Assessing Plant Community Structure in the Upper Las Vegas Wash Conservation Transfer Area

237 Pages·2017·6.86 MB·English

by  Amy A. Croft

UUttaahh SSttaattee UUnniivveerrssiittyy
DDiiggiittaallCCoommmmoonnss@@UUSSUU
All Graduate Theses and Dissertations Graduate Studies
5-2016
AAsssseessssiinngg PPllaanntt CCoommmmuunniittyy SSttrruuccttuurree iinn tthhee UUppppeerr LLaass VVeeggaass
WWaasshh CCoonnsseerrvvaattiioonn TTrraannssffeerr AArreeaa,, NNeevvaaddaa:: TThhee IInnflfluueennccee ooff BBiioottiicc
aanndd AAbbiioottiicc VVaarriiaabblleess
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
RReeccoommmmeennddeedd CCiittaattiioonn
Croft, Amy A., "Assessing Plant Community Structure in the Upper Las Vegas Wash Conservation Transfer
Area, Nevada: The Influence of Biotic and Abiotic Variables" (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 (I ) and standardized Morisita index (I ) results by
d p
size class.................................................................................................................82

3.5 Morisita’s index (I ) and standardized Morisita index (I ) results by
d p
soil map units .........................................................................................................84

3.6 Morisita’s index (I ) and standardized Morisita index (I ) results by
d p
vegetation association ............................................................................................86

3.7 Morisita’s index (I ) and standardized Morisita index (I ) results by
d p
geologic unit...........................................................................................................88

3.8 L-funtion results for Transect1 ..............................................................................90

3.9 L-funtion results for Transect 2 .............................................................................91

3.10 Morisita’s index (I ) and standardized Morisita index (I ) overall and
d p
size class results for the transect data ....................................................................92

4.1 Vegetation associations of the ULVWCTA ........................................................123

4.2 Soil map units of the ULVWCTA .......................................................................124

4.3 Geologic units of the ULVWCTA .......................................................................125