BIOLOGICAL NITROGEN FIXATION IN TROPICAL DRY FORESTS OF COSTA RICA: PATTERNS AND CONTROLS A Dissertation SUBMITTED TO THE FACULTY OF UNIVERSITY OF MINNESOTA BY Maria Gabriela Gei IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Jennifer S. Powers, Adviser June 2014 © Copyright by Maria Gabriela Gei, June 2014. All rights reserved. i Acknowledgements It would not have been possible to develop this thesis without the support of my mentors, colleagues, friends and family. Above all, I would like to thank my advisor Jennifer Powers for her constant support and encouragement. She was present and deeply involved at every single step along this way but she also allowed and motivated me to work independently. She encouraged me to seek funding from numerous sources and gave me immediate advice whenever I asked for it. Most importantly, she constantly reminded me of the exciting, fun and collaborative aspects of science. Her friendship and mentoring were essential to the success of this research! I would also like to acknowledge the members of my thesis committee: Sasha Reed, Sarah Hobbie, Rebecca Montgomery, and Deborah Allan. They provided insightful advice during the last six years that significantly improved my work. I am thankful that they were willing to accommodate my frequent travel to meet with me. They enriched my research experience significantly by helping me think about my ideas from numerous angles. I consider myself extremely lucky and honored by having had the example, mentorship and the opportunity to work with these five amazing scientists! I spent two years living and developing this research at the heart of Área de Conservación Guanacaste (ACG) in northwestern Costa Rica. Although this period included a great amount of hard work, I have only the happiest memories thanks to the many wonderful people who made it easy and above all, fun. I am most grateful to Milena Gutiérrez, Roger Blanco, and María Marta Chavarría in ACG for making both my research permits a smooth and easy ride. I would have not been able to endure such a long time living in a remote area without the help and friendship of Daniel Pérez, our field lab manager. I admire and thank Daniel for everything he taught me about the dry forests and the people of Guanacaste. He and his kind family made me feel at home and warmly included me in their lives like another family member. Many other wonderful guanacastecos helped me perform the fieldwork of this research, and I am very grateful to Dinia, Tomás, Gato, Raúl, Ronald, and Saúl. Geohanny Hernández from the nearby community of Cuajiniquil helped me collect most of the soil samples I analyzed for this dissertation. There was not a single day when he didn’t smile, make jokes, or find the most positive angle of every kind of situation. I also thank my neighbors, Luis Cantillano and his family, for always keeping an eye on me in this isolated place. Being able to work and share my time with to members of the Powers Lab made this experience even more enriching. Not only I am very grateful for their company, friendship and support but also many conversations with them helped and improved the quality of my work. I had a great time with you in Minneapolis and Guanacaste: Cathy, Justin, Joey, Heather, Sasha, Lisa, Yana, Kristen, Leland, Christina, and Cristina. I feel very lucky for having met so many close and lifelong friends that made my time in Minnesota the most enjoyable, including Ana, Rodrigo, María, Alejandro, Juan, Chepe, and Michelle. Six years ago, I would have never imagined calling home a place that is so many degrees of latitude away from the tropics and I owe this in great part to my dearest ii friend Stephanie Lee. I will always thank her for sharing with me her contagious passion and perseverance at seizing and enjoying every single day, even the ones during winter. I would like to take a moment to remember the loved ones who only saw me begin this experience and hold a very special place in my heart. Thank you Luis Diego Gómez Pignataro and Jorge Gómez Laurito for introducing me to the beauty and greatness of plants and to the most hidden and beautiful corners of Costa Rica, and for encouraging me to let myself be led by the passion for knowledge. “Conmigo vienen los de atrás”: my Tita Flory has been one of the most influential women in my life. She was the most supportive about any projects I would undertake in pursuit of my happiness, including this one. I find much strength and inspiration in thinking about her, Tito, Nonno, and Nonna. The hill at the end of the climb to the tallest mountain in Costa Rica is so steep that is called “los arrepentidos (those who regret)”. But to me, the last hill of this dissertation was the easiest because I had the love and support of Scott, my beloved life partner and I will always be grateful for his patience and encouragement. Muchas gracias, Maga iii Dedication This dissertation is dedicated to my parents, Orlando and Gabriela, for their unconditional love, support and encouragement. iv Abstract In tropical forests, new nitrogen (N) inputs fuel a large proportion of global net primary productivity. However, global estimates of tropical N fixation are biased towards wet forests and other areas such as tropical dry forests are understudied. In the dry forests of Guanacaste, Costa Rica, N fixing legume trees are highly abundant throughout forest successional stages, thus I hypothesized that in tropical dry forests legume trees are critical regulators of ecosystem level N dynamics. I addressed this question from multiple approaches that included a shade house experiment and field surveys of N fixing legume trees in plantations or in diverse secondary forests using a common set of species: Acosmium panamense, Dalbergia retusa, Enterolobium cyclocarpum, Gliricidia sepium, and Lysiloma divaricatum. Individual legume species had measurable influences on a number of soil properties, but this effect is more pronounced than the influence of legumes as a functional group. I observed species-specific variation in belowground foraging strategies and in the timing and degree of nodulation. In the shade house experiment, species differed in their nodulation effort and in how they regulated N fixation with respect to available resources. These five legume species could be arrayed along a continuum defined by strategies of nutrient conservation and nutrient acquisition, which coincided with degrees of fine-tuning of N fixation. In the field study, I did not find evidence of down-regulation of fixation with soil N. I hypothesized that the adjustment of N fixation to soil nutrients occurs indirectly and is mediated by water availability and its effects on nutrient pulses. My stand-level estimates N fixation by legumes showed that legumes are responsible for the largest contribution of new N inputs v to this ecosystem relative to other inputs such as free-living fixation or wet deposition, but which are modest relative to N recycling through leaf litter and fine root decomposition. Different legume strategies could represent different ways of dealing with the transient and seasonal water availability of this ecosystem. Collectively, my results suggest that the conceptual models of how N fixation works in tropical wet forests may not necessarily be the same in seasonally dry forests. vi Table of Contents Acknowledgements...........................................................................................................i Table of Contents.............................................................................................................v List of Tables .................................................................................................................viii List of Figures ..................................................................................................................x Introduction......................................................................................................................1 Chapter 1: Do legumes and non-legumes tree species affect soil properties in unmanaged forests and plantations in Costa Rican dry forests?................................12 Summary................................................................................................................12 Introduction............................................................................................................13 Methods..................................................................................................................16 Results....................................................................................................................22 Discussion .............................................................................................................26 Conclusions............................................................................................................31 Acknowledgements................................................................................................32 Figure Legends ......................................................................................................33 Bibliography .........................................................................................................41 Chapter 2: Surface fine root stocks are influenced by species and seasonality in plantations of four tropical legumes..............................................................................49 Summary................................................................................................................49