Award Date

December 2015

Degree Type


Degree Name

Master of Science (MS)


Biological Science

First Committee Member

Lawrence R. Walker

Second Committee Member

Todd C. Esque

Third Committee Member

Lloyd R. Stark

Fourth Committee Member

Daniel B. Thompson

Fifth Committee Member

Stephen M. Rowland

Number of Pages



Facilitation and competition among plant species, in addition to abiotic factors, play an important role in determining plant community structure in arid and semi-arid environments. I conducted a study in Dry Lake Valley, Lincoln County, Nevada, USA to investigate the importance of nurse plant associations to pre-reproductive Yucca brevifolia (Joshua tree). Dry Lake Valley lies within a transition desert between the Mojave and Great Basin Deserts with ecotonal plant communities consisting of species representative of both deserts. A vegetation survey described the communities in which this study was conducted as a Y. brevifolia woodland dominated by an understory of Ephedra nevadensis (mormon tea) and Grayia spinosa (spiny hopsage). Pairings of adult G. spinosa and pre-reproductive Y. brevifolia were selected as part of a removal field experiment to determine whether interactions between these species were facilitative or competitive and if continued growth of Y. brevifolia would result in the eventual exclusion of G. spinosa. Another survey investigated whether desert shrubs provide pre-reproductive Y. brevifolia protection from herbivores through associational resistance. Results did not indicate the occurrence of direct competition or facilitation between Y. brevifolia and G. spinosa. However, there was evidence that Y. brevifolia experience reduced herbivory damage by growing in proximity to desert shrubs. This study demonstrated that pre-reproductive Y. brevifolia are indirectly facilitated by nurse plants through associational resistance. This study also demonstrated the need to better understand the ontogeny of Y. brevifolia as it relates to soil niche development, and in turn, its interactions with its benefactor species.


association; competition; facilitation; herbivory; nurse plant; Yucca brevifolia


Biology | Botany | Environmental Sciences | Terrestrial and Aquatic Ecology

File Format


Degree Grantor

University of Nevada, Las Vegas




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