Abstract:

Aims: The three-dimensional structure of vegetation is crucial for the formation and spatial distribution of soundscapes in green spaces. Biophony, a key component of soundscapes, indirectly reflects regional biodiversity and ecosystem health. However, the complex relationship between vegetation structure and biophony in urban parks is not well understood. 

Methods: During the summer of 2024, we simultaneously collected high-resolution acoustic recordings and backpack LiDAR data from 52 sites in central Beijing parks. Six acoustic indices and 42 vegetation structural variables were calculated from these datasets. We used principal component analysis (PCA) and the XGBoost-SHAP model to identify and assess the importance of key vegetation variables influencing biophony. A generalized additive model (GAM) was then used to quantify the nonlinear relationships between these variables and biophony characteristics. 

Results: Our key findings are: (1) The power spectral density (PSD) of different biophony frequency bands showed distinct diurnal patterns. PSD at 2–4 kHz and 4–6 kHz exhibited similar circadian rhythms, while the 6–10 kHz band showed a staggered vocalization pattern. (2) Mean diameter at breast height (DBH), canopy relief ratio (CRR), and rumple index (RI) were key drivers of biophony across all frequency bands. Understory structure and canopy cover (CC) were dominant factors in regulating overall soundscape indices (ACI, ADI, and BIO). (3) Forest stands with a euphotic volume over 50% and medium-sized trees were more favorable for bird vocal activity. Increased CRR and canopy surface morphology significantly enhanced biophony, particularly insect sounds. (4) Excessive understory density and a large proportion of oligophotic volume were detrimental to the coexistence and propagation of multiple sound sources, which can reduce soundscape diversity. 

Conclusions: This study systematically reveals how three-dimensional vegetation structure influences biophony, identifies key structural factors for biophony patterns, and provides a scientific basis for soundscape optimization and biodiversity conservation in urban green spaces.

Key words: soundscape monitoring, acoustic index, LiDAR, urban forest, parks