Status of terrestrial vegetation and soils at San Bernardino National Wildlife Refuge

Hubbard, John; Hubbard, Tani; DeRango, Bethany

This report summarizes results of the initial terrestrial vegetation and soils monitoring in upland areas of San Bernardino National Wildlife Refuge (NWR) in Arizona. This project was conducted through a partnership between the U.S. National Park Service Sonoran Desert Network and the U.S. Fish and Wildlife Service Sonoran and Chihuahuan Deserts Zone. We established and sampled fifteen permanent field-monitoring sites in 2013. Our objectives were to determine the status of and detect trends in cover of vegetation, soil, and biological soil crusts; stability of surface soils; and frequency of vegetation. Despite at least a century of varied—and at times, intensive—land uses, terrestrial vegetation at San Bernardino NWR were generally within the expected range of natural variability. Twenty-five years of succession on fallow agricultural fields led to partial recovery of native sacaton grasslands—an important and regionally scarce vegeta-tion community that supports a variety of ecosystem functions and habitat for wildlife. More xeric upland vegeta-tion were also well within the spatial and temporal variance of the site and ecoregion. Upland areas on the western third of the refuge contained annual grass/tobosagrass grasslands over a shallow, dense claypan. Perennial plant diversity on these sites was very low – comparable with much more xeric low desert locations that we have sampled in the western Sonoran Desert. Although past land use may have been an important factor, we speculate that the unusually shallow claypan may dramatically reduce available soil water, limiting plant diversity and productivity beyond what would be expected based on precipitation. Although present throughout most of the refuge, non-native plants did not appear to have effectively invaded native plant communities. Encroachment by mesquite and other native woody plants was also limited, with the important exception of the recovering sacaton grasslands. Actively reducing mesquite on these sites should help maintain their integrity within the refuge. Unstable surface soil aggregates combined with relatively large areas of exposed soil indicated a high degree of soil erosion potential throughout the refuge. These conditions may reflect previous land uses (particularly intensive livestock grazing), although the extended period of recovery— more than 25 years—from those uses suggests that inherent soil factors may be a more important factor. Regardless, enhanced soil erosion could have tremendous direct impacts on ecosystem structure and function in upland ecosystems, as well as important consequences (through sediment and nutrient transport) for the aquatic systems that are the primary manage-ment focus and mission of San Bernardino NWR. We recommend managers focus on mitigating soil erosion from upland areas, and consider a site-specific soil inventory coupled with erosion modelling to more precisely assess soil erosion hazard and management options. We also recommend repeating this monitoring effort on 5-year cycles, beginning in 2018.

Status of terrestrial vegetation and soils at San Bernardino National Wildlife Refuge

Series