Graduate Thesis Or Dissertation


Associations Between Crested Wheatgrass and Native Vegetation in Southeastern Oregon : An Investigation of Co-establishment, Environment, Seeding History, and Livestock management Public Deposited

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  • Crested wheatgrass (Agropyron cristatum [L] Gaertm), an introduced bunchgrass, has been seeded on over 5 million hectares of degraded rangeland in western North America because it establishes more readily than native bunchgrasses. Because crested wheatgrass stands are associated with native species displacement and low biological diversity, there is substantial interest in re-establishing native species within seeded stands. However, efforts to reintroduce native grasses into crested wheatgrass stands have been largely unsuccessful, and little is known about the long-term dynamics of crested wheatgrass/native species mixes. This project was composed of two studies evaluating interactions between crested wheatgrass and associated native vegetation. In the first study, I examined the abundance of crested wheatgrass and seven native sagebrush steppe bunchgrasses which had been planted concurrently 13 years prior at equal densities. Thirteen years after planting, crested wheatgrass was the dominant bunchgrass with a ten-fold increase from its original planted density. Of the seven native bunchgrasses, four species: Idaho fescue (Festuca idahoensis Elmer); Thurber's needlegrass (Achnatherum thurberianum (Piper) Barkworth); basin wildrye (Leymus cinereus (Scribn. & Merr.) A. Löve) and Sandberg bluegrass (Poa secunda J. Presl) maintained their low planting density while three species: bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) A. Löve), needle-and-thread (Hesperostipa comata (Trin. & Rupr.) Barkworth), and squirreltail (Elymus elymoides (Raf.) Swezey), declined in density over the 13-year period. Results suggested that densities of native bunchgrasses planted concurrently with crested wheatgrass were unlikely to increase and that some species only persisted at low levels. However, the continued persistence of native bunchgrasses, even at low densities, suggests that co-planting of some native bunchgrasses may be a viable way of avoiding crested wheatgrass monocultures when this species is necessary for rehabilitation or restoration. In the second study, I sought to identify environmental, historic and livestock management factors associated with native vegetation occasionally found in seeded crested wheatgrass stands. Basal cover, density, species richness and diversity were measured in 2012 and 2013 on 121 sites previously seeded to crested wheatgrass in southeastern Oregon. Plant community composition of crested wheatgrass stands was variable; some of the seedings were monocultures of crested wheatgrass while others contained diverse native species. Functional group variability explained by environmental factors ranged from 0% of annual grass density to 56% of large native perennial bunchgrass density. Soil texture was significant and appeared to be an important environmental characteristic explaining functional group cover and density 10-50 years post seeding. Native vegetation was, for all functional groups, positively correlated with soils lower in sand content. Precipitation in the year following seeding of crested wheatgrass has long-term effects on plant community dynamics, especially for Wyoming big sagebrush. Higher precipitation in the year following crested wheatgrass seeding was associated with decreased shrubs, likely because crested wheatgrass seedlings were more successful and therefore sagebrush seedlings experience greater competition. Moderate grazing was associated with reduced crested wheatgrass monoculture characteristics relative to ungrazed sites. However, within spring grazed and spring-summer grazed sites, there was a negative relationship between increased stocking rate and native species cover and abundance. I speculate that was largely the effect of higher stocking rates being allotted to more productive crested wheatgrass seedings. Overall, my research suggested that pre-seeding treatment/disturbance on a site appears to have long-term implications for plant community dynamics. However, functional groups varied in response to different pre-seeding treatments. Results support the notion that crested wheatgrass is very competitive with native bunchgrasses in particular; and that introducing natives into crested wheatgrass stands may require high levels of disturbance and may be most successful in more finely-textured soils. The results of this study also suggest that management actions, both at the time of seeding and after seeding, can influence plant community characteristics.
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