Regeneration dynamics and resilience of unmanaged mountain forests in the Northern Limestone Alps following bark beetle-induced spruce dieback

Mountain forests provide many important ecosystem services including protection against avalanche and rock fall. These services often depend on a continuity of forest cover and therefore on ecological stability-resistance and resilience to disturbance. Natural regeneration following large-scale disturbance is an important component of forest resilience but in the mountain forests of the Alps is not well understood. This restricts management decision making, particularly in the face of an increased threat to the currently dominant Norway spruce from the bark beetle (Ips typographus). In light of this, we analyzed bark beetle infestation patterns, forest structure, selected tree regeneration characteristics and species composition on 96 study plots on a chronosequence of bark beetle-induced natural forest succession in unmanaged mountain forest ecosystems in southeastern Germany. The most advanced plots were twenty years on from the bark beetle-induced dieback of mature Norway spruce, and the majority were already quite densely stocked. The regeneration was clustered and was dominated by sycamore maple and Norway spruce. Most notably, the proportion of advance regeneration was negligible with the vast majority of the seedlings having germinated after the disturbance event. The findings suggest that these forests are naturally resilient in terms of regeneration if high browsing intensities do not prohibit the establishment of new seedlings. The bark beetle outbreaks appear to have generated an acceleration of development away from human influenced pure Norway spruce forests toward a more natural species composition. Nevertheless, Norway spruce will remain the dominant species for at least the next generation. These findings are important for managers intending to uphold protective forest functions. The integration of naturally evolving gaps as elements which diversify environmental conditions on the forest stand level as well as on the landscape scale is recommended for close-to-nature forest management.