Utah State University DigitalCommons@USU All Graduate Theses and Dissertations Graduate Studies 5-2016 Assessing Plant Community Structure in the Upper Las Vegas Wash Conservation Transfer Area, Nevada: The Influence of Biotic and Abiotic Variables Amy A. Croft Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/etd Part of the Ecology and Evolutionary Biology Commons Recommended Citation Croft, Amy A., "Assessing Plant Community Structure in the Upper Las Vegas Wash Conservation Transfer Area, Nevada: The Influence of Biotic and Abiotic ariables"V (2016). All Graduate Theses and Dissertations. 5005. https://digitalcommons.usu.edu/etd/5005 This Dissertation is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. ASSESSING PLANT COMMUNITY STRUCTURE IN THE UPPER LAS VEGAS WASH CONSERVATION TRANSFER AREA, NEVADA: THE INFLUENCE OF BIOTIC AND ABIOTIC VARIABLES by Amy A. Croft A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Ecology Approved: _________________________ _________________________ James A. MacMahon Janis L. Boettinger Major Professor Committee Member _________________________ _________________________ Thomas C. Edwards, Jr. Ethan P. White Committee Member Committee Member _________________________ _________________________ Eugene W. Schupp Mark R. McLellan Committee Member Vice President for Research and Dean of the School of Graduate Studies UTAH STATE UNIVERSITY Logan, Utah 2016 ii Copyright © Amy A. Croft 2016 All Rights Reserved iii ABSTRACT Assessing Plant Community Structure in the Upper Las Vegas Wash Conservation Transfer Area, Nevada: The Influence of Biotic and Abiotic Variables by Amy A. Croft, Doctor of Philosophy Utah State University, 2016 Major Professor: James A. MacMahon Department: Biology Ecological communities are complex, the structure of which is composed of interactions between multiple community characteristics and the abiotic and biotic factors shaping them. Because of this complexity, ecological studies are generally limited in scope and size, often dissecting communities into their component parts to examine them piece by piece. While this might be the most practical method to study communities, this approach often neglects other characteristics that, with their inclusion, would provide a more complete picture of community ecology. The studies described in this dissertation were conducted in an effort to synthesize the complexity that is inherent in ecological plant communities growing on a Mojave Desert bajada. Each study addresses a separate component of community structure, which, taken as a whole, provides a more thorough understanding of arid plant community dynamics. Overall, our results reveal the importance of substrate variables and their role in shaping plant community structure in arid environments. In addition, these investigations provide evidence of the strong role iv that facilitation plays on this bajada and possibly arid plant communities as a whole. The comprehensive approach described in this dissertation will enable ecologists to gain a more complete understanding of community dynamics and apply this knowledge to various climate change and land management scenarios. (236 pages) v PUBLIC ABSTRACT Assessing Plant Community Structure in the Upper Las Vegas Wash Conservation Transfer Area, Nevada: The Influence of Biotic and Abiotic Variables by Amy A. Croft By nature, ecological communities are complex. Communities are often composed of many different interacting species which, together, are influenced by the environment. Ecologists tend to study communities by breaking them into smaller parts and studying them piece by piece. While this might be the most practical method to study communities, this approach doesn’t provide a complete working picture of the entire community. The purpose of this dissertation was to investigate multiple working parts of plant communities growing in a portion of the Mojave Desert and integrate the studies for a greater understanding of how the communities function as a whole. Overall, our results show that soil properties have a strong influence on the amount of water that is available for plants to grow. This, in turn, has a large effect on plant size, how many plant species are present in one area, and how the plants are arranged on the landscape. Having a greater understanding of ecological communities as a whole will help scientists and land managers make appropriate decisions in the face of rapid urbanization and climate change. vi ACKNOWLEDGMENTS To begin, I would like to thank my major advisor, Dr. Jim MacMahon, for his continued support, patience, and guidance throughout this project. I would also like to express my appreciation to the members of my committee, Drs. Thomas C. Edwards, Janis L. Boettinger, Ethan P. White, and Eugene W. Schupp. I also thank Dr. Ron Ryel, who served as one of my initial committee members. They all provided valuable feedback and direction during the course of this study. This research was supported by the USDA Bureau of Land Management and the USU Ecology Center. Special thanks to Gayle Marrs-Smith at the Las Vegas Field Office for field assistance. I would like to extend special thanks to the many people who helped collect field data for this project, especially my fellow students, Dr. Lori Spears, Jesse Walker, and Dr. Mary Pendergast and members of the Boettinger and Edwards labs. Statistical direction was provided by Susan Durham and Drs. Jari Oksanen, David Roberts, Ethan White, and Andrew Rayburn. I would also like to recognize the USU Ecology Center and Biology Department for financial support. I have received a tremendous amount of personal support from many dear friends, family members, and the USU Biology Department faculty and staff. You have all provided constant reassurance and encouragement for which I am forever grateful. I extend much appreciation to my parents, Brent and Lori Howa, for their continued support in my education. Thank you to the MacMahon lab for your constant feedback and humor. vii Last but not least, many thanks to my husband, Scott Croft. Scott was exceptionally patient while I found my way through this project. I am deeply grateful for his continued love and support. My son, Benjamin Croft, provided constant light and encouragement. I hope this dissertation serves as a reminder that it takes hard work and dedication to reach your goals. Amy A. Croft viii CONTENTS Page ABSTRACT ...................................................................................................................... iii ACKNOWLEDGMENTS ................................................................................................. vi LIST OF TABLES ............................................................................................................. ix LIST OF FIGURES ........................................................................................................... xi CHAPTERS 1. INTRODUCTION ............................................................................................1 2. ABIOTIC INFLUENCES AND PLANT COMMUNITIES ON A MOJAVE DESERT BAJADA: A MULTIVARIATE ANALYSIS ......................................................................................................14 3. ABIOTIC INFLUENCE ON COMPETITION AND FACILITATION ON A MOJAVE DESERT BAJADA: A SPATIAL PATTERN ANALYSIS .................................................................46 4. BIOMASS AND ABUNDANCE PATTERNS AMONG PLANT COMMUNITIES IN THE MOJAVE DESERT: EXAMINING THE ROLE OF SELF-THINNING .........................................94 5. SUMMARY ....................................................................................................131 APPENDICES .................................................................................................................136 CURRICULUM VITAE ..................................................................................................219 ix LIST OF TABLES Table Page 2.1 Vegetation associations of the ULVWCTA ..........................................................36 2.2 Soil map units of the ULVWCTA .........................................................................37 2.3 Geologic units of the ULVWCTA .........................................................................38 2.4 Three-dimensional NMDS results .........................................................................39 2.5 CCA results ............................................................................................................41 3.1 Vegetation associations of the ULVWCTA ..........................................................79 3.2 Soil map units of the ULVWCTA .........................................................................80 3.3 Geologic units of the ULVWCTA .........................................................................81 3.4 Morisita’s index (Id) and standardized Morisita index (Ip) results by size class.................................................................................................................82 3.5 Morisita’s index (Id) and standardized Morisita index (Ip) results by soil map units .........................................................................................................84 3.6 Morisita’s