Date of Award:

5-2009

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Wildland Resources

Committee Chair(s)

Roger E. Banner

Committee

Roger E. Banner

Committee

Thomas A. Monaco

Committee

Steven A. Dewey

Abstract

Crested wheatgrass (Agropyron cristatum [L.] Gaertn.) is one of the most commonly seeded grass species in the western United States and dominates thousands of hectares in the Great Basin. Although many degraded Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) plant communities have been seeded with crested wheatgrass, successional pathways, influence of soil attributes, and cultivation history on the vegetation of these communities have not been fully characterized. I sought to identify community phases, vegetative differences, and soil attributes that explain variation among 35 Wyoming big sagebrush communities historically seeded with crested wheatgrass. All communities were more than 30 years old and had not experienced fire, or received subsequent chemical or mechanical treatments following their original seeding. Species richness, diversity, vegetation cover, and soil samples were measured in four 20 x 5 m intensive Modified Whittaker plots per community. Hierarchical clustering and principal component analysis of three indicator species (crested wheatgrass, Sandberg bluegrass, and Wyoming big sagebrush) identified four distinct community phases. Community phase 1 was dominated by crested wheatgrass and had the lowest species richness and cover of big sagebrush. Phases 2 and 3 had the highest species richness and cover of native species. Phase 4 was dominated by big sagebrush and had the lowest cover of crested wheatgrass. Community phases differed significantly for soil texture, soil nitrogen, and ground cover characteristics. Bare soil was almost double on loam-textured soils and rock cover was higher on clay loam texture soils (P < 0.05) as well as native plant cover. Communities previously cropped occurred on more coarse-textured soils and had 6-fold lower native species cover and double exotic herbaceous and crested wheatgrass cover. Cropping occurred on favorable, low rock, fine-texture soils, the same soils that favor crested wheatgrass production and reduce resilience of native plant composition. Delineation of community phases provided a new, empirically based state-and-transition model, while the characterization of soil attributes and disturbance history provided information about feedback mechanisms influencing dominant species that delineate community phases and effect community structure. This information can be used to assist in the development of management strategies in crested wheatgrass seeded communities.

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