Understanding the role of ecohydrological feedbacks in ecosystem state change in drylands L. Turnbull, 1 * B. P. Wilcox, 2 J. Belnap, 3 S. Ravi, 4 P. D’Odorico, 5 D. Childers, 6 W. Gwenzi, 7 G. Okin, 8 J. Wainwright, 9,† K. K. Caylor 10 and T. Sankey 11 1 Arizona State University Global Institute of Sustainability Tempe, AZ 85287, USA 2 Texas A&M University Department of Ecosystem Science and Management College Station, TX 77843, USA 3 US Geological Survey Southwest Biological Science Center Canyonlands Research Station, 2290 S. West Resource Blvd., Moab, UT 84532, USA 4 University of Arizona B2 Earthscience and UA Biosphere 2 Tucson, AZ 85721, USA 5 University of Virginia Department of Environmental Sciences Charlottesville, VA 22904, USA 6 Arizona State University School of Sustainability Tempe, AZ 85287, USA 7 University of Western Australia School of Plant Biology 35 Stirling Highway, Crawley, WA 6009, Australia 8 University of California Department of Geography Los Angeles, CA 90095, USA 9 University of Sheffield Sheffield Centre for International Drylands Research, Department of Geography Sheffield S10 2TN, UK 10 Princeton University Department of Civil and Environmental Engineering Princeton, NJ 08544, USA 11 Idaho State University Boise Center Aerospace Laboratory 322 E. Front Street Suite 240, Boise, ID 83702, USA ABSTRACT Ecohydrological feedbacks are likely to be critical for understanding the mechanisms by which changes in exogenous forces result in ecosystem state change. We propose that in drylands, the dynamics of ecosystem state change are determined by changes in the type (stabilizing vs amplifying) and strength of ecohydrological feedbacks following a change in exogenous forces. Using a selection of five case studies from drylands, we explore the characteristics of ecohydrological feedbacks and resulting dynamics of ecosystem state change. We surmise that stabilizing feedbacks are critical for the provision of plant- essential resources in drylands. Exogenous forces that break these stabilizing feedbacks can alter the state of the system, although such changes are potentially reversible if strong amplifying ecohydrological feedbacks do not develop. The case studies indicate that if amplifying ecohydrological feedbacks do develop, they are typically associated with abiotic processes such as runoff, erosion (by wind and water), and fire. These amplifying ecohydrological feedbacks progressively modify the system in ways that are long-lasting and possibly irreversible on human timescales. Copyright © 2011 John Wiley & Sons, Ltd. KEY WORDS ecohydrology; state change; drylands; feedbacks; state change; resilience Received 21 March 2011; Revised 31 August 2011; Accepted 30 September 2011 INTRODUCTION In this paper, we explore how ecohydrological feedbacks control ecosystem states in drylands. Ecosystem state change, which refers to a change in the structure and function of an ecosystem (Beisner et al., 2003), is typically induced by exogenous forces (environmental drivers and disturbances) that either cause a direct change in system state or trigger changes in the internal system feedbacks that control the dynamics of an ecosystem, including its resilience. Ecological resilience is the capacity of a system to respond to exogenous forces and reorganize while undergoing change, so as to retain the same function, structure, and identity – i.e. the same state (Walker et al., 2004). Until recently, the discourse regarding ecosystem state change has focused on ecological components of the system; consideration of ecohydrological feedbacks (among ecological, hydrological, and geomorphological processes) has been largely missing, even though they exert a profound influence on the structure and function of ecosystems (Huxman et al., 2005; Newman et al., 2006; Ravi et al., 2008; Turnbull et al., 2008; Wilcox et al., 2008; Turnbull et al., 2010a, b; Wilcox, 2010). It is imperative that future work in ecosystem state change adopt this more comprehensive, ecohydrological focus – which we argue will be critical for understanding the mechanisms by which exogenous forces cause ecosystem state change, and how they affect the dynamics of change, in particular, the crossing of critical thresholds. Such an understanding is critical to successful environmental management – either to reduce the likelihood that a system will transition to a less desirable state or to restore a system to a more desirable state. Thus, we explore how ecohydrological feedbacks control ecosystem state change in drylands by reviewing the current understanding of ecosystem state change in accordance with different types of exogenous forces, internal system feedbacks, and resulting dynamics of change. We then evaluate the significance of ecohydrolo- gical feedbacks by drawing upon five case studies, each of which addresses how exogenous forces affect ecosystem state, either directly or by altering the strength and/or type of ecohydrological feedback. Drawing upon these case studies, we explore different types of ecohydrological *Correspondence to: Correspondence to: L. Turnbull, Arizona State University, Global Institute of Sustainability, Tempe, AZ 85287, USA. E-mail: Laura.Turnbull@asu.edu † Current Address: Durham University, Department of Geography, Durham, DH1 3LE, UK ECOHYDROLOGY Ecohydrol. (2011) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.265 Copyright © 2011 John Wiley & Sons, Ltd.