Regeneration Dynamics in Response to Slash-And-Burn Agriculture in a Tropical
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AN ABSTRACT OF THE THESIS OF Daniela Roth for the degree of Master of Science in Rangeland Resources presented on January 5, 1996. Title: Regeneration Dynamics in Response to Slash-and-Burn Agriculture in a Tropical Deciduous Forest in Western Mexico. Redacted for Privacy Abstract approved: Patricia M. Miller Few studies have focused on the response of woody vegetation and seed banks to slash-and-burn agriculture in tropical deciduous forest ecosystems. To bridge this gap in knowledge, the following research project was undertaken. Pasture conversion fires killed 87% of the sampled woody plants that had sprouted following slashing of a tropical deciduous forest. Species richness was reduced by 40%. Prior to burning 96% of all sampled plants sprouted from above-ground tissues. Post-burn 98% of the survivors sprouted from below-ground tissues. During the first year following pasture conversion, survival of sprouting plants was high, 91%. Average height, elliptical crown area, and total area covered by sprouting plants tripled, average crown volume increased over 500% within a year. To document impacts on species composition, density, and structure prior to and following pasture conversion, transects were established in the intact forest in 1992 and following pasture conversion in 1994. Additional transects were established in an adjacent 10-year old pasture. The intact forest had an average density of 3115 ± 137 (3T± 1 SE) woody plants/ha (DBH z 2.5 cm), containing 59 species. Following pasture conversion the average density of woody plants/ha (stump diameter 1.0 cm) was 1338 ± 133. Thirty-six species were identified. The ten year old pasture had an average density of 1592 ± 105 woody plants/ha, containing 32 species. The average area covered by woody plants per 100 m2 transect sampled, as well as average elliptical crown area, crown volume, and height per plant were all lower at the new 1993 pasture than at the older one. Burning reduced the germinable seed density from 1003 seeds/m2 to 70 seeds/m2 immediately following the fires. Seventy-one percent of the germinated seeds from pre-fire soils were annual and perennial herbs, 18.4% were monocots, and 10.2% were trees, shrubs, and vines. Germination from post-fire soil sampling included a majority of herbs, and no monocots and or trees. One year following pasture conversion 3503 seeds/m2 germinated from the sampled soils, 96.9% of the total seed pool were monocots, 3.1% of the germinated seed pool were dicots. Germination from the adjacent intact forest soils sampled in 1994 produced a total of 703 seeds/m2. Life form distribution was similar to 1993 slashed forest distribution. °Daniela Roth January 5, 1996 All Rights Reserved REGENERATION DYNAMICS IN RESPONSE TO SLASH-AND-BURN AGRICULTURE IN A TROPICAL DECIDUOUS FOREST OF WESTERN MEXICO by Daniela Roth A THESIS submitted to Oregon State University In partial fulfillment of the requirements for the degree of Master of Science Completed January 5, 1996 Commencement June 1996 Master of Science thesis of Daniela Roth presented on January 5, 1996. APPROVED: Redacted for Privacy Major Professor, representing Rangeland Resources Redacted for Privacy Head of Department of Rangela Resources Redacted for Privacy Dean of Graduat chool I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Redacted for Privacy Daniela Roth, Author ACKNOWLEDGEMENTS The completion of this project took me over three years of my life. It has been one of the hardest things I have ever done. It would have never come to pass without the contributions of the following people. I would like to thank my major professor, Dr. Patsy Miller, for being my adviser and taking over when things got rough. I am also indebted to Dr. W.C. Krueger and the Dean of the Graduate School, Dr. Tom Maresh for their staunch support of my work. Without their help this thesis would have never been completed. Thanks also to my graduate representative Diane Carroll, and to my committee members, Dr. J. Kummerov, for his wisdom and insights, and Dr. D. Pyke for his thorough reviews of my drafts. Funding for this study came in part from student loans and in part from the National Science Foundation as part ofa larger three-year research project in cooperation with Universidad National Autonoma de Mexico (UNAM) and the University of Denver, Colorado. I like to thank the principal investigators Drs. Robert Sanford, Victor Jaramillo, and J.B. Kauffman for funding parts of my project through their grant. It was a growing experience. I am deeply indebted to Dr. J.B. Kauffman for setting an example of how not to do it, and for opening my eyes to what's going on behind closed doors. To some people scientific integrity is just a word. I like to thank my fellow students and friends Richard Case, Elmar deCastro, Barbara Lytjen, and and Ron Shea, Mack Barrington, Heather Miller, Joe Washington, Pat Dysart, Dana Nick Otting for their companionship and many spirited discussions. through my I would like to acknowledge my mother, Irmgard Roth, whose blood flows what. My veins, giving me the strength to stand up for what I know to be the truth, no matter takes no daughter, Freya Mariana Majalis Danieladotir, continuing in this line of spirited womym, I thank her for coming into being small part in the existence and completion of this project. don't know how through me, giving me the grounding power I needed to return to school. You with a constantly much you helped by being an exceptional child, showing incredible patience stressed out and broke mom, that hardly ever had any time to spare. Not to forget, much of my gratitude goes to her father, Olafur Brentmar, for being such an excellent and patient dad. Your love and support took a load of my back, especially in times when everything else around me seemed to collapse. Thanks to my sisters in arms, Maureen Mac Donell, Corinne Hammet, Jimi Vickers, Ann Brentmar, and Kat Beal for lending me a hand when I was in danger of drowning in the academic swamps. I would like to thank Senor Ramiro Pena for letting us work on his land and for showing so much patience when the scientific process collided with local agricultural practices. Thanks also to all the people at the biological station at Chamela, Mexico, for their help and support of this gringo project. Special thanks go to the cook, Senora Helena, whose love of people transcends racial and cultural boundaries. Your smile brightened up many a dark day. Thanks to Dr. Emily J. Lott who identified almost all the herbarium vouchers send to her from the field, and many thanks to Arturo Solis-Magallanes who helped with much of the identification of plants in the field. Without the help of these wonderful taxonomists this study as it is would not have been possible. Thanks for teaching me how to identify plants without a taxonomic key! At last I owe my heartfelt thanks to Pedro C. Gonzalez-Flores, without whom this project might have killed me. His loving support, no matter what, his relentless help during all days of treacherous field work battling fires, hurricanes, killer bees, angry brahman bulls, and many other demonic intrusions that combined to fight the completion of this project, as well as his patience in teaching me spreadsheet magic made this project possible beyond anything else. No words can express my gratitude. To my grandfather, Karl Mohler, who taught me love and respect for nature TABLE OF CONTENTS Page 1. INTRODUCTION 1 2. EFFECTS OF CUTTING AND BURNING ON THE SPROUTING CAPACITY OF WOODY SPECIES IN A TROPICAL DECIDUOUS FOREST OF WESTERN MEXICO 3 Abstract 4 Introduction and Literature Review 5 Site Description 7 Methods 9 Experimental Design 9 Field Procedures 10 Data Management 11 Results 13 Pre- and post-fire 1993 13 Pre-fire April, 1993 13 Post-fire August, 1993 16 Post-fire growth period August 1993 August 1994 19 Overall Survival 19 Growth of all plants combined 21 Survival of abundant species 25 Growth of the abundant species 26 Discussion 32 Conclusion 35 Literature cited 36 Appendices 39 Appendix A. Family and species distribution of 456 sprouting plants in eight experimental plots following slashing of a tropical deciduous forest in western Mexico, April, 1993 40 Appendix B. Average growth values of 456 sprouting plants in eight experimental plots of a slashed tropical deciduous forest in western Mexico, March-April, 1993 48 TABLE OF CONTENTS (continued) Page Appendix C. Family and species distribution in eight experimental plots following slash fires in a tropical deciduous forest in western Mexico, August, 1993 49 3. INFLUENCES OF SLASH-AND-BURN AGRICULTURE ON REGENERATION PROCESSES AND PLANT DIVERSITY IN A TROPICAL DECIDUOUS FOREST OF WESTERN MEXICO . 56 Abstract 57 Introduction and Literature Review 58 Site Description 61 Methods 63 Experimental Design 63 Field Procedures 64 Intact forest species composition, density, and structure 64 Woody plant composition, density, and structure following pasture conversion 64 Data Management 65 Results 65 Intact forest composition, density, and structure 65 Woody plant composition, density, and structure following pasture conversion 67 Intact forest vs. pasture comparisons 73 Discussion 74 Impacts on species diversity 74 Woody plant regeneration and density 75 Conclusion 77 Literature cited 79 Appendices 82 Appendix A. Woody plant families and species present prior to pasture conversion of an intact tropical deciduous forest during December, 1992. 83 Appendix B. Woody plant families and species present 1.5 years following pasture conversion of a tropical deciduous forest in western Mexico, August, 1994.