Barriers to invasive infilling by Brachypodium sylvaticum in Pacific Northwest forests

The success of invasive plants is often dependent on continued natural and anthropogenic habitat perturbations but it is less clear whether invasive species are capable of infiltrating undisturbed plant communities (range infilling). We studied the ecology of Brachypodium sylvaticum (slender false brome), a Eurasian bunchgrass that forms dense and expansive monocultures in forests of Oregon’s Willamette Valley with the objective of assessing its potential to infill undisturbed forests near the edge of its expanding range. Observations suggest that B. sylvaticum may be associated with sites that have a history of disturbance, but infestations appear to be spreading in forest understory habitats. We assessed the effects of leaf litter depth and texture (proportions of coniferous and deciduous litter) on seedling recruitment in the field and in a controlled experiment. Coniferous litter facilitated the establishment and growth while the presence of deciduous litter severely reduced B. sylvaticum recruitment. Survival and growth of established B. sylvaticum seedlings was highest in the coniferous litter. Leaf litter treatments contributed to the retention of soil moisture and seedlings had increased biomass and more tillering with deeper litter. Field surveys partially corroborated these results; recruitment occurs at the edge of existing B. sylvaticum populations where leaf litter depth is moderate, but B. sylvaticum is absent from regions of the understory characterized by deeper and less dense leaf litter. These results suggest that B. sylvaticum has the potential to invade native understory communities, but the progress of invasion will depend on the frequency and intensity of perturbations of the accumulated leaf litter.