Variation in Deadwood Microsites in Areas Designated under the Habitats Directive (Natura 2000)

Abstract

The continuing decline in biodiversity presents a major environmental protection challenge. The conservation of sufficiently extensive and diverse habitats requires an array of coordinated actions, often involving large areas. While a set of conservation objectives have been defined for the Natura 2000 network, no universal methods of accomplishing them have been specified, and so they must be designed by individual Member States. Deadwood volume and the density of large deadwood pieces are widely used for evaluating the quality of forest habitat types designated under the Habitats Directive. In the present study, data from 5557 sample plots were used to evaluate the mean values of the two deadwood indicators as well as the ratio of deadwood volume to living tree volume for each of the 13 habitat types in Poland. In addition, a logistic regression model was constructed to evaluate the effects of terrain, site, and tree stand characteristics as well as protection type on deadwood volume in Natura 2000 areas. Mean deadwood volume varied greatly between habitat types, with the lowest values found for Central European lichen Scots pine forests (91T0–2.5 m3 ha−1) and Old acidophilous oak woods (9190–4.4 m3 ha−1), and the highest for Riparian mixed forests (91F0–43.1 m3 ha−1) and Acidophilous Picea forests of the montane to alpine levels (9410–55.4 m3 ha−1). The ratio of deadwood volume to living tree volume ranged from approx. 1%–17%. Additionally, the presence of large deadwood differed among habitat types: in some, there were no deadwood pieces with a diameter of ≥50 cm, while their maximum density was 6.1 pieces ha−1. The logistic regression model showed that the likelihood of a habitat type to have a ‘favorable conservation status’ as defined by deadwood abundance (a threshold of at least 20 m3 ha−1 according to Polish manuals on habitat type evaluation) increased with sample plot elevation, site fertility, and moisture, as well as stand age and volume. Positive effects were also observed for forests under strict and active protection versus managed forests. Planned efforts are necessary to enhance the quality of habitats with insufficient deadwood, especially in managed forests. Special attention should be given to areas that are readily accessible due to gentle terrain and low site moisture. Furthermore, younger stands on less fertile sites may require intervention to promote deadwood accumulation. We recommend retaining a certain proportion of mature stands until natural death and decomposition. Increasing the density of large deadwood is currently one of the most pressing conservation needs in most habitat types.