Controlling invasive woody species presents a major challenge for restoration and management of fire-dependent ecosystems. Common buckthorn (Rhamnus cathartica) and honeysuckle (Lonicera spp.) are exotic invasive species that frequently degrade forest understories, edges, and grasslands in the Upper Midwest. Black (Quercus velutina) and Hill's oak (Quercus ellipsoidalis) are native species that can be invasive in prairie and barrens restoration projects. While prescribed fire is often used to manage these species, it can be difficult to inflict damage to larger stems under low-intensity fires and these species tend to vigorously resprout after being top-killed. While most prescribed burns occur in the early spring, there is evidence that burning during other times of the year can offer more effective shrub suppression. In this project, we ask how seasonality, fire temperature, and residence time interact to influence damage and resprouting of woody species. Fire intensity and residence times were controlled using a propane torch on individuals of Hill's/black oak, honeysuckle, and buckthorn in several locations across southern Wisconsin. Stems were burned for either 15-second or 30-second intervals and hot (>246° C) or moderate (125-175° C) temperatures during the early growing season, late growing season, and fall of 2017 and spring of 2018. We selected 20 plants per treatment per season plus 20 control plants (n=340 per species). Top-kill and resprouting were assessed in July 2018.
Results/Conclusions
Results suggest that, in general, higher temperatures and longer residence times are more effective at top-killing treated stems, and those stems that suffered greater damage due to higher temperatures and longer residence times typically had more resprouts. The role of seasonality varied among the three species. For the oaks and honeysuckle, treatment during the growing season was more effective at top-killing the plants than in the spring or fall, but seasonality had little effect on top-kill rates for buckthorn. The effect of burn season also varied among the species with respect to resprouting ability. Oaks showed the greatest resprouting in the early growing season, whereas honeysuckle resprouted most vigorously after the spring and fall burns. For buckthorn, there was no significant effect of burn season on numbers of resprouts. The findings from this study suggest that while variability in fire intensity can strongly impact the ability to control woody invasive plants, the efficacy of burning during the growing season or in the fall to control woody invasives could vary from species to species or based on site conditions.