THE EVOLUTION OF LEAF PHYSICAL DEFENSE IN THE SHADE OF A NEOTROPICAL FOREST By JARED W. WESTBROOK A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2009 1 © 2009 Jared W. Westbrook 2 To all those who paved the way to “tree thinking” 3 ACKNOWLEDGMENTS I thank Dr. Kaoru Kitajima for guidance at all stages of this project. Dr. Gordon Burleigh provided technical assistance with phylogenetic analyses and helpful comments in the preparation of this thesis. David Brasfield collected leaves, Eric Oriel measured leaf fracture toughness, and Mirna Sameniego ground the leaf samples. Dr. Karen Bjorndal generously allowed us to use her ANKOM fiber analyzer, and Alex Boulos and Kimberly Williams assisted me with the fiber analysis. I owe a debt of gratitude to Drs. S. Joseph Wright and Helene Muller-Landau for hosting me during my stay in Panama. My parents and my partner, Maribeth Latvis have supported me through this entire process. This project has been made possible in part by a grant from the Frank Levinson Family Foundation, a supporting organization of the Silicon Valley Community Foundation. The National Science and MacArthur Foundations have supported the BCI 50-ha plot censuses. 4 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...................................................................................................... 4 LIST OF TABLES ................................................................................................................ 6 LIST OF FIGURES .............................................................................................................. 7 LIST OF ABBREVIATIONS ................................................................................................ 8 ABSTRACT.......................................................................................................................... 9 CHAPTER 1 INTRODUCTION ........................................................................................................ 11 2 METHODS .................................................................................................................. 16 Study Site and Leaf Sampling .................................................................................... 16 Leaf Biomechanical Measurements ........................................................................... 17 Lamina Density and Thickness .................................................................................. 17 Foliar Fiber Analysis ................................................................................................... 18 Relative Growth Rate and Mortality ........................................................................... 19 Phylogeny Reconstruction .......................................................................................... 20 Phylogenetic Signal .................................................................................................... 20 Phylogenetically Independent Contrasts and Trait Correlations ............................... 21 Path Analysis .............................................................................................................. 22 Associations between Leaf Physical Defense and Demography.............................. 23 3 RESULTS.................................................................................................................... 24 Ahistorical Trait Distributions ...................................................................................... 24 Phylogenetic Signal in Leaf Physical Defense and Demography ............................. 24 Evolutionary Path Analysis of Leaf Physical Defense ............................................... 25 Relationships between Leaf Physical Defense and Demography ............................ 26 4 DISCUSSION & CONCLUSION ................................................................................ 37 APPENDIX: SUPPLEMENTARY TABLES, FIGURES, AND METHODS ...................... 42 LIST OF REFERENCES ................................................................................................... 62 BIOGRAPHICAL SKETCH................................................................................................ 67 5 LIST OF TABLES Table page 3-1 Phylogenetic signal leaf of physical defense traits and demography ................... 42 3-2 Evolutionary correlations between leaf physical defense traits ............................ 42 3-3 Associations between leaf physical defense traits and demographic rates ......... 42 A-1 List of species included in the analysis ................................................................. 42 A-2 Descriptive statistics for ahistorical traits............................................................... 47 A-3 The effect of outlier PICs on the phylogenetic signal of RGR .............................. 54 A-4 Testing the assumptions of PIC standardization and normality with equal branch lengths. ....................................................................................................... 56 A-5 A comparison between PIC (top values) and ahistorical correlations (in parentheses)........................................................................................................... 57 A-6 Multiple regression to test for additive effects of the hypothesized explanatory variables on lamina fracture toughness ................................................................. 58 A-7 Multiple regression to test for additive effects of the hypothesized explanatory variables on vein fracture toughness ..................................................................... 58 A-8 Multiple regression to test for additive effects of the hypothesized explanatory variables on lamina work-to-shear ......................................................................... 59 A-9 Predicted correlation matrix (top values) and residuals (in parentheses) from path analysis........................................................................................................... 59 A-10 Summary of PIC loadings on principal component axes 1–5 ............................... 60 A-11 Associations between principal component axes 1–5 and demographic rates ... 61 6 LIST OF FIGURES Figure page 3-1 Phylogenetic map of lamina fracture toughness. .................................................. 42 3-2 Phylogenetic map of median relative growth rate. ................................................ 42 3-3 Path model of leaf physical defense. ..................................................................... 42 3-4 Two independent evolutionary paths to tough leaves........................................... 42 3-5 Evolutionary principal component analysis of leaf physical defense.................... 42 3-6 Associations between leaf physical defense PC axes and demographic rates ... 42 A-1 Ahistorical trait distributions ................................................................................... 48 A-2 Phylogenetic trait maps .......................................................................................... 50 7 LIST OF ABBREVIATIONS BCI Barro Colorado Island, Panama PIC (plural PICs) Phylogenetically independent contrast J Joules m meters lamW S Lamina work-to-shear (J m-1) lamFT Lamina fracture toughness (J m-2) veinFT Vein fracture toughness (J m-2) lamD Lamina density (g cm-3) lamT Lamina thickness (mm) DBH Diameter and breast height of the main stem RGR Relative growth rate of the main stem (% year-1) PCA Principal component analysis GRH Growth rate hypothesis of Coley, Bryant, & Chapin (1985) 8 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science THE EVOLUTION OF LEAF PHYSICAL DEFENSE IN THE SHADE OF A NEOTROPICAL FOREST By Jared W. Westbrook December 2009 Chair: Kaoru Kitajima Major: Botany Leaf physical defenses effectively deter a broad suite of herbivores, but species with highly fortified leaves have slower growth rates than more poorly defended competitors in canopy gaps of tropical moist forests. The effect of these defenses on growth and survival in the deep shade, and their evolutionary history has received less attention; therefore, we characterized the leaf physical defense traits of 197 tree and shrub species, which co-occur within the shaded understory of the 50-hectare forest dynamics plot on Barro Colorado Island (BCI), Panama. Levels of leaf physical defense displayed significant phylogenetic signal; that is, the traits of closely related species were more similar than expected by chance. Path analysis with phylogenetically independent contrasts (PICs) revealed that tough leaves have evolved independently through increases in the foliar cellulose mass fraction, increases in lamina density, and increases in leaf thickness; however, only size-independent, material traits (lamina density, lamina fracture toughness, and vein fracture toughness) had significant negative correlations with mortality rates in the 1–10 cm diameter at breast height (DBH) class. The combination of eight leaf physical defense PIC distributions explained 18.5% of the phylogenetic variability in mortality rates, but only 4.9% of the variation in 9 median relative growth rates (RGR) in the 1–5 cm DBH class. Thus, physically fortified leaves were more strongly associated with enhanced survival than with reduced growth rates among shade tolerant