OOS 5-5
Above and belowground resilience in coastal sage scrub
Southern California coastal sage scrub (CSS) has been highly invaded by Mediterranean annual grasses, and has undergone vegetation-type conversion to exotic annual grassland in many areas across its range. Factors that promote type conversion include increased fire frequency, anthropogenic N deposition, and drought. Some 54% of CSS exceeds N deposition of 7.8 kg N/ha/yr, a value considered to be a critical load for loss of diversity. CSS does not recover through natural succession when it is impacted by a combination of these factors, and is undergoing widespread restoration efforts to conserve habitat and many sensitive species. Limited restoration success in some areas has called into question whether competition from invasive species alone, or their feedbacks on soil chemical and biological properties is responsible for the difficulty in restoration. A series of studies is reported that assessed the effects of above- and below-ground restoration efforts on the plant community, including control of invasive species, assessment of seedbank density, planting native species, observations of mycorrhizal fungi, and measurements of pools and fluxes of N and C.
Results/Conclusions
Invasion caused an increased rate of mineral N cycling, but did not change soil respiration or total soil N and C compared to restored stands of CSS. Depleted levels of extractable soil N recovered upon weeding. Exotic annual grasses and native forbs of CSS have both fine and coarse arbuscular mycorrhizae (AM), while shrubs have only coarse AM. However, the functioning of fine AM is not affected by N deposition, while high N reduces the mutualistic response of coarse AM. The exotic seedbank was high in CSS, more than 10,000 seeds/m2 even in sites with dominant shrub cover. Restored sites may be reinvaded, and following fire may revert back to exotic grassland. Some coastal sites have been restored successfully, but drought and elevated N promoted grass reinvasion. Root/shoot ratios of shrub seedlings were reduced with elevated N, and seedling densities were lower in the dry season. The soils of sites with low N deposition appear to be resilient because pools of total N and C did not change and fluxes were restored upon removal of exotics. However, the exotic seed bank is persistent and/or reinvades, and many sites, especially those under high N deposition, will be challenging to restore.