Many land management activities on Federal, State, and private lands involve the movement of vehicles and equipment along gravel roads and in off-road locations where plant propagules can be picked up and transported. When relocated to new areas, invasive and nonnative species of plants can become established and compromise the native ecosystem service and function. The objectives of this project were to quantify the capacity of different vehicle types to spread plant propagules, develop occupancy or risk maps of target invasive and non-indigenous species (NIS), evaluate the effectiveness of mobile wash units to clean different vehicle types, and use this information to develop a prioritization protocol to help prevent NIS spread within and between DoD CONUS (Continental U.S.) and OCONUS (Outside Continental U.S.) installations.

Adherence of soil to tracked, tactical wheel and civilian pattern vehicles and the effectiveness of mobile wash systems at removing soil were assessed in collaboration with the U.S. Forest Service. The rate at which seeds adhered to different types of vehicles was evaluated in collaboration with the military as part of training exercises. The rate of seed loss from a civilian pattern vehicle driven on paved and unpaved roads under wet and dry conditions was evaluated in a controlled experiment. Finally, sites were surveyed for invasive species and the data used to create occupancy maps.

All vehicle types gained considerable amounts of soil, with tracked vehicles gaining more than wheeled vehicles. The vehicle wash units removed approximately 80% of available soil matter, and 77% of seeds were destroyed by containment procedures. This demonstrated that mobile wash units can be used as a preventative tool to reduce the risk of propagule spread. Significantly more seed adhered to tracked vehicles than wheeled vehicles under both wet and dry conditions. For wheeled vehicles, there was a trend for more propagules to be gained on Humvees than tactical wheeled vehicles. And, as the distance Humvees travelled on unpaved roads and off-road increased, so did the number of propagules observed. In the more controlled study, researchers found that once soil and seed adhere to a vehicle, very little falls off under dry conditions when driving on paved or unpaved roads. In contrast, when the road surface is wet, soil and seed fall off, with the removal being more rapid on paved roads. These studies illustrate that vehicle type, road surface, and road condition need to be considered when evaluating the potential risk of vehicles becoming contaminated and spreading propagules.

Results of this project confirm that plant propagules adhere to, and can be spread by, vehicles, with these effects being amplified under wet conditions. Occupancy or risk maps of invasive NIS were generated that can be used to direct control practices and guide vehicle movement patterns away from concentrations of target species and consequently minimize invasive plant species dispersal. These maps can be used in conjunction with mobile wash units to help prevent the spread of invasive plant species within and between military installations and adjoining lands.