The Relative Importance of Abiotic and Biotic Factors for Seedling Establishment in the Colorado Desert, CA DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Natasha Nicole Woods Graduate Program in Evolution, Ecology and Organismal Biology The Ohio State University 2015 Dissertation Committee: Maria N. Miriti, Advisor Peter S. Curtis Kristin L. Mercer Copyright by Natasha Nicole Woods 2015 Abstract Drought combined with warmer temperatures has caused extensive mortality in US southwest desert ecosystems. Disproportional mortality among long-lived shrub species can have dire consequences for community regeneration if novel recruitment is dependent on facilitated establishment. However, regeneration niches can be under biotic or abiotic control. My dissertation research examines mechanisms that control juvenile recruitment in the Colorado Desert, CA. Specifically I examine, seed and seedling responses to resources by partitioning those resources that are under direct biotic control based on the production and consumption patterns of standing vegetation, and those that are under abiotic control due to the effects of temperature and precipitation. By examining the relative importance of abiotic and biotic factors that affect plant recruitment, I can anticipate community stability in the face of natural or anthropogenic disturbance and if necessary offer strategies to mitigate species loss by identification of relevant variables to promote recruitment of target species. In chapter 1 I characterize the spatial and temporal distribution of nitrogen for three years at a long-term study site in the Colorado Desert. In chapter 2, I examine the capacity for dominant shrubs to serve as plant facilitators for seed establishment and use a greenhouse experiment to identify the distinct roles of light and nitrogen on seed ii germination. In a third experiment, I examine the roles of neighboring species identity, frequency and density on seedling performance when water and light are limiting. I found that each focal species distinctly influenced nitrogen availability, which is generally higher near plant canopies. I also found higher levels of nitrogen under Ambrosia dumosa compared to the other focal species and the interspace. These results were highly dependent upon season and year (Chapter 1). Adult shrubs did not uniquely impact germination but beneficiary species identity was the main factor for germination. In the greenhouse, differences in percent germination were strongest at full light, and canopy light and nitrogen levels did not affect beneficiary germination (Chapter 2). Using average rainfall, temperature and nitrogen concentrations from canopy microhabitats of Ambrosia dumosa from 2011-2012, I found that competition influenced early performance of A. dumosa, and Eriogonum fasciculatum, but not Larrea tridentata. Reduced growth of L. tridentata seedlings was not attributable to competition (Chapter 3). Given these results, seed germination is not strongly regulated by abiotic or biotic factors; however, shrub canopies may be important for seedling growth and survival. If seedling persistence responds to nitrogen resources, A. dumosa may be instrumental in facilitating the establishment of perennial species. Nevertheless, all facilitators are important for providing increased heterogeneity in the landscape, which may increase regeneration niches for species not examined in my research. Nitrogen levels under shrub control would diminish without shrubs and subsequently, regeneration niches iii would also decrease. However, relative competitive abilities can influence future community composition at the study site. iv This document is dedicated to my brother, Lawrence C. Woods, my grandmothers, Mary L. Terry and Edna Mae Yates, and my brother-in-law Anthony Hardy who are no longer here to see me complete this goal. v Acknowledgements I would like to thank my advisor, Maria Miriti for her unfailing support since I moved to Columbus. Maria has not only trained me in plant community ecology but has supported me personally as well. Maria has always challenged me to think and read broadly. She is my role model and I hope that I can invest in other students all that she has invested in me. I would like to thank my committee members Peter Curtis and Kristin Mercer. I would like to thank Peter for allowing me to barge into his office without an appointment to discuss my research projects and for providing useful feedback on my manuscript. I would like to thank Kristin for inviting me to her lab meetings while Maria was on sabbatical and helping me with some of the initial experimental design for my field work. I also would like to thank Ralph Boerner for serving on my candidacy committee and teaching me how to do soil nitrogen extractions. I am grateful for the help of my lab mates, S. Rodriguez-Buriticá, C. Pieterson, T. Ibrahim and R. McCarthy who faithfully read and provided useful feedback on all of my chapters and/or supported me with my field work. I would also like to thank my work study students and volunteers T. Catrett, E. Salcido, M. Perry, N. Kim, and C. Etiel, for all of the work they helped me complete in the lab. I would especially, like to thank T. Catrett for all of his assistance inputting data and helping me with an untold number of vi nitrogen extractions. I would also like to thank my fellow EEOB graduate students who have helped me during the course of this research through loaning me necessary equipment or by reading my manuscripts, C. Lin (Chia), H. Chang (Chi), A. Fotis, K. Calinger-Yoak, R. Ricart, and K. Knight. This dissertation would not have been possible without support and funding from a number of different sources. This work was made possible by The Evolution, Ecology and Organismal Biology Department at The Ohio State University, and grants from The Desert Legacy Fund, (The Community Foundation Serving Riverside and San Bernardino Counties), the Robert E. Lee Graduate Student Grant (Lee Family Foundation and Joshua Tree National Park Association), Graduate Student Grant (Alumni Grants for Graduate Research and Scholarship, The Ohio State University) and Professional Development/Research Grant (Bells National Research Center, The Ohio State University). I would also like to thank Dr. Valerie Lee, Dr. James Moore III, Dr. Samuel Hodge, and Dr. Robert Bennett for all of their support during my dissertation. I would like to thank M. Umbel, M. Gerring and E. Zehr for being GREAT friends during my time in Columbus. I would also like to thank all of the members of University Baptist Church for all of their support and care. I would not have known that I could have a career in research had it not been for my advisor from my Master’s Degree program, Dr. LaJoyce Debro. Dr. Debro has been a friend and confidant for over ten years. I am grateful for her friendship. I would not be here without the support of my family and friends from Alabama. My dear family and friends, T. Christian, A. F. Christian, T. Toyer, K. Ward, G. Childs, vii J. Johnson, S. Smith, G. Webb, and J. Martin have supported me though this entire process. I would also like to thank my nieces, J. Hardy, L. Hardy, M. Toyer (my adopted niece), my nephew L. Hardy and my goddaughter (L. Ward). When the days got hard I thought of you and pushed on. Thank you for your encouragement. To my Mom and siblings, Frederica, Frederick and Lazarus, I am at a loss for words when it comes to expressing my appreciation for all of the prayers, the emotional and financial support you all have given me throughout my life. This is not just my accomplishment, it is ours. I thank God for all of you and for allowing me to complete in this awesome journey. viii Vita 1996………………………………………...Anniston High School 2001………………………………………...B.S. Biology, Jacksonville State University 2003………………………………………...M.S. Biology, Jacksonville State University 2003-2004…………………………………..Research Assistant, University of Alabama at Birmingham 2004-2008…………………………………...Laboratory Educator, Spelman College 2008 to present ..............................................Graduate Teaching Associate, Evolution Ecology and Organismal Biology, The Ohio State University Fields of Study Major Field: Evolution, Ecology and Organismal Biology ix Table of Contents Abstract ............................................................................................................................... ii Acknowledgements ............................................................................................................ vi Vita ..................................................................................................................................... ix Table of Contents ................................................................................................................ x List of Tables .................................................................................................................... xii List of Figures ................................................................................................................... xv INTRODUCTION .............................................................................................................. 1 Chapter 1: Shrub influences on nitrogen distributions in the Colorado Desert ............ 7 INTRODUCTION ........................................................................................................... 7 METHODS...................................................................................................................