Modelling dead wood islands in European beech forests: how much and how reliably would they provide dead wood?

Dead wood is an important element of forests both for biodiversity and ecosystem functions. Due to intensive silviculture, however, dead wood usually is strongly underrepresented in European forests. Forest reserves cannot fully compensate for this because they comprise only a small proportion of forested areas and are often isolated. Retaining a certain number of dead trees in managed forests is important, but may cause safety problems for lumbermen and visitors and still does not necessarily lead to an amount and incidence (i.e., probability of occurrence) of dead wood that might be required for many species and certain ecosystem functions. Our studies concentrate on a third and complimentary dead wood management strategy: dead wood islands, i.e. small unmanaged islands distributed throughout managed forests. As an example, we focus on European beech forests (Fagus sylvatica). An important question related to this strategy is: how do amount, quality, and incidence of dead wood depend on the island's size? To provide an answer, we use the spatially explicit, rule-based simulation model BEFORE-CWD that was developed to analyse dead wood dynamics in natural beech forests. This model and its predecessor, BEFORE, are well-verified and validated. They reproduce a suite of observed patterns and generate valid secondary and independent predictions. We found that islands that are too small, i.e. smaller than 0.33 and 0.08 ha for standing and lying dead wood, respectively, can fail to provide dead wood for several decades. The shape of the islands has only a minor effect. Extreme storm events temporarily increase and then decrease the amount of standing dead wood. In terms of the amount and incidence of dead wood, it makes no difference if one big or several small islands are set aside from management, unless the islands are not too small. We conclude that even relatively small unmanaged islands within managed forests can reliably provide dead wood and therefore should be considered as a management option. Our results can be used, for example by using metapopulation models of species of interest, to develop management plans for creating networks of dead wood islands.