The control of invasive woody species presents one of the greatest challenges for restoration and management of fire-dependent ecosystems. While the vast majority of prescribed burning occurs in the early spring, there is evidence that burning during other times of the year can offer more effective control of undesirable species. In this project, we asked how seasonality and fire residence time interact to influence stem damage and resprouting of two non-native invasive shrub species in southern Wisconsin. Common buckthorn (Rhamnus cathartica) and honeysuckle (Lonicera spp.) frequently invade forest understories, edges, and grasslands in the Upper Midwest. Managing these species with prescribed fire can be challenging given the inability to inflict damage to larger stems under low-intensity fires and their tendency to resprout after being damaged by burning. In spring 2016 we developed a “tree cooking” system with which we could reliably control fire intensity and residence times, and we burned the stems of small (1-2 cm diameter) and large (3-5 cm diameter) buckthorn and honeysuckle plants using a pair of propane burners. The stems were burned for either short (30-second) or long (120-second) durations derived from residence times of prescribed fires in this region within the same timeframe. We implemented the different treatments on individual plants (N = 480) during spring, early growing season, late growing season, and fall. Stem damage, top-kill, and resprouting were subsequently determined for each stem.
Initial results from the early growing season (end of June) and late growing season (end of August) burn treatments show that stem damage and top-kill were greater among the smaller stem size class and in those subjected to the longer fire residence time. However, stem damage and top-kill rates did not differ between the early and late growing seasons or between the two species. Stem resprouting was significantly greater among the stems subjected to longer fire residence time, but interestingly, resprouting was significantly lower for both buckthorn and honeysuckle stems burned in the early growing season compared to the late growing season. These results suggest that burning during the early growing season, when belowground carbohydrate stores are presumably at their lowest, could potentially help land managers control woody invasive plants such as buckthorn and honeysuckle by minimizing the number of resprouts on damaged stems following a prescribed burn.