Minimizing effort in restoring chaparral shrubs on type-converted slopes
In California, non-native annual grasses compete strongly with seedlings of woody plants. This competition poses a challenge to restoring native communities, such as chaparral, where too-frequent fire or other disturbance has left sites dominated by these grasses.
Previous work on establishing native chaparral shrubs on grass-dominated hillsides suggested that, without grass control, first-year survival of transplanted shrub seedlings in grassland was low (approximately 12%), but with grass control, it was much higher (approximately 50%). Seedlings in that study were watered periodically. Because post-planting care of seedlings is time-consuming and costly, we compared the effectiveness of different levels of post-planting care in promoting survival of seedlings of three chaparral species: Rhus ovata, Adenostoma fasciculatum, and Eriogonum fasciculatum. In February 2014, 54 plots, approximately 1.5 m in diameter, were established on a grassy hillside and treated with herbicide. One seedling of each study species was then transplanted into each plot. Planting holes were charged with 6 L of water at time of planting. Thereafter, one third of the plots were watered by hand every two weeks; one third of the plots were supplied with a cellulose-based hydrogel; and one third of the plots received no further supplemental water.
One year following planting, there were no significant differences in survival among treatments for any species. Survival ranged from 44% to 56% across treatments for Rhus ovata, from 56% to 78% for Eriogonum fasciculatum, and from 67% to 83% for Adenostoma fasciculatum. Furthermore, size of survivors (height and canopy volume) did not vary among treatments for Rhus ovata or Eriogonum fasciculatum. Only Adenostoma fasciculatum exhibited a treatment effect, with canopy volume being larger among survivors of conventionally watered plants than among survivors that received no further supplemental water after planting. These results suggest that in an annual grass-dominated site in a Mediterranean-type climate, transplant success of shrub seedlings may be relatively high without continued supplemental watering as long as grass competition is low. This study was conducted during a record drought, which helped reduce background grass competition outside the weeded plots. It is possible that additional effort to reduce grass competition might be required during years with higher rainfall and more vigorous grass growth.