Plant-soil feedbacks and native coastal sage shrub recolonization of California exotic annual grasslands

Understanding the processes that contribute to plant species invasiveness is important for protecting agricultural and unmanaged lands from the harmful effects of exotic plant invasions.  Exotic plants that alter soil microbial communities or nutrient cycling in ways that favor their own growth, spread, and impact are predicted to be particularly competitive invaders.  The research presented here focuses on how plant-soil feedbacks constrain native shrub reestablishment in exotic annual grasslands of the California Channel Islands.  Although the grazers that initially converted the islands from shrubland to grassland have been removed, native shrubs, such as Artemisia californica and Eriogonum arborescens, have been slow to recolonize into grasslands dominated by the annual exotic Avena fatua.  To study the role of plant-soil feedbacks in regulating this dynamic, we examined how exotic grass effects on nutrient cycling differ from those of the native shrubs, and how these differences feedback to alter plant performance.  This was done by following soil nutrient properties over the course of one year to look for species effects on soils, a field fertilization experiment to test for nutrient limitation, and a soil transplant experiment in the field to test for feedbacks.  Artemisia tended to have higher nitrogen and phosphorus than either Avena or Eriogonum.  Avena, as shown by the fertilization experiment, is highly nutrient limited and in the soil transplant experiment obtained higher biomass on Artemisia soils than its own.  Net species pair feedbacks suggest that plant-soil feedbacks may contribute to the ability of Eriogonum to invade alien annual grasslands, while keeping Artemisia from doing so.