Recent studies have found that exotic, invasive species are capable of disrupting the vital mutualisms upon which many native species depend. One such mutualism is between plants and mycorrhizal fungi. Mycorrhizal fungi develop symbiotic relationships with the fine roots of their host plants and supply the host with soil nutrients in exchange for photosynthate. In this study, we tested the hypothesis that tamarisk (Tamarix spp.) negatively affects the arbuscular mycorrhizal fungal (AMF) and ectomycorrhizal fungal (EMF) communities of neighboring cottonwoods (Populus fremontii) in an Arizona riparian ecosystem. The negative effects of tamarisk on ecosystems have been widely investigated, but few studies have determined if tamarisk influences the mycorrhizal fungal associations of its native plant neighbors. Understanding the potential impact tamarisk may have on these fungal mutualists is crucial because Fremont cottonwood, a threatened, riparian foundation species, relies on them for establishment and growth. We tested our hypothesis by comparing the AMF colonization and EMF colonization and community composition of cottonwoods with and without tamarisk neighbors in the field and the greenhouse.

Results/Conclusions

Several key patterns emerged from this research; 1) In the field, EMF colonization and species richness was significantly lower in cottonwoods with tamarisk neighbors than in cottonwoods with native plant neighbors. 2) The presence of tamarisk also was associated with a shift in the EMF community composition of neighboring cottonwoods. 3) Similarly, cottonwoods with tamarisk neighbors had significantly lower AMF colonization than cottonwoods with conspecific or willow neighbors. Instead, the cortex of roots of cottonwoods with tamarisk neighbors was dominated by an unidentified fungus that may be pathogenic. 4) Preliminary results from experiments in which cottonwoods were grown with tamarisk or conspecific neighbors in the greenhouse corroborate findings from the field. Cottonwood growth was severely reduced in the presence of tamarisk neighbors, and cottonwoods did not benefit from inoculation with mycorrhizal fungi. In contrast, relative to non-inoculated controls, cottonwoods with conspecific neighbors showed a growth benefit following inoculation with either EMF alone or EMF and AMF combined. These results provide further evidence that the success of some invasive species is advanced through the disruption of mutualisms critical to native species.