Wetland plant community composition can be influenced by factors such as hydrology, surrounding land use, propagule sources and dispersal capabilities (e.g. Casanova & Brock 2000; Smith & Haukos 2002). Wetland vegetation composition and cover responds to hydrology and concomitant changes through time (Euliss et al. 2004). In addition, the surrounding vegetation matrix may influence the plant community of a habitat patch (Wiser & Buxton 2008). For example, land use adjacent to wetlands (e.g. 250–300 m) influences plant diversity and species richness via the abundance and distribution of propagules and the route by which propagules disperse (Houlahan et al. 2006). Surrounding habitat patches with more edges (e.g. roads and ditches) may increase the likelihood of introducing invasive species by providing corridors (Wilcox 1989; Parendes & Jones 2000). However, wetland seed banks are important for species that do not disperse readily. At a landscape scale, wetland area, wetland isolation and surrounding land use may influence plant species composition via the rate at which propagules are generated (Matthews et al. 2009a) and dispersed (Boughton et al. 2010). Wetland area also has been shown to be somewhat predictive of wetland plant richness as larger wetlands may hold water longer (Smith & Haukos 2002). Determining the strength of these factors in dictating wetland plant communities is important in selecting potential future restoration endpoints. If vegetation (e.g. richness, composition) is to be used as a restoration endpoint, an understanding of how wetlands and their surrounding landscape attributes influence the plant community is necessary in decision making.
Loss of wetlands reduces landscape connectivity for wetland-dependent species relying on these habitats for reproduction, productivity and dispersal. This, in turn, can decrease landscape level biodiversity (Semlitsch & Bodie 1998; Gibbs 2000; Houlahan & Findlay 2003). In the conterminous United States, more than 53% of historical wetland area has been lost to filling, draining and modification of wetlands (Dahl 1990). Degradation has resulted further in functional and physical loss of wetland area and species (Holland et al. 1995; Davis & Froend 1999). Species will succeed or fail in impacted wetlands as dictated by local and landscape factors coupled with species dispersal and tolerance of environmental gradients (van der Valk 1981; Wright, Flecker & Jones 2003). Conservation and restoration efforts require that we understand how wetland degradation impacts biota and the function and longevity of wetlands (Kentula 2000).
Approximately 25 000–30 000 playa wetlands in the Southern High Plains (SHP, c. 82 000 km2), USA, serve as focal points of biodiversity for plants, invertebrates, birds and amphibians (Haukos & Smith 1994). Playas are shallow, circular, depressional wetlands that formed and are maintained through combined processes of wind, waves and dissolution (Smith 2003). Playas average 6·3 ha (Guthery & Bryant 1982) and comprise c. 2% of the SHP landscape (Haukos & Smith 1994). Native prairie surrounding playa wetlands has mostly been converted to row-crop agriculture. Over the past 80 years (Smith 2003), intensive cultivation of the SHP has resulted in significant erosion of playa watersheds and unsustainable sediment accumulation in playas. Through enrolment in the Conservation Reserve Program (CRP; the dominant conservation program in the SHP, designed in part to conserve highly erodible soils throughout the US), unsustainable sedimentation from cropland surrounding playas should be curtailed. Playas embedded in the cropland landscape have lost more than 100% of their hydric-soil defined volume (Luo et al. 1997; Tsai et al. 2007). Playas with more cropland within their watersheds have shorter hydroperiods and higher water loss rates (Tsai et al. 2007, 2010). Playas surrounded by cropland also have fewer perennials and more exotic plant species than playas embedded in native grassland (Smith & Haukos 2002). However, influences of hydrology and landscape variables on vegetation communities have not been studied.
Our objectives were to test the influence of local and landscape variables on plant species richness, diversity and composition to determine their relative influence on plant communities in wet playas. We included local factors such as water depth, playa area, sediment depth and percentage playa volume loss that were hypothesized to impact plant presence and cover. While other studies have simply discussed land use as an influencing factor on wetland plant communities, we investigated more specific landscape scale factors such as percentage urban area, percentage CRP area and tilled index (ratio of tilled to untilled land within each watershed) to determine the impact of various anthropogenic land disturbances. We also incorporated edge density (i.e. total length of edges per area) to represent potential exotic source locations or dispersal routes (e.g. ditches along roads) and number of playas as potential source populations. We hypothesized that land use and surrounding playas are the dominant landscape factors determining plant species richness, diversity and composition (e.g. Smith & Haukos 2002; Houlahan et al. 2006; Matthews et al. 2009a).