Effects of fire season and temperature on a spotted knapweed (Centaurea stoebe) infested grassland

Invasive species, including the non-native forb Centaurea stoebe (spotted knapweed), constitute an imminent threat to degraded and restored native prairies. Considering the major threat that C. stoebe poses to imperiled prairie ecosystems, I examined the effectiveness of fire as a control agent of C. stoebe and (±)-catechin. I conducted a 2-year experiment in part of a restored tallgrass prairie community at Pierce Cedar Creek Institute in Barry County, Michigan between May and August of 2016 and 2017. My experiment consisted of individually burning 60 1-m² plots with a propane torch to achieve high (316° C) and low (103° C) temperatures across spring and summer seasons over two years, then planting and seeding six native prairie plant species to monitor their establishment after burning. I compared the effects of the different burn treatments on the plant community by estimating percent cover and biomass of all species within each plot at the end of the field season in August 2017. I also examined the effects of the simulated burn treatments on (±)-catechin levels in the soil, which I quantified using High Performance Liquid Chromatography. On average, burned plots had 22 percent less C. stoebe cover and only onefifth as much C. stoebe biomass when compared to unburned control plots. Summer-burned plots had 16 percent less C. stoebe cover and less than one-third as much C. stoebe biomass when compared to spring-burned plots. Differences in burn temperature failed to produce significantly different results. Planted native grass biomass increased almost three grams more on average after spring burns than after summer burns. Preliminary findings also suggest that burning at high temperatures in spring may indirectly reduce soil (±)-catechin levels. Overall, these results indicate that prescribed burning is an effective tool for controlling C. stoebe and promoting native species establishment in restored tallgrass prairies