Is deadwood helping regeneration? Natural regeneration dynamics in a stand replacing windthrow area.

Forests provide many important ecosystem services. Natural disturbances, such as wildfires, pest outbreaks and windrows, are the main phenomena shaping forest ecosystems. Due to both climate and global changes, extreme events are increasing in frequency and forests are thus increasingly being affected by stand replacing disturbances. Mountain forest dynamics and ecosystems services are critically influenced by disturbances, in particular storm. In this framework, is crucial to understand these environmental modifications, finding the best management strategies to restore or maintain ecosystem services provided by forests. After large disturbance, there are two different issues to deal with: the large amount of deadwood on the ground, and the needs of  regeneration in order to re-establish the forest cover. To face these problems different management strategies can be adopted. Salvage logging (total or partial) and no-intervention are the two opposite approaches to handle the large amount of deadwood. Natural regeneration or reforestation, instead, are the main strategies to consider to re-establish forest cover. In this study we focused on post-windstorm conditions, in particular concerning large windthrows caused by the Vaia storm, occurred in October 2018 on Eastern Italian Alps. After such large-scale event, natural regeneration is the most convenient strategy to regenerate forest. This process should take place in an area with a high amount of coarse wood debris (CWD). For this reason is crucial to understand the interaction between windthrown timber and regeneration dynamics. In this study we analyzed how CWD is able to create a favorable regeneration microsite enhancing seedling establishment probability. In particular, we focused on two different facilitative mechanisms provided by CWD: microsite amelioration and seedling protection. The former has been analyzed measuring temperature and SWC in the proximity of seedling planted in the surrounding of deadwood elements, the latter by recording browsing evidence at the end of the season.  In order to infer the CWD contribution, control sites have been established in empty sites (no CWD presence in the surrounding). Our results showed that in southern slopes, microsite with significative lower temperature are found northern to the logs, decreasing water stress for saplings. The ameliorative function of logs and CWD in general contribute to decrease the transplanting shock, increasing the probability to establish for saplings. Moreover, the presence of lying deadwood decreased significantly the browsing on saplings. The result of our study highlighted the importance of favorable regeneration microsites provided by deadwood, both for natural regeneration dynamics and for increasing the survival probability for planted saplings. Favorable microsites and nurse biological legacies should be considered in defining post-disturbance management strategies, promoting only a partial salvage logging or non-intervention  approaches as much as possible.