Prescribed burning may produce refugia for invasive forb, Oncosiphon pilulifer

Prescribed burning is a common management technique to reduce non-native grass cover and seed availability in temperate forests and grasslands; however, its effectiveness in reducing non-native forbs is unclear. Litter of invasive forbs like Oncosiphon pilulifer are not consumed by fire like invasive grass litter is, resulting in residual singed stands and high heterogeneity in the postburn landscape. We investigated the potential for this incomplete burning to alter postfire establishment of native and non-native plant species by conducting a field experiment in a prescribed burn in Lake Perris State Park, CA. We investigated the role of microclimate and seed availability on establishment for 2 years following a prescribed burn in both singed stands and completely burned patches by adding or removing litter and adding native seed in a factorial design. Litter presence reduced soil temperatures and light availability, while singed stands had lower soil moisture and temperature. Litter present treatments had 5.6 +/- 5.9% (mean +/- SE) greater Oncosiphon cover yet doubled Oncosiphon viable seeds in the seedbank. Singed stands had 22.6 +/- 4.9% greater Oncosiphon cover and more than doubled Oncosiphon viable seeds. Native seed addition did not influence native or Oncosiphon cover. These results suggest that residual singed stands within the prescribed burn landscape can create a favorable microclimate and allow Oncosiphon to retain seed, increasing reinvasion. Our experiment suggests that litter increased establishment of non-native species as these species may better utilize postburn establishment opportunities impacting overall community recovery. Management of invasive forbs with prescribed burns may require secondary treatments to reduce reinvasion.

Automated summary

Prescribed burning is commonly used to reduce non-native grass cover and seed availability, but its effectiveness in reducing non-native forbs remains unclear. Residual singed stands of invasive forbs can create high heterogeneity in postburn landscape and increase reinvasion. A field experiment in a prescribed burn area found that litter presence reduced temperature and light availability, while singed stands had lower soil moisture and temperature. The presence of litter increased Oncosiphon cover and viable seeds, and singed stands had even greater Oncosiphon cover and viable seeds. The addition of native seed did not affect native or Oncosiphon cover. These results highlight the importance of considering secondary treatments to reduce reinvasion when using prescribed burns for managing invasive forbs.