Facilitative Interactions Among Native Perennial Shrubs and Native and Exotic Annuals in Recovering Coastal Sage Scrub

Facilitative interactions can have a powerful influence on the structure of plant communities and must be accounted for in efforts to restore disturbed and invaded habitats, such as the now rare coastal sage scrub (CSS) of California. In this study, I tested for evidence of facilitative effects by the native shrubs Eriogonum fasciculatum var. foliolosum and Artemisia californica on germination and survival of the CSS native annual forb Phacelia distans. P. distans seeds were planted near patches of native shrubs, both under the shrub canopy where any facilitative effects should be strongest, and 0.5 m away in the more exposed grassland. To determine whether the shrub-forb relationship was affected by the presence of invasive annuals, E. fasciculatum sites were assigned either invasive annual removal or non-removal control treatment; no removal treatments were carried out for A. californica. P. distans had significantly higher germination but lower survivorship under the canopy of E. fasciculatum. The results showed no overall effect of invasive removal, but there was a weak interaction effect with location; in shrub-canopy plots, invasive species further lowered survivorship. A. californica showed neither facilitative nor negative effects of this shrub species on either germination or survivorship of P. distans, in contrast with the results for E. fasciculatum. Although E. fasciculatum appears to facilitate the germination of native forbs under its canopy, it also seems to have a negative effect on survival. The weak interaction between location and removal to further decrease survival under the canopy when invasives are present, and the abundance of grasses growing under the canopy, suggest that native shrubs may facilitate the growth of invasive annuals as well as the germination of natives. Consequently, controlling invasive grass abundance may be necessary to capture the benefits of shrub facilitation for the restoration of native CSS herbs