As a result of climate change, interactions between forest disturbances are becoming more and more frequent. These so-called compound disturbances can alter the dynamics of post-disturbance recovery compared to an individual disturbance alone, leading to unpredictable outcomes, therefore, they represent a threat for forest resilience. A recent example of such interacting disturbances is an extremely severe ice storm (i.e. rain at a temperature below 0°C which freezes upon impact with the surface) which happened in Slovenia during the winter 2014. This event was followed by a bark beetle outbreak, which affected spruce trees, and their subsequent salvage logging.
In order to analyse forest resilience to such compound event, we sampled forest stands in central-western Slovenia, which featured a gradient in disturbance severity, mainly due to the varying proportions of spruce in the stands. We assessed multiple dimensions of resilience, including forest structural and species diversity, growth, and post-disturbance regeneration, each with focus on some ecological driver that can be influenced by forest ecosystem management.
The proportion of spruce before the disturbance was the main driver of both structural and species diversity after the compound event, even when accounting for the differences in pre-disturbance diversity. More interestingly, structural diversity abruptly dropped above 50 % of spruce in the stand composition, highlighting the greater resilience of mixed forests in contrast to spruce monocultures.
When comparing radial growth of canopy trees that survived the compound event, we found that their growth over a period of 6 years after the ice storm was equal or greater than in the period before the event. In particular, broadleaf trees with the crown damaged up to 75 % did not show evidence of consistently lower growth than undamaged trees.
Finally, the negative effect of ungulates browsing on post-disturbance regeneration was evident from our exclosure experiment. Regeneration of highly palatable species had significantly lower heights outside the exclosures compared to inside, which underscores that, if we aim to recruit these species into the canopy, thus maintaining species diversity in the stands, interventions to reduce deer abundance are needed in areas with particularly high populations.
For more information check the full article published in Forest Ecology and Management:
Cerioni, M., Klopčič, M., Roženbergar, D., & Nagel, T. A. (2025). Multiple dimensions of forest resilience to compound disturbances in a mixed sub-montane forest landscape. Forest Ecology and Management, 577, 122400.