A test of genetic divergence and local adaptation for Brachypodium sylvaticum, an invasive grass in Oregon

An important goal for invasion biology is to determine how genotypic plasticity and genetic divergence/adaptation contribute to the success of invading species. Brachypodium sylvaticum (false brome) is an invading grass that arrived in Oregon’s Willamette Valley sometime before 1939, and has subsequently spread across Oregon west of the Cascades. Using a common garden, we measured traits for B. sylvaticum plants collected from throughout Oregon (as well as the native range in Europe) to assess whether Oregon populations have genetically diverged. We used climate modeling and GIS data analysis to identify habitat characteristics for the sites sampled in Oregon, and then determined if plant variation observed in the common garden correlated with the local environment of the source populations. We measured traits that other researchers reported were different among European populations with different light environments (marginal leaf hair density, leaf shape, and chlorophyll a/b ratio). We found Oregon populations to be genetically diverged for traits measured in the common garden, including marginal leaf hair density (F=1.99, p=0.048), leaf shape (F=2.03, p=0.043), and flowering time (F=6.03, p<0.0001). The means for Oregon populations for several traits were significantly correlated with abiotic habitat characteristics (e.g., leaf hair & elevation, r=0.71, p=0.018; leaf shape & ave. daily temperature r=0.67, p=0.031), consistent with a role for natural selection in shaping the divergence among Oregon populations.