Locality: Southeastern Arizona, Cochise County, from the Mexican border to St. David, AZ (31.676400, -110.136000)

Abstract: The San Pedro Riparian National Conservation Area (SPRNCA) is located among the 'sky islands'of southeastern Arizona, a region of considerable environmental heterogeneity and biological diversity. The SPRNCA is approximately 69 km (43 mi) long, and 3-8 km (2-5 mi) wide, encompassing over 19,000 hectares (~74 mi2). The average elevation is 1200 m (4000 ft). A botanical inventory was conducted from February 2001 through November 2003 documenting 625 taxa from 92 families, including a new species of Eriogonum; five new state records, Calystegia sepium (Convolvulaceae), Carlowrightia texana (Acanthaceae), Mancoa pubens (Brassicaceae), Psilactis brevilingulata (Asteraceae), and Tagetes minuta (Asteraceae); and numerous under-collected and rare entities. The best represented families are Asteraceae (18%), Poaceae (17%), and Fabaceae (7%). Eleven percent of the flora is non-native. Fifteen percent are wetland plants. The flora of the SPRNCA is a unique expression of local climatic, hydrological, geomorphic, edaphic, and biotic conditions. Several factors contribute to species richness of the study site including perennial surface water amid arid surroundings, unregulated flood regimes, influences from surrounding floristic provinces, and variety in habitat types. The six major vegetation types are Chihuahuan Desertscrub, cottonwood-willow riparian corridors, mesquite terraces, sacaton grasslands, rocky outcrops, and cienegas.The San Pedro River is a fragile and rare ecosystem that is threatened by increasing demands on the groundwater that sustains its high levels of diversity. Addressing the social, cultural, and economic driving forces causing groundwater loss in the region presents significant challenges for land managers.