There is evidence that mature trees are valuable in all sorts of ways. Here is a brief summary. Some of the ideas are mentioned in the article 'The natural capital of City trees' By Kathy Willis & Gillian Petrokofsky In Science, 356, 374-377, 28 April 2017.
- They remove substantial amounts of particulate matter from the air. For example, a study in Strasbourg estimated that tree in public spaces removed 88.23 tonnes of particulate matter from the air in a year.
- They evaporate vast quantities of water vapour. Cutting down mature trees will make the ground more waterlogged
- On the other hand, by shading the surrounding soil they reduce the evaporation of water. Their shading, however, provided an ideal habitat for many species of characteristic plant and animal species which will disappear if the trees are removed.
- There is solid evidence that trees improve human physical health. A study in Toronto, Canada, suggested that planting 10 more trees per city block would save > $10,000 Canadian dollars per household in health-related costs. An US-wide study suggests that trees significantly improve human heart and lung health.
- There is solid evidence from Wisconsin, USA and London UK that trees improve human mental health, e.g. reduce prescription of antidepressants.
- Removing mature trees obviously removes nesting sites and cover for many bird and bat species and some mammals. What may be less obvious is the number of different insect species, many food for birds, which depend on trees. These numbers come from a classical study 50 years ago: Oak 284 insect species, willow 266 species, birch 129 species, hawthorn 149 species, poplars 97 species, apple 91 species, ash 44 species and so on… This biodiversity takes many tens of years to re-establish itself when trees are cut down.
- Beneath them, dead, decaying wood accumulates, providing food for a wide variety of fungi and invertebrates, which in turn provide food for birds and so on.
- When mature trees of a variety of species and ages are removed and replaced by trees of an even age, this reduces the species diversity of the insects and birds which specialise on them.
- They absorb substantial amounts of carbon dioxide and produce substantial quantities of oxygen
Tim King
25 September 2018