ectomycorrhizal fungal (EMF) associations with host plants, yet little is known about the effect of host plant species composition and hybridization on rates of EMF colonization, diversity, and community composition.  In a common garden, where environmental factors were held constant, we examined the hypothesis that two dominant and hybridizing tree species, narrowleaf and Fremont cottonwoods (Populus angustifolia and P. fremontii, respectively), supported different EMF communities and that hybrid communities would be intermediate between the two parental species. Three findings have emerged:  1) the highest EMF colonization levels occurred on Fremont and differed only among Fremont and the two hybrid crosstypes; 2) narrowleaf crosstypes had the greatest EMF diversity; and 3) EMF community diversity on hybrids was lower than that for the parental species. These results suggest that host plant identity can play an important role in determining EMF species composition. Furthermore, conserving EM fungal diversity may require conserving closely related, hybridizing parental host-plant species of threatened cottonwoods populations.