Lack of native species recovery following severe exotic disturbance in southern Californian shrublands.
The effects of severe exotic disturbance (construction, heavy-vehicle activity, landfill operations, soil excavation and tillage, occurring 1-71 years previously) on shrub communities in southern California were investigated. These disturbances led to the conversion of indigenous shrublands to exotic annual communities with low native species richness. Nearly 60% of the cover on disturbed sites consisted of exotic annual species, while undisturbed sites were primarily covered by native shrub species (68%). Annual species dominant on disturbed sites included Erodium botrys, Hypochaeris glabra, Bromus spp., Vulpia myuros and Avena spp. The cover of native species remained low on disturbed sites even 71 years after initial exotic disturbance ceased. Native shrub seedlings were also very infrequent on disturbed sites, despite the presence of nearby seed sources. Only two native shrubs, Eriogonum fasciculatum and Baccharis sarothroides, colonized some disturbed sites in large numbers. Although some disturbed sites had lower total soil nitrogen and percentage organic matter and higher pH than undisturbed sites, soil variables measured in this study were not sufficient to explain variations in species abundances on these sites. Non-native annual communities observed in this study did not recover to a pre-disturbed state within typical successional time (<25 years), supporting the hypothesis that altered stable states can occur if a community is pushed beyond its threshold of resilience.