Annual Species in Native Prairies of South Puget Sound, Washington

Abstract

Although it is well documented that the vast majority of native prairies in western Washington have disappeared, it is less clear to what extent the remaining fragments have been modified by the loss of native taxa. In this study, we focus on one group of plants—native annuals—that are notably lacking in prairies today. Based on current and historical records, we compiled a list of 190 native herbaceous plant taxa (species, subspecies, and varieties) with high or moderate fidelity to prairie and oak habitats that have been recorded in upland prairies in the South Puget Sound region. Eighty (42%) of these are annuals, a proportion that is considerably higher than what occurs in these prairies today (average 18%). In addition, most native annuals (75) are forbs. These data suggest that in the past, native annual forbs may have comprised a significantly greater proportion of the diversity in the prairies. Although it is impossible to determine how widespread these species were historically, several measures suggest significant declines have occurred. Of the 80 total native annuals observed, 39% have not been recorded in recent inventories of the floras in any prairies in the region, and another 21% are known from only one site. Data from recently burned prairies suggest that both cover and species richness of annuals may double after a single fire. To avoid further loss of native annuals, we urge the inclusion of species from this functional group in future prairie restoration in this region.