Brachypodium sylvaticum, a shade-tolerant, forest dwelling, and aggressive invasive grass native to Eurasia, is a noxious weed in California, Oregon, and Washington. This species could cause ecosystem collapse by altering forest fire regimes. To examine interactions with fire, we divided two Willamette National Forest sites into eight units and randomly selected half for treatment with prescribed fire in spring 2011. We assessed the effect of B. sylvaticum on fire (severity and intensity) as well as the effect of fire on B. sylvaticum (cover, seedling emergence, and dispersal). We found that B. sylvaticum cover decreased fire severity but had no effect on intensity. Furthermore, fire severity influenced B. sylvaticum cover; in areas receiving low-severity fire, the grass increased from 21 ± 15.05 to 34 ± 15.81%, but in areas of high-severity fire, cover remained consistently around 0% (0 ± 0% cover in yr 1 to 0.2 5± 0.25% in yr 3). In the field, prescribed fire decreased seedling emergence by 32% compared to controls, but not in an associated greenhouse experiment. However, in the greenhouse, severely burned plots had zero emergence, compared to 0.29 ± 0.14 seedlings low-severity m−2 plot. Fire severity also influenced dispersal in the field; we monitored plots with < 0.5% cover B. sylvaticum initially; when these plots experienced low severity fire, they had greater B. sylvaticum cover (increasing 1,200%), suggesting increased dispersal with less severe fires. High-severity dispersal plots did not experience increased cover. High severity fires have the potential to control the grass, but low-severity fires will likely increase its cover.
Nomenclature: False brome, Brachypodium sylvaticum (Huds.) Beauv.
Management Implications: In the conifer forests of the Pacific Northwest, the nonnative grass Brachypodium sylvaticum (false brome) is forming "lawns" in forests that have never had continuous grass cover. Can prescribed fire be used to control this invasive grass that decreases tree germination and growth? Our results suggest that if a uniform, high-severity fire can be achieved and sustained, this method of treatment can be used to control B. sylvaticum. Unfortunately, fires do not often burn uniformly, but instead usually create mosaics of varying severity across a landscape, thus rendering this objective challenging. The stakes are high given that we showed that low-severity burns actually increase the seedling emergence, apparent dispersal, and cover of B. sylvaticum. Therefore, it is difficult to prescribe fire to control the grass. However, if prescribed burns are being used anyway for other reasons, or if natural fires occur, then there is opportunity for controlling B. sylvaticum by adding in on-the-ground control methods (such as herbicide or mechanical removal) in the less-severely burned zones.