Climate is widely recognised as an important determinant of the latitudinal diversity gradient. However, most existing studies make no distinction between direct and indirect effects of climate, which substantially hinders our understanding of how climate constrains biodiversity globally. Using data from 35 large forest plots, we test hypothesised relationships amongst climate, topography, forest structural attributes (stem abundance, tree size variation and stand basal area) and tree species richness to better understand drivers of latitudinal tree diversity patterns. Climate influences tree richness both directly, with more species in warm, moist, aseasonal climates and indirectly, with more species at higher stem abundance. These results imply direct limitation of species diversity by climatic stress and more rapid (co-)evolution and narrower niche partitioning in warm climates. They also support the idea that increased numbers of individuals associated with high primary productivity are partitioned to support a greater number of species.

Biology Department Wilfrid Laurier University Waterloo ON Canada

Center for Conservation and Sustainability Smithsonian Conservation Biology Institute National Zoological Park Washington DC USA

Center for Ecological Research Northeast Forestry University 150040 Harbin

Center for Theoretical Study Charles University Academy of Sciences of the Czech Republic Praha Czech Republic

Center for Tropical Forest Science Forest Global Earth Observatory Smithsonian Tropical Research Institute Panama City Panama

College of Forestry South China Agricultural University 510642 Guangzhou

Conservation Ecology Center Smithsonian Conservation Biology Institute National Zoological Park Front Royal VA USA

Departamento Ecologia Universidade de São Paulo Instituto de Biociências Cidade Universitária São Paulo SP Brazil

Department of Biology and Tyson Research Center Washington University in St Louis St Louis MO USA

Department of Botany National Museum of Natural History Washington DC USA

Department of Ecology and Evolutionary Biology University of California Los Angeles Los Angeles CA USA

Department of Ecology Faculty of Science Charles University Praha Czech Republic

Department of Ecology State Key Laboratory of Biocontrol School of Life Sciences Sun Yat sen University 510275 Guangzhou

Department of Environmental Studies University of California Santa Cruz CA 95064 USA

Department of Forest Ecology Silva Tarouca Research Institute Brno Czech Republic

Department of Natural Resources and Environmental Studies National Dong Hwa University 97401 Hualien

Department of Plant and Microbial Biology University of Minnesota St Paul MN USA

Department of Plant Science and Technology University of Jos Jos Nigeria

Department of Plant Sciences University of Oxford Oxford OX1 3RB UK

Department of Renewable Resources University of Alberta Edmonton Alberta T6G 2H1 Canada

ECNU Alberta Joint Lab for Biodiversity Study Tiantong National Station for Forest Ecosystem Research East China Normal University 200241 Shanghai

Field Museum of Natural History Chicago IL USA and Morton Arboretum Lisle IL USA

Forest Ecology Group Smithsonian Environmental Research Center Edgewater MD USA

Guangdong Chebaling National Nature Reserve 512500 Shaoguan

Guangxi Key Laboratory of Plant Conservation and Restoration Ecology in Karst Terrain Guangxi Institute of Botany Guangxi Zhuang Autonomous Region and Chinese Academy of Sciences 541006 Guilin

Harvard Forest Harvard University Petersham MA USA

Heilongjiang Forestry Enginerring and Environment Institute 150040 Harbin

Hydrological Ecological Interactions Branch Earth System Processes Division Water Mission Area U S Geological Survey Reston VA USA

Institut de Recherche en Ecologie Tropicale Centre National de la Recherche Scientifique et Technologique Libreville Gabon

Institute for Tropical Ecosystem Studies University of Puerto Rico San Juan Puerto Rico 00936 USA

Institute of Botany Minzu University of China Chinese Academy of Sciences 100093 Beijing

Kadoorie Farm and Botanic Garden Tai Po Hong Kong SAR

Kellogg Biological Station and Department of Integrative Biology Michigan State University Hickory Corners Michigan 49060 USA

Key Laboratory for Plant Diversity and Biogeography of East Asia Kunming Institute of Botany Chinese Academy of Sciences 650201 Kunming

Key Laboratory of Aquatic Botany and Watershed Ecology Wuhan Botanical Garden Chinese Academy of Sciences 430074 Wuhan

Key Laboratory of Forest Ecology and Management Institute of Applied Ecology Chinese Academy of Sciences 110016 Shenyang

Key Laboratory of Tropical Forest Ecology Xishuangbanna Tropical Botanical Garden Chinese Academy of Sciences 650223 Kunming

Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems South China Botanical Garden Chinese Academy of Sciences 510650 Guangzhou

Lijiang Forest Ecosystem Research Station Kunming Instituted of Botany Chinese Academy of Sciences 674100 Lijiang

Research Institute of Tropical Forestry Chinese Academy of Forestry 510000 Guangzhou

School of Biological Sciences The University of Hong Kong Pokfulam Road Hong Kong SAR

School of Biological Sciences University of Aberdeen Aberdeen UK

School of Biological Sciences University of Canterbury Christchurch New Zealand

Southeast Asia Rainforest Research Partnership Danum Valley Field Centre PO Box 60282 91112 Lahad Datu Sabah Malaysia

State Key Laboratory of Vegetation and Environmental Change Institute of Botany The Chinese Academy of Sciences 100093 Beijing

The Nigerian Montane Forest Project Taraba State Nigeria

Wildland Resources Department Utah State University Logan UT USA

Zhejiang Tiantong Forest Ecosystem National Observation and Research Station School of Ecology and Environmental Sciences East China Normal University 200241 Shanghai

References provided by Crossref.org

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