Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/2004
Publication Date: 10/30/2004
Citation: Williamson, T.N., Graham, R.C., Shouse, P.J., 2004. Effects of a chaparral to grass conversion on soil physical and hydrologic properties after four decades. Geoderma. 123:99-114. Interpretive Summary: Chaparral is a fire-prone plant community found on steep slopes where erosion is of major concern. Often post-fire rehabilitation in these plant communities includes seeding of non-native grass species in order to decrease the possibility of rain induced soil erosion. In 1960, a major wildfire burned through most of the San Dimas Experimental Forest in southern California. Some areas were converted to grassland vegetation and some were left to re-grow natural chaparral. This unique vegetation experiment gave us an opportunity to evaluate the impact of vegetation conversion on soil physical properties and related hydrology. Sufficient time has elapsed for the difference in vegetation to alter soil properties (A-horizon thickness and carbon distribution) and associated hydrologic conditions (bulk density, hydrophobicity, and the natural wetting front) in the SDEF watersheds. Accumulation of organic matter around the dense, fibrous, grass roots resulted in deeper topsoil that provide a gradual transition from the surface to subsoil. Transitional horizons are absent from chaparral watersheds. Soil morphologic changes in converted areas originated adjacent to grass plants and spread downward and laterally removing the layered effect that was observed in the chaparral soils allowing spatially homogeneous infiltration, distribution, and storage of soil water.
Technical Abstract: Forty years after conversion from chaparral to perennial veldt grass in the San Dimas Experimental Forest, we compared land surface and soil properties between areas of the two vegetation types. Our objective was to evaluate the impact of this vegetation conversion on the soil physical properties likely to impact zero-order watershed hydrology. In three watersheds of each vegetation type, surface cover and soils were described within five watershed elements. Surface cover is approximately 90 percent in both vegetation types, but the frequency of individual plants is significantly higher in converted watersheds, leading to significantly lower variability in surface cover between grass watersheds. In chaparral watersheds, very fine to very coarse roots extend laterally, and downwards, in all directions. Only very fine roots emanate from grass, forming a dense fibrous mass that is concentrated below individual plants. In areas converted to grass vegetation, A-horizon bulk density is significantly higher due to the development of transitional AB horizons that are absent from chaparral areas. Vertical changes between the surface soil and subsoil are more gradual under grass than under chaparral. The significantly higher frequency of grass plants caused concurrent transformation of much of the converted areas, removing the layered effect that is observed in the chaparral soils and allowing spatially homogeneous infiltration, distribution, and storage of soil water.