Effects of a chaparral-to-grass conversion on soil physical and hydrologic properties after four decades

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% 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. D 2004 Elsevier B.V. All rights reserved.