The data

Vegetation information is from the Vegetation Type Map (VTM) survey of California. Conducted in the 1920's and 1930's, it remains the most extensive systematic sampling effort of California's vegetation. A total of 4288 VTM records (all the plots featuring oak species) are organized in a database including the data listed on this page.

The VTM survey plots were 0.081 hectare rectangles chosen to be representative of the vegetation map units being delineated in the field. Vegetation information consisted of the number of stems per diameter class of overstory tree species and the percent cover of each understory plant species. Original environmental information includes plot location, elevation, slope, aspect, soil surface texture and parent material.

Temperature, length of growing season and evapotranspiration data for each plot were derived by Evett (1994) from a series of county level isoline maps drawn by C. R. Elford, the California state climatologist, in the 1960's and 1970's.

Precipitation data originated from an isohyetal map (1:250,000) of thirty-year annual average precipitation (1950 to 1980) drawn from approximately 4100 stations by the California Department of Water Resources.

Average available soil water capacity was computed from the State Soil Geographic (STATSGO) GIS soils database (1:250,000) for California (USDA Natural Resources Conservation Service, National Cartography and GIS Center, P. O. Box 6567, Fort Worth, TX 76115).

Climatic zonations were derived from: (1) the generalized plant climate map of California [scale ca. 1:1,440,000], compiled by Sunset Books and published by Pacific Gas and Electric Co., San Francisco, CA. (1989); and (2) a map of modified Koeppen climates [scale ca. 1:2,000,000] drawn by James (1966).

Ecological zonation information is from a map of ecological units of California [scale 1:1,000,000] by Goudey and Smith (1994). Values for individual plots were interpolated and/or extracted from the maps using the GIS program Arc/Info (ESRI, Redlands, CA.).

Distance to the closest point on the coast was computed for each plot using Arc/Info and a FORTRAN program based on equations from Robinson et al. (1978). Potential cloudless solar radiation on a tilted surface (in MJ/m2) was computed as a function of latitude, altitude, slope and aspect using a FORTRAN program written by T. Rumsey (personal communication) based on formulas given by Duffie and Beckman (1980).

The GIS database of California's hardwood rangelands was developed by Pillsbury and others (1991) and revised by Pacific Meridian (1994) using remotely sensed data for the California Department of Forestry and Fire Protection.

(A complete list of references can be found in Vayssieres and Plant, 1998.)

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