Where there is human activity, there is usually noise. A particular type of human activity, which is that of vehicular travel, is a principle means of noise pollution, and wherever the highway supporting such vehicular travel may reside (in an urban, rural, or largely isolated area), there are adverse consequences to the noise pollution (for human health and ecosystem health). Therefore, it is important that we can understand what tree species are best-placed to buffer the most amount of noise, as particularly for urban locations it may influence how buffer planting schemes may be designed. Beyond the urban setting, recognising how the impacts of noise from a road will be dampened by constituent tree species is also important, as it may potentially influence exactly how such a site is managed.
In the study that is the focus of this post, the authors investigated how deciduous and coniferous tree species influence noise levels at staggered distanced away from roads in the Sonbolrood forest, Iran. The forest is home to the coniferous tree species Cupressus sempervirens var. horizontalis, Juniperus spp., Taxus spp., and Thuja orientalis, and the deciduous tree species Alnus spp., Fagus orientalis, and Platanus spp. For the purposes of this study, 25 plots containing predominantly coniferous species, and another 25 plots containing predominantly deciduous species, were identified, with each one measuring 20m x 50m. All plots were located adjacent to a road, and also had their total tree populations counted.
At each plot location, in the adjacent roadway, a trumpet was sounded four times (with an approximate sound level of 100 decibels), and the decibel level was measured (at an elevation of 1.8m) at distances of 20m, 100m, and 300m (the latter two were to understand how noise is dampened over much greater distances) for each of the four sounds. Therefore, a total of 600 measurements were taken.
In terms of what the authors found, they first recognised that the more trees present within a plot the greater the noise reduction. However, there were observed differences between the effectiveness of deciduous and coniferous stands in dampening such sound, with deciduous trees reducing sound more significantly at densities of up to 40 trees when compared to coniferous trees. However, once coniferous stands reached beyond 40 trees in the sample areas, they dampened sound more effectively than their deciduous counterparts (as shown by the two graphs below). This, the authors allege, is because the crowns of deciduous trees are generally broader and less regularly shaped, thereby meaning they dampen sound waves more readily (as they have more matter with which to buffer against the waves). Furthermore, though also applicable for some constituent coniferous species such as Thuja orientalis, the form of deciduous trees sees them adopt, in general, a lower H:D ratio (height:diameter), and such larger trunks have more mass with which to reflect or refract sound waves. Of course, because deciduous trees abscise their foliage during winter, they may potentially dampen noise less effectively than coniferous species when their foliage is absent. In this sense, a mixed stand may well be most optimal for noise buffering.
Interestingly, but probably unsurprisingly, there was also a near uniform decrease in the decibel level measured at all three distances away from the sound’s origin, and this applies for both predominantly broadleaved and coniferous plots. Of course, for the two distances beyond the plots themselves, stand composition and density was not measured, though such a feat would have been wonderfully impressive at the 300m distance, in particular. Again, the two graphs below demonstrate this sound level reduction associated with tree density.
In relation to the graph displaying sound level reductions in coniferous plots, it is actually interesting to note that the decibel measurements at 20m, 100m, and 300m were all relatively similar, and only when these plot stands had tree densities of over 26-29 individuals was there sufficient dampening of the trumpet’s sound. With regards to broadleaved stands, the authors suggest that shade tolerant species including Fagus orientalis may, because they can persist in the lower canopy and form dense branching structures, effectively dampen sounds and thereby supplement, particularly in summer, the effectiveness of deciduous stands (or, if they exist underneath stands of conifers, those also).
Drawing upon the results provided, we can begin to understand, albeit in a rather basic sense, exactly how trees can aid with noise level reductions. Admittedly, this study was undertaken in a forest, and it is almost impossible that an urban highway would be bordered by 100m+ of tree belt, but if focussing on the results from 20m it may perhaps be best to select deciduous species principally, though consider planting conifers within, at high densities. Beyond the scope of this study, but something worth considering too, is whether deciduous species that actively sprout from the base, such as Tilia spp., will aid with the dampening of sound yet further.
Source: Nasiri, M., Fallah, A., & Nasiri, B. (2015) The effects of tree species on reduction of the rate of noise pollution at the edge of Hyrcanian forest roads. Environmental Engineering and Management Journal. 14 (5). p1021-1026.
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