Diameter distribution models for thinned Taiwania (Taiwania cryptomerioides) plantations

Taiwania (Taiwania cryptomerioides), which grows mostly in
Taiwan, is a highly valued species and consequently requires
intensive management. We developed diameter distribution models
that incorporate the effects of thinning at different
intensities at different ages to aid forest managers with
meeting their management objectives. Our data consists of tree
diameter at breast height measurements from four thinning
treatments (control, light, medium, and heavy thinning) at ages
6, 11, 17, 25, 31 and 40. We entertained four potential
functions for our distribution model: a truncated normal, a
generalized Weibull, a four-parameter logit-logistic (LL), and a
two-parameter logit-logistic. The four-parameter LL function
resulted in the best diameter distribution model for our data.
We then modelled the parameters of the fitted LL diameter
distribution models as a function of time and thinning regime.
This allows the diameter distribution to be reconstructed for
various combinations of ages and thinning regimes. The
truncated normal function fit the unthinned diameter
distributions well. However, the four-parameter LL function
also fit the unthinned data reasonably well but was also able to
capture the effect of thinning on the diameter distributions.
We discuss some potential applications of the models.