Occurrence, fate, transport, and ecological effects of aerially applied herbicides in the effort to control invasive buffelgrass (Pennisetum ciliare syn. Cenchrus ciliaris) in Saguaro National Park

Science Center Objects

The Sonoran Desert lands that the Saguaro National Park (SAGU) has been tasked to protect are facing an unprecedented threat from buffelgrass (Cenhrus ciliaris), an invasive perennial grass that was added to Arizona’s noxious weed list in 2005. The buffelgrass invasion has been so pervasive that the U.S. Department of Interior issued a declaration in 2010 which highlighted a need for a “war on buffelgrass.” 

Buffelgrass management is the highest natural resource priority for the Park. Buffelgrass competes with native plant species for nutrients and water, produces dense monoculture infestations, and greatly increases the connectivity of dry fuels that quickly carry wildfires. A buffelgrass fueled fire will kill many of the native species while promoting regrowth of buffelgrass and opening space for other non-native plants to invade. Application of herbicide has been shown to be the only management tool effective at the scales requiring treatment, and due to physical and safety constraints, aerial application is the most promising treatment method under consideration for difficult-to-reach areas. However, knowledge of the potential biological effects of the herbicide, its degradation products, and adjuvant components of the treatment mixture is essential for resource managers who strive to sustain a balanced and healthy Sonoran Desert ecosystem at the park and in the surrounding environments.

Objective:

The initial phase of sampling will inform the park about the fate and effects of previous spraying applications. Remnant herbicide concentrations and occurrence will be useful for understanding the movement through the ecosystem and their persistence in the context of a semi-arid environment. If contaminants are present, these concentrations can be used to analyze associations between observed effects in canyon tree frogs and assess potential risk to the possible reintroduction of lowland leopard frogs.  The second phase of the study will be constrained to a controlled watershed and the focused application will provide the necessary data to understand the transport and loads of compounds and degradates through the watershed with various flow events. This phase of the study will also provide the much need information about exposure and effects to tadpoles and frogs. The sampling will carry over a few generations informing the Park about extended effects that could be affecting the population viability. The results of the study will provide the Park with an in-depth understanding of the occurrence, fate, transport, and ecological effects of aerially applied herbicides. 

 

Approach:

The study is a comprehensive approach for understanding the occurrence and distribution, and fate and transport, of glyphosate in surface-water and groundwater derived from past, recent, and ongoing aerial herbicide applications. Sediment and water will be sampled for glyphosate and degradate, aminomethylphosphonic acid (AMPA). Watersheds that have been sprayed will be compared to adjacent and similar areas that have not been sprayed. Concentrations found in water and sediment will be related to ecologically sensitive organisms using risk assessment techniques and reproductive biomarkers. The information gained through the study will allow the NPS to adjust or change the specifics of the herbicide application strategy by identifying unanticipated side effects of the application program and mitigating negative impacts of the program on the broader SAGU aquatic ecosystem. The broader ecosystem implications will be important to other agencies when they initiate their own buffelgrass remediation strategies.

 

Relevance and Benefits

Results from this project will provide Park staff with information needed to better understand the potential ecosystem effects of aerial herbicide treatments in Sonoran Desert environments. The information gained through the study will allow the National Park Service to modify the herbicide application strategy by identifying potential side effects of the application program and guiding the development of approaches to mitigate negative impacts of the program on the broader SAGU aquatic ecosystem. The broader ecosystem implications will be extremely relevant to other agencies in the Sonoran Desert when they initiate their own buffelgrass remediation strategies. The growing public awareness has prompted many questions about Park’s buffelgrass management strategy. The investigation will also supply the Park with the necessary information to properly educate the visiting public and park neighbors about the effects of the aerial treatment program.