The effects of thinning management on bats and their insect prey in temperate broadleaved woodland

Abstract

Trees, woods, forests and associated biodiversity are being affected by anthropogenic climate breakdown, and need management to maintain delivery of a wide range of ecosystem services. Wood harvested from sustainably managed woodlands can be used to mitigate greenhouse gas emissions through carbon substitution, directly using biomass for bioenergy to replace fossil fuels or indirectly through the use of wood products instead of higher carbon footprint materials such as concrete and steel. However, it is also important to understand how managing woodlands to mitigate climate change affects biodiversity.We tested the hypotheses that thinning woodland benefits bats and their insect prey by measuring bat species/species group richness and activity, and insect species/species group richness and biomass in 27 pairs of managed and under-managed broadleaved woodlands, and explored temporal responses to time since management. Sixteen woodland characteristics were measured to investigate how management affected woodlands, and to assess the relative importance of these characteristics to bats and their insect prey.Woodland thinning significantly reduced five woodland characteristics known to be important for woodland-dwelling bats. Standing dead trees were three times more abundant, and tree cavities five times more frequent in under-managed woodland compared with managed paired sites.Woodland thinning significantly increased bat richness and activity. Common and adaptable bat species, and those that forage along woodland edges (e.g. Pipistrellus pipistrellus), were positively affected by management, presumably exploiting less cluttered woodland interiors. Rarer bat species, and species that roost predominantly in trees (e.g. Barbastella barbastellus) were negatively affected by management, which reduced roosting opportunities. Overall bat activity and species richness were relatively low in woodland that had not been thinned for 30 years before increasing. Insect biomass peaked after 30 years of no thinning.We recommend minimum intervention management to conserve rare bat species in woodlands, although common and adaptable bat species may benefit from intermediate to heavy thinning. Sustainably thinned woodland could be greatly improved for all bats by retaining or mimicking habitat characteristics that are more representative of old growth woodland such as (i) standing dead trees, (ii) tree cavities, (iii) heterogeneous canopy architecture, and (iv) an overall uncluttered below-canopy vegetation with pockets of densely cluttered shrubs.