Substrate requirements of red-listed saproxylic invertebrates in Sweden

To evaluate which qualities of dead wood have the highest conservational value for invertebrates, substrate requirements were recorded for all 542 saproxylic (wood-living) red-listed invertebrates in Sweden. Preferred qualities of dead trees were described for each species in seven category variables: tree species, stage of decay, type of main stem, coarseness of stem, part of tree, light preference and microhabitat. For each quality we quantified the number of red-listed insect species using it and the number that are specific to the quality. The number of species associated with different tree genera ranged from five to 202. Species-rich genera had a higher proportion of monophagous species. To what extent different tree genera have faunas complementary to each other was also analysed. As the wood decays, the host range broadens and the flora of decay fungi will probably take over as the most important factor determining the saproxylic fauna. Because there are many different types of rot required by different invertebrates, an abundance of similar looking dead trees, logs and snags are probably needed. Some 59% of the invertebrate species can live in sun-exposed sites. Most of these are favoured when the forest burns and at least 29 of them are more strongly associated with fires. For the former category, leaving dead wood on clear cuts should be an effective way to increase the amount of breeding substrate. However, some species, especially those living in the last successional stages, are dependent on shaded sites. Hollow tree trunks are another important microhabitat; 64 of the 107 species living there are specialists. To maintain all saproxylic species we need a diversity of substrate types and management methods. Our data can help to decide how to optimize the conservation measures taken during forest operations.