Site Quality: What Will Grow Best?
Being able to determine the tree species that grow best on a particular site is a powerful tool for land managers. Conventional methods for determining site quality use tree age and height relationships that are difficult and time consuming to compute-and often result in inaccurate estimates when applied to upland hardwood forests.
Research forester Henry McNab, project leader David Loftis, and others have developed an alternative method that defines site quality based on the tree species in a stand. "Site quality is defined by where you are on certain gradients, for example, the gradient of dry to moist," says McNab. "There are characteristic sets of species that grow on different parts of the moisture gradient."
McNab and fellow researchers collect data on more than 500 permanent and temporary plots, correlating tree growth to climatic, geological, topographical, soil, and vegetational factors. For a study in North Carolina, the researchers ranked all tree species in relation to their relative moisture requirements and assigned an index value to each species, developing a system that can effectively classify site quality as high, medium, or low.
"Forest managers and private landowners can easily apply the system," says McNab, "If they can identify the tree species present in a stand, they can calculate a moisture index based on the relative moisture values we've determined for each species. From that, they can decide which tree species are best suited to the stand and estimate their growth rate."