Soil Types Impact Invasive Annual Plant Densities in California’s Chaparral and Sage Scrub

RM 216

Invasive annual plants displacing native plants in California’s chaparral and California sage scrub habitat is of increasing concern. With the deterioration of these natural ecosystems, examining factors that account for the incursion of invasive plants is crucial. In this study we examined the correlation between invasive annuals and soil color, which is indicative of its chemistry, mineralogy, carbon content, and genesis pathways. Various metal oxides and organic matter content are typically responsible for the color of a soil. Our study compares sites with red soils to sites with lessred soils. The red coloration of red soils is likely caused by hematite, an iron oxide, and the less-red soils colored by another iron oxide-- likely goethite. The different iron oxides not only affect soil color, but also play a role in plant nutrients. Hematite and goethite have been shown to react differently with cations and anions depending on the pH of the soil, which may impact critical plant nutrients. With soil chemistry differing with soil color, we also expected to see a difference in plant ecology associated with soil color. A survey measuring invasive annual plant density determined that red, hematite-influenced soils have fewer invasive annuals than less-red soil sites of similar slope and aspect. An initial study that compared important nutrient and mineral parameters in the two soils types did not show differences that could account for the differences observed in invasive annuals. Further studies will focus on the supply rate of nutrients to the soils, infiltration rates of the soil, and pH measurements.