Abstract
The Austrian climate and energy strategy plans a massive expansion of wind energy production. In Styria, most of the planned sites for wind farms are in montane forests. Studies on the intersections of bat activity and wind energy production have been conducted mostly at lowland sites. Recent studies in the Alps show surprisingly high bat activity at high elevations. As data on bat activity at nacelle height is virtually absent for Austrian montane forests, the present study aims to close this gap.
Bat activity was studied at five wind farms situated in Styrian montane spruce forests (1,200–1,700 m a.s.l.) by using batcorders (ecoObs) in nacelles from at least May to mid-September. We recorded bat calls at all sites. In addition, wind speed was measured at four study sites and air temperature at two study sites at nacelle height. All data were collected in the context of planned wind energy projects. All study sites have a low rotor-ground distance of up to 50 m.
Eight species were detected in the monitored wind park sites. The recorded species included bats from local populations like Pipistrellus pipistrellus and Eptesicus nilssonii as well as long-distance migratory species like Nyctalus noctula, Vespertilio murinus and P. nathusii. We recorded calls from the genus Myotis at a single study site. 85.2 to 97.4 % of recordings were assigned to Nyctaloids and 2.5 to 14.7 % to Pipistrelloids.
Variability in the number of recordings was very high between study sites, between years within study sites, and between wind turbines within the same wind park. Bat activity varied substantially not just between months, but also between days. We measured the highest activities of bats from June to September. In general, April and May featured a low acoustic activity of bats; at two sites relatively high activities were recorded still in October. The high variability underscores both the importance of multi-annual nacelle monitoring studies and of several sampled nacelles within a wind farm.
Bat activity was recorded at wind speeds of up to 6.5 to 15 m/s. As expected, bat activity decreased with increasing wind speeds, but this decrease varied strongly between sites and within sites between years. At some sites, bats were still active at freezing point temperatures.
These results indicate that bats in montane forests may be encountered at higher wind speeds and lower temperatures than in the lowlands. Further differences appear in the annual phenology; high activity occurs as early as June, i.e. preceding bat autumn migration periods. These differences should be taken into consideration when planning wind farms in montane forests. Curtailment algorithms require critical assessment if inferred from studies conducted at lowland sites. Due to differing activity patterns the application of the ProBat-tool is not recommended for the time being. As a result of the low rotor-ground distance of most of the wind energy plants in montane forests in Styria there should be case-to-case evaluations of the effectiveness of curtailment algorithms by means of fatality searches beginning at least in June. Further studies are needed to increase the evidence of systematic differences between montane forests, lowlands and other study sites.