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Purpose of the Review The concept of tree-related microhabitats (TreMs) is an approach to assess and manage multi-taxon species richness in forest ecosystems. Owing to their provision of special habitat features, TreMs are of special interest as a surrogate biodiversity indicator. In particular, in retention forestry, TreMs have gained attention over the past decade as a selection criterion for retained structural elements such as habitat trees. This review seeks to (a) address the suitability of TreMs as biodiversity indicator in the context of retention forestry, (b) summarize drivers of TreM occurrence and the status quo of the implementation of TreM-based retention concepts in forest management, and (c) discuss current and future challenges to the use of TreMs as biodiversity indicator. Recent Findings The TreM concept originated in Europe where it is now increasingly implemented. Most studies of the quantity, quality, and diversity of TreMs are focused on tree species from this region, although it is increasingly applied in other contexts. In addition to tree species, tree dimensions and live status have been identified as the main drivers of TreM occurrence. One major remaining research challenge is to verify relationships between the occurrence and abundance of forest-dwelling species from different taxonomic groups and TreMs to improve the evidence basis of this concept and thus increase its integration in forest conservation approaches. Summary TreMs are not the “silver bullet” indicator to quantify biodiversity of forest dwelling species, but they provide an important tool for forest managers to guide the selection of habitat trees for the conservation of the associated biodiversity.

Continuously suppressed S.torminalis trees were found to be highly sensitive to release cuttings. After release diameter growth increased drastically, the crown redensified from the inside and seeds were produced during the first growing season. Sorbustorminalis is valued for its timber characteristics and ecological role in cultural landscapes. Due to its increased drought tolerance, the species is of growing interest in the context of silviculturally driven climate change adaptation measures. Due to its comparably slow growth, it often forms a second canopy layer below other deciduous tree species. With regard to its ecological and economic value, forest practitioners need to know whether releasing these continuously suppressed individuals from competition is worth it. Therefore, the objective of this study was to examine the effect of release cuttings on the crown growth and radial increment of S.torminalis heavily suppressed by surrounding sessile oaks (Quercuspetraea). We observed changes of shoot and diameter growth within the first year following release cuttings. The overall average annual shoot growth more than tripled when compared to pre-release growth patterns (from 1.9 to 6.2 cm). The greatest release effects were found in the inner part of the crown. The majority of the shoots forming the crown periphery developed fruits and showed only restrained increment. Crown redensification from inside was accompanied by the development of epicormic branches in previously branch-free bole sections. Following release cuttings, diameter growth increased by several orders of magnitude. Altogether, release cuttings seem to have a very positive effect on the vitality of continuously suppressed S.torminalis trees. All released trees grew vigorously and developed fruits throughout the crown surface. Thus, release cuttings may facilitate species propagation and reduce inbreeding effects.