Natürliche Waldentwicklung unter dem Einfluss des Borkenkäfers im Nationalpark Berchtesgaden

Abstract

Disturbances are an integral part of forest ecosystems. Integrating single-tree dynamics to large-scale disturbances as windthrows, fire or insect calamities, disturbances influence natural species composition and regeneration processes of forest stands. Management-related high shares of pure secondary coniferous stands and changing climatic conditions have increased the impact of large-scale disturbances over the last century and it is likely that they are further going to increase in the future. Ecological knowledge about the impact of these disturbances on species composition and natural forest succession is essential for a sustainable forest management. Due to steady interventions with salvage-logging, planting and thinning activities, natural dynamics can hardly be studied in managed forests. However, large strictly protected areas enable such investigations. Therefore, natural forest succession following bark beetle-induced spruce dieback and the related changes in stand structure and biodiversity were studied in the Berchtesgaden National Park (Germany) in the Northern Limestone Alps. Centuries of intensive timber extraction, mainly due to salt mining, and intentionally high ungulate populations heavily altered the forest structures and species composition of the Berchtesgaden National Park from natural mixed mountain forests dominated by European beech, silver fir and Norway spruce towards homogenous stands consisting purely or predominantly of Norway spruce. Following the establishment of the national park in 1978, severe bark beetle infestations occurred especially after the winter storms Vivian/Wiebcke in 1990 and Kyrill in 2007. Evaluations of aerial photographs revealed a scattered and rather small-scale development of the bark beetle infestations. Mean gap sizes comprised 0.07 ha (1990-1997) and 0.29 ha (2007-2012) and total infestation areas covered 30 ha (1990-1997) and 260 ha (2007-2012). Applying a chronosequence of 140 study plots of undisturbed secondary spruce stands (mature stage), stands affected by bark beetles within the last five years (initial early-seral stage) and stands infested in the 1990s (advanced early-seral stage) forest succession could be surveyed in montane to subalpine altitudinal zones, on south- and north-facing slopes. The bark beetle-induced dieback of the mature spruce stands caused a significant reduction of stand volumes and crown cover, but initiated a huge emergence of standing deadwood. Large shares of the standing deadwood broke down due to decomposition during the first two decades of succession. Humus degradation and significant changes in mesoclimatic conditions could not be detected. The significantly increased shares of direct radiation at the forest floor induced by the spruce dieback, increased cover and height of the ground vegetation. Albeit the intensified competition with the ground vegetation, the gaps got regenerated rather fast. Around 5.000 regeneration individuals (> 50 cm height) could be found in median per hectare in the montane zone two decades after the bark beetle infestation. This development was slightly protracted in high montane zones, where the importance of deadwood as growing substrate increased with altitude and decay stage. Natural regeneration was dominated by Norway spruce, sycamore maple and rowan. European beech and silver fir would naturally dominate the mixed mountain forests, but where found in very little shares as seed trees and as regeneration individuals in this study, due to historical forest management. The disturbance and the scattered patterns of the post-disturbance regeneration initiated an increase of structural heterogeneity on stand and landscape level. The importance of advance regeneration for the regeneration process was almost negligible. Contrary to the expectations, more than 90 % of the seedlings did germinate after the disturbance event. This indicates a high resilience of the mountain forests after medium-scale disturbance, if present ungulate densities enable the survival of the natural regeneration. The studied epigaic species groups showed no changes (Coleoptera, Arachnida, Mollusca) as well as decreasing species densities (Collembola) due to missing litter supply after the disturbance. Contrary, the light, nutrient and dead wood dependent species did profit from the temporary gap conditions and revealed a significant increase in species densities during succession. The findings suggest that in unmanaged forests after bark beetle attack, a structurally complex phase prior to tree canopy closure can last several decades, and that many aspects of early-seral biodiversity and ecosystem function only fully develop given this extended time period. The mosaic of the different small-scale successional stages revealed a high gamma diversity, especially for wood-inhabiting fungi and saproxylic beetles. The species compositions of vascular plants did not vary significantly among the successional stages on landscape scale. Differences among the stages got rather visible as shifts in species dominances. Salvage-logging and planting in the infested stands of the bark beetle management zone did not lead to higher regeneration densities than in unmanaged stands during the initial early-seral stage. However, the planting activities increased the shares of European beech and silver fir in the managed parts of the national park. Mollusks, true bugs and pollinating beetle species showed a positive response on the slightly higher radiation amounts in the salvage-logged gaps and revealed higher species densities on these sites. At the same time, the logging changed the species composition of many species groups and significantly decreased the species densities of the saproxylic beetles inside the bark beetles management zone. Where management of moutain forests includes objectives for sustaining biodiversity, accommodating the protracted early-seral stage is important to supporting the full range of organisms and functions associated with canopy-opening disturbances. European beech and silver fir need to be planted in managed forests, where involvement of these species is of prime importance and seed trees are missing. However, under comparable post-disturbance conditions, the high resilience of the mountain forests is expected to lead to fair natural regeneration densities, if ungulates are managed accordingly.