The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis-based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.

Center for Tree Science Morton Arboretum 4100 Illinois Rt 53 Lisle IL 60532 USA

Centre d'Ecologie Fonctionnelle et Evolutive Université de Montpellier CNRS EPHE IRD Univ Paul Valéry Montpellier 3 Montpellier 34293 France

Departamento de Conservación de Suelos y Agua Centro de Edafología y Biología Aplicada del Segura Consejo Superior de Investigaciones Científicas Murcia 30100 Spain

Department of Biological Sciences Macquarie University North Ryde NSW 2109 Australia

Department of Earth and Environmental Sciences The University of Manchester Manchester M13 9PT UK

Department of Ecology Evolution and Behavior University of Minnesota St Paul MN 55108 USA

Department of Forest and Soil Sciences University of Natural Resources and Life Sciences Vienna 1190 Austria

Department of Functional Ecology Institute of Botany CAS Dukelska 135 Trebon 37901 Czech Republic

Department of General Botany Institute of Experimental Biology Faculty of Biology Adam Mickiewicz University Uniwersytetu Poznańskiego 6 Poznań 61 614 Poland

Environmental Biology Department Institute of Environmental Sciences CML Leiden University Leiden 2333 CC the Netherlands

Environmental Sciences Division and Climate Change Science Institute Oak Ridge National Laboratory Oak Ridge TN 37831 USA

Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague Prague 165 00 Czech Republic

Forest Dynamics Swiss Federal Research Institute WSL Zürcherstr 111 Birmensdorf 8903 Switzerland

Forest Soils and Biogeochemistry Swiss Federal Research Institute WSL Zürcherstr 111 Birmensdorf 8903 Switzerland

Functional Forest Ecology University of Hamburg Haidkrugsweg 1 Barsbüttel 22885 Germany

Geobotany Faculty of Biology University of Freiburg Schänzlestr 1 Freiburg 79104 Germany

INRA AMAP CIRAD IRD CNRS University of Montpellier Montpellier 34000 France

Institute of Applied Ecology Chinese Academy of Sciences Shenyang 110016 China

Institute of Dendrology Polish Academy of Sciences Parkowa 5 Kórnik 62 035 Poland

Institute of Environment Department of Biological Sciences Florida International University Miami FL 33199 USA

Institute of Terrestrial Ecosystems ETH Zurich Zurich 8092 Switzerland

Laurentian University 935 Ramsey Lake Road Sudbury ON P3E 2C6 Canada

Noble Research Institute LLC 2510 Sam Noble Parkway Ardmore OK 73401 USA

Odum School of Ecology University of Georgia 140 E Green Street Athens GA 30602 USA

Plant Ecology and Nature Conservation Group Department of Environmental Sciences Wageningen University and Research PO box 47 Wageningen 6700 AA the Netherlands

Plant Ecology University of Goettingen Untere Karspüle 2 Göttingen 37073 Germany

Plant Sciences Forschungszentrum Jülich GmbH Jülich D 52425 Germany

School of Agriculture Policy and Development University of Reading Reading RG6 6EU UK

School of Science and Engineering University of Dundee Dundee DD1 4HN UK

Soil Biology Group Wageningen University Wageningen 6700 AA the Netherlands

Station d'Ecologie Théorique et Expérimentale CNRS 2 route du CNRS Moulis 09200 France

Systematic Botany and Functional Biodiversity Institute of Biology Leipzig University Johannisallee 21 23 Leipzig 04103 Germany

The James Hutton Institute Invergowrie Dundee DD2 5DA UK

Université Clermont Auvergne INRAE UREP Clermont Ferrand 63000 France

UR 1115 PSH Centre PACA site Agroparc INRAE Avignon Cedex 9 84914 France

USDA ARS Water Management and Systems Research Unit 2150 Centre Avenue Bldg D Suite 320 Fort Collins CO 80526 USA

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