The indicator side of tree microhabitats: A multi‐taxon approach based on bats, birds and saproxylic beetles

Abstract

National and international forest biodiversity assessments largely rely on indirect indicators, based on elements of forest structure that are used as surrogates for species diversity. These proxies are reputedly easier and cheaper to assess than biodiversity. Tree microhabitats—tree‐borne singularities such as cavities, conks of fungi or bark characteristics—have gained attention as potential forest biodiversity indicators. However, as with most biodiversity indicators, there is a lack of scientific evidence documenting their quantitative link with the biodiversity they are supposed to assess. We explored the link between microhabitat indices and the richness and abundance of three taxonomic groups: bats, birds and saproxylic beetles. Using a nation‐wide multi‐taxon sampling design in France, we compared 213 plots located inside and outside strict forest reserves. We hypothesized that the positive effect setting aside forest reserves has on biodiversity conservation is indirectly due to an increase in the proportion of large structural elements (e.g., living trees, standing and lying deadwood). These, in turn, are likely to favour the quantity and diversity of microhabitats. We analysed the relationship between the abundance and species richness of different groups and guilds (e.g., red‐listed species, forest specialists, cavity dwellers) and microhabitat density and diversity. We then used confirmatory structural equation models to assess the direct and indirect effects of management abandonment, large structural elements and microhabitats on the biodiversity of the target species. For several groups of birds and bats, the indirect effect of management abandonment and large structural elements on biodiversity was mediated by microhabitats. However, the magnitude of the link between microhabitat indices and biodiversity was moderate. In particular, saproxylic beetles’ biodiversity was poorly explained by microhabitats, large structural elements or management abandonment. Synthesis and applications. Tree microhabitats may serve as indicators for bats and birds, but they are not a universal biodiversity indicator. Rather, compared to large structural elements, they most likely have a complementary role to biodiversity. In terms of forest management and conservation, preserving diversity of microhabitats at the local scale benefits several groups of both bats and birds.

Predicting Tree-Related Microhabitats by Multisensor Close-Range Remote Sensing Structural Parameters for the Selection of Retention Elements

“…Forests are enormously important for the conservation of biodiversity and the provisioning of habitats within forests is closely related to their structural richness or complexity. Forest structure, therefore, is an important driver for biodiversity among other forest ecosystem services [1][2][3]. Consequently, forest biodiversity conservation has shifted from a focus on single-species protection towards understanding and conserving multi-taxon as well as structural indicators of forest biodiversity [1,[4][5][6][7] and forest taxa on different scales including fine-scale structures at the tree-level [1].…”

Nothing else matters? Tree characteristics have more effects than biogeoclimatic context on microhabitat diversity and occurrence: a nationwide analysis

“…Once dead, standing trees are affected by decomposition processes that initiate and develop microhabitats [14,32,33]. Such trees could also constitute privileged foraging grounds for a number of species [5,7,19], including for example woodpeckers [33,34]. In particular, insect larvae or ants that live below the bark of more or less recently dead trees constitute a non-negligible part of some birds’ diet [7,35,36].…”

“…As a consequence, conserving and promoting large trees in daily forest management is likely to enhance the structural heterogeneity at the stand scale [20,53], including a variety of tree-borne microhabitats, that could further help to better conserve specific forest biodiversity [5,54]. Despite the fact that the diameter effect seems consistent across different conditions, promoting a variety of large trees of various species may further increase the effect on biodiversity [19], since the succession dynamics of microhabitats as well as their formation speed may vary with tree species [10,12]. Successional patterns and long-term dynamics or microhabitats remains largely unknown [10], long term monitoring at the tree and stand scales are still needed to better understand their dynamics and the underlying processes at play [5].…”

“…tree species) or abiotic (e.g climatic) factors. Since microhabitats are on the potential biodiversity indicator list [7,16,19], it is also important to better understand the factors of influence at various scales, including the tree scale.…”

Evaluating the effectiveness of retention forestry to enhance biodiversity in production forests of Central Europe using an interdisciplinary, multi‐scale approach

“…Recommendations to date, that is, for the required amount of deadwood and habitat trees, have been based mostly on single taxonomic groups such as forest birds, saproxylic insects, lichens, or fungi (Müller & Bütler, ; Sandström et al, ). Studies investigating a wide spectrum of species including multiple taxa and trophic groups in the same study system are rare (Franklin, Macdonald, & Nielsen, ; Müller & Bütler, ; Paillet et al, ; Ranius & Fahrig, ; Vítková et al, ). Further, relationships between forest structure and biodiversity have not been studied comprehensively in a landscape context (Mori, Tatsumi, & Gustafsson, ).…”