Evaluating the Influence of Topographic Variables for Predicting Productivity of Tree Species

Research Issue

Forest site productivity (expressed by site index, which is tree height at 50 years of age) in the southern Appalachian Mountains is primarily influenced by the availability of water for use by vegetation during the frost-free growing season. Height growth of most tree species is associated with topographic variables (e.g. elevation, aspect) and soil characteristics (texture, depth), which are surrogates for moisture availability. Prediction models based on those factors can be applied using GIS to map estimated site productivity in mountainous terrain for management purposes and possible climate change scenarios.

Existing productivity models for certain species have limited applicability at landscape scales because the relationships were developed at small watershed scales, where environmental conditions are relatively homogeneous. Information is needed generally more important than mapped soil series for assessing site quality in mountainous lands using digital data, but those variables account for relatively little variation of tree growth, height growth.

Our Research

In a cooperative study using NRCS field data collected from 41 stands throughout the southern Appalachian Mountains of western North Carolina, we evaluated the influence of six common soil and topographic variables on site index of two commercially valuable mesophytic tree species: northern red oak (Quercus rubra) and yellow-poplar (Liriodendron tulipifera). We found only one variable, landform index, had a significant effect on site index, but response varied by species: northern red oak had a lower predicted site index compared to yellow-poplar (see figure below). Similar research is planned for several xerophytic species.

Northern Red Oak and Yellow-Poplar Graph

Northern Red Oak and Yellow-Poplar Graph

Expected Outcomes

Publications in peer-reviewed journals and prediction models that can be applied using digital elevation data sets with GIS.

Research Results

  • McNab, W.H. 2010. Effects of landform on site index of two mesophytic tree species in the Appalachian Mountains of western North Carolina, USA. International Journal of Forestry Research. 2010: 1-7.

Research Partners and Collaborators

  • Natural Resources Conservation Service