Edaphic Factors and Invasive Annuals in Chaparral and Coastal Sage Scrub

The displacement of the Californian coastal sage scrub, an ecosystem marked by drought-deciduous shrubs, and hard chaparral, an ecosystem dominated by evergreen shrubs, to invasive annual species has been a growing issue in Southern California. Attempts to restore these shrublands on degraded sites have led to the conclusion that the presence of invasive annual plants present a major obstacle to shrubland restoration. Red soil patches in local shrublands appear to support fewer invasive annual plants and suggest that some edaphic factor in these red soils may suppress annual plant growth. Soil color is typically imparted by various iron oxides which also play an important role in nutrient sorption and redox reactions. During vegetation surveys conducted in April of 2018, it was confirmed that red soil sites have less invasive plant cover than the less-red soils, but nutrient analysis of the soils could not explain why. In the winter of 2019, I conducted studies to determine if Bromus rubens, an invasive grass, had lower germination rates on red soils than on less-red soils. Field germination trials suggested the germination success did not differ between the two soil types. Differential growth and/or the differential occurrence of safe-sites for seed deposition, therefore, might account for the observed differences of grass density on the two types of soils