Modeling diameter distributions of mixed-species forest stands

This study compares three diameter distribution models to fit mixed-species forest stands using four example plots with two or three species components in Daxing'an Mountain, PR China. The methods include (1) a finite mixture model (FMM) to fit two or three species components simultaneously, (2) a single Weibull function to fit the whole plot only, and (3) a single Weibull function to fit each species component separately and the summation of the individual species produced the whole plot. Our results indicated that Method 2 is only suitable to regular and unimodal diameter distributions with a balanced reversed J-shape. Method 3 may be able to fit each species component well if its frequency distribution is known and available in the data. However, Method 3 ignores the interspecies relationships within a given plot. Thus, the summation of the species components may not produce a good fit for the whole plot. In contrast, Method 1 (FMM) fits the species component distributions simultaneously with the constraint that the individual components add up to the whole plot, without requiring the observed frequencies for each species across the diameter classes. The FMM models are more flexible to describe highly skewed and irregular diameter distributions for the whole plot, as well as provide the acceptable estimation for each species component and the mixing proportions. Thus, the FMM models can be a useful tool for effectively managing mixed-species forest stands.