Diameter Growth of Silver Fir (Abies alba Mill.), Scots Pine (Pinus sylvestris L.), and Black Pine (Pinus nigra Arnold) in Central European Forests: Findings from Slovenia

Abstract

The main objectives of the study were to (1) determine the response of the diameter growth of silver fir, Scots pine, and black pine in Central European seminatural forests to tree, stand, and environmental factors and (2) test for differences in their growth rate on different soils. Based on 26,291 permanent sampling plots (500 m² each), we developed a linear mixed-effects model of the diameter increment for each of these tree species. The models explained 32%–47% of the total diameter increment variability. The models differ in the set of predictors. All models suggested a non-linear effect of tree diameter on diameter increment. Nine predictors were common to all three models (stand basal area, quadratic mean diameter, basal area of overtopping trees, the proportion of beech in the stand volume, inclination, elevation, mean annual temperature, mean diurnal range, and soil unit), and six predictors were specific for one or two models (tree diameter, logarithm of tree diameter, proportion of other broadleaves, site productivity, rockiness, eastness index). Tree diameter was the most important variable for fir growth, while climatic variables explained most of the variability in pine diameter growth. The soil unit contributed from 5.3% to 7.5% to the explained diameter increment variability. Although the developed models are only locally accurate and cannot be used outside the study area without validation, the model predictions can be compared to those in other stand growth simulators and other geographical regions.