Cascading processes in a headwater catchment: unraveling Vaia windstorm's Influence on stream Morphology and Large Wood Dynamics

Large Infrequent Disturbances (LIDs) reshape river basins, triggering sediment changes and channel network alterations. The impacts of the Vaia windstorm (27th – 31st October 2018, Northeast Italy) underscored the urgent need for local and global strategies to address and adapt to the changing climate. This study focuses on investigating the impacts of the Vaia event and subsequent cascading effects (i.e., hillslope stability and bark beetle outbreaks) on a small Alpine stream, emphasizing the importance of understanding large wood dynamics and sediment interactions for effective management. The study area is the headwater catchment of Malgonera, a sub-basin of Tegnas Torrent, located in the Northeast Italian Alps. Even-aged Norway spruce forests and pastures cover the basin. The stream area of interest, covering 6.025 m², features boulder-cascade and step-pool bed forms with a 19% slope. During the windstorm, a significant number of windthrown trees started interacting with the morphology and sediment fluxes. The study exploited remote sensing (UAVs) and field data to achieve its objectives, shortly after the Vaia storm (i.e., 2020-2023). Two Dem of Difference (DoDs) were computed to monitor the channel evolution during the study period (2020-2021 and 2021-2023). Large wood (LW) surveys, integrating field and remote sensing data, classified and computed LW loads for the years 2020, 2021, and 2023 and (ii) budgets for 2021 and 2023, providing a comprehensive understanding of the ongoing geomorphic dynamics and LW patterns over the study period. The study indicates minimal changes in the channel settings, with the exception of a significant mass movement observed between 2020 and 2021. Turning to the dynamics of LW, its overall volume increased progressively from 58.00 m³ (96.3 m3 ha-1) in 2020 to 61.50 m³ (102 m3 ha-1) in 2021 and further to 63.44 m³ (105 m3 ha-1) in 2023. Interestingly, the average volume per LW element consistently held at 0.21 m³ across the three years. The count of LW elements gradually rose from 279 (463 N° ha-1) in 2020 to 292 (485 N° ha-1) in 2021 and reached 294 (488 N° ha-1) in 2023. The average length of LW elements displayed minor fluctuations, measuring 3.44 m in 2020, 3.41 m in 2021, and 3.48 m in 2023. Meanwhile, the average diameter of LW elements maintained a steady value of 0.23 m throughout the observation period demonstrating the presence of even-aged spruce forests. The number of LW jams increased steadily from 29 in 2020 to 30 in both 2021 and 2023, while the number of single elements increased from 6 in 2020 to 8 and 10 in 2021 and 2023, respectively. Overall, the LW budget between 2020 and 2021 amounted to +3.50 m³, and this is ascribable primarily to cantilever failures in small portions of the unstable forested hillslopes and to the previously mentioned mass movement process. The subsequent positive budget of LW between 2021 and 2023 (+ 1.94 m³) is attributable to compound factors. These factors include both mechanical failure of the riverbanks, lack of root stability, and biological stress, which refers mainly to the widespread bark beetles attack further weakening the surrounding forests. Despite small fluctuations in wood load and neglectable morphological changes, our analyses demonstrate that headwater channels can be strongly affected by both primary and secondary effects of LIDs. Even-aged forests can be significantly influenced by disturbances that generate widespread instabilities. In connection to this, the ongoing secondary processes related to bark beetle infestation and the subsequent production of snags seem to be of great importance in long-term LW budgeting analyses. These analyses can enhance forest cover management to prevent or mitigate massive LW recruitment, which could pose a risk after LIDs.