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Diameter distribution models for thinned taiwania (Taiwania cryptomerioides) plantations

Summary 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 assist forest managers to meet their management objectives. Our data consist 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 y. We entertained four potential functions for our distribution model: a truncated normal, a generalised Weibull, a four-parameter logit-logistic (LL), and a two-parameter logit-logistic. The four-parameter LL fonction 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 fitted the unthinned diameter distributions well. However, the four-parameter LL function also fitted the unthinned data reasonably well and was also able to capture the effect of thinning on the diameter distributions. We discuss some potential applications of the models.