A high proportion of norway spruce in mixed stands increases probability of stand failure

The decline of pure spruce forests in the Beskydy Mountains in the Czech Republic and Slovakia is mainly driven by bark beetle attacks and storm events. Salvage-harvest records based on timber volume on the forest-stand level document the extent of stand failure processed by salvage logging. On the basis of these records, the stand failure proportion was expressed as the proportion of timber volume processed by salvage harvest divided by the standing timber volume over a period of 10 years (sf). Two null hypotheses to be tested are that sf is not influenced by (H1) the size of trees expressed by mean stand age (age) or alternatively by mean stand diameter (dbh); and (H2) the volume proportion of spruce within the stand (spp). The dataset was evaluated as a total and separately for the three site types, each for spruce, beech, fir, larch, pine, and other broadleaved species. The dataset was analysed using a binary logistic regression. The sf increased with age, dbh, and the spp. Pure spruce stands with high age and high dbh on mountain sites are associated with the highest sf. However, a slight admixture of 25% tree species other than spruce was shown to stabilise the stands significantly. Species other than spruce in a minor admixture to spruce stands were destabilised by the breakdown of spruce stands. For a realistic financial evaluation of forest-stand management, managers should consider the risk of stand failure, and reduce the risk by increasing species admixture in the spruce forest stands.