In urban areas, tree cover increases species richness and shapes species composition of saproxylic beetles along with the microclimate

Urbanisation constitutes a major threat to biodiversity due to the reduction and fragmentation of natural habitats. However, with appropriate conservation measures, cities can harbour valuable microhabitats for biodiversity such as hollow trees and dead wood. We tested the effects of environmental filters, such as tree cover (reflecting habitat amount), density of urban structures (reflecting dispersal barriers), and microclimate on species richness, abundance and species composition of saproxylic beetles along urbanisation gradients. According to the urban homogenisation hypothesis, we expected more homogeneous communities across highly urbanised sites than across sparsely urbanised sites. Furthermore, we assessed whether communities at highly urbanised sites are more dominated by thermophilous species due to the urban heat island effect. In six cities in southern Sweden, we placed ten wood mould boxes emulating hollow trees along urbanisation gradients and sampled saproxylic beetles inside those boxes. To determine to what extent communities were dominated by thermophilous species, we calculated the Community Temperature Index (CTI). Species richness of saproxylic beetles decreased with the density of urban structures, whereas the abundance increased with tree cover. Microclimate and tree cover affected saproxylic beetle species composition significantly, but we did not find support for the urban homogenisation hypothesis. Despite an observed urban heat island effect and a cooling effect of tree canopies, CTI did not increase with the density of urban structures or decrease with tree cover. We conclude that it is possible to promote saproxylic insects in cities by maintaining and creating tree cover with varying canopy densities, leading to a variety of microclimatic conditions. Artificial microhabitats, such as wooden boxes, have similar microclimatic conditions as hollow trees and can be used to increase the supply of habitats for saproxylic species in cities.