Influence of clear-cutting on the risk of wind damage at forest edges

Abstract

The objective was to predict the risk of wind damage at forest edges in Scots pine ( Pinus sylvestris), Norway spruce ( Picea abies), and birch ( Betula spp.) dominated stands in Central Finland. The method used here was to integrate a mechanistic wind damage model and an airflow model with forest database containing information at the tree, stand, and regional levels. Analyses were made for the current forest edges (Case I) and for situations in which new forest edges might be created through clear-cutting: (i) whenever a stand were to reach the minimum acceptable mean diameter and/or stand age (Case II); or (ii) whenever the stand age were to exceed 100 years (Case III). These case studies were used to analyse the number of stands and total area at risk, and length of vulnerable edges for different critical wind speeds and risk probabilities. It was evident that new clear-cuttings did increase the high wind speeds at forest edges, especially in Case II, as compared with the wind speeds at current forest edges (Case I). This local effect was however compensated at regional level by the more intensive cuttings in Case II, since the old stands, which were more vulnerable, were cut and the average tree size at the regional level decreased relative to Case I or III. The overall risk of wind damage at the regional level therefore decreased in Case II in terms of the number of stands and the total area at risk, and also in terms of the length of vulnerable edges. On the other hand, the risk increased at the regional level in Case III relative to Case I or II, because there were still a lot of vulnerable old stands left at the newly created edges. The sensitivity analyses of wind conditions also showed that Norway spruce was more vulnerable in overall than Scots pine under current conditions and birch the least vulnerable (i.e. Norway spruce has risks even in slow wind speeds, while Scots pine and birch have risks at higher wind speeds). Furthermore, Norway spruce was more sensitive than Scots pine under more windy conditions.