Predicting height to live crown increment for thinned and unthinned loblolly pine plantations

Abstract

Several nonlinear, individual tree crown height increment equations were tested for their ability to predict annual crown height increment in loblolly pine plantations. The selected model contained tree height (HT), tree crown ratio (CR) raised to the one-half power, age (A), and a competition index (CI) for the distance-dependent model and the ratio of quadratic mean diameter to tree dbh (DR) for the distance-independent model. The distance-dependent and the distance-independent models were the same form, except for the expression for competition.

Hypothesis tests revealed that thinning, both its intensity and the elapsed time since its occurrence, had a significant effect on crown height increment. A thinning variable, THIN1, which accounted for thinning intensity and the interval since thinning, was developed and incorporated into the final individual tree increment models. Predictions of crown height increment were improved using models with the THIN1 variable as compared to those with no thinning allowance.

In another approach, existing crown height equation was modified to account for the effect of thinning on crown recession. Another thinning variable, THIN2, similar to THIN1, was added to the crown height model. This model yielded better results than its counterpart with no thinning variable; however, the improvement was not as great as for the increment models.

The individual tree increment models were also used to form a stand level crown height increment model. The independent variables were collapsed to stand-level statistics; the final model contained average height of dominants and codominants (HD), average crown ratio (R), age (A), and the THIN1 variable. Unlike the individual tree models, raising the average crown ratio to .5 did not improve the fit; however, including THIN1 did improve the results.

From this study it was concluded that better data, a standard definition of height to the live crown, and other crown variables, such as crown diameter, will be required to produce more refined individual tree crown height increment models.