Seasonal prescribed fire variation decreases inhibitory ability of Poa pratensis L. and promotes native plant diversity

Abstract

Global biodiversity is threatened by invasive plant species. Without a thorough understanding of effective management strategies, minimizing their impacts while improving native species diversity will be challenging. Burning in fire-prone landscapes has been successful for managing invasive species and increasing native biodiversity, but it is unclear how specific fire regimes improve restoration practices in novel ecosystems where invasive plants have a similar growth phenology to native plants. We investigated fire as a restoration practice in the Northern Great Plains to decrease the cover of Kentucky bluegrass Poa pratensis (bluegrass), a perennial cool-season invasive grass phenologically similar to dominant native cool-season grasses, by 1) evaluating season of burn (early-growing season, late-growing season, and dormant season) in a field experiment to test differences in plant community composition and 2) manipulating fine fuels (3000–5000 kg ha−1) in an experimental approach to determine the effects of fire on plant survivability of selected native grasses and bluegrass. Bluegrass cover decreased 27% on all field burned plots the first year post-fire. Three years post-fire, late-growing season and dormant season treatments had 35% less bluegrass, whereas the early-growing season treatment was not significantly different from the control. Overall, fire altered the native plant community, with native plants more associated with burned plots. However, native plant community changes were only evident three years post-fire in late-growing season and dormant season burn treatments. In the experimental approach, native grasses and bluegrass experienced high rates of mortality (40–50%) at fuel loads above 4000 kg ha−1 and heat dosages above 30,000 °C·sec, features commonly associated with early-growing season burns. Therefore, early-growing season burns meant to reduce bluegrass may also impact native plants dominant in the region. Invasive species management is complicated in novel ecosystems when invasive plants have similar growth phenologies to native species. Control efforts should consider how different management strategies impact invasive and native plants alike. We found that late-growing season and dormant season burns, along with higher fuel loads, decreased bluegrass cover. Determining mechanisms of control in fire-prone landscapes is a crucial step to improving invasive plant control and increasing native biodiversity.