The processes causing the latitudinal gradient in species richness remain elusive. Ecological theories for the origin of biodiversity gradients, such as competitive exclusion, neutral dynamics, and environmental filtering, make predictions for how functional diversity should vary at the alpha (within local assemblages), beta (among assemblages), and gamma (regional pool) scales. We test these predictions by quantifying hypervolumes constructed from functional traits representing major axes of plant strategy variation (specific leaf area, plant height, and seed mass) in tree assemblages spanning the temperate and tropical New World. Alpha-scale trait volume decreases with absolute latitude and is often lower than sampling expectation, consistent with environmental filtering theory. Beta-scale overlap decays with geographic distance fastest in the temperate zone, again consistent with environmental filtering theory. In contrast, gamma-scale trait space shows a hump-shaped relationship with absolute latitude, consistent with no theory. Furthermore, the overall temperate trait hypervolume was larger than the overall tropical hypervolume, indicating that the temperate zone permits a wider range of trait combinations or that niche packing is stronger in the tropical zone. Although there are limitations in the data, our analyses suggest that multiple processes have shaped trait diversity in trees, reflecting no consistent support for any one theory.

Center for Theoretical Study Charles University Prague and Academy of Sciences of the Czech Republic 110 00 Praha Czech Republic;

Centre d'Ecologie Fonctionelle et Evolutive Unité Mixte de Recherche 5175 Centre National de la Recherche Scientifique Université de Montpellier Université Paul Valéry Montpellier École Pratique des Hautes Études 34293 Montpellier France;

Department of Biology University of Maryland College Park MD 20742;

Department of Biology University of North Carolina at Chapel Hill Chapel Hill NC 27599;

Department of Computer Science and Information Systems Bradley University Peoria IL 61625;

Department of Ecology and Evolutionary Biology University of Arizona Tucson AZ 85721;

Department of Ecology and Evolutionary Biology University of Arizona Tucson AZ 85721; Center for Macroecology Evolution and Climate Copenhagen University 2100 Copenhagen Denmark;

Department of Ecology and Evolutionary Biology University of Arizona Tucson AZ 85721; iPlant Collaborative Tucson AZ 85721;

Department of Ecology and Evolutionary Biology University of Arizona Tucson AZ 85721; iPlant Collaborative Tucson AZ 85721; Santa Fe Institute Santa Fe NM 87501

iPlant Collaborative Tucson AZ 85721; National Center for Ecological Analysis and Synthesis University of California Santa Barbara CA 93106;

Landcare Research Lincoln 7640 New Zealand;

Missouri Botanical Garden St Louis MO 63166;

National Center for Ecological Analysis and Synthesis University of California Santa Barbara CA 93106;

New York Botanical Garden Bronx NY 10458; and

Section for Ecoinformatics and Biodiversity Department of Bioscience and

Section for Ecoinformatics and Biodiversity Department of Bioscience and Center for Massive Data Algorithmics Department of Computer Science Aarhus University DK 8000 Aarhus Denmark;

Sustainability Solutions Initiative and

Sustainability Solutions Initiative and School of Biology and Ecology University of Maine Orono ME 04469;

Yale NUS College Republic of Singapore 138614;

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