LATE SUMMER WATER STATUS OF SOILS AND WEATHERED BEDROCK IN A GIANT SEQUOIA GROVE

The natural occurrence of giant sequoia (Sequoiadendron giganteum) is restricted to a mid-elevation zone on the west slope of the Sierra Nevada, California, where summer rainfall is negligible. This study measured the late summer water status of regolith (soil + weathered bedrock) profiles in the Packsaddle Grove on the Sequoia National Forest to assess the distribution of available water by geomorphic position at the end of the dry season. Soil and weathered granitic bedrock samples were collected by auger during September 7–12, 1993, at 30–cm intervals to a depth of 270 cm on triplicated sideslope, swale, and drainage positions for both giant sequoia and non-sequoia microsites throughout the grove. The samples were stored in air-tight cans and were used for water potential and water content determinations in the laboratory. Soil and weathered bedrock water potentials ranged from 0 to −2.1 MPa, generally increasing (becoming less negative; indicating moister conditions) with depth and in drainage positions. No consistent differences between water potentials under the two vegetation types were apparent for any of the landscape positions. The Cr (weathered bedrock) horizons held a substantial quantity of plant-available water. A representative 270–cm thick profile consisting of 90 cm of soil underlain by 180 cm of weathered bedrock is calculated to hold about 44 cm of plant-available water at field capacity, 27 cm of which is stored within the weathered bedrock. Assuming 35% runoff losses, it is estimated that annual precipitation of <68 cm may fail to fully recharge the regolith profile on upland (non-drainage) sites and result in late season water stress. Such conditions of low precipitation have existed in 3 of the past 10 years. Giant sequoia growing in drainages receive additional water from runoff and throughflow; thus they are less likely to experience water stress.