Effects of timing of prescribed fire on the demography of an invasive plant, spotted knapweed Centaurea maculosa

1. Prescribed burning is a key tool used in prairie and savanna restoration projects. The timing and frequency of prescribed fires are important variables to consider because they can differentially affect the survival of individual plant species, including noxious exotic species. 2. We used annual censuses and population matrix analysis to evaluate the demographic consequences of burning on populations of the invasive species spotted knapweed Centaurea maculosa in a Michigan, USA, prairie restoration experiment. We compared spring, summer and autumn burns at two frequencies: annually (2000-03) and in alternate years (2001, 2003). We examined the effects of different seasons and frequencies of prescribed fire on the survival, growth and reproduction of C. maculosa populations, and used life-table response experiments and elasticity analyses to determine how season of fire affects population growth rates. 3. We found that annual summer burning was the only treatment that significantly reduced overall population growth rates. 4. The life-table response experiment analysis indicated which life stages contributed the most to changes in population growth rates under the different treatments. The summer burn treatments reduced population growth by reducing reproduction. 5. The different burn treatments altered the elasticity structure of populations, but in general reproduction of adults as well as survival of non-reproductive adults and juveniles remained important in all treatments. This indicates that future attempts to control C. maculosa should focus on combinations of treatments targeting these life stages. 6. Synthesis and applications. Management decisions regarding prescribed burning in grassland restoration projects should consider the responses of individual species already present in the system. In particular, it is important to consider the effects of burning on noxious invasive species that may or may not have different life histories and responses to fire compared with native species. Population matrix modelling provides a more complete picture of the effects of management on population responses compared with other measures based only on community composition and relative abundance. Depending on the goals of a restoration, non-traditional management options, such as summer burning in mid-west USA prairies, may be effective management tools for the control of invasive species.