Road impacts to sheetflow-dependent ecosystems in the Sonoran Desert

Abstract

Roads affect ecological functioning at local to landscape scales, but little is known about the indirect effects on hydrology and vegetation outside road corridors. These impacts are of particular concern in arid landscapes such as the southwestern US, where road networks have rapidly expanded in recent decades. This study quantified impacts to vegetation where roads eliminated downslope sheetflow runon and identified how road and landscape characteristics affect the probability of sheetflow disruptions at Organ Pipe Cactus National Monument, in the Sonoran Desert of southern Arizona, USA.The abundance of perennial grasses, drought-deciduous subshrubs, and trees were significantly reduced in downslope plant communities where entrenched graded roads and above-grade paved roads diverted or impounded sheetflow. The remaining vegetation was dominated by annual herbaceous plants and the drought-resistant evergreen shrub creosotebush (Larrea tridentata [de Candolle] Coville), resulting in lower species and functional group richness. Sheetflow runon reductions also significantly decreased the growth, vigor, and survival of woody plants, particularly for the winter-deciduous tree velvet mesquite (Prosopis velutina Wooten).Hydrologic alterations affected the density and extent of nearby vegetation at 25% of 484 locations where roadways intersected sheetflow paths. Primitive roads such as off-highway vehicle routes produced the smallest topographic changes and were the least likely to disrupt runon, while graded roads had a 48% cumulative probability of ecohydrological impacts due to their tendency to become entrenched below ground. Paved roads were most likely to alter hydrologic connectivity due to embankments and roadside ditches and berms. Geologic substrate and soil type affected the probability of sheetflow impacts, which were most frequent on Quaternary alluvium and young soils lacking restrictive horizons. These results were used to map landscape sensitivity to sheetflow path disruption by roadways throughout Organ Pipe Cactus National Monument and to provide recommendations for sustainable management of existing and future roads.