Who is using tree-related microhabitats (TreMs)?

Abstract

Tree-related microhabitats (TreMs) are well-delineated tree structures that provide important resources and refuge to thousands of species. For these reasons, TreMs have been recommended as an indirect indicator of biodiversity in European forests. We conducted a literature review to understand the scale at which different taxonomic groups are associated with TreMs in Europe. Across a pool of 54 papers, we found that TreMs were most frequently studied at the scales of sampling plots (n = 28, 58 %) and individual trees (n = 18, 33 %), and less frequently at the stand scale (n = 5, 9 %). Two studies addressed the landscape scale in addition to the plot and stand scale (n = 1). Birds, saproxylic beetles and bats were the most frequently studied taxonomic groups overall and at the plot scale, whereas amphibians were only studied at the scale of individual trees. Tree cavities were the most frequently studied TreM-form at all scales and were often associated with bats and saproxylic beetles. Crown deadwood and fungal fruiting bodies were associated with saproxylic beetles, and tree injuries were frequently associated with bats. Sample sizes across all TreMs-taxa combinations were low, allowing only descriptive analyses and interpretation. The largest sample size for saproxylic beetles investigated at the plot scale (n = 15) across different TreMs, were found to be significantly associated more than 20 % of the times with tree cavities (33 %), tree injuries (21 %) and fungal fruiting bodies (20 %). We recommend future studies to evaluate species-TreM associations for the poorly studied TreMs such as epiphytic and epixylic structures, excrescences, and exudates, or poorly studied taxa such as meiofauna, and to evaluate multiple scales and taxonomic groups when possible. Our review could also serve as a starting point to consider which taxonomic groups would benefit from those management practices that enhance TreM-associated biodiversity.