Response of mountain Picea abies forests to stand-replacing bark beetle outbreaks: neighbourhood effects lead to self-replacement.
Large, severe disturbances drive many forest ecosystems over the long term, but pose management uncertainties when human experience with them is limited. Recent continent-scale outbreaks of bark beetles across the temperate Northern Hemisphere have raised major concerns as to whether coniferous forests will regenerate back towards pre-outbreak condition and meet possible reforestation objectives. To date, however, analyses of post-outbreak regeneration across broad spatial and temporal scales have been rare, and entirely lacking for many regions. Following a series of large, severe (∼99% overstorey mortality) outbreaks of spruce bark beetles Ips typographus in Central Europe, we capitalized on an extensive forest inventory data set (n=615 plots across ∼7000 ha) to evaluate regeneration dynamics in Norway spruce Picea abies forests across the Bohemian Forest Ecosystem (spanning Germany and the Czech Republic). We asked whether neighbourhood effects (conspecific advance regeneration of spruce) would support prompt regeneration back to spruce forest, or whether the rapid, severe canopy mortality would overwhelm this influence and promote pioneer and broadleaf species. We tracked 15 years of post-outbreak regeneration dynamics (occupancy, density, height, composition) of all tree species and evaluated initial variations in successional pathway and structure. Median tree regeneration density increased from ∼400 trees ha-1 at the time of outbreak to ∼2000 trees ha-1 within a decade, and occupancy increased from 58% to 76%. The increases were driven by spruce, which primarily recruited from advance regeneration, gradually occupying greater height classes. Only one broadleaf/pioneer species increased in relative proportion, for a brief (<3-year) period before declining again. Nevertheless, both pure spruce and spruce-broadleaf stands were common and, coupled with wide variations in density and height, contributed to diverse early-successional structure. Synthesis and applications. Contrary to common expectations, spruce beetle outbreaks in Central Europe effectively promoted their host in the long term. Outbreak-affected forests are naturally self-replacing even after severe canopy mortality, when positive neighbourhood effects of conspecific advance regeneration lead to rapid replacement of the dominant species. Thus, natural regeneration may be considered among the most effective ways to meet possible reforestation objectives in forests destroyed by beetles.