RESEARCH ARTICLE Return of Fire to a Free-Flowing Desert River: Effects on Vegetation Juliet C. Stromberg, 1,2 Tyler J. Rychener, 1 and Mark D. Dixon 3 Abstract After a long period in which fuel loads were sparse, fire recently has occurred with high frequency in the ungrazed riparian zone of the Upper San Pedro River in southern Arizona’s Chihuahuan Desert. We studied four accidental fires that occurred during 1994–2003 (two in different years at the same site). Woody vegetation was contrasted between three burned sites and matched spatial controls, and before and after the most recent fire. Herbaceous veg- etation was sampled in multiple years producing a chrono- sequence of time since fire (from 4 months to 8 years). Riparian fire was associated with reductions in woody plant species diversity and canopy cover. In contrast, fire caused a short-term (2 year) pulse of herbaceous plant diversity, driven by annual species, and persistent increase in herbaceous cover. Path analysis indicated that the increase in herbaceous cover was mediated in part by the reduction in tree canopy cover. Ordination (nonmetric multidimensional scaling) and regression analysis also indicated that canopy cover and/or fire played a role in structuring the herbaceous community, although its effects were secondary to that of hydrologic factors (stream flow rate, seasonal flood size). By converting riparian forests to grasslands and savannahs, fire may be shifting structure of the Upper San Pedro floodplain vegetation closer toward conditions present during past centuries when fire was frequent in the upland desert grasslands and embedded riparian corridor. Key words: disturbance, diversity, dryland, fire, flood- plain, herbaceous, plant community, restoration, riparian, river, semiarid, stream flow, vegetation. Introduction Ecological disturbances are stochastic events that have a characteristic timing, intensity, and frequency. If the dis- turbance regime changes, the plant community structure changes in tandem. For example, where flood patterns have been altered by river damming, flow regulation, or shifts in watershed land cover, riparian plant communities have undergone changes in composition, diversity, and successional stage (Johnson 1998; Nilsson & Berggren 2000; Uowolo et al. 2005). Although flooding is the pre- dominant disturbance in riparian ecosystems, many also periodically burn (Timoney et al. 1997; Gom & Rood 1999; Dwire & Kauffman 2003). In the southwestern region of the United States, as in many regions throughout the world, fire patterns have var- ied through time in response to changes in climate and human cultural practices (Floyd et al. 2004). Fire was com- mon along some rivers in the Desert Southwest prior to the 1800s or 1900s (Humphrey 1974; Turner 1974; Davis 1982; Swetnam 1990; McPherson 1995; Davis et al. 2002), as a result of lightning strikes and cultural ignitions. Subse- quently, fire became less frequent in desert grasslands and other upland vegetation types of western North America, as a result of active fire suppression and the introduction of cattle, which reduced grassy fuel loads (Reynolds & Bohning 1956; Humphrey 1958, 1974; Bahre 1985; Fule & Covington 1999; Van Auken 2000 Veblen et al. 2000). A similar dynamic presumably was at play in the embedded riparian zones given that riparian zones in dry climates or along dry river reaches often burn at approximately the same frequency as the upland vegetation (Olson & Agee 2005; Charron & Johnson 2006). More recently, a suite of factors has contributed to high frequency of fire in riparian zones (Busch 1995). First, conservation-linked removal of livestock has led to increased grass cover in some riparian zones (Sarr 2002; Krueper et al. 2003); in some others, vegetation density (thus fuel loads) has increased because of river damming and flood suppression (Ellis 2001; Shafroth et al. 2002). Second, because the Desert South- west is rapidly urbanizing, human activities are encroach- ing into wildlands and fire ignitions are increasing (Wells et al. 2004; Franklin et al. 2005). Changes in fire regimes can bring about many shifts in plant community structure. In dryland and wetland sys- tems alike, fire suppression can allow woody vegetation to increase in abundance (Heisler et al. 2003; Lee et al. 2005; Heyerdahl et al. 2006). Decline in fire frequency may be one of the factors that contributed to historic shifts 1 School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, U.S.A. 2 Address correspondence to J. C. Stromberg, email jstrom@asu.edu 3 Department of Biology, University of South Dakota, Vermillion, SD 57069, U.S.A. Ó 2007 Society for Ecological Restoration International doi: 10.1111/j.1526-100X.2007.00347.x MAY 2009 Restoration Ecology Vol. 17, No. 3, pp. 327–338 327