Multi-taxa responses to forest disturbances through a multi-disciplinary approach

Natural disturbances, such as windstorms and insect outbreaks, are complex phenomena with multiple effects on forest ecosystems. Unfortunately, these disturbances are expected to increase in the coming years, due to the climate change. Therefore, understanding the multi-facet impacts of forest disturbances is an urgent objective for targeting appropriate management strategies. During my PhD thesis, I focused on three main goals: (I) understanding the effects of a large-scale wind disturbance on forest biodiversity, (II) identifying the predisposing factors for the tree susceptibility to bark beetle attacks, and (III) evaluating the ecological role of post-disturbance sites to protect biodiversity at the landscape scale. In the first part of the thesis, I focused on the effects of wind disturbance on three groups of organisms: ground-dwelling arthropods, microarthropods, and oomycetes. Our results highlighted that wind disturbance had different consequences depending on the taxonomic and the functional traits. Moreover, underlying ecological gradients, such as topography and climate, modified the impact of windstorm. In particular, we found that stronger impacts usually occurred at high elevations and drier sites. In the second part, I studied biotic disturbances focusing on the predisposing factors of the European spruce bark beetle (Ips typographus L.) outbreaks. Both local growing conditions of trees and landscape features are important predictors of the risk of bark beetle outbreaks. At the early stage of a large-scale outbreak, forest stands on flat terrains with high water availability, and high host cover exhibited the highest high-risk of attack. In the third part, I focused on the long-term response of post-disturbance sites. Using spiders as target group, we found that temporary abandoned patches played a pivotal role in enhancing the spider diversity at the landscape scale. These transition habitats created by disturbance or abandonment enhanced the landscape environmental heterogeneity providing novel ecological niches compared to homogenous forest landscapes. Overall, two general conclusions can be derived from this thesis. First, since forest disturbances are extremely complex phenomena, studying the interactive effects between disturbance and underlying ecological gradients might help to successfully understand the impact of these natural processes. Second, because forest disturbances are multifaced events affecting both biotic and abiotic dimensions of forest ecosystems, complementary skills are needed to study them. Here, we suggested that different investigation approaches should be used depending on the spatio-temporal scale and the target organisms. Finally, the high complexity of such phenomena and the variety of their outcomes should be considered when designing conservation and management actions, and when planning future forest landscapes. In conclusion, my PhD work contributed to emphasise the urgent need for a multidisciplinary and a holistic approach to shape more resilient forests under global change.