Water source utilization and seasonal root dynamics of Pinus jeffreyi and Arctostaphylos patula on thin soils over weathered bedrock (California)

Much of the upland forested areas of California contain relatively thin soils that are underlain by weathered granitic bedrock that may be many meters thick. Plant roots inhabit bedrock fractures and, during summer drought, plants rely on moisture stored within the weathered bedrock to avoid moisture stress. The goal of this research was to evaluate the soil-like functions of weathered granitic bedrock in such a forested ecosystem in the Sierra Nevada, California. Using stable isotopes, water source utilization was determined for co-occurring Pinus jeffreyi and Arctostaphylos patula three times over both the 1997 and 1998 growing seasons in the Sequoia National Forest, where soils averaged only 75 cm thick but were underlain by up to 5 m of weathered granitic bedrock. During this same time period, minirhizotrons and a video camera system were used to observe seasonal root dynamics within the soil and underlying weathered bedrock at the site. Volumetric water content was determined simultaneously with stable isotope sampling and minirhizotron root observations, in 25-cm depth increments, so that patterns of water source utilization and root growth could be compared to patterns of, water depletion. Water source utilization for both pine and manzanita generally followed a similar trend as for moisture depletion, with moisture being primarily used first from surface soil zones and from progressively deeper zones, including weathered bedrock to a depth of several meters, as the overlying soils dried. Pine, however, had lower δD values than manzanita early in the growing season of 1997 (−81‰ compared to −77‰) suggesting a slightly lower functional rooting depth. Later in the 1997 growing season, manzanita δD values reflected opportunistic water uptake from summer rainfall, while pine δD values reflected water uptake from deeper weathered bedrock. In 1998, when precipitation patterns resulted in a very moist profile during much of the growing season, δD values between pine and manzanita were not significantly different and both plants exploited bedrock-derived water as soil water was depleted.; Minirhizotrons were not useful for quantitatively measuring root dynamics within the soil and underlying bedrock. Low accuracy in determining root length density was achieved due to high variability among minirhizotrons and inordinately low numbers of root observations in the soil (0–75 cm). Observed RLD was not strongly correlated with water content of the substrate, although r2 values generally increased with depth in the substrate. Although RLD did not vary significantly within each 75-cm depth increment over time, root growth appeared to continue within bedrock until a threshold water content approximately equal to 0.05 cm3 cm−3 was reached. Below 300 cm, RLD increased over the course of the study, since water content was consistently >0.05 cm3 cm−3 . The same trend was observed in overlying soil, except the threshold water content may have been slightly lower than for bedrock. This may reflect a higher proportion of active manzanita roots in surface soil horizons later in the growing season compared to pine, with manzanita shrubs exploiting soil water at lower water potentials. Both studies demonstrated the importance of weathered bedrock in meeting plant moisture requirements over summer drought.