Modelling Diameter Distribution in Near-Natural European Beech Forests: Are Geo-Climatic Variables Alone Sufficient?

Diameter distribution is an important indicator of stand structure and an input for many forest growth models. It is commonly modelled using theoretical functions, in which distribution parameters are expressed as a function of stand, geo-climatic and other predictors. However, modelling diameter distributions in near-natural forests remains limited, and the influence of geo-climatic factors has not been systematically assessed. Using data from 6759 sample plots, our aims were (i) to develop models of the scale (b) and shape (c) parameters of the two-parameter Weibull function for near-natural beech forests in Slovenia; (ii) to examine whether diameter distributions can be reliably modelled using only geo-climatic variables; and (iii) to determine whether separate models are required for different beech forest types. A broad set of stand, geo-climatic and forest management variables was considered in the modelling procedure. The results indicate that stand variables had the strongest influence, while geo-climatic variables were included in the best-performing models, but had negligible effects. The importance of stand-level variables over geo-climatic variables was highlighted. Models based solely on geo-climatic predictors performed poorly and are unsuitable for practical forestry applications. Model performance did not differ substantially across forest types, suggesting that separate models for forest types are unnecessary.