Soil and Weathered Bedrock

Virtually all of the commercial forests in the southern Sierra Nevada are on granitic terrain, where bedrock may be weathered to depths >15 m while soils are <1 m thick. Because plant‐available water is depleted in these thin soils by midsummer, study objectives were to characterize the edaphic role of the weathered bedrock relative to the soil. The site was a 30‐yr‐old Jeffrey pine ( Pinus jeffreyi Grev. & Balf.) plantation growing on relatively thin soils (75 cm in depth) overlying weathered granitic bedrock. The average depth to hard bedrock was 350 cm. A trench was excavated and physical and chemical properties of the soil and bedrock were evaluated. Cation‐exchange capacities (CEC) were lower in the weathered bedrock (Cr1 horizon = 4.6 cmol kg −1 ) than in the soil (A horizon = 13.4 cmol kg −1 ), but pH values were similar (4.6–5.5). Organic C content was negligible in the weathered bedrock matrix (<0.1%), but was higher within joint fractures (3.7%), where roots were concentrated, than within the soil A horizon (2.7%). Carbon/N ratios were much lower in the soil A horizon (19.6) than in the bedrock fractures (62.0). Saturated hydraulic conductivities ( K sat ) of the soil and the weathered bedrock were similar and high (8–11 cm h −1 ). Mean root length density (RLD) was greater within the joint fractures than within the soil, but on a whole rock basis bedrock RLD was much lower (<0.08 cm cm −3 ). Total plant‐available water storage capacity of 48.8 cm was calculated for the 350 cm thickness of regolith, with 14.7 cm (30%) contributed by soil and 34.1 cm (70%) by weathered bedrock. Weathered bedrock underlying soils is critical to the survival of forest ecosystems, particularly with regard to water supply, and should not be neglected in ecosystem site evaluations and models.