Habitat requirements and preservation of the beetle assemblages associated with hollow oaks

Abstract: One of the most endangered assemblages of species in Europe is saproxylic beetles associated with old trees. To be able to conserve these species there is a need of developing methods to survey the fauna and to evaluate the conservation value of different oak habitats, develop instruments for landscape planning and detailed knowledge of species habitat requirements. The results are based on a data set from window and pit-fall trapping of saproxylic beetles at 94 different sites spread over four regions in southern Sweden. Additionally, 48 wooden boxes with artificial wood mould, consisting mainly of oak saw dust and oak leaves and some varying additional substrates, were studied for three years at three of the sites and their vicinities.A comparison of three methods to assess species richness and composition of the saproxylic beetle fauna in standing hollow oaks showed that all trapping methods were effective in detecting species, but as they partially target different assemblages of species it is profitable to combine the methods. Window traps gave most species but wood mould sampling is the cheapest method to sample the fauna. It was possible to predict the conservation value individual oak patches with sets of indicator species of saproxylic beetles with regard to number of species or presence of conservation priority species. Indicator sets of species effectively caught with pitfall traps gave the overall best predictions. When comparing different treatment of species indata, the explanatory power of predictions was strongest for presence/absence data. Predictions of species number and an index worked well within the same regions but gave varied result for three other regions, which shows that transferability of indicators between regions may be doubtful. Species richness was greatest in stands with large, free-standing trees. Among individual trees, large girth as well as low canopy cover, increased frequency of occurrence for several species. Forest regrowth was found to be detrimental for many beetle species. An evaluation of to what extent artificial habitats, mimicking the conditions in hollow oaks, can be exploited by saproxylic beetles showed that nearly 70% of the species found in hollow oaks was found in artificial wood mould boxes. A dead hen added to the artificial wood mould gave a higher number of beetle specimens. The number of species associated with tree hollows in oak decreased with distance from sites with hollow oaks.An analysis of species assemblages at 38 sites and positions of 33 000 large/hollow oaks showed that different beetle species dependent on a single substrate, hollow oaks, responded to different scales. The total species richness responded to a scale of 859 m and the characteristic scale of response for single species varied between 52 m and 5284 m. Several species were sensitive both to smaller and larger scales.As most sites with endangered beetles living in old oaks are small and isolated, ongoing management directed to keep old oaks free standing and sun exposed and to ensure the recruitment of young oaks, and the restoration of abandoned pasture woodlands should have a high priority in nature conservation. Artificial habitats may in critical areas be created to fill gaps in old oak habitat for parts of the species assemblage. To preserve the saproxylic beetle fauna dependent on old oaks, it is important to retain and create suitable habitats both in local stands and at the landscape level, from single hectares up to hundreds of hectares depending on the species. In some landscape, creations of new oak areas in the fragmented landscape are crucial for long-term survival of sensitive species.