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Interactions between Bromus tectorum L. (cheatgrass) and native ruderal species in ecological restoration




Stube, Cassandra J. Kieffer, author
Paschke, Mark, advisor
Brown, Cynthia, committee member
Meiman, Paul, committee member

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Bromus tectorum L. is an invasive annual grass that dominates much of the rangeland in western North America. It has shown an impressive ability to invade ecosystems, causing substantial changes in the composition and function of native plant and soil communities in ways that promote further exotic invasion through displacement of native plant species and slowing or halting of succession. B. tectorum has several characteristics, including high reproductive rates, affinity for disturbed sites, and the ability to create positive feedback conditions in invaded sites, which make it highly successful at invading new sites, and extremely challenging to remove during revegetation efforts. Traditional control methods including herbicide application, grazing, and burning have largely proven unsuccessful at preventing establishment and spread of B. tectorum. Similarly, restoration seed mixes often consist of native perennial grass species, which tend to be slower growing and less robust in disturbed sites, and therefore provide little competition against B. tectorum and do not promote the reestablishment of native plant communities. In addition, seed mixes are often planted at a fraction of the rate of annual B. tectorum seed production, giving them a distinct disadvantage. Native ruderal species share many traits with B. tectorum and could potentially compete with this invader if used at high, competitive seeding rates in restoration efforts, and may alter site characteristics in ways that promote succession of the native plant community. One key characteristic that may be closely associated with community development is the development and composition of the arbuscular mycorrhizal fungi (AMF) community. AMF are important for resource acquisition by a majority of plant species. They are particularly important for late-seral plant species, which typically exist in low available nutrient conditions. Bromus tectorum causes shifts in the mycorrhizal community that could lead to a loss of AMF species richness and abundance in a very short time period, resulting in conditions that are difficult for late-seral species to colonize, due to a lack of access to resources through host-specific plant-AMF relationships. Utilizing native seed mixes composed of species selected for specific functional and competitive traits, and mycorrhizal status, and creating seeding rates designed to increase interspecific competition with B. tectorum may provide the missing link for successful restoration of B. tectorum-invaded sites. A study was conducted in northern Colorado to determine whether native ruderal species could suppress B. tectorum establishment and persistence in a disturbed site, and how these effects compared to similar effects by sterile wheat. In this study, B. tectorum was seeded with and without a high rate native ruderal seed mix and a sterile wheat species (QuickGuardTM) used in revegetation efforts for erosion control. Bromus tectorum biomass and density data were collected, as well as biomass and density for all seeded native species and sterile wheat. All treatments were seeded in the fall of 2010, and vegetative data collection occurred during the summers of 2011 and 2012. To assess the immediate effects of the establishing plant community on the AMF community, soils were collected from three of the field study treatments after one year of growth: 1) B. tectorum, 2) a mixture of native early-seral species, or 3) B. tectorum plus native early-seral species. Three mycorrhizal host plant species (Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths, Ratibida columnifera (Nutt.) Woot. & Standl., Sorghum bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet & Harlan) were grown in these soils under greenhouse conditions. Roots were harvested after 30 days and analyzed for AMF colonization. After one growing season, the native ruderal mix significantly reduced B. tectorum biomass in the field. After the second growing season, the effect was no longer detectable in biomass measurements, but was still observed as a reduction in density of B. tectorum in the native ruderal plots. The sterile wheat reduced B. tectorum biomass after one growing season, but to a lesser extent than the native mix, and had a positive effect on density. In 2012, effects on both biomass and density disappeared. Results from the AMF colonization analysis indicated that the presence of the native species had an impact on AMF richness or abundance within the soils and that B. tectorum alters the AMF community in a way that is unique in comparison to weedy native vegetation. Soils from beneath native early-seral plant species had much higher colonization of the host plant species relative to soils from beneath B. tectorum. In addition, the native host R. columnifera had much higher rates of colonization than the non-native host, S. bicolor, indicating that there may be some host-dependent plant-AMF relationships that are more beneficial to the native plant than the non-native plant. The results of these studies could have important practical field applications for restoring invaded sites, particularly when the goal is to create conditions that promote development of late-seral native plant communities. Utilization of native ruderal species in revegetation mixes could provide a critical missing link for facilitation of late-seral, native plant communities through suppression of B. tectorum, as well as rapid facilitation of AMF communities that successfully colonize native late-seral host species. Continued monitoring and assessment of this study site could lend further insight to the long-term dynamics of the native ruderal plant community with B. tectorum and development of a late-seral plant community.


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