Empirical diameter distributions and predicted yields of thinned loblolly pine plantations
A flexible method to approximate diameter distributions in forest stands was developed. Different functions, each in the form of a modified Weibull cumulative distribution function (cdf), were joined together to form a segmented cdf. Five percentile points (the 0th, 25th, 50th, 75th, and l00th percentiles) were used to determine a segmented cdf. Based on the one-sample KolmogorovSmirnov goodness-of-fit statistic, the segmented cdf was found to perform almost as well as the Weibull distribution for unthinned data and to be superior in case of thinned stands, especially when the data were irregular.
A growth and yield model for thinned loblolly pine plantations was developed based on data from 128 0.2-acre permanent plots in the Virginia Piedmont and Coastal Plain. The segmented cdf which characterized the diameter distribution of a stand was searched for such that the resulting total basal area and average dbh estimates were identical to those predicted from stand variables using regression equations. Program THINNED was written in standard FORTRAN to provide stand and stock tables for thinned old-field loblolly pine plantations.
Trials with several thinning intensities at ages 15, 20, and 25 and a final harvest at age 30 showed that, for average sites (site index 60 feet at base age 25 years), residual basal areas of 95 square feet per acre resulted in approximately the same total volume (volume removed in thinnings plus final harvest volume) production as unthinned stands. Heavier thinnings (i.e. residual basal areas of less than 95 square feet) resulted in a slight decrease in total volume production, whereas residual basal areas of more than 95 square feet showed a slight increase in total volume production.
Estimates from the new model was compared with those from Coile and Schumacher (1964). Total volume production predicted by program THINNED was little affected by thinning, as confirmed by many researchers. The Coile and Schumacher model, however, predicts higher stand response to thinning and consequently its predicted total volumes were consistently higher than those from program THINNED. On the other hand, total volumes resulting from periodic thinnings predicted from the new model agreed closely with observed yields reported by Goebel et al. (1974).