Diameter Distributions in Even-aged Stands of Shade-tolerant and Midtolerant Tree Species

Abstract

Characteristics of diameter distributions in 28 even-aged northern hardwood (Acer-Betula-Tsuga) and upland oak (Quercus spp.) stands were investigated to determine if even-aged stands of shade-tolerant and midtolerant (intermediate) species can be consistently identified without direct age determinations. The diameter distributions of midtolerant species consistently approximated a normal distribution. Those of tolerant species were highly variable, ranging from unimodal to steeply descending in form. Distributions of tolerant species that departed strongly from a unimodal form were those of stands having a large admixture of midtolerant species. Confusion with all-aged stands in such cases can be avoided if classification of age structure is based on the diameter distributions of midtolerant species. Even-aged and all-aged stands can generally be distinguished on the basis of diameter distributions, but even-aged stands cannot be consistently distinguished from multi-aged stands in which young trees are sparse or absent. In such cases, a number of direct age determinations would be necessary. INTRODUCTION Data on age structure of forest stands are often needed in ecological studies, especially for quantifying the role of natural and man-caused disturbances in forested landscapes. The only feasible nondestructive technique for direct interpretation of age structure requires the extraction of cores from trees in sample plots. This procedure is time-consuming and is not feasible when extensive areas must be inventoried. Accuracy in age determination of large trees is limited by the frequent occurrence of heart rot and by the fact that the number of rings above the butt swell does not indicate the total age of the tree. Indirect assessment of age structure has therefore been frequently attempted, of which the most promising method has been the interpretation of diameter frequency distributions. To give a reasonable indication of age structure, diameter distributions must be plotted separately for individual species from small homogeneous stands (Hough, 1932). Even-aged stands of many species typically have unimodal diameter distributions that may show varying degrees of skewness at a young age but gradually approach a more symmetric normal distribution with time (Baker, 1923; Meyer, 1930; Hough, 1932; Schnur, 1934; Nelson, 1964; Mohler et al., 1978). All-aged stands of shade-tolerant species with only light past disturbance, on the other hand, are wellknown to have steeply descending, monotonic diameter distributions that can be approximated by the negative exponential and negative power functions (de Liocourt, 1898; Hough, 1932; Assmann, 1970; Leak, 1973, 1975; Hett and Loucks, 1976; Tubbs, 1977; Lorimer, 1980). A third type of age structure may occur in stands having several age classes that may or may not be of equal prominence. Diameter distributions in such stands may vary from near normal to irregular negative exponential in form depending upon the dispersion of age classes and the degree of shade tolerance of the species (Hough, 1932; Henry and Swan, 1974; Leak, 1975; Oliver and Stephens, 1977; Harcombe and Marks, 1978; Lorimer, 1980, 1983a). Some stands of this latter type are technically uneven-aged or all-aged but nevertheless developed after heavy destruction of the overstory, and the wide range in age merely reflects the range of ages among smaller trees present prior to the disturbance (Henry and Swan, 1974; Lorimer, 1983a). In other cases, the restricted or unbalanced age distribution may be due to other external