Natural and human-related determinants of dead wood quantity and quality in a managed European lowland temperate forest

Abstract

Much is known about the minimum thresholds for dead wood that would ensure the persistence of saproxylic organisms and maintain the overall biodiversity of forest ecosystems, but reaching those thresholds is still a challenge in forests where nature conservation is not the sole goal (e.g., Natura 2000 forests managed for timber). Understanding what determines the quantity and quality of dead wood in managed forests is a prerequisite for developing methods to increase its retention without decreasing a forest’s production. We surveyed the volume, number, stage of decay, and species identity of standing and lying coarse woody debris (CWD) in 183 stands in a complex of lowland Natura 2000 forests in southern Poland, and then determined the relationships between these parameters and tree stand structure, intensity of previous silviculture interventions, accessibility of terrain, and local community impact. CWD volume varied from 0.1 to 66.3 m3 ha−1 (average 6.9 m3 ha−1), which is up to several times less than the recommended thresholds. Large-sized CWD and CWD of species known to support rich communities of saproxylic organisms (e.g., Carpinus betulus) were clearly underrepresented. Wood of economically less valuable species was often left in the forest, which boosted the diversity of CWD microhabitats. CWD quantity and quality was significantly shaped by both natural and human-related factors. Among the latter, harvesting intensity and proximity to human settlements affected CWD negatively, while terrain-related difficulties (distance to roads, watercourse density) affected it positively. Surprisingly, the site accessibility effect was important even though the studied area is level and covered by a dense road network. The results suggest that cost-effective CWD management plans for Natura 2000 forests, or, more broadly, multifunctional forests, should discourage some practices accompanying timber harvesting (e.g., fragmenting of pieces of aged CWD), promote practices that positively affect CWD (e.g., not harvesting low-value timber), and set aside small areas to accelerate CWD retention. These things should be done wherever their effects are unlikely to be reversed, i.e., at sites difficult for forestry vehicles to access and away from human settlements.