Impacts and underlying factors of landscape-scale, historical disturbance of mountain forest identified using archival documents

• Historical maps provide insight into forest historical range of variability (HRV).
• Large scale disturbances are in HRV of central European forests.
• We found unusually high amount of old-growth forests before disturbance.
• Stand age is the most important factor for disturbance severity prediction.

Large areas of temperate mountain Norway spruce (Picea abies) forests in Central Europe have been disturbed by windstorms and subsequent bark beetle (Ips typographus) outbreaks in recent years. The impact of these events has been severe, with millions of canopy trees dying in nature reserves, where salvage logging was not conducted. The occurrence of these windstorms has raised the question of whether such events were within the historical range of variability (HRV) of forest dynamics in Central Europe, where disturbances were traditionally perceived as non-natural and without strong relevance for forest development. To answer this question, we analyzed the available historical forest management maps documenting large-scale disturbance resulting from windthrow events in the years 1868–1870 in spruce- and beech- (Fagus sylvatica) dominated forests in the Bohemian Forest region (Šumava Mts., Czech Republic and Bayerischer Wald, Germany). We created a cross-boundary forest database covering 54 974 ha and containing information about tree species composition and age, and the severity of the 1868–1870 disturbance, considering the cumulative effect of the windthrow, subsequent bark beetle outbreak, and salvage logging. The age structure of the forests before the disturbances was unbalanced, with stands of 80–120 years underrepresented, and covering only 9% of the area, and stands older than 120 years, historically classified as old growth, covering 26% of the area. Within the decade that included the windstorms and their aftermath, 40% of the stands in the mountain range were at least partly disturbed, with significant effect on the oldest stands. To identify important factors responsible for the severity of disturbance, we constructed regression models relating severity to two groups of explanatory variables: forest stand characteristics and environmental attributes (mainly topographic factors). Overall, stand age was identified as the most important driver of disturbance severity across the landscape, with the oldest trees most susceptible. The high importance of age for disturbance severity showed the role of forest age structure in determining the scale of disturbances resulting from windstorms and associated bark beetle outbreaks. Nevertheless, despite the documented occurrence of frequent large disturbances during the two centuries that preceded the 1868–1870 events, old growth accounted for 26% of the area, making it clear that both large scale disturbance and old-growth forest are within the HRV of mountain spruce forest dynamics in Central Europe.

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