Conifer is the top tree in an urban sound absorption test

larch tree bark

Scientists say trees have a role to play in combating noise pollution in urban environments and have identified the best species for the job.

The larch was found to be the most effective tree when it comes to absorbing noise with its bark. The conifer was the most effective out of 13 tree species in a laboratory-based sound absorption test. The researchers say the findings can help urban planners use trees for noise control. The results have been published in the Applied Acoustics journal. The study assessed 76 samples from 13 tree species that displayed a variety of different bark characteristics.

 

conifers

Sound ‘silencer’

Co-author Jian Kang, from University College London (UCL), said: “Beside emphasising the effects of vision and shade, urban greening should be considered as well to achieve noise reduction during propagation.” He told BBC News: “Using plants as a potential ‘silencer’ of urban noise could combine environmental protection and landscape business.”

The samples were selected by using a range of criteria, including bark thickness, tree age and trunk diameter. Disks of the trunks were collected from recently felled trees. “The main goal was to have a sufficient variety of species, including broadleaved and coniferous,” Prof Kang observed. In the laboratory tests, the team tested species that were often found in urban areas, such as cherry, pine, beech, willow, poplar and alder trees.

Comfort of conifers

The team found that the sample of larch was the most effective species, while conifers acted more effectively when it came to absorbing sound than broadleaved trees. “The influence factors on noise reduction by tree bark are bark thickness, tree age, and bark roughness,” explained Prof Kang. “Tree age and bark roughness seemed [to be] the parameters with the most predictive powers.” He said that the small changes in the sound absorption characteristics of the bark could influence the effectiveness of dense tree belts. “So, selecting species with slightly more absorbing barks can effectively [reduce] noise pollution and possibly mitigate the adverse effects of traffic and industrial noise.”

Prof Kang added that the trees could be used as natural “silencers” to limit the impact of noise from traffic in towns and cities. “As a result of the fact that the barks of conifers absorb sound slightly better than those of broadleaved trees, conifers could be used more in urban green spaces”, he said. “Moreover, tree density is important for noise reduction of a tree belt, and species will also influence the densities that can be obtained in a tree belt.”

How to plant a tree

Planting a tree with the best results is surprisingly easy, but needs proper planning and preparation. The time and effort taken to plant your tree correctly will pay off handsomely when you have a beautiful, well-structured and healthy tree in your garden. A properly planted tree will grow faster, stronger and healthier.

planting a tree

Remember that the roots of all trees spread out horizontally and grow into the first 80 or 90cm of soil. This is because they need oxygen to feed and below this depth there is very little or no oxygen.

  • Dig a wide shallow saucer-shaped hole that is as deep as the height of the root ball of your tree (contrary to the old way of digging a huge square hole). It is especially necessary to dig the hole wide if the soil is compacted. The loosening of this soil will encourage horizontal growth of the roots. The roots can spread as much as twice the height of the tree.
  • Identify the trunk flare, which is the area where the trunk starts to flare and the root and stem bark meet. This area must be exposed to the air and not buried. Place the tree in the hole at the correct height with the trunk flare above the soil line. If planted too deeply the roots will suffocate and thus not grow well and the tree may even die, as it impedes its ability to move carbohydrates up and down from the canopy.
  • Never pick a tree up by the trunk but always by the root ball.
  • Straighten the tree in the hole – back fill to one third and tamp down. Gently scratch the root ball to loosen fine roots to encourage them to grow into the new soil. Then back fill the rest of the hole and tamp down.
  • If your tree does not stand upright stake it according to the diagram. Beware of tying anything around the tree that will damage the bark. A piece of hosepipe threaded with wire will do. Place the hosepipe covered wire around the tree and make a figure of eight around the stake.
  • Immediately after planting, mulch with wood-chips, leaf mould or compost in a large area around the tree. This layer of mulch should not be more that 10cm deep and should never rest against the stem of the tree.If you think it is necessary you can sprinkle with 5:1:5 or 2:3:2 slow release fertilizer before you cover with the mulch.
  • Keep the soil moist but not soaked. This may mean watering once or twice a week until the tree is settled well in its new home. Perhaps for the first two weeks to a month, depending on weather and environmental conditions. 

Queries in specific suburbs

For queries in specific suburbs, please contact the following: 

          Newlands                          Clare Burgess                   082 546 7938

          Lynfrae                               Eleonore vd Horst           083 701 7634

          Wynberg                            Alison Coutras                  082 822 5550         

          Kenilworth                        Henk Egberink                  082 371 6989

          Bishopscourt                   Jane Prinsloo                     076 733 2318

          Harfield                             Gail Brown                          082 897 2972

          Rondebosch                     Heleen Louw                      072 236 6146

          Hout Bay                          Jill Mackay                           072 775 0750  

Tree Alert procedure

  • Introduce yourself to the contractor who is pruning or cutting the tree - indicate your interest
  • Establish who has contracted him and ask to see the permit from Recreation & Parks - all work on trees in public areas must have a permit
  • If no permit can be produced, ask him to stop work until he can show it
  • Obtain the contractor's name and take photos of him and vehicle, the tree being pruned or felled, and the entrance gate of the house with number
  • Send pictures to Neil.Fortes@capetown.gov.za and Treekeepers and ask whether a permit was granted and for what reasons
  • If no reply is received within two days, follow up.

Emergency procedure

As per procedure above but also phone Neil Fortes on 021 444 1694 or 082 561 3619, or Johan Herholdt on  021 4444 1692 or 084629 3257.  Also phone Clare Burgess on 082 546 7938.

How to prune a tree the right way

Trees for Bees

In Honour of World Bee Day, we are sharing 15 trees for the bees of the world! 🐝

1. African teak (Pterocarpus angolensis)
2. African wattle (Peltophorum africanum)
3. Apple-leaf (Philenoptera violacea)
4. Black thorn (Acacia mellifera Subsp. detinens)
5. Wild pear (Dombeya rotundifolia)
6. Buffalo thorn (Ziziphus mucronata)
7. Sweet thorn (Acacia karroo)
8. Tree fuscia (Halleria lucida)
9. Dogwood (Rhamnus prinoides)
10. False olive (Buddleja saligna)
11. Forest elder (Nuxia floribunda)
12. Coastal coral tree (Erythrina caffra)
13. Sumach-bean (Elephantorrhiza burkei)
14. Spekboom (Portulacaria afra)
15. Waterberry (Syzygium cordatum)

Bee on flower