Thesis (M.S.)--Brigham Young University. Dept. of Plant and Animal Sciences, 2006. / Includes bibliographical references (p. 14-21).

Thesis (M.S.)--Brigham Young University. Dept. of Integrative Biology, 2005. / Includes bibliographical references (leaves 20-24).

Herbivory and plant competition affect sexual reproduction of plants in various ways. Exclusion of mule deer (Odocoileus hemionus) and cattle, removal of plant competition (both inter- and intraspecific), and all combinations of the above treatments were used to examine the individual and combined affects on Artemisia tridentata ssp. wyomingensis (Wyoming big sagebrush) reproduction. Reproduction of Artemisia tridentata ssp. wyomingensis was divided into hierarchical levels of the number of: (1) modules per current-year ' s growth (CYG), (2) nodes per module, (3) inflorescence heads per node, (4) achenes per inflorescence head and (5) percent viable achenes. Counts at hierarchical levels were made to determine the level affected by the treatments. Deer herbivory significantly reduced reproduction at the reproductive-module-per-CYG-vegetative-biomass hierarchical level, while plant competition (both inter- and intraspecific) significantly reduced reproduction at the nodes-per-reproductive-module level and at the inflorescence-heads-per-node level. Cattle presence had neither a beneficial nor detrimental influence on reproduction during this two-year study. The combined effects of release from deer herbivory and from plant competition on reproduction was more than additive because these biotic interactions affected nested hierarchical levels.

Plant spatial patterns have been studied to gain insight into plant interactions such as competition and facilitation (positive plant interactions). The stress gradient hypothesis predicts that as environmental stress increases facilitation dominates, while competition dominates in less stressful conditions. Beneficial plants (nurses) can create favorable abiotic conditions for subanopy plants. Additionally, palatable herbaceous species growing under nurse shrub canopies benefit from physical protection. I investigated spatial associations between Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) and three native grasses (Poa secunda, Elymus elymoides, and Pseudoroegneria spicata) across a rainfall gradient in the Great Basin, USA. I also explored the effect of grazing on grass-shrub spatial associations. I hypothesized that positive shrub-grass spatial associations would become more frequent at lower rainfall levels; I further hypothesized that 1) at intermediate levels of stress, positive grass-shrub spatial associations would dominate and 2) at extreme levels of stress, positive grass-shrub spatial associations and interactions would no longer dominate. At high moisture stress, the addition of grazing stress may limit the nurse’s ability to provide to benefits to subcanopy plants. Cover of P. secunda was greater in shrub canopy microsites than interspaces at low to moderate levels of rainfall. Cover and density of E. elymoides were greater in sagebrush canopies over most rainfall levels. Elymus elymoides and P. spicata were taller and narrower in basal width and less likely to be grazed in canopy versus interspace microsites. I next investigated the effects of grazing intensity over a rainfall gradient and found a significant interaction of rainfall and microsite on P. secunda cover. Poa secunda formed positive interactions with A. tridentata at lower rainfall levels, regardless of grazing intensity. Its cover was significantly greater in interspaces at high rainfall compared to low rainfall sites. Elymus elymoides density was greater in canopy vs. interspace microsites, regardless of rainfall level or grazing intensity. Plant spatial associations can indicate which nurse microsites are favorable to plant growth and may improve seeding or planting success during ecological restoration. My results suggest that exploiting sagebrush canopy microsites for restoration of native perennial grasses would improve plant establishment, growth or survival particularly in drier areas.

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.

The sagebrush steppe is a particularly sensitive ecosystem that is easily disturbed by fires, oil and gas extraction, woody-plant encroachment, and overgrazing. The natural regeneration of native species following a disturbance within this system is typically slow and sporadic, which allows invasive grasses to occupy the landscape. Attempts to assist the recovery of these landscapes through direct seeding is commonly met with poor success rates, particularly in lower elevation, drier sites. Novel seed enhancement technologies and planting techniques that mitigate limiting factors impairing restoration efforts may improve the likelihood of restoring these degraded areas. For chapter 1, we evaluated a solid-matrix priming technique, where bluebunch wheatgrass (Pseudoroegneria spicata) and Lewis flax (Linum lewisii) were primed and then the priming matrix and seed were pelleted together. We evaluated primed seed that had been incorporated into pellets at two field sites against seed that was pelleted but been left unprimed, and untreated seed (control). These three seed treatments were planted in the spring (mid-march) in shallow (2-cm) and deep (15-cm) furrows, in a complete factorial design. We found that primed seeds generally produced higher plant densities than control seed at the beginning of the growing season; however, its influence diminished towards the end of the growing season. We also found that deep furrows increased plant density throughout the growing season and even into the following year. The combination of priming and deep furrows outperformed control seed in shallow furrows in all measured metrics. For chapter 2, we evaluated a seed conglomeration technique for improving Wyoming big sagebrush (Artemisia tridentata ssp. Wyomingensis) emergence and survival under fall and winter plantings. The trial was implemented at five sites across Utah and Nevada in a randomized complete block-split-split plot design, with site, and planting season, comprising the split-plot factors. Each site and season combination was seeded with conglomerated and control seed. We found that in most cases, a fall seeding of Wyoming big sagebrush was either the same or more successful compared to planting on the snow in the winter, which is the current suggested practice. Our results also demonstrated that seed conglomeration produced higher plant densities compared to control seed throughout the growing season. The higher density of plants produced from conglomerates combined with the improved seed delivery provided by the conglomeration technique was estimated to offset the cost in producing conglomerates and reduce overall restoration costs by 41%.