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Pedology

Pedogenesis–Terrain Links in Zero-Order Watersheds after Chaparral to Grass Vegetation Conversion

^a Dep. of Geosciences, Univ. of the Pacific, Stockton, CA 95211
^b Dep. of Forest Resources, Univ. of Idaho, Moscow, ID 83844
^c USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507
^d Dep. of Environmental Science, Univ. of California, Riverside, CA 92521

^* Corresponding author (twilliam{at}pacific.edu)

Four decades after conversion from chaparral to grass, zero-order watersheds were compared to identify differences in topography and its relation to soil characteristics. Three watersheds of each vegetation type were topographically mapped and sampled at random points for depth to weathered bedrock and soil water content. Stepwise regression was used to explain spatial variability in terms of terrain variables. In chaparral watersheds, convex slopes result in widespread infiltration and significantly higher storage of water on the slopes. Topography of watersheds converted to grass is more concave, resulting in higher upslope contributing areas. This favors water convergence in the subsurface and results in significantly lower soil water content in grass watersheds. In chaparral watersheds, upslope average slope gradient best explains variability in depth to weathered bedrock. In contrast, slope gradient best explains depth to weathered bedrock in grass watersheds, suggesting that the uniform plant distribution localizes erosional processes. Soil water content is explained by depth to weathered bedrock and slope aspect in both vegetation types; however, a positive relation with profile curvature is the third indicator in chaparral watersheds, compared with an inverse relation with upslope average slope gradient in grass watersheds. The result is that grass watersheds drain water downslope, creating similar processes and forms in watersheds of various sizes. For both depth to weathered bedrock and soil water content, prediction using the regression models is only successful in grass watersheds. Thus terrain variables may be ineffective predictors of soil characteristics in shrublands where a dense canopy hides a nonuniform erosional environment.