Modelling tree mortality across diameter classes using mixed-effects zero-inflated models

Abstract

The mortality of trees across diameter class model is a useful tool for predicting changes in stand structure. Mortality data commonly contain a large fraction of zeros and general discrete models thus show more errors. Based on the traditional Poisson model and the negative binomial model, different forms of zero-inflated and hurdle models were applied to spruce-fir mixed forests data to simulate the number of dead trees. By comparing the residuals and Vuong test statistics, the zero-inflated negative binomial model performed best. A random effect was added to improve the model accuracy; however, the mixed-effects zero-inflated model did not show increased advantages. According to the model principle, the zero-inflated negative binomial model was the most suitable, indicating that the “0” events in this study, mainly from the sample “0”, i.e., the zero mortality data, are largely due to the limitations of the experimental design and sample selection. These results also show that the number of dead trees in the diameter class is positively correlated with the number of trees in that class and the mean stand diameter, and inversely related to class size, and slope and aspect of the site.

Keywords

Tree mortality / Mixed forest / Zero-inflated model / Hurdle model / Mixed-effects

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