Abstract
Wetlands are vulnerable to plant species invasions and there is evidence that publicly owned wetlands may be at an even greater risk due to insufficient monitoring and control. Perennial pepperweed (Lepidium latifolium) has invaded thousands of hectares of marshland in the San Francisco Estuary, California, USA, while control options remain poorly studied. In this study, a novel method of control was tested along a gradient of increasing L. latifolium density in brackish tidal marshes of Suisun Marsh, within the Estuary. Chlorsulfuron (Telar®), restricted to at least temporarily non-flooded areas above the mean high tide mark, was applied at 0.104 kg ai /ha, following label instructions, to half of each of six L. latifolium-invaded tidal wetland sites during prolonged periods of lower tides and no precipitation in May 2006. Thought to inhibit successful re-establishment of natives, the thatch layer of dead L. latifolium from previous years' growth was removed in one-half of plots used to collect vegetation data within each site in December 2006. Chlorsulfuron was effective at reducing L. latifolium by 85% overall one year following treatment; however, two years following treatment, L. latifolium was reduced by 60% in older denser areas and 53% in more recently invaded areas due mainly to reinvasion from untreated edges. Though species richness did not increase significantly, resident plant cover did significantly increase in all chlorsulfurontreated plots, with some variability in change (increase or decrease) between individual species. Thatch removal did not affect any plant species, including L. latifolium, nor did it affect species richness or chlorsulfuron efficacy. There was evidence of a switch from a non-competitively structured plant community to a competitively structured one in older colonies treated with chlorsulfuron. Soil nutrients were not significantly different among treatments; however, several soil nutrients were significantly different among colony ages. These results provide important information pertinent to restoration and management of tidal marshes in Suisun and elsewhere in the San Francisco Bay and Estuary.