Abstract
In Germany, wind turbines are increasingly installed in forests. Many bat species are highly dependent on forests for roosting and foraging. It is widely assumed that bats may suffer from increased collision risks and habitat loss when wind turbines are erected and operated in forests. Accordingly, measures have to be adjusted to survey bat populations and to mitigate emerging conflicts to guarantee that the installation of wind turbines in forests does not come at the expense of bat conservation.
Comparative acoustic surveys at wind turbines as well as at wind masts show that the same bat species occur above forest canopies and at sites in open landscapes, i.e. Pipistrellus pipistrellus, P. nathusii, P. pygmaeus and the similar calling species Nyctalus leisleri, N. noctula, Vespertilio murinus, Eptesicus serotinus und E. nilssonii. Similar to open landscapes, the acoustic activity of bats at forest sites depends on wind speed, temperature, time of year and time of night. Bats of the genera Myotis and Plecotus have rarely been encountered above the canopy. Also Barbastellus barbastellus has rarely been observed above the forest canopy. Therefore, a high collision risk is not anticipated for these species under normal circumstances. Special activity patterns might occur close to roosts e.g. close to mating roosts of N. leisleri. In general, current evidence suggests that the collision risk for bats at wind turbines is similar in forests and in open landscapes. Thus, standard curtailment algorithms can be developed and applied as mitigation schemes for wind turbines at forest sites when based on acoustic monitoring at the nacelle height. Yet, special attention has to be given to daytime roosts of species with high collision risk when activity patterns deviate form the normal scheme. The number of accepted bat fatalities per turbine should be chosen as low as possible for an efficient protection of bat populations. Turbine specific curtailment algorithms have to be implemented for all wind turbines following the current state of knowledge since elevated collision risks can be expected for bats at all sites in forests as well as in open landscapes.
The loss of forest habitats is relevant for all species that roost in trees. Especially maternity colonies are at risk when turbines are placed next to roosting areas, leading to the destruction of used and potential roosts. Furthermore, species which prefer closed forests for foraging, such as Myotis bechsteinii, may loose essential hunting areas due to forest clearance. Currently, it is investigated if disturbances caused by wind turbine operation and microclimatic changes caused by the opening of formerly closed forests may lead to the deterioration of areas adjacent to clearings. Roosting and hunting areas of forest bats should be intensively surveyed prior to the erection of wind turbines to prevent habitat loss as much as possible. Old forests with many potential bat roosts must be completely avoided as potential sites for wind park projects. Repeated mist-netting in suitable ambient conditions and radio-tracking of several reproductive females over a longer period of time are necessary to identify important roosting and foraging areas. Turbines should be installed at a minimum distance of 200 m to roosting areas and essential foraging habitats. For compensation, it is recommended to permanently abandon the commercial use of forest patches with known daytime roosts, including valuable adjacent areas, in combination with the installation of bat boxes.