Fate of Hexazinone and Picloram after Herbicide Site Preparation in a Cutover Northern Hardwood Forest

Herbicides show promise to improve the efficiency and economics of forest stand conversion and regeneration. However, the impacts of herbicides on forest ecosystems and the ultimate fate of these chemicals are not completely understood. A major problem in pine regeneration in northern mixed hardwood forests is competition from fast-growing and easily sprouting species like red maple, northern red oak, aspen, and white birch. Three commonly used forest herbicides were applied to a cutover stand in Baraga County, Michigan, to study the ecological consequences of stand conversion and the movement and dissipation of herbicides in sandytextured spodosols. A mixture of hexazinone (1.7 kg/ha), 2,4-D (1.1 kg/ha), and picloram (0.3 kg/ha) was aerially sprayed in July 1982. Picloram and hexazinone residues were analyzed in soil solution and streamflow samples collected over a 4-month period after herbicide application. Both chemicals are widely used in forestry, are low in toxicity to fauna, and have soil as well as foliar phytotoxic activity. Picloram and hexazinone are highly soluble and thus have the otentfa'l to mOYe offsi;ze* Picloram is more persistent in the environment than hexazinone or 2,4-D, the other herbicjde used. Picloram moving through the soil profile peaked 2 weeks after application (40 ug/L) and hexazinone peaked (102 ug/L) 2 weeks later. Presence of an Oe horizon was instrumental in reducing herbicide residue movement deep into the soil by factors of 4 to 40. Streamflow contained only traces of hexazinone during the day of the application. Samples over the next 4-month period did not contain detectable residues. Picloram was never detected in streamflow. Water quality of streams adjacent to herbicide application areas was not affected. 1 Authors are, soil scientist, USDA Forest Service, Southeastern Forest Experiment Station and Soil Science Department, University of Florida, Gainesville, FL 32611; and research forester and chemist, USDA Forest Service, Southern Forest Experiment Station, Auburn University, AL 36849, respectively.