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.

Three‐dimensional stratification pattern in an old‐growth lowland forest: How does height in canopy and season influence temperate bat activity?

“…Free space within forest gaps, which is restricted in size and differs space in gaps at mid and high heights probably offered highly profitable occurrences in lepidopteran prey (Rydell et al, 1996;Sierro & Arlettaz, 1997). Burford et al (1999) and Carr et al (2020), however, found moth species richness, occurrence, and abundance to be positively related to vegetation clutter, but Barbastelle bats-as large and rather fast flying ESFs-are precluded from cluttered vegetation. In the forest interior, their activity was four times lower compared to canopy gaps and restricted to the lower two layers where limited hunting possibilities were available.…”

Bat responses to changes in forest composition and prey abundance depend on landscape matrix and stand structure

“…As expected, P. kuhlii/nathusii, and P. pygmaeus / M. schreibersii were negatively affected by vegetation clutter. These edge-specialist foragers are known to avoid foraging or commuting in highly cluttered situations 6 , 9 , 12 , 37 . Stands with open canopy and low shrub cover may therefore represent optimal foraging habitats for these species.…”

“…There is strong support for bat activity in forest ecosystems being more related to habitat structure than to prey abundance 37 , 38 , 51 , 52 . Yet, the interactive effect between prey abundance and habitat structure has been largely overlooked to date.…”

“…To go beyond the basic comparison between pure deciduous and pure coniferous stands, it is crucial to investigate how bats respond to deciduous-coniferous mixture gradients at multiple spatial scales and how these scales may interact to better understand the ecological processes affecting bats in forests 36 . Another process that may explain the effects of forest attributes on bat activity is the bat-prey relationships 36 , which are expected to be complex and to strongly vary between species 37 , 38 . Moths constitute the main prey items of many temperate insectivorous bats 39 .…”

Roosting ecology of endangered plant‐roosting bats on Okinawa Island: Implications for bat‐friendly forestry practices

“…Studies generally recommend preserving old‐growth stands or retaining mature trees and snags in mixed‐age stands since many tree‐roosting bats appear reliant on these habitats (e.g., Burgar et al., 2015 ; Perry & Thill, 2007 ; Webala et al., 2010 ). Thinning trees or clearing understory foliage to promote the growth of remaining trees may benefit open‐space and edge‐foraging bats while negatively impacting clutter‐foraging species (e.g., Patriquin & Barclay, 2003 , Carr et al., 2020 ). For example, species including the open‐space foraging Lasiurus seminolus prefer roosting in relatively open forests (Perry et al., 2007 ), while the clutter‐foraging Barbastella barbastellus prefer roosts in unmanaged forests with dense understory (Russo et al., 2004 ).…”