Microhabitats in lowland beech forests as monitoring tool for nature conservation

We investigated the occurrence of 20 precisely defined structural microhabitats on trees in mature (>120 years old) lowland beech forests in Germany that represented 12 managed, 5 recently unmanaged and 2 reference (>100 years unmanaged) stands. To promote naturalness assessments in forest inventories and for biodiversity monitoring, we analysed (1) which microhabitats on individual trees are characteristic for reference stands, (2) the heterogeneity in types and frequency of microhabitats, and (3) the link between structure-based indicators and threatened species of saproxylic beetles.The number and diversity of microhabitats was significantly higher in the reference stands. We found there on average about 250 microhabitats/ha with almost 7 of the total of 20 different microhabitat types/circular plot of 500 m2. Fourteen individual microhabitat types were significantly (P < 0.01) more frequent in the reference stands.Multi-habitat trees with up to seven different microhabitat types on an individual tree were more common in the reference stands. We found a strong linear correlation between the number of microhabitats and the dbh of the habitat tree in the references but not in the managed or recently unmanaged stands. Consequently, the architecture of large trees in the reference stands reflects the ecological continuity of these forests.We also analysed the frequency of threatened saproxylic beetles in relation to the presence of tree microhabitats. Only recently unmanaged stands showed a clear correlation between the habitat supply with microhabitats and the threatened saproxylic beetles. The number of threatened saproxylic beetles was high in the reference and low in the managed stands without a correlation with the number of microhabitats.We propose that microhabitats are a reliable monitoring tool for assessing and reporting on biodiversity of forests that naturally have a high structural heterogeneity and show examples for implementing microhabitats into forest inventory and monitoring as a measure of nature conservation.