Buffelgrass is a perennial grass that is invasive to the Sonoran Desert of the Southwest, where it threatens desert ecosystems by out-competing native plants and altering fire regimes, with the potential to transform the Sonoran Desert ecosystem from a diverse assemblage of plants to a grassland monoculture. The USGS has teamed up with the National Park Service (NPS), the USA National Phenology Network (USA-NPN), and the Sonoran Desert Buffelgrass Coordination Center (SABCC) to map where buffelgrass is located and detect when it is green. The ability to map the location and growing status of buffelgrass will enable the strategic application of herbicide, which is most effective when the vegetation is photosynthetically active. USGS scientists integrated ground observations of buffelgrass phenology collected by citizen scientists and professionals, a map of buffelgrass distribution in Saguaro National Park, climate information and MODIS imagery to understand the dynamics and relationships between these variables. Statistical analyses reveal strong correlations between observed buffelgrass greenness and current MODIS vegetation indices as well as between observed buffelgrass greenness and lagged precipitation values. In particular, buffelgrass has a dramatically stronger and more rapid response to precipitation events than native vegetation. Using this knowledge, USGS scientists have developed strategies to operationally 1) map buffelgrass locations regionally using historic MODIS and precipitation data, 2) predict buffelgrass green-up using current precipitation data and 3) confirm buffelgrass green-up using current MODIS data. Scientists are refining these strategies in collaboration with NPS personnel at Saguaro National Park to enhance invasive buffelgrass management activities at the park.
Figure: Photographs a buffelgrass plant taken monthly over the 2014 monsoon growing season. For scale, the rock in the lower right hand corner is about 6 inches long. Scientists exploit the rapid response of buffelgrass to rainfall using citizen science observations, remote sensing imagery, and precipitation data to map and monitor greenness timing and location.