Unconventional Estimation of Site Productivity from Tree Species Composition

Research Issue

The productive capacity of forest stands strongly influences their response to silvicultural treatments. Site index (the average total stand height at 50 years of age) is the method most used to evaluate site quality in eastern upland hardwood stands. Estimation of site index in mixed hardwoods is often problematic, however, because suitable sample trees are usually lacking in stands receiving a prescription for management, particularly on sites of intermediate or lower quality where oaks predominate. Estimated site index may be biased for many oak stands and result in erroneous classifications of productivity and response to management activities. An alternate method for evaluating site quality is needed for use in stands where oaks are important.

Our Research

Forest site productivity in the southern Appalachian Mountains is associated primarily with the availability of soil moisture during the growing season; stands on moist sites tend to produce greater amounts of woody biomass than stands on dry sites. Because the occurrence of many tree species in this region tends to be associated with a characteristic soil moisture regime, we hypothesized that the species composition of a sample plot could be used as the basis for assessing its moisture regime and predicting site productivity.

We developed a prototype method for predicting site productivity based on species composition for sites in the Bent Creek Experimental Forest. In this watershed of 6000 acres, site index of oak (Quercus spp.) dominated stands may be estimated with a mean square error of +/- 8.1 feet. The simplicity of such a site classification system is appealing because it can be easily applied, it is easily adapted to other ecosystems with their associated species and its underlying basis is easily understood by a range of audiences with different knowledge backgrounds. As illustrated in the figure below, we found oak site index was associated with an index calculated from tree species present on sample plots.

Expected Outcomes

The species composition method of estimating site quality has been developed and evaluated only in the Bent Creek Experimental Forest. Additional testing at other locations is desirable before the method can be recommended for general use. Results of this research will provide professional foresters and landowners with a field guide for classifying a forested site by its soil moisture regime and a prediction model for estimating site index for upland oaks.

Research Results

• McNab, W.H. 2010. Estimating site index from tree species composition in mixed stands of upland eastern hardwoods: Should shrubs be included? In: Jain, Theresa B.; Graham, Russell T.; Sandquist, Jonathan. Integrated management of carbon sequestration and biomass utilization opportunities in a changing climate: Proceedings of the 2009 National Silviculture Workshop; 2009 June 15-18; Boise, ID. Proceedings RMRS-P-61. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. P. 187-197.
• McNab, W.H.; Loftis, D.L. In Press. A preliminary test of estimating forest site quality using species composition in a southern Appalachian watershed. In: Guldin, James. Proceedings of the 2009 Biennial Southern Silviculture Research Conference. Hot Springs, AR. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. P. xxx-xxx.
• McNab, W.H.; Keyser, T.L. xxxx. Predicting site index from tree species composition in a Southern Appalachian upland hardwood forest. [Submitted to Southern Journal of Applied Forestry, Oct. 2012]

Research Partners and Collaborators

• North Carolina Wildlife Resources Commission