SUBSOIL WEATHERED GRANITIC BEDROCK IN CALIFORNIA: PEDOLOGY AND ECOSYSTEM FUNCTION

Granitic bedrock underlies 20% of California's land area and is generally widespread in the western United States. Throughout much of this area, the transition from soil to hard rock consists of a weathered bedrock zone that is sufficiently friable to be excavated with a shovel, but differs from saprolite in that the feldspars and other weatherable minerals are not thoroughly altered to clay minerals. This weathered bedrock retains rock structure and fabric, but has acquired key soil-like properties that make it a critical component of ecosystems. In particular, the development of porosity opens the bedrock to infiltrating meteoric water and to the biological world. Water can move rapidly down joint traces and fractures (often 1-20 mm wide) and more slowly through matrix microfractures (30-500 µm). Capillary pores from dissolution pits, clay mineral masses, and small microfractures retain water in a plant-available state. Weathered bedrock has a plant-available water capacity (10-15%) that approaches that of overlying soils, but the weathered bedrock zone is often several times thicker (e.g., 4 m vs. <1 m), so it holds more water overall. Roots of conifers, oaks, and chaparral shrubs have been found to grow many meters deep in joint fractures, but they are too large to penetrate most matrix microfractures. Water extraction from the weathered bedrock matrix is facilitated by the symbiotic mycorrhizal fungi that infect the roots and have hyphae >2 m long, but narrow enough (<20 µm) to penetrate the microfractures. The hyphae exploit the porous weathered rock matrix and conduct water back to the roots confined within the joint fractures. In Mediterranean climates, plants rely on water stored within the substrate to survive the annual summer drought. The weathered bedrock zone is a key component of this substrate, often supplying plants with several times as much water as the overlying soil. Currently, neither geological nor soil survey databases supply sufficient information on weathered bedrock to support realistic ecosystem and hydrologic models.