A preliminary assessment of the impact of forest thinning on bat activity: Towards improved clutter-based hypotheses
The management of native regrowth forests for biodiversity has become increasingly important in recent years, as most old growth forest on mainland Australia is now protected and logging operations occur primarily in regrowth. Regrowth forest typically has higher stem density than old growth; therefore, bats (Microchiroptera) adapted to foraging along edges and in open spaces are likely to be less active in regrowth forest. Thinning is an integral component of regrowth management and could reduce structural clutter to a level suitable for bats with a range of clutter tolerances; yet little is known about the effects of thinning on bat activity, especially in Australia. In a preliminary study, we compared the activity levels of all bats, one open-space and three edge-space aerial foraging bat guilds in unthinned (n = 3) and thinned (n = 3) regrowth, in forests managed for timber production on the south coast of New South Wales. We measured bat activity at three heights in the forest (understorey, subcanopy and canopy). The number of volant insects (prey items) was also measured at each site/height combination, while 20 structural vegetation variables were measured at each site. A total of 263 bat passes was recorded in 18 detector-nights. We found high variability in bat activity for all bats and guilds between regrowth treatments and among heights, and no significant difference between levels of these factors.There was no significant interaction between logging and height factors. Vegetation structure varied significantly between unthinned and thinned sites for just two variables: the shrub layer had a higher percentage cover, and the vertical gap between canopy and understorey trees was halved, in thinned regrowth. Multivariate analysis of vegetation structure suggested just a small increase in the distance between canopy and understorey tree stems and slightly less cover in the canopy in thinned sites, changes that would represent a reduction in clutter. With the highest density of stems, the understorey of both regrowth types represented the most cluttered stratum for bats and overall bat activity was lower there (though not significantly). However, total bat activity did not indicate a clear response to insect abundance in the less cluttered subcanopy and canopy. Although the variability in bat activity within our regrowth treatments was too high to unequivocally state that thinning had no effect, results obtained from older regrowth of the same forest type suggest that overall activity is low for bats in dense regrowth forest, and that the level of thinning conducted at our sites was not sufficient to increase levels of activity consistently. We review the scientific literature on thinning effects for bats and formulate more specific hypotheses for future testing. Appropriate measures of clutter need to be identified to incorporate into indices of stand structural complexity that are relevant for bats.