Cheatgrass (Bromus tectorum L.) interactions with arbuscular mycorrhizal fungi in the North American steppe: Prevalence and diversity of associations, and divergence from native vegetation

Cheatgrass (Bromus tectorum L.) is a highly invasive winter annual grass that has caused significant changes to the steppe ecosystem of western North America. Cheatgrass is considered a facultative host of arbuscular mycorrhizal fungi (AMF), and has been shown to reduce AMF density in invaded soils and reduce AMF diversity in roots of neighboring grasses. However, specific information about interactions between cheatgrass and AMF remains unknown, as well as how these interactions differ from native vegetation. The research presented here addresses these knowledge gaps.;To determine when cheatgrass is colonized by AMF and the magnitude of colonization, two dense cheatgrass patches were identified in invaded shortgrass prairie in Colorado. Individuals were excavated every three weeks, from six weeks after germination through senescence. Roots were collected from individuals, cleared, stained, and observed for AMF colonization. Roots were colonized by AMF at every sampling date, but percent colonization of roots declined dramatically when soil temperatures dropped below 0° C, and colonization remained low from late January through March. Peak colonization occurred in May (15.3%), when florets appeared on the cheatgrass shoots, and colonization dropped in June, once seeds were produced and senescence began. Although mycotrophic, cheatgrass is a poor host for AMF throughout its life, as evidenced by low AMF root colonization. Severe, lasting invasions by cheatgrass could have a negative impact on the AMF community.;Cheatgrass invasion is most severe in the sagebrush steppe of western North America, which is dominated by the native shrub big sagebrush ( Artemisia tridentata Nutt.). As cheatgrass replaces big sagebrush, it is important to know how this shift affects the AMF community. Two studies were conducted to identify and compare AMF species associating with these two host plant species. Three sites (in Colorado, Utah and Wyoming) were selected where coexisting cheatgrass and big sagebrush populations were interspersed. Soil and root samples from underneath sixteen individuals of each species were collected at each of the sites.;In the first study, in which AMF species associating with big sagebrush and cheatgrass were identified, soil and root material was seeded with Sudangrass (Sorghum bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet & Harlan), a promiscuous AMF host, and grown in the greenhouse for three consecutive culturing cycles. A total of 32 AMF species were identified from the trap cultures. Alpha diversity of AMF associated with big sagebrush was higher across all study sites compared to AMF associated with cheatgrass, although differences were only statistically significant across all sites. Gamma diversity was similar and beta diversity was higher in AMF associated with cheatgrass compared to big sagebrush. These results indicate that big sagebrush individuals associate with more AMF species than cheatgrass and the sagebrush-associated AMF communities are more similar from one individual host to the next when compared to cheatgrass. Indicator species analysis identified two AMF species (Archaeospora trappei and Glomus viscosum) that were significantly more frequent in association with big sagebrush than cheatgrass across multiple sites. Identification of specific changes to the AMF community due to invasion, as suggested here, could lead to improved understanding of key plant-AMF interactions necessary for native plant recovery and restoration.;In the second study, AMF DNA was isolated from root and soil subsamples from source material for each of the trap cultures. A total of 27 unique AMF sequences were isolated from roots and soils. Although AMF diversity did not differ between host plants, AMF community composition in roots was significantly influenced by host, most likely due to half of the sixteen AMF species isolated from roots colonizing only one of the host species. This finding has important implications for invasion success and restoration of invaded soils, as alterations to the AMF community could provide positive feedbacks on the invader and decrease successful establishment of native plant species dependent on AMF.;In the final study, the interactions between early- and mid-seral native plant species and an AMF community associated with cheatgrass invasion were investigated. Plant responsiveness was measured using field soil with and without AMF. Soils used in the plant responsiveness study that contained AMF were collected and grown with a bioassay plant. AMF density was then measured by observing AMF colonization of the bioassay roots in the trained soils. Plant species studied were highly variable in their interactions with AMF, and mutualisms, parasitisms, amensalisms and commensalisms were all observed. The presence of certain AMF facilitators may have a strong founder effect on plant communities and, where such feedbacks exist, identifying and utilizing these key interactions might facilitate the restoration of degraded ecosystems.