The Influence Of Wastewater Subsidy, Flood Disturbance And Neighbouring Land Use On Current And Historical Patterns Of Riparian Vegetation In A Semi-Arid Watershed

Riparian systems are among the most diverse and most threatened ecosystems in the western USA and consequently the focus of much conservation and restoration effort. Detailed mapping of existing riparian vegetation and quantitative description of historical riparian dynamics can inform and direct ecosystem conservation and restoration efforts. Using historical aerial photography, satellite imagery and field methods, we examine and compare historical riparian vegetation dynamics within catchments of varying land-use/water-use intensity along a 51-km stretch of the Santa Cruz River in Arizona, USA. We mapped current (2006-2007) vegetation to a formation class level (dominant life form) through aerial photo-interpretation and refined the map to an alliance level (dominant species) with extensive field data. We mapped historical vegetation to the formation level using six dates of aerial photography (1936-2004) and quantified rates of land-use and vegetation change over the 70-year period with spatial analysis techniques. Our results indicate that the current amount, distribution and diversity of vegetation alliances are linked to effluent supply. Sites receiving effluent since 1972 experienced a rapid increase in area of riparian forest and woodland from 1984 to 2004, followed by retrogression to herbaceous-dominated types from 2004 to 2006, the result of an extensive cottonwood tree die-off. Sites with no effluent subsidy are currently dominated by a mixture of riparian shrub and non-native herbaceous alliances but tend to have higher overall alliance diversity and exhibit more stability over time. These results suggest that the use of effluent for riparian restoration may have variable long-term effects depending on catchment land-use history.