Evolutionary potential in a newly invasive grass in spite of severe phenotypic and genetic bottlenecks

Brachypodium sylvaticum, or slender false brome, is a Eurasian perennial bunchgrass introduced into Oregon near Eugene or Corvallis OR, sometime before 1939. Within the last few decades it has become widespread in northwestern Oregon, and in several locations it is an aggressive invasive weed. Moreover, it very recently colonized San Mateo County California and may pose a threat to native Redwood forests. Brachypodium sylvaticum’s recent range expansion in Oregon, and now California, presents a unique opportunity to study the spread and evolution of a newly invasive species. Low genetic diversity and uniform phenotypes throughout the invasive range suggest thatB. sylvaticum has been through a severe bottleneck and may not have sufficient additive genetic variantion to respond to selection. Yet little is known about the nature of additive genetic variation in this or other invasive species. We have been monitoring 12 native and 11 invasive populations in a common garden experiment for two years. We measured performance and life history traits including growth, flowering phenology, and biomass allocation traits. We estimated narrow sense heritability h² for Oregon populations using a half-sib design. Overall, native and invasive populations differ significantly for growth and several other traits (F_{1, 31.4} P < 0.0002). Populations nested within region also differed (F_{21, 946} P < 0.0001) for growth, tiller number, flowering time, and dichogamy. Contrary to expectation the invasive populations do not always outperform natives. However, within the invasive range significant family within population effects (F_{121, 708} P < 0.01) for growth, flowering time, and allocation to fruit, indicate these differences are heritable. If the measured traits are adaptive then differences among invasive populations suggest that selection may facilitate the invasion of B. sylvaticum in a variety of habitats.