Restoration of degraded rangelands in southern California: Testing techniques on a landscape scale
Much of the heavily ranched land in California has been invaded by non-native species, often to such an extent that plant communities have undergone type-conversion to one consisting almost entirely of non-native annual grasses and ruderal species. Ecological studies have demonstrated the importance of competitive interference from these species, likely explaining their persistence after grazing has been removed. Although there is much interest in restoration, there are few projects designed to provide scientific information important to improving techniques, and likewise, there are few experiments conducted at the scale and with the tools practitioners utilize when implementing restoration. Restoration of coastal sage scrub and native grassland was initiated across a 25 hectare site, with some areas subdivided and seeded over multiple years. The following questions were studied over a four year period: 1) Do subsequent years of weed control reduce or change the composition of the germinating non-native seed bank? 2) Are certain seeding techniques superior, or favor certain species groups? 3) Does seeding species in functional groups result in greater cover or density compared to a single mix?, and 4) Does weeding at 4, 6, or 8 week intervals result in different levels of native cover?
Abundance of germinating non-native species varied more by year than the number of years of weed control, but the identity of species changed with control over time. Areas treated with goats, followed by mowing, had fewer germinating non-natives than areas treated with either goats or mowing, followed by herbicide. Native species density and cover, with weeding during establishment, had the opposite response. Native grasses established at greater density when drill seeded vs. broadcast, raked, and tamped, whereas coastal sage scrub had the opposite response. Coastal sage scrub functional groups seeded separately had greater cover than when seeded together, but this effect was not significant for the grassland community functional groups. Weeding at 4 or 6 week intervals resulted in greater native cover than at 8 week intervals. While one year of weed control was important in reducing non-native grasses, weather conditions in a given year were more important than additional years of weed control. Separating species into functional groups can allow one to optimize both the seeding method and subsequent growth, in addition to the option of incorporating selective herbicides post-seeding. Maintenance weeding post-seeding is important, occurring at a minimum of every six weeks during the growing season.