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

To determine if the temporal and spatial pattern of alien plant invasion and native plant expansion can be observed using 100 years of herbarium data from Oklahoma, USA, and to eliminate herbarium collection biases in such analyses. Oklahoma, USA. 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). Maps of the collection records of invasive and expansive taxa illustrated no discernible spatial invasion or expansion pattern. The slopes of the linear regression for alien invasive taxa were significantly steeper than those of their associated native non-expansive counterparts, indicating an increase in abundance. Juniperus virginiana, L. japonica and Tamarix spp. exhibited one or more periods during which they were collected at a disproportionately higher rate than their native non-expansive counterparts. Patterns of species invasion and expansion in Oklahoma were detected using techniques developed for regions with longer collecting plant histories. The proportion curve analysis eliminated some biases inherent in herbarium data by reducing the effect of collecting effort. Both the regression model and proportion curve analyses illustrate the temporal invasion patterns of alien invasive species. The native species did not show a clear expansion pattern. The information found in recently established herbaria may not be sensitive enough to detect the increase in abundance of native species.