An ecosystem’s invasibility is influenced by changes in biotic and abiotic resistances, which often occur due to shifts in the prevailing disturbance regime. The susceptibility of a community to intrusion by nonnative species may interact with propagule pressure to determine the extent of a biological invasion. We examined how propagule pressure, forest community structure, and disturbance interact to influence the invasibility of temperate Pacific Northwest forests by the newly invasive grass, perennial false-brome (Brachypodium sylvaticum). Our goal was to identify factors enabling shifts from establishment to population growth in B. sylvaticum populations at the edge of its expanding range. Ecological sampling methods were used to identify patterns in B. sylvaticum habitat. An inverse relationship between the amount of B. sylvaticum and all perennial vegetation types and soil litter depth was found, suggesting that disturbance might play a role in B. sylvaticum population establishment or growth. An experimental study was then performed to test the effects of disturbance, propagule pressure, and habitat on B. sylvaticum seedling establishment in sites where B. sylvaticum was already naturalized. We found evidence that disturbance of the soil and vegetation led to increased B. sylvaticum seedling recruitment within naturalized sites, especially where conditions of high propagule pressure and deciduous forest canopy existed. In contrast, B. sylvaticum populations dominated by coniferous forest canopy were much more invasible than deciduous forests and did not show increased seedling recruitment in response to our disturbance treatments. Our study shows how propagule pressure and plant community dynamics interact to alter the invasibility of Pacific Northwest forests allowing B. sylvaticum to transition from establishment to population growth thus allowing this weed to cause greater negative impacts on the ecosystem.
Nomenclature: Perennial false-brome, Brachypodium sylvaticum (Huds.) P. Beauv. BRSY.
Management Implications: The results of this study reinforce the growing consensus that avoiding or minimizing disturbance of native vegetation is one of the best ways to prevent exotic plant invasion. Native vegetation growing near Brachypodium sylvaticum populations or in areas at risk of B. sylvaticum introduction should be harmed as little as possible during human activities, whereas disturbed areas should be replanted with appropriate native plants as soon as possible after the disturbance. Our research also shows that propagule pressure (the amount and frequency of reproducible units entering a site), in this case seed rain and soil seed bank, is primarily responsible for the severity of B. sylvaticum invasion. Thus B. sylvaticum population growth and spread could be minimized by discouraging B. sylvaticum seed-set by mowing, weed-whacking, or hand-pulling the flowering culms before seeds have matured (typically before mid-August). Care should be taken to limit disturbance of intermixed native vegetation while doing this. Consistently cleaning shoes, clothing, and equipment that have come in contact with B. sylvaticum populations should also greatly help discourage its spread into new sites. Informational signage and cleaning stations positioned at trailheads frequented by the public where B. sylvaticum is prevalent may also help educate citizens of the role they can play to help conserve natural area biodiversity and facilitate their participation. Finally, while our study did not show hemlock mulch to suppress B. sylvaticum establishment, deciduous leaf cover (big-leaf maple, red alder, and black cottonwood)was associated with lower germination and establishment, and therefore may be a fruitful material for experimental