Date of Award:

5-2015

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Wildland Resources

Committee Chair(s)

Kari E. Veblen

Committee

Kari E. Veblen

Committee

Thomas A. Monaco

Committee

Peter B. Adler

Abstract

As environmental stress increases, positive plant interactions dominate, resulting in clumped, plant-plant spatial patterns. Positive plant-plant interactions have received more attention in recent years due to their importance to the structure and function of arid ecosystems. Certain plants (nurses), provide benefits that help neighboring plants grow or survive.

I investigated spatial associations between Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) and three native grasses (Poa secunda, Elymus elymoides, and Pseudoroegnaria spicata) across a rainfall gradient in the Great Basin, USA. We also explored the effect of grazing on grass-shrub spatial patterns. I hypothesized that positive grass-shrub spatial associations would become more frequent at lower rainfall levels; we also hypothesized that : 1) at intermediate levels of stress (i.e. low rainfall, low grazing intensity conditions or high rainfall, high grazing intensity conditions), positive grass-shrub spatial associations would become more frequent and 2) at extreme levels of stress (low rainfall, high grazing intensity conditions), positive grass-shrub spatial associations and interactions would no longer dominate. At extreme levels of moisture stress, the addition of high grazing stress may cause positive interactions to break down.

I sampled perennial grass cover, density, height, basal width, grazing status, and reproduction in sagebrush canopies and interspaces at 32 sites over a rainfall gradient. I found that cover of P. secunda was higher in canopy microsites than interspaces at low to moderate levels of rainfall. Cover and density of E. elymoides were higher in sagebrush canopies at almost all rainfall levels. Both E. elymoides and P. spicata plants were taller, narrower, and were less likely to be grazed in canopy microsites.

To investigate the effects of grazing, I sampled 5 site pairs over a rainfall gradient. I did not find any significant effects of grazing, consistent with the results of other studies. For P. secunda, shrubs were more facilitative at low rainfall compared to high rainfall sites, regardless of grazing intensity. E. elymoides was facilitated regardless of rainfall level or grazing intensity

Plant spatial associations can indicate which types of microsites are favorable for plant growth, which could in turn help improve seeding or planting success during ecological restoration of the Great Basin. My results suggest that using sagebrush canopy microsites for restoration of native perennial grasses would likely improve plant establishment, growth, especially in drier areas.

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