Soil seed banks of invasive plant species of the Lower Mississippi River Delta

Abstract

The invasion routes of non-native species into estuaries can be difficult to determine. Purple loosestrife (Lythrum salicaria) is an invasive species in North America and occurs sporadically in the remote freshwater marshes of the Lower Mississippi River Delta in Louisiana, U.S.A. The origin of L. salicaria in this region, however, is not known. This species has been documented in the Lower Mississippi River in Louisiana since the 1980s but these observed individuals have disappeared and reappeared in the vicinity in subsequent years. We collected soil samples (i.e., soil seed banks) from all known past and present individuals at the time of sampling and attempted to germinate them in a greenhouse under different water regimes, but we did not find germinable seeds of L. salicaria. The lack of a persistent soil seed bank suggests that the establishment of new populations may be from plant fragments originating in either local or upstream sources in the river (e.g., Wisconsin glaciation region). Alternatively, the individuals could be spread from locally developed seeds that do not persist for long periods of time in the soil seed bank. We found only one other non-native species in the soil seed banks of this estuary (Cyperus difformis), although many non-native species exist in the standing vegetation. Knowledge of soil seed bank composition can give insight into the origin of regenerative material for vegetative establishment and inform management approaches to control non-native species.