From ground to above canopy—Bat activity in mature forests is driven by vegetation density and height

Abstract

For several decades, forest managers have considered the effects of logging on the habitat quality of forests for bats. Concern about bat activity above the canopy has now been raised owing to rapidly increasing demands for wind energy and the ensuing placement of wind turbines over forests. We investigated the little-explored vertical stratification of bat activity in forests at ten sites on ten nights using five simultaneous bat-call recorders placed from near ground up to above the canopy. The vegetation-free space at each recorder position was measured with terrestrial laser scanning. We predicted that (1) the activity of Pipistrellus, nyctaloids and the open-habitat foraging guild will increase in mature forests with increasing height above ground, independent of local vegetation density and temperature, and (2) the activity of Myotis and the edge-habitat-foraging guild will decrease with height but increase with local low vegetation density. Our generalized linear mixed model indicated that nyctaloids, Pipistrellus and open-habitat foragers were increasingly active in higher strata, independent of temperature and local vegetation density. Activity of Myotis and Pipistrellus species and the edge-habitat foragers was higher along interior edges of forests. The activity of single species in the above-canopy stratum could be explained well by their Europe-wide wind-turbine risk assessment. Thus, we conclude that open-habitat bat species and Pipistrellus species not only forage regularly in clearings or forest meadows, but also above the canopy of closed mature stands, behaviour that may put them at risk from turbines.