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Title: REGOLITH WATER IN ZERO-ORDER CHAPARRAL AND PERENNIAL GRASS WATERSHEDS FOUR DECADS AFTER VEGETATION CONVERSION

Author
item WILLIAMSON, TANJA - UNIV OF THE PACIFIC, CA
item NEWMAN, BRENT - LANL, LOS ALAMOS, NM
item GRAHAM, ROBERT - UC RIVERSIDE, CA
item Shouse, Peter

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2004
Publication Date: 3/1/2004
Citation: Williamson, T.N., Newman, B.D., Graham, R.C., Shouse, P.J. 2004. Regolith water in zero-order chaparral and perennial grass watersheds four decads after vegetation conversion. Vadose Zone Journal. 3:1007-1016.

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 measure differences in soil water flux between grass and chaparral. During the study, we found that under average climatic conditions the grassland vegetation type used less water than the chaparral. This means more water was available to recharge groundwater or stream flow under grassland than chaparral. During dry climatic conditions we found no differences between vegetation types.

Technical Abstract: In 1960, areas of chaparral were converted to perennial grass after fire burned most of the San Dimas Experimental Forest in southern California. This conversion provided an opportunity to compare regolith moisture patterns of zero-order watersheds under native chaparral to those under non-native veldt grass. We collected data as a function of vegetation type and watershed element in order to test the hypothesis that conversion from chaparral to grass altered water distribution in the vadose zone as a result of changes in the physical environment, including rooting depth and soil horizonation. Patterns in vadose zone water distribution during the dry season, including soil water potential and residual flux, were significantly different in converted areas, reflecting the different rooting habits of the two vegetation types. In chaparral areas, there was no significant change in soil water potential between the surface and 150-cm depth; soil water potential was consistently below -1.5 MPa, reflecting the extensive root system. In grass areas, soil water potential was most negative close to the surface, where grass roots were most abundant. Plant available water was present below 100-cm depth, suggesting that recharge to the groundwater may occur under grass in average or wetter years. Under both vegetation types, the largest differences in residual water fluxes were near the soil - weathered rock contact. However, there was a significant relation between minor differences in fluxes and soil horizon boundaries, confirming the effects of vegetation conversion on soil properties and vadose zone soil water.