Propagule Pressure and Disturbance Drive the Invasion of Perennial False-Brome (Brachypodium sylvaticum)

Abstract

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.

Inverted invasions: Native plants can frequently colonise urban and highly disturbed habitats

“…). Our findings parallel the negative relationship between disturbance and the abundance and cover of native species that is typically found by studies of non‐native invasion of natural ecosystems (Lake & Leishman ; Taylor & Cruzan ). However, our study was the first to quantify this relationship for native species in novel habitats.…”

Common mycelial networks impact competition in an invasive grass

“…Seeds of Brachypodium sylvaticum and its native competitor Bromus vulgaris were collected from several sites within Milo McIver State Park, Estacada, Oregon. Within Milo McIver, B. sylvaticum is distributed along elevational, moisture, and light gradients (Taylor and Cruzan, 2015) ranging from dry or drier upland, coniferous forests to riparian habitats and deciduous, lowland forests. The native competitor B. vulgaris was chosen due to its similar environmental preference and proximity to B. sylvaticum within the majority of sites sampled.…”

“…Slender false brome is ideal for use in this study because its performance and competitive ability has been found to be influenced by mycorrhizal colonization (C. Lee, Portland State University, personal communication; R. Workman, unpublished data). It is also ideal for use in examining invasive infilling processes—the potential for the invasive to invade intact native plant communities—not the likelihood of primary invasion success or how the exotic plant might establish in a new environment (Taylor and Cruzan, 2015; Taylor et al, 2015). Understanding the potential roles of CMN and root interactions on competitive outcomes is important for predicting the infilling success and impacts of B. sylvaticum on native plant communities.…”

Selective differentiation during the colonization and establishment of a newly invasive species

“…In native regions, plants can become infected with an endophytic fungus which inhibits seedling survival (Roy et al ., ), while the infectious endophyte does not occur in invasive regions. Additionally, B. sylvaticum invasion is associated with sites that have a recent history of physical disturbances, such as logging, that aid primary colonization (Taylor & Cruzan, ). Moreover, several mechanisms have been found to reduce the effects of bottlenecks in the invasive region for B. sylvaticum .…”