Root distribution and seasonal water status in weathered granitic bedrock under chaparral

Abstract

Soils in mountainous terrain are often thin and unable to store sufficient water to support existing vegetation through dry seasons. This observation has led to speculation about the role of bedrock in supporting plant growth in natural ecosystems, since weathered bedrocks often have appreciable porosity and, like soil, can store and transmit water. This study, within a chaparral ecosystem in southern California, was designed to determine the extent of rooting within weathered granitic bedrock and to measure the relative contributions of soil and weathered bedrock to water-use by chaparral shrubs ( Adenostoma fasciculatum Hook and Am., Arctostaphylos glandulosa Eastw., and Ceanothus greggii A. Gray). The rooting pattern was mapped from the wall of a trench excavated into the weathered bedrock. Water contents of soil-weathered bedrock profiles were measured at one to four week intervals for two years using a neutron probe. Chaparral roots penetrate deeply ( ≥ 4 m) into the weathered bedrock and are largely confined to joint traces. During summer dry seasons, the shrubs extracted 39.4 cm of water from a 2.9-m-thick zone of weathered bedrock -- accessing nearly ten times as much water as from the 0.35-m-thick soil (Typic Xerorthents). Although it is commonly neglected in ecological inventories and analyses, weathered bedrock can be an essential ecosystem component, particularly where soils are thin and seasonal drought occurs.

Citation

Sternberg, P. D.; Anderson, M. A.; Graham, R. C.; Beyers, J. L.; Tice, K. R. 1996. Root distribution and seasonal water status in weathered granitic bedrock under chaparral. Geoderma 72:89-98.