Floral Diversity in Relation to Playa Wetland Area and Watershed Disturbance

Abstract

Abstract: There are 25,000–30,000 playa wetlands in the intensively cultivated Southern Great Plains of the United States. Knowledge of area and watershed influences on wetland flora are needed to guide their conservation. We surveyed plant‐community composition in 224 playas over 360,000 km2 and examined the relationships of species richness and diversity ( Shannon's) to playa area and watershed disturbance. The relationship of increasing species diversity to increasing area is most often hypothesized to be associated with an increasing number of habitats and/or larger populations as area increases. Watershed disturbance ( perennial grassland vs. annual cropland) was included to determine its relationship to floral diversity and bioinvasion by exotic species. For all (terrestrial and wetland) plant species and all playas, there were only marginal relationships (r 2 < 0.1) between area and richness and diversity. Analyses supported the hypothesis that the number of habitats affects playa plant diversity because number of habitats changed little as playa area increased. Tests on only wetland plant species suggested stronger (r 2≅ 0.2) relationships between richness and area. This relationship also was not likely related to increased population size, because large playas remain flooded longer than small playas, and therefore wetland plant species have a greater opportunity to become established. Finally, playas with cropland watersheds had more ( p < 0.05) exotic species, higher diversity, and fewer perennial species than playas with grassland watersheds. Because watershed cultivation has altered playa hydroperiod and increased frequency of disturbance, playas are associated with a flora dominated more by annuals and exotics. Conservation efforts aimed at preserving playa wetland plant diversity and native communities should focus not only on the area of the wetland but also on the condition of its watershed.