HOW DO LEAF FUNCTIONAL TRAITS VARY ACROSS ECOLOGICAL SCALES? Julie Messier Biology Department McGill University, Montréal, Québec, Canada February 2009 A thesis submitted to McGill University in partial fulfillment of the requirements of the degree of Masters of Science © Julie Messier 2009 Library and Archives Bibliothèque et Canada Archives Canada Published Heritage Direction du Branch Patrimoine de l’édition 395 Wellington Street 395, rue Wellington Ottawa ON K1A 0N4 Ottawa ON K1A 0N4 Canada Canada Your file Votre référence ISBN: 978-0-494-56854-5 Our file Notre référence ISBN: 978-0-494-56854-5 NOTICE: AVIS: The author has granted a non- L’auteur a accordé une licence non exclusive exclusive license allowing Library and permettant à la Bibliothèque et Archives Archives Canada to reproduce, Canada de reproduire, publier, archiver, publish, archive, preserve, conserve, sauvegarder, conserver, transmettre au public communicate to the public by par télécommunication ou par l’Internet, prêter, telecommunication or on the Internet, distribuer et vendre des thèses partout dans le loan, distribute and sell theses monde, à des fins commerciales ou autres, sur worldwide, for commercial or non- support microforme, papier, électronique et/ou commercial purposes, in microform, autres formats. paper, electronic and/or any other formats. The author retains copyright L’auteur conserve la propriété du droit d’auteur ownership and moral rights in this et des droits moraux qui protège cette thèse. Ni thesis. Neither the thesis nor la thèse ni des extraits substantiels de celle-ci substantial extracts from it may be ne doivent être imprimés ou autrement printed or otherwise reproduced reproduits sans son autorisation. without the author’s permission. In compliance with the Canadian Conformément à la loi canadienne sur la Privacy Act some supporting forms protection de la vie privée, quelques may have been removed from this formulaires secondaires ont été enlevés de thesis. cette thèse. While these forms may be included Bien que ces formulaires aient inclus dans in the document page count, their la pagination, il n’y aura aucun contenu removal does not represent any loss manquant. of content from the thesis. - ACKNOWLEDGEMENTS - First and above all, I would like to give my dearest thanks to my supervisors, Brian J. McGill and Martin J. Lechowicz, whose constant support, guidance and encouragements have been indispensable. Thanks to the National Science and Engineering Research Council (NSERC), the Smithsonian Tropical Research Institute, an aid-in-research grant from the Neo program of McGill University and the Organisme Québec-Amérique pour la Jeunesse for funding this research. I am also grateful to Ricardo Cossio for help with field work and to Andy Gonzalez, Omar Lopez, Joe Wright, Mirna Samaniego, Reinaldo Uriola and Jose Barahona for advice and invaluable technical help along the bumpy road of graduate studies. I would also like to thank Mélisanne Loiselle-Gascon, Vanessa Ward, Jaclyn Paterson and Kyle Simpson, for technical help in processing samples. I would like to give special thanks to two beloved close friends (Maryse & Gerardo), to wonderful lab mates (Volker, Richard, Peter & Sergio) and to my family (Danielle, Mario & Simon). You have supported me through hurdles and tribulations and fueled my motivation all along this winding road. Without you, I would not have made it through these challenging two years. ii - CONTRIBUTION OF AUTHORS - JM & BJM conceived the study question and sampling design; JM carried out field work and data collection with guidance from BJM & MJL; BJM conceived the data analysis and carried it out with JM; BJM, JM and MJL collaborated on interpreting the data and writing chapter 1 of the manuscript: “How do leaf functional traits vary across ecological scales?”; JM wrote the remainder of the thesis with guidance from BJM and MJL. iii - TABLE OF CONTENTS - ACKNOWLEDGEMENTS . ii CONTRIBUTION OF AUTHORS . iii TABLE OF CONTENTS . iv ABSTRACT . vi RÉSUMÉ . vii LIST OF TABLES . viii LIST OF FIGURES . ix GENERAL INTRODUCTION . 1 Traits at the Heart of Ecology & Evolution The Role of Functional Traits The Study of Functional Trait Variation Ecological Questions Addressed Through Trait Variation . A - Trait Plasticity . B - Correlations among traits . C - Functional Classifications . D - Trait Variation & Community Composition . E - Trait Variation & Environnemental Gradients . F - Trait Variation & Ecosystem Processes . G – Trait Variation & the Niche . Interdependence of Research Topics Integrating and Structuring Trait Variation: How Do Traits Vary Across Ecological Scales? Research Objectives & Hypotheses METHODS . 24 The Traits iv The Study Design Study Site Details Trait Measurement Protocol Statistical Analyses CHAPTER 1 - How Do Traits Vary Across Ecological Scales? . 30 Body of the Text Method Summary GENERAL CONCLUSION . 40 Endorsing Contrasting Viewpoints Intraspecific Trait Variability as “Noise” Trait Based Environmental Filters Future Directions: Are Species Distinct Entities in Trait Space? REFERENCES . 44 TABLES & FIGURES . 51 APPENDICES . 58 Appendix I. - Species List Appendix II. - Chapter 1. Supplementary Material . Methods . R Code Used in Partitioning of Variance . Results of Variance Partitioning Without Species Level . Calculation of Confidence Intervals v - ABSTRACT– Functional traits, measurements of adaptive aspects of the phenotype, are increasingly used for the study of plant community ecology. Despite their importance, we do not know which ecological scales contain the most variation in a given trait, which hampers assessment of the wider relevance of findings from studies conducted at only one scale. To address this deficiency, I studied the variance distribution of two key leaf functional traits (leaf mass per area - LMA and leaf dry matter content - LDMC) across six nested ecological scales (site, plot, species, tree, strata, leaf) in lowland tropical rainforests of Panama. Variance in both traits is uniformly distributed across all scales except the plot level, which shows virtually no variance despite high species turnover among plots. This contradicts the widely held belief that species-level variation predominates in organizing species distribution and abundance and indicates that communities regulate plant ensembles by filtering on leaf functional traits regardless of species. vi - RÉSUMÉ - Les traits fonctionnels, attributs indicatifs des aspects adaptatifs du phénotype, sont de plus en plus utilisés pour étudier l’écologie des communautés végétales. Malgré l’importance des traits fonctionnels, nous ne savons pas à quelle échelle écologique un trait donné varie le plus, ce qui nous empêche de mettre dans un contexte général les découvertes des études conduites à une seule échelle. Pour combler cette lacune, j’ai étudié dans les forêts tropicales humides tropical du Panama la distribution de la variance de deux traits fonctionnels foliaires clés (la masse par surface foliaire - LMA et le contenu foliaire en matière sèche - LDMC) à travers six échelles écologiques emboitées (le site, la parcelle, l’espèce, l’arbre, la strate et la feuille). La variance de ces deux traits est uniformément distribuée à travers toutes les échelles, sauf à l’échelle de la parcelle qui ne présente aucune variance malgré la forte différence de composition d’espèces entre les parcelles d’un même site. Ces résultats vont à l’encontre de la croyance populaire selon laquelle la variation interspécifique joue un rôle prédominant dans le contrôle de la distribution et l’abondance des espèces. Ils indiquent plutôt que les communautés régulent l’assemblage des végétaux en exerçant un filtre sur les traits fonctionnels, indépendamment de l’espèce. vii - LIST OF TABLES - Table 1. Summary of the seven main research areas involving p.42 functional traits. Table 2. Possible expected outcomes of partitioning the variance in p.44 traits across ecological scales according to different research paradigms. Table 3. Descriptions of study sites p.44 Table 4. Variance partitioning of the full nested linear models on p.45 leaf mass per area (LMA) and leaf dry matter content (LDMC) across six nested ecological scales. Table A1. List of tree species sampled with number of leaves p.60 sampled per species and site. Table S1. Variance partitioning of an alternative nested linear p.67 model of LMA and LDMC across five nested ecological scales (Leaf, Strata, Tree, Plot and Site), leaving out the species scale. viii - LIST OF FIGURES - Figure 1. Location of study sites. PNSL – Parque Natural San p.46 Lorenzo. BCI – Barro Colorado Island. PNM – Parque Natural Metropolitano. Map from Google Earth. Figure 2. Boxplot of LMA and LDMC values for the 17 most p.47 abundant species (species for which 30 or more leaves were measured). Figure 3. Histogram of values for LMA and LDMC by site and by p.48 plot. Solid histograms represent the sites: PNM (red), PNSL (black) & BCI (blue). Lines represent the histograms of individual plots for each site, colors coded accordingly. Figure S1. LMA & LDMC variance partitioning across ecological p.68 scales for the full model (with species level included) and the alternative model (without species level). ix - GENERAL INTRODUCTION - Traits at the Heart of Ecology & Evolution 5 The distribution of life on the planet is extremely patchy; most species are absent from almost everywhere (Begon et al. 1996). Different organisms live in different environments because each species is adapted