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Will Local or Commercial Native Plants Succeed Where Exotic Invaders Fail? Cheatgrass Die-offs as an Opportunity for Restoration in the Great Basin, USA
AdvisorLeger, Elizabeth A.
Natural Resources and Environmental Science
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The exotic annual <italic>Bromus tectorum</italic> (cheatgrass) commonly occurs in dense, near-monocultures in the Great Basin, U.S.A. after diverse native plant communities have been mostly extirpated. Efforts to reestablish native species via direct seeding, typically with commercially produced, non-local seeds, are often unsuccessful. In addition to abiotic factors that often limit establishment, <italic>B. tectorum</italic> competition can impede native establishment in highly invaded communities, and commercially produced seeds may differ from local genotypes in their responses to these limitations. The phenomenon of complete <italic>B. tectorum</italic> stand failure, or `die-off', can leave areas within near-monocultures devoid of growth for one or more years. Such areas may represent restoration opportunities if native seeds can establish within them. In October 2012, local and nonlocal (commercial) sources of <italic>Poa secunda</italic> (Sandberg bluegrass) and <italic>Elymus elymoides</italic> (bottlebrush squirreltail) were precision-planted in a recent die-off and adjacent near-monoculture (control) in northern Nevada under six treatments: litter removal, fungicide application, and no treatment; each with and without added simulated precipitation. I addressed the following questions: 1) Can native species be successfully restored in recent <italic>B. tectorum</italic> die-offs, and is establishment related to seedbed treatments and competition with <italic>B. tectorum</italic>? 2) Do local and nonlocal materials differ in their performance as restoration material, and if so, are these differences consistent across seedbed treatments and in and out of a recent die-off? Seeded plots were monitored nine times throughout two growing seasons for emergence and seedling activity, and seedling growth was measured in May of the first season. Emergence of native seeds was significantly lower in die-off plots, but there were significantly more actively growing seedlings of both species in die-off plots than in adjacent control plots by the end of the first growing season, and seedlings in the die-off exhibited more leaves, increase late season vigor, and, for <italic>E. elymoides</italic>, increased height. Second year survival patterns also suggested greater establishment in die-off plots. Local <italic>P. secunda</italic> demonstrated improved performance over nonlocal material (`Mt. Home' germplasm) in both seasons, whereas nonlocal <italic>E. elymoides</italic> (`Toe Jam Creek' germplasm) demonstrated aspects of higher performance than the local collection in the first year but not the second. Litter removal had a positive influence on seedling activity for both species, but only affected the survival of <italic>P. secunda</italic>. Late autumn (early growing season) water addition affected emergence timing for both species and resulted in more <italic>E. elymoides</italic> seedlings, but this affect was equal across controls and die-offs. Although these results are representative of conditions at only one site, they suggest that <italic>B. tectorum</italic> die-off may support increased establishment of native species regardless of seedbed treatment, and may therefore represent valuable opportunities for restoration. Also, they indicate that local and nonlocal seeds differ in performance in important but idiosyncratic ways, which supports the belief that seed source should be considered as a factor affecting restoration success but contradicts the idea of generalizable local vs. nonlocal performance patterns.