Can herbarium records be used to map alien species invasion and native species expansion over the past 100years?

Methods Using herbarium records from the Oklahoma Vascular Plants Database from 1903 to 2004, we reconstructed the spatial and temporal collection history of two alien invasive taxa (Lonicera japonica and Tamarix spp.) and three native expansive species (Ambrosia psilostachya, Amphiachyris dracunculoides and Juniperus virginiana). To compare the overall collecting trend, groups of native non-expansive taxa were selected as counterparts. We recorded the year of the first collection in each township in Oklahoma for all taxa. The cumulative number of occupied townships was log-transformed, plotted against time and modelled with linear regression. The slope of the linear regression represented collection trend over time for the non-expansive counterpart group. However, for the invasive and expansive species, the regression slope represented the collection effort plus the invasion or expansion rate. We calculated the proportion of invasive and expansive species to non-expansive species by dividing the cumulative number of townships for each invasive or expansive species by the cumulative number of townships occupied by the counterpart group (proportion curve).