Seasonal timing and intensity of fire alters the competitive relationship between the invasive legume, Lespedeza cuneata, and co-occurring native species

Background/Question/Methods

Exotic species can often invade and subsequently dominate communities in which they are not native, particularly when the disturbance regime of the native habitat has been significantly altered.  For example, in many areas throughout the world, humans have altered the fire regime in natural communities (typically by fire suppression), and this often leads to dominance by non-native species.  Although fire is frequently reintroduced to these systems under the guise of restoring communities to their native state, often little is known about the effects of fire on invasive species, making it difficult to predict how communities will respond.  We predict the success of an invasive plant following fire should depend on the invasive plant’s response to fire, aspects of the fire regime, and presence of co-occurring fire-adapted plants, such as native perennial grasses.  In old-fields near Saint Louis, Missouri, we examined how Lespedeza cuneata, an invasive legume, and the native plant community responded to prescribed fires during different phases of L. cuneata and the native community’s development  (early June, mid-July, October, January).  Within plots, we used a split-split design to also test for effects of dominant prairie grasses, Schizachyrium scoparium and Androgogon gerardii, and increased fire intensity (i.e., increased fuel loading).  

Results/Conclusions

Lespedeza cuneata resprouted after fires in all seasons and at all intensities with and without co-occurring perennial grasses.  Plant size before burning was positively correlated with likelihood of surviving and resprouting regardless of treatment.  A higher proportion of plants resprouted after high intensity burns compared to low intensity burns.  Seedlings were abundant in burned plots and rare in unburned controls immediately after fires in all seasons and at all intensities.  Time-since-fire was positively correlated with reproductive effort; plants produced more seeds after early growing season fires and in long unburned controls.  Less intense fires during the early growing season and the presence of perennial grasses decreased fecundity of L. cuneata and increased recruitment of native forbs from the seed bank.  Overall, these data indicate that the invasive L. cuneata is most successful after high intensity burns in dormant and early growing seasons where perennial grasses are absent.  Without the presence of established, perennial grasses, L. cuneata is more likely to survive and regenerate quickly following fires and out-compete slower-growing native forbs that resprout or reseed.   It is important to consider the life histories and population responses of invasive species to fire before using prescribed burns to restore degraded habitats.