A quantitative description of the interspecies diversity of belowground structure in savanna woody plants FRANCES C. O’DONNELL, 1,4,  KELLY K. CAYLOR, 1 ABINASH BHATTACHAN, 2 KEBONYE DINTWE, 3 PAOLO D’ODORICO, 2 AND GREGORY S. OKIN 3 1 Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544 USA 2 Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia 22904 USA 3 Department of Geography, University of California, Los Angeles, California 90095 USA Citation: O’Donnell, F. C., K. K. Caylor, A. Bhattachan, K. Dintwe, P. D’Odorico, and G. S. Okin. 2015. A quantitative description of the interspecies diversity of belowground structure in savanna woody plants. Ecosphere 6(9):154. http:// dx.doi.org/10.1890/ES14-00310.1 Abstract. The relative importance of resource competition and disturbance in limiting woody cover is one of the most basic questions in savanna ecology. Modeling approaches that seek to address this question are limited in their ability to accurately represent resource competition, which occurs belowground, by the limited detail of existing data on root system structure. Using compressed air, we excavated individual trees and shrubs and mapped their coarse roots on a three-dimensional grid system up to 1.5 m depth. We excavated four woody savanna species at three sites spanning a climate gradient on the Kalahari Transect in southern Africa and determined functions to describe the distribution of root biomass within the root systems. Overall, most of the variation in both large-scale and small-scale characteristics of root system structure was related to species. The species excavated fell into two groups that coexisted across the climate gradient. Acacia mellifera and Terminalia sericea had straight roots in a laterally extensive system that was shallow relative to the aggregate root profile for woody plants at the sites. Boscia albitrunca and Ochna pulchra had sinuous roots that were mostly concentrated beneath the canopy and were more prevalent in deep than near-surface soil layers, departing from the conventional model of decreasing root abundance with depth. Based on our results, we suggest a reframing of the theoretical characterization of the root zone used in modeling approaches to consider diversity within the woody plant community. Key words: Acacia mellifera; belowground competition; belowground zone-of-influence; Boscia albitrunca; coarse roots; excavation; Kalahari Transect; Ochna pulchra; savanna; Terminalia sericea. Received 8 September 2014; revised 18 December 2014; accepted 18 March 2015; final version received 2 May 2015; published 24 September 2015. Corresponding Editor: S. Ke ´fi. Copyright: Ó 2015 O’Donnell et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. http://creativecommons.org/licenses/by/3.0/ 4 Present address: School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011 USA.  E-mail: frances.odonnell@nau.edu INTRODUCTION Determining the primary limitation on woody plant cover in mixed tree-grass systems is one of the most important—but still unanswered— questions in savanna ecology, despite its conse- quences for rangeland management decisions and prediction of future ecosystem state transi- tions (House et al. 2003, Asner et al. 2004). The three limiting factors that are generally cited are rainfall, fire, and herbivory (Bucini and Hanan 2007, Bond 2008). Many studies have offered support for one or a combination of these factors, and they generally consist of one of three types of v www.esajournals.org 1 September 2015 v Volume 6(9) v Article 154