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In southern California more and more restoration projects involve control of non-native species, because most mitigation lands are degraded from their original condition, and invaders are good at taking advantage of disturbed habitat. |
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For example, fennel (Foeniculum vulgare) is an invasive species that dominated portions of the San Clemente site and the upcoming South Parcel Nature Park. It has deep roots, is very difficult to kill and has an abundant seedbank. A contrasting but still difficult problem is that of ripgut brome (Bromus diandrus) which dominates portions of Lagoon Island to the near exclusion of native species. It is an annual species that is easily pulled up by hand but is extremely abundant, making hand removal a laborious process. It also can readily regenerate from the seedbank. While control of species such as these can be achieved with persistent effort via mechanical or chemical treatments, long term control and project sustainability really relies on creating ecological resistance in the invaded site. Ecological resistance can be provided by creating a competitive array of native species that slow down or largely halt re-invasion of the site.
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Fennel plant in degraded field. Dead
stalks are flowering shoots from
previous year. Photo by J. Abraham. |
Dense ripgut brome grass (Bromus diandrus) with
scattered wild radish (also introduced) on Lagoon island
in area undergoing restoration. Photo by Alice Levine.
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Research into the sources and strength of ecological resistance has been ongoing in my lab for the last two decades and provides support for activities of the restoration practitioner. For example, in my research on highway iceplant (Carpobrotus edulis, from South Africa) invasion of coastal habitats in the 1980s, I demonstrated that although resistance is never complete and all the coastal sites I studied have the potential for some iceplant invasion, grazing of iceplant seedlings by native rabbits and competition from existing vegetation could greatly slow down invasion. Thus, restoration of a well established plant community along with its associated animals is the key to keeping this invader under long-term control. More recently I, along with students and postdocs in my lab, have studied sources and strength of ecological resistance in grassland communities in northern California and now here in Santa Barbara County. An important point that emerges from this work is that California native perennial grasses generally are very good at suppressing many non-native grassland species. Getting through the bottleneck of establishment, however, can be difficult for the native perennial grasses because non-native species likely outnumber native grasses in the seedbed by 5,000:1.
In two different northern California grassland sites, we tried using sawdust to promote microbial immobilization of available nitrogen prior to planting native grasses. Our working hypothesis was that non-native annual grass species, because they are much faster-growing than native grasses would be more hurt by the low soil nitrogen in sawdust addition plots than native grasses. While we did find some evidence that sawdust reduced soil nitrogen, and initially it reduced the competitive suppression of native perennial seedlings by exotic annual grasses, overall at the application rates we used, sawdust was not an effective tool for tipping the balance to natives. Other people have tried adding sugar or mixes of sugar and sawdust to the soil at high rates, and found that the growth of fast-growing weeds can be effectively reduced thereby benefiting slower growing, native species.
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