Identifying Colluvial Soils in the Ridge and Valley to Refine Site Quality Estimates in the Upland Hardwood Forests.

The oak-dominated hardwood forests of the central Ridge and Valley physiographic province in southwestern Virginia are found on long parallel mountains that typically have steep sideslopes and shallow soils.  As a result, the species composition of stands is dominated by xeric and subxeric species such as scarlet, chestnut, black, and white oaks. Site quality tends to be fairly low overall.  However, colluvial soils occur at some locations on the sideslopes of the mountains. There are distinct differences in topography, soil attributes, site quality, species composition and tree quality on colluvial soils compared to residual soils on sideslopes in the Ridge and Valley. Although soils on colluvium deposits are distinctly different from adjacent soils formed in residuum, they are frequently not mapped as separate map units on existing published soil surveys of the region.  Therefore, it is often difficult to locate colluvium deposits unless extensive field surveys are conducted.  These are time consuming and require experience.  If the signature of colluvial soils could be identified using field data and geospatial modeling, land managers could target silvicultural treatment to the higher quality, more productive sites.  Our objective for the current study was to develop methods, based on field data and available digital data, to identify colluvial soils in the central Ridge and Valley of southwestern Virginia.  Two hypotheses were tested.  First, on the linear sideslopes of the Ridge and Valley, where site quality is typically low in stands with subxeric to xeric moisture regimes, we proposed that vegetation and topography could be used to predict colluvial soils.  Second, we felt that a topographic signature of colluvial soils could be identified geospatially with a digital elevation model.  We found the Moisture Regime Index and the Terrain Shape Index could be used to predict the presence or absence of colluvium deposits in the Ridge and Valley.  The basal area of yellow-poplar was associated with colluvial soils in the study area.   Mean GIS modeled slope change values of the upper boundary of the colluvial soils increased as neighborhood size increased.  No such change was seen within areas of residual soils.  This information will be used to refine modeled site quality estimates of the area at a scale that is sufficient for stand-level forestry management.  In the absence of accurate maps of site quality, these indicators, which are field and GIS-based, will be useful in delineating stands for site-specific management.