Incorporating thinning response into a loblolly pine stand simulator

Effects of thinning, such as increased diameter growth and decreased mortality in the residual stand, are largely the result of increased tree vigor induced by a decreased level of competition. These relationships are reflected in the models that are central to PTAEDA2, a growth and yield simulator developed for loblolly pine (Pinus taeda L.) plantations. A long-term thinning study served as a basis for attempting to improve the predictive output of PTAEDA2. The growth models were refit using data from plots that received different thinning intensity treatments. Height increment and mortality models were found to need no additional refinement to account for response changes due to effects of thinning. The refit diameter increment and crown ratio models in PTAEDA2 could not account for thinning effects in their original form, and thinning response functions that exhibit proper behavioral response were added to these models. Models were evaluated individually and in combinations in a reduced growth simulator that contained the growth subroutines from PTAEDA2. Results showed significant improvements in predictive ability when using the diameter increment model with thinning response variable. There were no significant improvements in crown ratio prediction when the crown ratio model with thinning response variable was utilized. The thinning response modifications resulted in unanticipated model behavior when incorporated into the stand simulator, indicating that careful evaluation of the behavior of stand-level prediction is needed. South. J. Appl. For.