Fall-Prescribed Burn and Spring-Applied Herbicide Effects on Canada Thistle Control and Soil Seedbank in a Northern Mixed-Grass Prairie

Abstract

Prescribed burning in Theodore Roosevelt National Park has played an important role in maintaining a natural ecosystem. However, changes in plant community dynamics caused by burning may have led to an invasion of weedy species such as Canada thistle (Cirsium arvense L.). The objectives of this research were to evaluate the effect of a fall burn before spring herbicide application on Canada thistle control and to evaluate the soil seedbank within Canada thistle infestations. Canada thistle stem densities initially were higher in the burned compared with the nonburned areas because plants were slower to emerge in the nonburned treatments. However, the effect was short-lived, and Canada thistle densities were similar in the burned and nonburned treatments by the second season following the prescribed burn. Canada thistle control averaged 78% 60 days after treatment with clopyralid, clopyralid plus triclopyr, or picloram when spring applied whether or not application was preceded by a prescribed burn. Control declined to less than 60% by 363 days after application. Grass cover increased from an average of 5% before treatment to 37% and 46% 60 and 425 days after herbicide application, respectively, regardless of burn treatment. Forb cover increased following a prescribed burn but was unaffected by herbicide treatment. Overall the number and variety of species in the soil seedbank was not affected by a prescribed burn. A total of 74 species (56 forbs, 13 grasses, and 5 other mesic species) were found in the soil seedbank. However, the majority of the soil seedbank consisted of nondesirable low seral and invasive species including Canada thistle and Kentucky bluegrass (Poa pratensis L.), which accounted for over 80% of the total germinated seed. Although a prescribed burn caused an initial increase in Canada thistle density and cover, the greater long-term concern may be the lack of desirable species present in the seedbank to replace Canada thistle once the weed is controlled.