Herbicide treatment of invasive Vinca major growing with endangered Galium buxifolium, an island endemic

Abstract

Galium buxifolium E. Greene [Rubiaceae] (sea-cliff bedstraw) is a small shrub restricted to San Miguel and Santa Cruz Islands, in the California Channel Islands. Almost all of the 26 known populations grow on vertical north-facing sea cliffs in native scrub, sandwiched between the sea below and non-native annual grasslands on the terraces above. A notable exception is a popula?tion at Pelican Bay on Santa Cruz Island, growing on the cliff and on thin terrace soils above the cliff in a stand of coastal bluff scrub that is recovering from more than a century of sheep grazing (Figure 1). Ironically, this stand is near the location of the historic Eaton Resort, a charismatic inn frequented by Hollywood glitterati in the early 1900s. Several landscape ornamentals planted there persist today in the area of the Galium population. Italian stone pine (Pinus pinaea) and eucalyptus (Eucalyptus spp.) tower over the site while the invasive groundcover Vinca major L. [Apocynaceae] (greater periwinkle) forms dense mats at the cliff edge that are spreading into the developing native plant stand. Wherever the Galium is found it is intermixed with other native scrub plants in dense communities on moist ocean bluffs. Historic notes and herbarium collections indicate that it may have been more widespread on nearshore terraces adjacent to sea-cliff populations before conversion to grassland. Since Pelican Bay is the only site that we know of where Galium is spreading onto the nearby terrace, we wanted to know whether the Vinca posed a roadblock to upslope population expansion. In 2005 we mapped Vinca and Galium at the site. We also measured Galium individuals to see where the smaller, younger plants were to better understand where the population is expanding. We observed that 1) both the Galium and the Vinca appear to be spreading from the cliff face upslope onto a series of rock outcrops, stone walls and benches, 2) the native scrub community is recovering at the site and 3) the Vinca appears to be moving into the native scrub where it overtops small plants, including those Galium in the smallest size classes. This observed pattern of Vinca displacement of native vegetation has been noted in other places, where it is treated as an invasive weed. We concluded that Vinca may pose a threat to the expansion of both the native scrub and the Galium population that it supports. Therefore, we worked with the US Fish and Wildlife Service and others to develop a research project investigating 1) best techniques for killing Vinca within the boundaries of an endangered plant population and 2) demographic response of Galium to the treatment. Our intent is to push Vinca back to the vertical cliff face to give the natives a chance to establish a vigorous stand. Our conservation goal is to encourage natural establishment of new Galium plants on the terrace along with expansion of the native coastal bluff scrub and Galium population. Our immediate treatment objective is to reduce live Vinca cover by 90 % on the accessible upslope portions of the habitat. Our recovery objective is no net loss of Galium plants 2006-2016.