- Flood pulsing and metacommunity dynamics in desert riparian ecosystems - 373 Journal of Vegetation Science 19: 373-380, 2008 doi: 10.3170/2008-8-18377, published online 11 March 2008 © IAVS; Opulus Press Uppsala. Flood pulsing and metacommunity dynamics in a desert riparian ecosystem Boudell, Jere A. 1* & Stromberg, Juliet C. 2 1 Department of Natural Sciences, Clayton State University, Morrow, GA 30260-0285, USA; 2 School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USA; E-mail jstrom@asu.edu; * Corresponding author; E-mail jboudell@clayton.edu Abstract Questions: 1. Does lood pulsing drive metacommunity dynam- ics and provide insurance against catastrophic looding in desert southwestern riparian ecosystems? 2. Do upland and wetland species in the loodplain differ in their dynamics? Location: Southwestern USA. Methods: We sampled vegetation and propagule banks in four communities along a loodplain hydrogradient. Plant species were classiied as wetland or upland and community wetland indicator scores were calculated. ANOVA tested for differ- ences between data categories amongst communities (extant vegetation and propagule banks) and soil depths. Sørensen’s similarity coeficient was calculated to determine compositional similarity between vegetation and propagule banks. Results: Community propagule banks had high similarity indicating broad dispersal by lood waters. Wetland propagules were present in soils from channel bars to loodplain edges, despite declines in wetland vegetation with distance from chan- nel. Wetland communities in propagule banks were dissimilar from those in vegetation except on channel bars. Upland species (vegetation and propagule banks) increased with distance from channel. Propagules of upland species were most abundant in the litter, and were compositionally similar to upland species in most communities. Conclusions: Flood pulsing is one mechanism that drives spatiotemporal metacommunity dynamics in dynamic desert riparian ecosystems. The homogenized regional propagule bank created by lood pulsing provides wetland species with a mechanism to escape local extinction by allowing for recoloni- zation after looding creates suitable establishment conditions. Upland species are able to germinate from in-situ sources after small-scale looding or rainfall moistens soil. In luctuating environments, these dynamics sustain biodiversity in the face of ongoing environmental change. Keywords: Community dynamics; Disturbance dynamics; Population dynamics; Seed bank; Seed dispersal. Nomenclature: Kearney & Peebles (1960). Abbreviations: PAR = Photosynthetically active radiation; WIS = wetland indicator score. Introduction A metacommunity is a network of communities linked by dispersal of potentially interacting species (Hanski & Gilpin 1991; Wilson 1992; Holyoak et al. 2005). Metacommunity dynamics describe species dispersal between constituent communities and the consequent interactions between species within these communities (Holyoak et al. 2005). The theory that describes metacommunity dynamics gives ecologists a framework with which to understand and predict how landscape fragmentation impacts biodiversity and ul- timately ecosystem functioning (Holyoak et al. 2005). Much of the work on metacommunity theory has focused on modeling (Loreau & Mouquet 1999; Klausmeier 2001; Mouquet & Loreau 2002, 2003) and small ield experiments (Kolasa & Romanuk 2005; Miller & Kneitel 2005). Few attempts have been made to link metacom- munity theory to empirical knowledge by examining dispersal patterns at the landscape scale (Condit et al. 2002; Tuomisto 2003; Chase et al. 2005). Southwestern riparian ecosystems in the USA provide an excellent model in which to study metacommunity dynamics. The constituent communities of these spatially structured ecosystems are arrayed along a moisture and disturbance gradient and are connected by lood puls- ing (Patten 1998). In this arid environment, the steep gradients in depth to groundwater and declines in lood intensity and frequency that occur with distance from the channel produce strong vegetation patterns (Stromberg et al. 1996). The extreme lood regime of rivers in arid regions changes abiotic and biotic conditions within the lood- plain and drives rapid hierarchical patch dynamics (sensu Pickett & White 1985; Stromberg et al. 1993, 1997). Communities may not re-establish in the same location after lood scour and may be replaced by a different community type altogether. In this luctuating environ- ment, annuals and short-lived perennials comprise the majority of plant species (Wolden et al. 1994; Makings