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A Trait-based approach to restoring southern California habitats invaded by the Mediterranean grass Bromus madritensis ssp. rubens (Poaceae)
Non-native grass invasion is a major concern for land management and native plant conservation. In the southern San Joaquin Valley of California, human-disturbed environments that are heavily invaded by non-native grasses often contain some native annual species. These “weedy” native species apparently have the functional traits and life history characteristics to compete and persist in disturbed and invaded landscapes. We categorized several native plants into either a weedy or non-weedy habit based on several criteria, and confirmed this habit designation using a field trial where we controlled for other factors likely to influence species abundance under field conditions. We then measured functional traits for a group of five non-weedy species that occur locally but are not abundant in invaded areas, to a group of four weedy plant species that included the invasive grass red brome. We hypothesized that a suite of functional traits would define the weedy plant habit. We examined this hypothesis in a specific community, where we compared functional traits between weedy and non-weedy plant habits, and between native species and red brome. A principal components analysis (PCA) identified three distinct ecological clusters among the analyzed species (weedy native forbs, non-weedy native forbs, and grasses), and suggested the presence of a weedy functional type. Weedy species were different from non-weedy species in several traits (larger leaf area investment, larger root biomass allocation, heavier diaspores, and dormant seeds requiring cold-stratification for germination). The PCA suggested complementarity as an explanation for the co-occurrence of weedy native species and red brome, but individual trait similarities and a competition experiment suggested that competition can occur for some resources. Further understanding of the traits shared among weedy native plant species may lead to cost-effective approaches to restoring invaded landscapes.
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