Synchronised Disturbances in Spruce- and Beech-Dominated Forests Across the Largest Primary Mountain Forest Landscape in Temperate Europe

Abstract

Understanding temporal and spatial variations in historical disturbance regimes across intact, continuous, and altitudinally diverse primary forest landscapes is extremely important to help forecast forest development and adapt forest management in an era of rapid environmental change. Because few complex primary forest landscapes remain in Europe, previous research has largely described disturbance regimes for individual forest types and smaller isolated stands. We studied the largest, still relatively unexplored mountain primary forest landscape in temperate Europe, the Făgăraș Mountains of Romania. To describe historical disturbance regimes and synchronicity in disturbance activity and trends between two widespread forest community types, dominated by Norway spruce ( Picea abies (L.) Karst.) and European beech ( Fagus sylvatica L.), we established 191 permanent study plots (70 beech; 121 spruce) across 11 valleys, thereby providing information at both stand and landscape levels. We used a dendrochronological approach to reconstruct and describe the spatiotemporal patterns of historical disturbances. We observed a diverse spectrum of disturbance severities and timing across the forest landscape. High-severity disturbances created periods of synchrony in disturbance activity at the landscape scale, while moderate- and low-severity disturbances were asynchronous and random in both spruce- and beech-dominated primary forests. We detected a peak of canopy disturbance across the region at the end of the nineteenth century, with the most important periods of disturbance between the 1890s and 1910s. At the stand scale, we observed periods of synchronised disturbances with varying severities across both forest types. Average severities in these two primary forest types were comparable. The level of disturbance synchrony varied widely among the stands. The beta regression showed that spruce forests had significantly higher average synchrony and higher between-stand variability of synchrony than the beech-dominated forests. Synchronised disturbances with higher severity were infrequent, but they were critical as drivers of subsequent forest development pathways and dynamics across both forest types.Our results provide insight into future resilience to climate-driven alterations of disturbance regimes in spruce- and beech-dominated forests. We suggest that management efforts should recognize protecting large continuous and altitudinally diversified forest landscapes as a necessary measure to tackle climate change and ensure temporal and spatial structural heterogeneity driven by a wide range of disturbances. The diverse and synchronous disturbance activity among two interconnected forest vegetation types highlights the need for complex spatiotemporal forest management approaches that emulate disturbance synchronicity to foster biodiversity across multiple forest vegetation types within forest landscapes.

Keywords: dendroecology, beech-fir forest, spruce forest, natural dynamics, primary forest, Southern Carpathians