Testing the soundscape response to silvicultural interventions in a controlled before-and-after experiment

Abstract

In forest ecosystems, soundscape recordings are increasingly used to measure loss or shifts in biodiversity. In particular, acoustic indices can quantify the response of biodiversity to human interventions. However, most soundscape studies only use observational data, which precludes causal conclusions. To improve the use of acoustic indices as tools for conservation, experimental validation is essential. In a before-after experiment, we manipulated the canopy structural complexity of 59 broadleaf forest patches (50 m × 50 m) in Germany through silvicultural interventions, recorded their before-after soundscapes and calculated four well-established acoustic indices. The manipulation entailed a variation in logging intensity (0–33.3 % of basal area), different spatial arrangements (aggregated as gaps vs. distributed as thinnings), and the creation of standing deadwood. Linear mixed models controlling for pre-treatment levels showed increasing Soundscape Saturation and Acoustic Complexity with logging intensity particularly during the day, probably as a response of increased bird vocalization. During the night, these indices were lower in gaps, probably as a response of lower canopy insect activity in the absence of trees. We found more Events per Second, a measure of acoustic activity, in open canopy forests. None of the small-scale interventions altered the before-after synchrony of the circadian soundscape pattern, indicating no drastic shifts in the vocalizing community. Our results demonstrate that enhancing forest complexity by interventions can increase soundscape diversity in commercial forests and highlight the importance of recording the whole diurnal cycle for comprehensive insights into temperate forest soundscapes.