Understory Revegetation Enhances Efficacy of Prescribed Burning after Common Buckthorn (Rhamnus cathartica) Management

The use of prescribed burns to suppress woody invasive species like common buckthorn (Rhamnus cathartica) in temperate deciduous forests is often limited by fine fuel availability. This is particularly problematic in the period following mechanical removal of buckthorn, when fire has the greatest probability of preventing buckthorn from re-establishing dominance through remaining small individuals, resprouts, or seeds. Here, we test whether revegetating by seeding C3 grasses and forbs enhances fine fuel availability and subsequent spread and impact of prescribed burns in two semi-open forests (8-24% tree canopy light transmission) in Minnesota. We found seeding increased cover of grass litter by more than 12-fold and decreased bare ground by 73%. Consequently, seeded areas enhanced fire spread by 85% and resulted in a three-fold increase in the proportion of wood pyrometers fully consumed. One year after burning, seeded plots had 72% less woody cover compared to adjacent unseeded plots, and burned subplots had 33% less woody cover compared to adjacent subplots that were not burned. Our findings support the use of herbaceous seeding (particularly of Elymus grasses) in buckthorn removal projects. The positive effects of seeding on burn performance via increased fine fuel quantity outweighed potential negative effects of understory phenology on fuel moisture content and flammability. Thus, a combined approach of seeding and burning is likely to offer enhanced control of small buckthorn stems compared to either passive restoration or seeding alone.