Legacy earthen berms influence vegetation and hydrologic complexity in the Altar Valley, Arizona

Abstract

Across the American Southwest water development has played a critical role in managing rangelands. Earthen berms have been constructed throughout US rangelands to manage runoff and reduce erosion. The berms altered runoff patterns to increase soil moisture with positive local vegetative response. However, altered runoff patterns can be considered a disturbance that affects broader scale vegetation patterns. We hypothesized that the hydrologic impacts of earthen berms in semiarid rangelands will be reflected in contrasting upslope and downslope vegetation patterns. A supervised classification of grass, shrubs, and bare soil was performed using orthographic imagery taken in June 2016 to quantify the effects of 181 earthen berms in the uplands and floodplain of the Altar Valley in Southern Arizona, US. Intact berms blocked runoff, creating downslope runoff “shadows” within which the precipitation is the dominant water input. We documented more bare soil downslope of intact berms in comparison with upslope. Grass and shrub cover affected by berms were related to soil texture. Grass and shrub cover were not different above and below intact berms on fine textured soils, but on sites with coarser textured soil, grass cover was lower downslope of berms. Where breaches occurred on coarser textured soils, the up and downslope differences in grass cover diminished. This study points to the role of conservation structures in adding additional complexity to already heterogeneous landscapes by creating patchwork assemblages of vegetation and bare soil proximal to earthen runoff and erosion control berms.