COS 201-3 - Post burn heterogeneity contributions to recruitment of invasive forb Oncosiphon piluliferum

Background/Question/Methods

Prescribed burns are an effective management tool to reduce the litter and soil seedbank of invasive annual grasses in Southern California; however, their effectiveness on invasive forbs are unclear. Forb litter might not be consumed by prescribed fires, leaving singed stands behind. These stands may result in differential post burn recruitment by forming small refugia within the burn area, promoting reinvasion. These refugia might alter the processes of recruitment after fire through the facilitative effect of litter on microclimates or via higher seedbank survival. Here, we investigated how post burn heterogeneity influences recruitment of an invasive forb, Oncosiphon piluliferum. To test our prediction that singed stands would act as refugia, we set up a factorial experiment of burn patch (bare/singed stand) and litter (present/removed) throughout a two-hundred-acre prescribed burn area in the Lake Perris State Park, California. We measured plant composition at peak biomass over two years and evaluated seedbank composition in a greenhouse study to assess how post-fire conditions influence Oncosiphon post fire recruitment.

Results/Conclusions

Our results indicate that the heterogeneity in post burn conditions significantly contributes to Oncosiphon recruitment for at least two years after fire. There was 23% greater cover of Oncosiphon within the intact singed stands regardless of litter treatment, with a greater amount of Oncosiphon seeds present in the seedbank. There was 6% greater cover of Oncosiphon with litter than without regardless of stand status. Community composition was significantly influenced by Oncosiphon stands, litter treatment, and year. We also observed that cover of two exotic species, Erodium cicutarium and Bromus madritensis, were inversely related to Oncosiphon, suggesting they may be competitors that limit Oncosiphon post burn recruitment. Our findings suggest that the incomplete consumption of noxious invasive forbs during prescribed burns may yield refugia from which these forbs may spread both through retention of litter and through increased seed availability.