Skip to page content
USDA Forest Service

Research & Development Treesearch

Treesearch Home
About Treesearch
Contact Us
Research & Development
Forest Products Lab
International Institute of Tropical Forestry
Pacific Northwest
Pacific Southwest
Rocky Mountain
Southern Research Station
Help - We Participate  Government Made Easy

Global Forest Information Service

US Forest Service
P.O. Box 96090
Washington, D.C.

(202) 205-8333

You are here: Home / Search / Publication Information
Bookmark and Share

Publication Information

View PDF (0 bytes)

Title: Contributions of water supply from the weathered bedrock zone to forest soil quality

Author: Witty, James H.; Graham, Robert C.; Hubbert, Kenneth R.; Doolittle, James A.; Wald, Jonathan A.;

Date: 2003

Source: Geoderma. 114: 389-400

Publication Series: Scientific Journal (JRNL)

Description: One measure of forest soil quality is the ability of the soil to support tree growth. In mediterranean-type ecosystems, such as most of California's forests, there is virtually no rainfall during the summer growing season, so trees must rely on water stored within the substrate. Water is the primary limitation to productivity in these forests. Many forest soils in California are relatively thin, but are underlain by thick zones of weathered bedrock. Weathered granitic bedrock, the most prevalent lithology, has available water capacities (0.12-cm water/cm rock) that approach those of soils (0.2-cm water/cm soil) and, because the weathered rock zone is usually so much thicker (several meters) than the soil (<1 m), it almost always constitutes the larger storage reservoir for plant-available water. The weathered bedrock retains the original rock fabric and is friable and easily excavated, but the primary minerals are not thoroughly altered to clay minerals, so it is not considered saprolite. Roots of ponderosa pine (Pinus ponderosa) seedlings penetrate through the soil and encounter weathered bedrock within the first 2 years on many sites. Thus, the influence of the weathered bedrock zone on plant growth begins early. Root access to the weathered bedrock is restricted to fractures, which are often spaced about 50 cm apart. Water is extracted from the intervening rock matrix through unsaturated flow toward the root mat in the fractures and by mycorrhizal fungal hyphae that penetrate the rock matrix. At one site in the Sierra Nevada, 30-year-old Jeffrey pine (P. jeffreyi) depleted the soil-held water by mid-June and relied on water stored in the weathered bedrock until the rainy season began in October. In this case, the weathered bedrock supplied at least 70% of the water used by the trees during the growing season. In the same area, we found that thin soils are not a detriment to pine productivity when they are underlain by a thick zone of weathered bedrock. In mediterranean-type ecosystems, the weathered bedrock zone is an essential component of the plant water supply system and is thus an important contributor to forest soil quality.

Keywords: Ponderosa pine, Jeffrey pine, Site productivity, Regolith, Granitic rock, Roots

Publication Notes:

  • We recommend that you also print this page and attach it to the printout of the article, to retain the full citation information.
  • This article was written and prepared by U.S. Government employees on official time, and is therefore in the public domain.



Witty, James H.; Graham, Robert C.; Hubbert, Kenneth R.; Doolittle, James A.; Wald, Jonathan A. 2003. Contributions of water supply from the weathered bedrock zone to forest soil quality. Geoderma. 114: 389-400


 [ Get Acrobat ]  Get the latest version of the Adobe Acrobat reader or Acrobat Reader for Windows with Search and Accessibility

USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.