PREDICTING CROWN-HEIGHT INCREMENT FOR THINNED AND UNTHINNED LOBLOLLY-PINE PLANTATIONS

Several nonlinear, individual tree crown-height increment equations were tested for their ability to predict annual crown-height increment in thinned and unthinned loblolly pine plantations. The selected model contained tree height (HT), tree crown ratio (CR) raised to the one-half power, age (A), and a measure of competition. The distance-dependent and the distance-independent models were the same form; however, the competition index (CI) in the distance-dependent model was replaced by the ratio of quadratic mean diameter to tree dbh (DR) for the distance-independent model. The individual-tree increment models were used as a basis for a stand-level crown-height increment model. The independent variables were collapsed to stand-level statistics; the model contained average height of dominant and codominant trees (HD), average crown ratio (R), and age (A). Unlike the individual-tree models, raising the average crown ratio to 0.5 did not improve the fit. Hypothesis tests revealed that thinning, both its intensity and the elapsed time its occurrence, had a significant effect on crown-height increment. A thinning variable, T, which accounted for thinning intensity and the interval since thinning, was developed and incorporated into the final individual-tree and stand-level increment models. Predictions of crown-height increment were improved using models with the T variable as compared to those with no thinning allowance.