Modelling the cone yields of Korean pine

Abstract

Korean pine is the most important plantation tree species in northeast China. Besides timber, it produces edible seeds. Economic comparisons between alternative management schedules of Korean pine plantations require information on tree growth but also on seed yields since Korean pine seeds generate significant economic returns. This study developed models for the cone yields of Korean pine. Data were collected from 24 permanent sample plots during 2004–2018. Model types suitable for modelling count data with a high proportion of zero counts, namely zero-inflated and hurdle models, were tested as alternatives to those types of Poisson and negative binomial models that did not include a sub-model for the excess of zeros. Correlations between observations collected from the same plot and tree were taken into account by fitting mixed-effects models. There was much annual variation in seed production, which was modelled by using indicator variables for different years. The other predictors of the cone models were tree diameter at the breast height, stand basal area, competitive position of the tree in the stand (basal area in larger trees) and site index. It was concluded that the best model types for predicting cone yields in simulation and optimization studies were negative binomial hurdle model and Poisson hurdle model. The same models were evaluated to be the most recommendable also in cases where probabilistic predictions are needed, for instance in stochastic optimization.