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

Authors: WILLIAMSON, TANJA - UNIVERSITY OF PACIFIC; GESSLER, PAUL - UNIVERISTY OF IDAHO; Shouse, Peter; GRAHAM, ROBERT - UC RIVERSIDE

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2006
Publication Date: 10/27/2006
Citation: Williamson, T.N., Gessler, P.E., Shouse, P.J., Graham, R.C. 2006. Pedogenesis-Terrain Links in Zero-Order Watersheds after Chaparral to Grass Vegetation Conversion. Soil Science Society of America Journal. Vol 70:2065-2074

Interpretive Summary: Soil and watershed development are explained by the complex interaction of environmental factors including parent material, topography, vegetation, and other organisms, and climate all working through time. Our goal was to look at the effects of vegetation type on the development of soils and watershed characteristics over a 40+ year period of time in the mountains of Southern California. We found that vegetation type can act as an independent landscape component that can affect topography, water retention and runoff and the spatial variability of soil factors including organic carbon, water storage, and redistribution of chemical nutrients. The impact of our specific findings will be an increased understanding of the role of vegetation in the control of soil formation and the relation of vegetation to water storage and runoff in our mountainous watersheds.

Technical Abstract: Four decades after a vegetation conversion from chaparral to grass, zero-order watersheds were compared in order 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 the spatial variability of depth to weathered bedrock and soil water content in terms of terrain variables. This research showed significant differences in topography and the spatial variability of soil characteristics between watersheds with native and converted vegetation, indicating that vegetation can act as an independent landscape component. In chaparral watersheds, convex slopes result in widespread infiltration and significantly higher storage of water on the hillslopes. Topography in watersheds converted to grass is more concave, relative to that in chaparral watersheds, 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. Stepwise regression showed that soil water content is controlled 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. The result is that grass watersheds drain water downslope, creating similar process and from in watersheds of varying sizes. For both depth to weathered bedrock and soil water content, prediction using the regression models is only successful in grass watersheds. This analysis indicates that terrain variables may be ineffective predictors of soil characteristics in dense shrublands where a dense canopy hides a non-uniform erosional environment. Vegetation effects on rainfall disposition suggest that if topography and soil characteristics are to be assessed at a resolution of 1m2, assessment of plant distribution, not simply canopy cover, might improve our understanding of soil characteristics.