Abstract:

Aims: Traffic noise is one of the major sources of urban noise that seriously harms residents’ physical and mental health. Roadside green belts can effectively reduce traffic noise. Studies have shown that green belts with more complex vertical structures or mixed conifer–broadleaf plantings tend to achieve greater noise reduction. However, whether tree species diversity itself makes an independent contribution remains unclear. This study aimed to investigate the independent effect of tree species diversity on traffic noise attenuation and its pathways. 

Methods: We selected 120 plots (60 pure and 60 mixed forest plots with varying richness) along five arterial roads in Harbin, China, for in-situ acoustic measurements. Single-tree morphological attributes were extracted using laser point cloud scanning. Covariance analysis, hierarchical regression, and Bootstrap robustness tests were combined to examine the independent effect and pathways of tree species diversity. 

Results: (1) Pure and mixed forests plots exhibited significant inter-group differences in physical volume and spatial arrangement. After correcting for these differences, mixed forests plots still achieved significantly greater noise reduction than pure forests plots. (2) Tree species richness demonstrated an independent and positive contribution to noise attenuation. This contribution was not mediated by morphological diversity, nor did species evenness exhibit an independent effect. (3) The noise reduction gain from increased diversity showed a diminishing marginal effect. 

Conclusions: This study provides preliminary scientific evidence for tree species selection and configuration in noise-abatement-oriented urban roadside green belt design. The primary gain in noise reduction was found to arise from the transition from pure to mixed forests, suggesting that introducing even one or two companion tree species may be an effective strategy.

Key words: urban roadside green belt, tree species diversity, traffic noise attenuation, hierarchical regression, independent effect