Polymorphic site index curves for two hardwood species in west Tennessee : sweetgum (Liquidambar styraciflua L.) and southern red oak (Ouercus falcata michx)

Present forest management systems require more and more accurate information on site productivity. The classical method of site evaluation has failed to provide the desired accuracy. Because of the inadequacy of classical site evaluation techniques, newer and more accurate techniques of site evaluation have been developed. The foremost of these new techniques is the construction of site index curves under polymorphic assumptions and through the use of stem analysis. This study was conducted to determine if locally constructed polymorphic site index curves were significantly different from published curves, since large differences could cause changes in yields and productivity classification, based on site index. Site index curves were constructed for southern red oak (Quercus falcata Michx.) and sweetgum (Liquidambar styraciflua L.) at Ames Plantation. The data were collected by stem analysis. Using a sigmoid growth model, the data were fitted by an iterative, nonlinear, least-squares procedure. Site index curves were constructed by averaging the curves of the individual trees in each ten-foot site class, and corrections were applied by use of a proportional ratio to force the curves through the proper height at index age. It was found that sweetgum exhibited polymorphism, while southern red oak did not. Published curves were found to overestimate site index in stands of less than 50 years of age and to underestimate site index in stands of over 50 years of age in the study area. There was a large divergence between the locally constructed curves and regional curves published by the U. S. Forest Service. These differences could lead to misclassification of site productivity at various ages and at various levels of site quality.