WATER-HOLDING CHARACTERISTICS OF WEATHERED GRANITIC ROCK IN CHAPARRAL AND FOREST ECOSYSTEMS

Many chaparral and conifer species in California have root systems that penetrate to >6 m below the lower boundary of soil into fractured bedrock. Because soils in these ecosystems often are coarse textured, shallow, and extremely dry during much of the year, the underlying weathered rock is a potentially important source of plant-available water. We measured the water retention characteristics of weathered granite rocks from the San Bernardino and San Jacinto Mountains to assess their capacity to hold plant-available water. Rock samples were segregated by weathering class (WC), ranging from WC 2 (very weakly weathered) to WC 6 (well weathered). After saturating the samples on a wet sand table, volumetric water contents (theta(v)) at potentials to -1500 J/kg were determined using a pressure plate apparatus. Vapor equilibration was used to evaluate theta(v) at -5000 J/kg. Available water capacity (AWC) was considered to be the water retained between matric potentials of -10 and -1500 J/kg. Rocks in WC 2 to 4 (cannot be broken by hand) had AWCs of almost-equal-to 0.01 m3/m3, while rocks in WC 5 and 6 (can be broken in bare bands) had mean AWCs of 0.04 and 0.07 m3/m3, respectively. Soils in the area have similar AWCs, but are often <0.3 m thick, while the zone of weathered rock is generally several meters thick. Thus, weathered rock mantle is often capable of holding more water within a matric potential range accessible to plants than are the overlying soils.