Modelling height to diameter ratio – an opportunity to increase Norway spruce stand stability in the Western Carpathians

Norway spruce is one of the most widespread and economically important coniferous species in Europe and is also one of the tree species most mechanically sensitive to windthrow and consequently, is frequently damaged by storms. As height-to-diameter (HD) ratio has long been identified as an indicator of stand static stability, the main factors affecting the ratio in Norway spruce stands in the Western Carpathians were analysed to assess their general susceptibility to storm damage. A model was developed to assist forest managers in deciding on levels of thinning intensity for increasing the stability of such spruce stands, while still maintaining productivity. Data from the National Forest Inventory (260 plots) and previously existing research projects (48 plots) were used. Plots were distributed over the major part of the Western Carpathian range covering most variability in forest site factors. The final model incorporated the most relevant characteristics; those indicating ecological conditions: altitude and site index; and others related to stand properties: crown ratio, stand density, and mean stand height. The results indicated that intensive early thinnings must be applied, specifically on very rich sites at low altitudes, to decrease the HD ratio to a more stable range