One of the good things to have come out of the world-wide lockdown during this ‘Time of Covid‘ is that we can hear bird song and other sounds of nature now that much of the human-generated sounds – aka ‘noise’ – that usually masks such things is largely absent. And hearing such sounds can help to soothe the human spirit – which salving is sorely needed during these testing times.
Having just praised the joys of nature’s soundscape it may seem strange that this Cuttings item looks at a system for blocking out sound. But these are strange times, and this piece is very much forward-looking, to the days when this colossal case of coronic irritation is behind us and we return to some sense of normality. So, on with the plant-based research…
Whilst natural sounds can be soothing for people, many human-generated sounds can have the opposite effect. A nature-inspired solution to reducing the amount of noise pollution experienced by humans is to plant ‘tree-belts’ [“a strip between the sidewalk and curb of a road, planted with grass and sometimes with shade trees”] between the noise source and people (e.g. Chih-Fang Fang & Der-Lin Ling, Landscape and Urban Planning 63(4): 187-195 (2003); https://doi.org/10.1016/S0169-2046(02)00190-1; Timothy Van Renterghem, Ecological Engineering 69: 276-286 (2014); https://doi.org/10.1016/j.ecoleng.2014.04.029). Potentially, the number of different tree species that could be used for such sonic barriers is rather large [for example, world-wide there are 60,065 tree species], and each species is likely to differ in the degree to which it can reduce sound – quite apart from considerations of their suitability to be grown in the particular noise-polluted environment.
With over 60,000 tree species to choose from, we’d probably all appreciate some help in selecting the best tree for the job of noise-reduction. Thankfully, some guidance is provided by Mengmeng Li et al. (Applied Acoustics** Volume 165, August 2020, 107328; https://doi.org/10.1016/j.apacoust.2020.107328). Their laboratory investigation concentrated upon the sound-absorption characteristics of trees and investigated factors such as bark thickness, roughness, tree age, and moss coverage. They examined 3 conifer (Picea abies, Larix kaempferi, and Pinus sylvestris) and 10 broadleaved (Robinia pseudoacacia, Juglans regia, Prunus avium, Betula pendula, Populus nigra ‘Italica’, Salix alba, Fagus sylvatica, Alnus glutinosa, Salix caprea, and Populus tremula) species. Barks of the conifers were found to “absorb sound slightly better” than those of the broadleaves, with Larix kaempferi (Japanese larch) the top performer.
Several aspects of the study are recognised as being limitations by the authors, e.g. the fact that the data were based on “random sampling of fallen trees in Flanders, the northern part of Belgium”. Although the relevance of work on fallen trees to a living tree-belt will need to be checked, as will applicability of Belgian trees’ data to other parts of the world, Mengmeng Li et al. are optimistic that their study will indicate which bark characteristics have promise in the search for more sound-shielding trees. Until then, we could do worse than plant that fast-growing conifer Leyland Cypress (× Cuprocyparis leylandii), leylandii for short. As a conifer it should provide some sound-shielding from the outside world as we are forced to live in our domestic lockdown ‘bubbles’, even if it does have notoriety as the neighbour–annoying ‘scourge of suburbia’. As a botanical bonus, as well as any sound-proofing, the tree’s dense foliage should also shield you from the view of any no(i)sey neighbours; two important ecological /ecosystem services in one species.
* For those not in the know, this item’s title is an acknowledgement of one of Monty Python’s Flying Circus’ evergreen, coniferous comedy sketches.
** The journal whose research paper is here showcased – Applied Acoustics – is not one that I normally scan for plant tales; finding one there goes to show that botanical stories can be found in the most unlikely-sounding of places. Regardless of its phytonewsworthiness, the journal is mentionable because its Editorial Board – the group of worthy academics, etc. who advise on what is to be published in that esteemed organ – is affectionately known as the ‘sounding board’. OK, it’s probably not, but methinks it might be from now on – and I’m hoping that a little levity may help us to get through this COVID-19 pandemic with our humour – and, hopefully, humanity also – intact. Stay alert (because Britain needs lerts…).