YELLOW BUSH LUPINE INVASION IN NORTHERN CALIFORNIA COASTAL DUNES - I - ECOLOGICAL IMPACTS AND MANUAL RESTORATION TECHNIQUES

We studied the ecological effects of the invasion of coastal dunes by Lupinus arboreus (yellow bush lupine), an introduced species, and used the results to develop manual restoration techniques on the North Spi t of Humboldt Bay. Vegetation and soil data were collected in five veg etation types representing points along a continuum of bush lupine's i nvasive influence. We collected data on the number and size of shrubs, vegetation cover, and soil nutrients. One set of plots was subjected to two restoration treatments: removal of lupine shrubs only, or remov al of all nonnative vegetation and removal of litter and duff. Treatme nts were repeated annually for four years, and emerging lupine seedlin gs were monitored for three years. Prior to treatment, ammonium and ni trate were found to increase along the lupine continuum, but organic m atter decreased at the extreme lupine end. Yellow bush lupine was not the most significant variable affecting variation in soil nutrients. A fter four years, nonnative grasses, including Vulpia bromoides, Holcus lanatus (velvet grass), Bromus spp. (brome), and Aira spp. (European hairgrass), were significantly reduced in those restoration plots from which litter and duff was removed. Native species increased significa ntly in vegetation types that were less influenced by lupine. By the t hird year, soil variables differed among vegetation types but not by t reatment. Bush lupine seedling emergence was higher, however, in plots receiving the litter and duff removal treatment. Based on these resul ts, we conclude that bush lupine invasion results in both direct soil enrichment and indirect enrichment as a result of the associated encro achment of other nonnative species, particularly grasses. Although tre atment did not affect soil nutrients during the period of this study, it did reduce establishment of nonnative grasses and recruitment of ne w bush lupine seedlings. Restoration should therefore include litter a nd duff removal. In areas that are heavily influenced by lupine and co ntain few native propagules, revegetation is also required.