Acoustic indices enable the discrimination of temperate forest types and better predict differences in bird species composition than in bird species richness

Acoustic indices have been proposed as a tool to estimate animal biodiversity and abundance, as well as habitat types and vegetation structure. However, recent studies have reported inconsistent relationships between acoustic indices and environmental metrics. In this study, we recorded soundscapes across different forest types and calculated six of the most commonly used acoustic indices to evaluate their effectiveness in predicting bird species richness, bird community composition and temperate forest characteristics. We observed that bird species richness did not significantly differ between forest types, soil moisture, forest age, the Normalized Difference Vegetation Index (NDVI), the area covered by standing dead trees, or forest protection. In contrast, acoustic indices showed significant differences between forest types, soil moisture levels, forest age, and NDVI. In general, acoustic complexity increased with the progression of forest productivity and moisture, was the lowest in the youngest forests, decreased with increasing NDVI, did not differ between protected and unprotected areas, and was not affected by the amount of standing dead trees. The dissimilarity of the acoustic structure of the environment correlated more strongly with dissimilarity of bird community composition than with bird species richness, but it best explained the differences in forest characteristics obtained from satellite data. These correlations were weak, in contrast to the moderate dependency observed between differences in bird community composition and satellite data describing forest characteristics. Our study revealed that in temperate forests, soundscape enables discrimination between forest types, based on productivity, moisture, or age. Furthermore, the acoustic structure of the environment better explains differences in characteristics of the forest obtained from satellite data than differences in bird community composition or bird species richness. This suggests that soundscape complexity depends on forest characteristics and is shaped by species richness, community composition, and the abundance of individual bird species. Therefore, the soundscape can serve as an indicator of temperate forest quality and disturbance levels.