The frequency and intensity of windthrow disturbances of forest stands in Europe and in the world have been increasing since the late 20th century (Nilsson et al. 2004; Senf and Seidl 2021). Accordingly, research is now targeting not only the consequences of stand disturbance, but also various ways to support stand recovery (Schönenberger 2002; Hotta et al. 2021; Wohlgemuth et al. 2017). The effects of a windthrow event depend on the force of the hurricane, species composition and height of the forest stand, soil type, soil depth, soil waterlogging, slope inclination and other factors (Dobbertin 2002; Bouchard et al. 2009). Windthrow disturbs stands over large areas, producing a patchwork of different degrees of disturbance severity, which entails different rates of stand regeneration in individual plots (Skłodowski 2020). In particularly severely disturbed stands (when all or nearly all tree crowns are broken), windthrow reverts succession to early developmental stages where pioneering tree species are dominant (Christie and Armesto 2003), while in stands showing less disturbance, with more surviving trees, established seedlings of tolerant species are gpossible, taking forest stand recovery to late successional stages (Girard et al. 2014).
Stand renewal most often starts with removal of broken and upturned trees. This salvage logging makes it easier to plant saplings, which achieve a greater density and height than emerging saplings from seeds present in the soil (Schönenberger 2002; Wohlgemuth et al. 2017). However, salvage logging causes the forest ecosystem to revert to an earlier successional stage and thus impedes recovery (Taeroe et al. 2019). Salvage logging also exerts a moderate negative effect on the ecosystem and its services (Leverkus et al. 2020) and negatively affects the species number and richness of invertebrates (Thorn et al. 2018). Consequently, the structure of invertebrate assemblages undergoes significant changes consisting in a decreased proportion of forest specialists (late successional fauna) and an increased proportion of the early successional fauna of non-forest species (Otte 1989). By affecting abundance and distribution patterns of resources on macro- and micro-habitat scales in the disturbed forest stand, a windthrow event impacts the abundance and species richness of insect species (Bouget and Duelli 2004).
Among the many invertebrate families, the ground beetles (Coleoptera, Carabidae) are often used in ecological research owing to the widespread presence of many of the species, their well-established ecology and easy trappability (Lӧvei and Sunderland 1996; Koivula, 2011). The carabid response to various types of disturbance and stress (e.g., clear-cutting, forest fires, anthropogenic pollution, etc.) has often been decreased abundance of late successional fauna, and increased abundance of early successional fauna, producing an increase in species richness (Beaudry et al. 1997; Magura et al. 2001, 2015, 2017; Koivula and Niemelä 2003; Paquin 2008; Nagy et al. 2016; Kędzior et al. 2018; Kosewska et al. 2018). The response of carabid assemblages to a windthrow disturbance to a forest stand is similar (Duelli et al. 2002; Bouget 2005; Gandhi et al. 2008; Šustek and Vido 2013; Kašák et al. 2017; Skłodowski 2017a). Windthrow events in forest stands followed by salvage logging produce a marked reduction in the abundance of carabid forest species, whose recovery will commence in a few years as a new stand begins to grow. Conversely, in windthrown stands where all broken tress were left in place (there was no salvage logging), the proportion of forest fauna in carabid assemblages remained quite high during the first three years after a disturbance to become reduced only in the 4th year, which may have been associated with possible survival of forest species under the fallen crowns of broken pine trees (Skłodowski 2017a).
Indeed, the crowns of windthrown pines retain their needles for several months, the needles falling off gradually over 3 years (personal observation). Every such crown with needles offers protection against excessive sunlight and overdrying of at least a dozen or so square metres of litter around the trunk. These are appropriate conditions for sheltering forest fauna (Bouget 2005). Similarly, logging residue and stumps on clear-cutting sites support overwintering, egg lying and hiding of forest carabid species (Cobb et al. 2007). Stacks of branches also support the survival of late successional carabid fauna even in the centre of a clear-cut during the first 2 years after clear-cutting (Skłodowski 2017b), whereas extensive slash removal impoverishes the species richness of ground-active beetles (Gunnarsson et al. 2004). Some authors suggest that slash harvesting produces a short-lived effect; as early as one year after stump-harvesting, it had no effect on assemblage composition (Shevlin et al. 2017). At the same time, other studies have shown that salvage logging may have long-lasting effects on the composition and structure of carabid assemblages; even 5 years after cutting, the stand numbers of carabid species were significantly higher in clear-cuts than in sites where slash was left on the ground (Nittérus et al. 2007). Slash removal caused a decline in forest species and an increase in generalist species (Nittérus et al. 2007).
Carabid species can have a high or low dispersal power. The former group comprises non-forest species that efficiently colonise disturbed habitats owing to their ability to fly, while the latter group consists of wingless forest specialists that are only able to move along on the ground and thus do not cover long distances. In a windthrown stand, members of this latter group, representing late successional fauna, probably seek shelter in canopies of broken pine trees lying on the ground. As the occurrence of these species is supported by a greater degree of canopy closure (Magura et al. 2006; Taboda et al. 2008), the shade of pine crowns lying on the ground should facilitate survival of forest species. It appears that the protective effect of canopies of fallen trees on forest and late successional species should be more pronounced in more disturbed stands (with all or nearly all trees broken) than in less disturbed ones (with half of the trees intact). However, as needles fall off in successive years, the protective effect presumably weakens. Two hypotheses were put forward: (1) the more severe the disturbance, the higher the abundance and species richness, and the lower the proportion of late successional fauna in carabid assemblages, (2) crowns of pine trees lying on the ground support a higher proportion of late-successional carabid species, which effect should be more pronounced in more severely disturbed stands, but will decrease as needles fall off over the next 3 years. An additional objective was to identify the environmental variables that are the most conducive to the survival of late successional carabid fauna under crowns of fallen pine trees in windthrown stands in consecutive years.