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

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. WHAT WAS ONCE PERCEIVED as random soil variability has now been linked to our incomplete understanding of the relation between pedogenesis and landscape development (Daniels et al., 1985; Kachanoski, 1988). Soil characteristics vary across the landscape, interacting with hydrologic, geomorphic, and biologic processes. Terrain analysis enables the modeling of the spatial variability of these processes, and their interactions with soils, by providing a means to integrate topography with attribute data observed in the field (Moore et al., 1991; McSweeney et al., 1994; Slater et al., 1994; Evans, 1998; Montgomery et al., 1998). The ability to link data spatially allows an iterative analysis of landscapes, integrating regional data on geology, climate, and other landscape parameters with field and lab analysis at the mesoand microscale (McSweeney et al., 1994). Integration of topography with attribute data is most effective when small catchments and slopes are the basic unit of study (Kachanoski, 1988; Dietrich et al., 1995), providing the opportunity to focus on discreet hypotheses that relate topographic form to landscape process (Montgomery et al., 1998). Primary and secondary terrain attributes are recognized as standard means of linking topographic form to watershed process (Table 1; for complete reviews, see Moore et al., 1991; McSweeney et al., 1994; Montgomery et al., 1998). Primary attributes are calculated directly from a digital elevation model (Gallant and Wilson, 1996) and describe characteristics that control slope hydrology (Selby, 1982). Secondary attributes are derived from a combination of two or more primary attributes, and characterize the spatial variability of specific landscape processes (Gallant andWilson, 1996). A secondary attribute used in many studies to compare soil physical and hydrologic characteristics to local topographic form is ln(AS/tanS), where AS is the upslope contributing area, or the compound topographic index or CTI, and S is slope (Table 1). The CTI, also known as the topographic wetness index, has been combined with primary topographic attributes to explain patterns in A-horizon thickness, solum depth, and soil water content (Sinai et al., 1981; Hanna et al., 1982; Gessler et al., 2000; Chamran et al., 2002). Soil and watershed development are commonly explained by the interaction of environmental factors, including topography, parent material, vegetation and other organisms, climate, and time (Horton, 1932; Jenny, 1941). Vegetation is a factor that reflects local geology and climate, but is not normally an independent component of the landscape. Vegetation effects on soil genesis include increasing organic matter in the soil, controlling subsurface water via evapotranspiration and preferential flow, and physical disruption of bedrock structure (Joffre and Rambal, 1993; Canadell et al., 1996; Martinez-Meza andWhitford, 1996; Quideau et al., 1998). Effects of vegetation on landscape processes include protection of the surface from erosion by vegetative canopy and plant litter and increasing slope stability due to root cohesion (Schumm and Lichty, 1965; Reneau and Dietrich, 1987). In the USA, .1500 nonnative vegetation species have been established (Vitousek et al., 1996). Some of these species were introduced into natural ecosystems as part of land management practices. In areas of vegetation conversion, vegetation effects can be studied without being compounded by changes in geologic parent material or climate that might occur with different geographic locations. The SDEF (San Dimas Experimental Forest) provides an opportunity to evaluate the effect of vegetation conversion on terrain and soil characteristics. After a widespread fire in 1960, vegetation was manually conT.N. Williamson, Dep. of Geosciences, Univ. of the Pacific, Stockton, CA 95211; P.E. Gessler, Dep. of Forest Resources, Univ. of Idaho, Moscow, ID 83844; P.J. Shouse, USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507; R.C. Graham, Dep. of Environmental Science, Univ. of California, Riverside, CA 92521. Received 3 Dec. 2003. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:2065–2074 (2006).