4.1 Overall forest dieback effect
Given the scale of the expected increase in the frequency and spatial extent of forest dieback and decline in the future, assessing their current and long-term consequences on forest communities is paramount (e.g. Anderegg et al. 2013; McDowell et al. 2020; Sallé et al. 2021). As expected, we showed that forest dieback had an overall positive effect on the taxonomic diversity of saproxylic beetles, at both local and landscape scales. This probably results from the accumulation of deadwood and tree-related microhabitats associated with increased canopy openness, since both environmental factors generally have positive effects on the species richness and abundance of saproxylic beetles (Müller and Bütler 2010; Lassauce et al. 2011; Bouget et al. 2014; Thorn et al. 2018; Godeau et al. 2020; Sallé et al. 2020, 2021; Haeler et al. 2021).
4.2 Large-scale effects of forest dieback
Our results highlight the relevance of the landscape scale (i.e. from 500 m to 1500 m) when considering the effect of forest dieback on local saproxylic beetle communities. With the exception of the 200 m scale, which was strongly correlated with the local scale, we noticed that the best landscape scales (i.e. with the lowest AICc) for measuring the impact of dieback on saproxylic beetle diversity were the larger scales, i.e. 1100 m and 1500 m (except for wood-eating species richness; Tab. 2 & 3). It has already been shown in previous studies that the accumulation of deadwood, both at local and landscape scales, has positive effects on saproxylic beetles (Økland et al. 1996; Gibb et al. 2006; Franc et al. 2007; Sverdrup-Thygeson et al. 2014; Haeler et al. 2021). Likewise, local canopy openings as well as an interconnection within the landscape forest matrix of fine patches of open habitats, i.e. gaps, is also beneficial to saproxylic beetle biodiversity (Bouget and Duelli 2004; Bouget et al. 2014; Seibold et al. 2016; Kozel et al. 2021). Furthermore, our results showed positive responses to forest dieback at the landscape scale for most of our variables (except for rare species richness, wood-eating species abundance, phylogenetic diversity metrics, and CWM and FDis of canopy closure preference and mean size body; Tab. 2 & 3). Many species of saproxylic beetle are considered highly mobile and therefore only slightly dispersal-limited; they therefore potentially respond to large scales of effect (Jackson and Fahrig 2015; Janssen et al. 2016; Thorn et al. 2018). Indeed, they may cover long distances, within a limit of roughly ≥10 km (Komonen and Müller 2018), to find suitable habitats and/or resources (related to the deadwood size, decay stage, tree species and position; to tree-related microhabitats; or to the presence of open areas; Grove 2002; Stokland et al. 2012).
Furthermore, we observed a significant landscape effect of forest dieback on the abundance of both cavicolous and fungicolous species (Tab. 2). The increasing severity of forest dieback in the surroundings of the local sites induced a spill-over effect on the abundance of these two substrate guilds. These results suggest that forest dieback does not just increase deadwood amount and canopy openness but also favours the development of tree-related microhabitats such as the fruiting bodies of saproxylic fungi and cavities (e.g. rot-holes; Ojeda et al. 2007; Larrieu et al. 2018; Speckens 2021).
We did not observe any significant interaction between local and landscape forest dieback (Tab. 2). According to Seibold et al. (2017), the lack of interaction between these two spatial scales should support the habitat-amount hypothesis since the amount of habitat at both scales is merely additive (Fig. 4). This is further supported by the fact that local and landscape effects alone cancelled each other out in our multiplicative models, while most of the univariate-model effects for taxonomic diversity were significant (Tab. 2; Fig. S2). The habitat-amount hypothesis predicts that “species richness in a sample site is independent of the area of the particular patch in which the sample site is located (its local patch)” (Fahrig, 2013). Therefore, in our study, local scale alone (i.e. without any dieback areas in landscape) should not have been sufficient to detect dieback effects on saproxylic beetle biodiversity, even if it appears as the potentially scale of effect. Nevertheless, the opposite is also true: forest dieback surrounding an undisturbed forest patch cannot contribute to local biodiversity, since the habitat or resource of interest is not locally present, unless there is a spill-over effect (see cavicolous and fungicolous species, Tab. 2; Bouget and Parmain 2016).
Forest dieback at the landscape scale also had effects on the functional diversity metrics, mainly FDiv and FEve which did not respond to local forest dieback (Tab. 3). In another study, based on data from the same Pyrenean plots and from plots located in Bavarian mountain forests (Cours et al. 2021), we hypothesized that the severity of the local forest dieback in Bavaria was correlated to dieback severity at the landscape scale, as sudden, large-scale mortality occurred in the area following a major bark beetle outbreak (mean bark beetle gap size = 6.8 ha; Müller et al. 2008). In contrast, the drought-induced dieback in the Pyrenean forests caused gradual mortality in discrete patches across the landscape; in this case, the local conditions did not necessarily reflect large-scale conditions (Andrew et al. 2016; Cours et al. 2021). These variations in the immediacy and scale-intensity of the dieback may explain (i) why the local forest dieback in the Pyrenean mountains did not affect functional trait metrics in our previous study (Cours et al. 2021) and (ii) why, in this study, the same metrics only responded to landscape-scale conditions. Our results therefore suggest that studying local conditions alone may be insufficient to detect the functional response of saproxylic beetles to forest dieback when tree mortality occurs in discrete patches, and that landscape conditions can act as a strong filter on trait diversity (Tab. 3; Gámez-Virués et al. 2015; Cours et al. 2021).
4.3 Contrasting responses of different biodiversity dimensions to forest dieback
We did not detect any response to forest dieback for phylogenetic diversity. However, PSE was negatively correlated with FDiv and positively correlated with FEve, both of which were influenced by landscape forest dieback (Tab. 3; Fig. S1-2). Therefore, forest dieback did not seem to induce any loss or gain in the range of evolutionary history occupied by saproxylic beetle assemblages, or if so, only indirectly by influencing functional diversity. Nonetheless, the use of DNA barcodes alone may be insufficient to estimate real phylogenetic diversity and the inclusion of multigene phylogenies may better estimate phylogenetic diversity and its response to ecological processes (Liu et al. 2019). In contrast, taxonomic and functional diversities were influenced by forest dieback (Tab. 2 & 3). Consequently, the diversity and quantity of habitats and resources released by forest dieback increased species richness and more heterogeneous functional assemblages, as suggested by the more-individuals and the habitat-heterogeneity hypotheses (Seibold et al. 2016), without significantly increasing phylogenetic diversity. Furthermore, phylogenetic response to disturbance may be such a long-term process that the effects of forest dieback on this component could not be observed in our study (Purschke et al. 2013). However, Kozák et al. (2020) showed that phylogenetic diversity of saproxylic beetles was positively affected by canopy openness, which in turn was positively influenced by recent disturbances.
4.4 Functional responses of assemblages to forest dieback: heterogenisation and specialisation
Forest dieback increased beetle functional richness (FRic), at both local and landscape scales (Tab. 3). In our study, the increase in total species richness seemed to be associated with this increase in FRic. This indicates that the range of functional traits is quite broad in the functional space of disturbed stands and within disturbed landscapes; in other words, functional traits have more extreme values in declining forests (Mouillot et al. 2013). This is congruent with previous observations showing that the functional richness of saproxylic beetle assemblages is positively influenced by the local amount and diversity of deadwood, as well as by canopy openness (local = 0.1 ha, R ≈ 18 m; Thorn et al. 2018), features which are typically favoured during forest decline and dieback (Thorn et al. 2018; Sallé et al. 2021; Cours et al. 2021). Here, we demonstrate that this probably stands true at larger spatial scales.
We observed a positive response to forest dieback for functional divergence (FDiv) and a negative response for functional evenness (FEve), but only at the landscape scale. When FDiv increases with the severity of forest dieback, this indicates that the dieback has made the arrangement of traits in the functional space wider (Mouillot et al. 2013). In parallel, when FEve decreases, it means that forest dieback leads to a less equal arrangement of traits in the functional space filled by the community (Villéger et al. 2008; Mouillot et al. 2013). Consequently, our results suggest that forest dieback at the landscape scale clustered the functional traits of local saproxylic beetle assemblages (decrease in FEve) into the extreme values of the functional spaces filled by these assemblages (increase in FDiv and FRic), and therefore led to local assemblage specialisation (Mouillot et al. 2013). Our results also suggest that forest dieback clustered the assemblages even more when it was severe at both the local and landscape scales, even if local dieback had no effect in the univariate and multiplicative models (positive synergistic effect in the multiplicative model; Fig. 4, Tab. 3). Therefore, forest dieback, especially at the landscape scale, seemed to promote and enhance local functional heterogeneity and thus diversify the functional niches of saproxylic beetles, at our study sites in the Pyrenean mountains. Intensive management generally leads to functional homogenisation, which is often driven by the decline of specialist species in favour of generalists (Clavel et al. 2011). Our plots were managed, and we found that forest dieback induced a functional heterogenisation accompanied by a specialisation of the studied assemblages at the boundaries of the functional space. We hypothesize that the functional heterogenisation was driven by the high resource availability and habitat diversification subsequent to the forest dieback. At the landscape scale, the dieback logically resulted in a matrix of remaining live trees, acting as disturbance refugia (Krawchuk et al. 2020), and discrete patches of open woodlands with standing dead trees and snags, logs, large deadwood, tree-related microhabitats, etc. Ultimately, this promoted the coexistence of a wide variety of ecological niches (Swanson et al. 2011), allowing the co-occurrence of functionally diverse saproxylic beetle assemblages (Thorn et al. 2018; Kozák et al. 2020).
In addition, we observed functional specialisation in species preference for deadwood diameter: when forest dieback increased at the landscape scale, local assemblages preferred larger deadwood and functional dispersion was lower (CWM and FDis Diameter; Tab. 3). A previous study showed that the functional specialisation of saproxylic beetles towards large-diameter and well-decayed deadwood occurs when the overall amount of deadwood increases (Gossner et al. 2013). This functional specialisation might not account for the needs of species that prefer small-diameter deadwood. Nevertheless, these species still benefit from a relatively high amount of deadwood and are also less sensitive to intense forest management in the surrounding area (Gossner et al. 2013). Moreover, we observed that forest dieback at the landscape scale led to an increased preference of the saproxylic beetle assemblages for more decayed deadwood, along with a higher functional dispersion than in healthy forests (CWM and FDis Decay; Tab. 3). Therefore, our results suggest that iterative forest dieback events had released a large amount of large-diameter deadwood in varying stages of decay, which is invaluable for biodiversity (Similä et al. 2003; Gossner et al. 2013; Lachat et al. 2013; Bouget et al. 2013; Kozák et al. 2020).
4.5 Application and conclusions
Our study revealed that the taxonomic and functional diversity of saproxylic beetle assemblages in Pyrenean mountain fir forests significantly benefitted from forest dieback, at both local and landscape scales, mainly thanks to landscape heterogenisation, to a large build-up of deadwood and to more canopy openings (Bouget et al. 2014; Thorn et al. 2017, 2018; Sallé et al. 2020; Sallé and Bouget 2020). Our results lead us to consider unharvested declining forest stands as potentially relevant sites for biological conservation (Müller et al. 2010, 2019; Hlásny et al. 2021) because they favour the functional diversity, abundance and richness of saproxylic beetle species otherwise threatened in conventionally managed stands (Grove 2002). In line with the habitat-amount hypothesis supported by our results, local clusters of forest dieback alone may be insufficient to maintain diverse communities of saproxylic beetles. It is also necessary to maintain areas of forest dieback in the landscape, i.e. at scales of at least 1100 m and 1500 m (Tab. 2 & 3). Furthermore, the discrepancies we found in the response of various biodiversity dimensions call for a multidisciplinary integrative approach and studies on wide species communities in disturbed forests (Sallé and Bouget 2020; Sallé et al. 2021; Sire et al. 2021)