No quiet after the storm: Emergency forestry operations put Alpine forest biodiversity at risk 5 years after major windstorm

In forest ecosystems, birds influence forest structure and functioning through seed dispersal, pollination, predation and/or ecosystem engineering (Fraixedas et al., 2020). Among forest birds, woodpeckers are key species of forest ecosystems: the cavities they build for nesting are used by many other species including birds, mammals and many insects (Martin, 2015; Edworthy et al., 2018). Woodpeckers are ecosystem engineers that modify the forest environment, supporting richer and complex biological communities, limiting demographic explosions of insects and enhancing sap availability for other organisms (Martin, 2015). Thus, they foster biodiversity, especially in cold environments like the alpine forests where conifers, which normally lack natural cavities originating from decay, are dominant. Among European woodpecker species, the black woodpecker, Dryocopus martius, is particularly relevant for forest biodiversity, since it provides large cavities (Gorman, 2011). These holes are essential for large and mid-size secondary cavity nesters, for example, the boreal owl Aegolius funereus, a species that occurs as a relic of a colder past in the Alps, where its distribution range is shrinking towards higher elevation (Brambilla et al., 2020). Recent trends have led to an improvement in the European forests' management techniques: some key forest elements, such as dead wood and tree-related microhabitats (Martin et al., 2022), are now preserved in different areas (including in the Trento province where our study area is located), favouring forest biodiversity. Large-scale natural disturbances, such as fires, windstorms and insect outbreaks, represent some of the main factors influencing the composition and structure of temperate forests (Fischer, Marshall, & Camp, 2013; Senf & Seidl, 2021). In October 2018, large portions of the forests covering North-Eastern Italy were severely damaged by the windstorm Vaia. Immediately after the storm, forest owners, in agreement with the local forestry authorities, removed an enormous amount of wood. In the subsequent years, there occurred the rapid diffusion of the European spruce bark beetle Ips typographus: the amount of windthrown stands on the ground triggered an epidemic cycle of the parasite, also favoured by mild winters and extremely warm and dry summers and autumns (Marini et al., 2012). At the time of writing (September 2024), the bark beetle has damaged more than 2 million m3 of wood in the Trento province (which is among the areas most severely hit by the Vaia storm), resulting in at least half the amount of damage caused by Vaia (Servizio Foreste Provincia Autonoma Trento, 2023). To reduce the impact of the bark beetle outbreak, dried-out forest patches have been generally salvage-logged, sharply increasing clear-cut areas. Salvage-logging, in response to bark beetle outbreaks, had already jeopardized the ecological value of the main European temperate forest (Mikusiński et al., 2018). Here, we focus on the fate of trees with woodpecker holes (especially those of the black woodpecker) in the Paneveggio-Pale di San Martino Natural Park (Dolomites, Trento province, NE Italy), hit by Vaia and then by the bark beetle outbreak. In 2007, a still ongoing conservation project was started throughout the province, with the aim of identifying woodpecker nesting trees and marking them with a red ‘P’, thus preserving them from cutting. Two thousand five hundred trees were marked over the Trento province, 166 of which were in the Paneveggio Park territory, where c. 5000 ha of subalpine forests dominated by Norway spruce, Picea abies, had been regularly investigated for woodpecker cavities before and after the Vaia storm. Within the surveyed area, 50% of the trees with black woodpecker cavities were destroyed by Vaia. However, the black woodpecker appeared to be resilient towards the impact of both the storm and the bark beetle outbreak. Data on the density and reproductive biology of the woodpecker 5 years after the storm (2023) indicated a stable population, which progressively moved from cavities built on living trees (pre-Vaia condition) to cavities built on dead stands. In May 2024, 60% of the breeding population nested on beetle-killed spruce trees which can provide optimal nesting sites for decades. We are now witnessing that both the black woodpecker trees with nesting cavities not affected by the windstorm and those that, after the storm, had been excavated in beetle-killed trees, are being logged during forestry operations. Moreover, these activities are undertaken in the breeding season of woodpeckers and many other species, in exception to pre-Vaia protection measures. The salvage-logging of beetle-killed trees is progressively removing all the breeding sites of the ecologically most important forest species of Alpine valleys, potentially creating an ecological trap for the species (Fig. 1). In fact, the black woodpecker nests in spruce trees only when they are at least 80 years old (LM personal observation). Therefore, until 2100, the species will be able to nest only on the already existing trees, and thus, there is an urgent need to preserve nesting trees for this and all the species dependent on its cavities. To mitigate the possible dramatic impacts of post-disturbance salvage-logging, based on our direct experience, we propose to exclude some spruce woods from the logging, preserving groups of 20–30 trees (both dead and/or alive) around the trees with cavities (Fig. 1). Those groups of trees could guarantee the availability of some tens of cavities, which are essential for the reproduction of black woodpeckers and of all the species reliant on their nest cavities, until the end of the current century. In conclusion, the local population of black woodpecker and the species utilizing its cavities, after overcoming the effects of the windstorm Vaia, have a new challenge to face: due to the salvage-logging performed to contain the bark beetle outbreak, the speed of the removal of woodpecker holes has exceeded the capacity of the species to build new nests. Adjusting the current management is key to the survival of forest biocenoses in the unique landscapes of the Dolomites; new management maintaining plots with woodpecker holes is currently starting in the State Forest inside the Paneveggio Park. This simple and relatively low-cost conservation measure represents an easy-to-implement strategy that can be applied in similar circumstances elsewhere. Considering the current knowledge on black woodpecker ecology in the Alps, and the habits of the species typically associated with its cavities, such a measure would promote biodiversity by making available tens of cavities for decades, also favouring the natural antagonists of the bark beetle. Further research is required into the detailed responses of the black woodpecker, and the associated species, to this conservation measure, exploring in particular the effects of the number and cover of dead and live trees, the size and shape of the uncut tree patches and their spatial relationships with other forest and non-forest habitats. Such knowledge would help identify the most effective configuration of the patches to be preserved from salvage logging to maintain forest animal biodiversity and key ecosystem processes in forested landscapes hit by bark beetle outbreaks. We are very grateful to the Editor I. Gordon and E. Rantanen for helpful comments on the manuscript.