Reconstructing the historical spread of non-native plants in the North American West from herbarium specimens

The aim of this study is to demonstrate that the historical spread patterns of 10, in part poorly investigated exotic plant species can be reconstructed from herbarium records, using methods to account for herbarium collection bias. We collected herbarium specimen data of native and non-native plants from several North American herbaria, digitized and examined them at county scale. The year of the first collection in each county of the research area was recorded for all taxa. Cumulative number of specimen locations was plotted vs. time to construct invasion curves. For native species, these curves represent the spatiotemporal sampling distribution for herbarium specimens in the research area as well as the history of knowledge about geographical species distribution over time. Deviations from this curve provide information on the spread rates of non-native species over time. Periods of invasiveness become visible if the ratio of cumulative number of (a) locations of each exotic species and (b) locations of all native species for every year on record is plotted vs. time (proportion curves). We could identify main plant collection periods in the late 1930s, the 1970s and between 2007 and 2011. Starting in the 19th century, the intensive collection period of these plants was between 1980 and 2010. Invasion curves of three of the investigated alien species showed significant deviations from the native plant collection curve. Several highly variable periods of invasiveness could be detected for all studied non-natives, many of these periods occurring in the late 19th century. The calculation of proportion curves successfully revealed periods of invasiveness for the first time in the research area and minimized effects of collection bias.