The purpose of this research was to quantify diameter distributions of stands using the Weibull
probability density function for 4 conifers, namely China fir (Cunninghamia lanceolata), Japanese
cedar (Cryptomeria japonica), Taiwan red cypress (Chamaecyparis formosensis), and Taiwania
(Taiwania cryptomerioides). These 4 conifers are widely planted in mountain areas of Taiwan,
and we focused on plantation forests. The study area included all of Taiwan, and the data were
from a system of permanent sample plots of the National Forest Management Plan Revision.
We utilized the Weibull function to model diameter distributions based on 3 approaches of the maximum likelihood estimator (MLE) and 2 percentile estimators [PE(I) and PE(II)]. After
examining the goodness-of-fit for the observed data and the Weibull distribution estimated
using the 3 approaches with the Kolmogorov-Smirnov (K-S) test, we found that PE(II) was
superior to the other approaches, and the respective pass rates of samples were
100, 96.6, 85.5, and 97.2% for China fir, Japanese cedar, Taiwan red cypress and Taiwania.
A similar trend of pass rates of samples indicated that PE(II) > PE(I) > MLE for each species.
The results implied that PE(II) was the best estimator among the 3 approaches for these 4
conifers.