Paleoecological and historical records of range expansion and invasive tendencies in Typha species of eastern North American wetlands

Typha latifolia, T. angustifolia and T. x glauca have recently begun to display invasive tendencies in many wetlands of eastern North America. Typha angustifolia is often assumed to be highly invasive, although it is not known which species is spreading most aggressively. Various hypotheses have been put forth for the observed expansion of Typha species, but little use has been made of historical data to better understand these biogeographic changes. Collection dates and localites for these species were recorded from 1320 herbarium sheets to capture changes in spatial and temporal distributions since the mid-19^th century. Records of Typha­-type pollen grains were also extracted from the North American Pollen Database (NAPD) to provide information on changing abundances of Typha species over the past 1000 years. Proportion curves comparing the relative increase of T. angustifolia to T. latifolia reveal a period of range increase in T. angustifolia in the early to mid-20^th century. From the mid-20^th century onwards, however, the two species increase at the same rate. Pollen data indicate that almost all relevant sites had an increase in the abundance of one or more Typha species over the past 1000 years, however, there was no significantly greater increase in one species compared to another. The results confirm that the range increase was considerably larger in T. angustifolia than in T. latifolia since 1880, but this period of expansion was confined to a thirty-year period of invasiveness in the first half of the 20^th century. Both Typha spp. have been increasing at the same relative rate since then.