PLANETARY HEALTH WEEKLY

Milner on Biodiversity

Blog #11 “Urban Tree-planting”

Planetary Health Weekly

Edward Milner, February 9, 2023

In midwinter the prospect of a long hot summer may look inviting, but I advise caution: be careful what you wish for. While the thrust of international gatherings like recent COP meetings in Egypt and Canada and headline writers of the popular press are all focussed on far distant dates like 2030 or even 2050. There seems to be little discussion of what needs doing right now, in 2023. Too many ominous trends are already evident upon us; there is no time to waste. Take London for example, my home city. What measures need putting in place starting today?

Climate change will clearly have a big impact, but perhaps there are ways in which we could consider ‘future-proofing’ the metropolis – with implications for all urban areas.  Inevitably we must expect not only heatwaves and prolonged periods of drought, but also increasing turbulence; more storms producing heavier and more irregular rainfall, causing local flooding, transport disruption and even threats to life and limb. And that’s on top of some existing problems like urban pollution and rocketing energy use which are both set to worsen.

In too many cities air pollution reduces the life expectancy of large numbers of citizens – even killing some of them directly. Two years ago, when Ella Kissah-Debra (8 years old) of Lewisham, South London, died of an asthma attack, air pollution was recognised as a direct cause of her demise. New studies by Kings College, London and University of British Columbia have shown links between levels of air pollution and the prevalence of long-term illness, even ‘impairment’ of brain function. Ella’s family lived close to a major road where air quality had been far below legal minimum standards for years. City authorities frequently express their (overdue) concern about the quality of city air, but at present action seems to be restricted to the wringing of hands and easy measures like penalising users of road vehicles and planning a highly controversial ULEZ (Ultra Low Emission Zone) schemes. Like other highly polluted places, the centre of Lewisham is virtually bereft of trees or other greenery, but so far urban tree-planting and hedge installation are not part of the single-track anti-pollution drive.

Deciduous trees in particular can have an enormous ameliorating effect on air pollution; in London we are fortunate in having large numbers of mature London Plane trees (Platanus x acerifolia) which are well adapted to the urban setting, being robust and resistant to soil compaction, drought and air pollution as well as growing into magnificent and beautiful specimens. Most were planted in the last century when they were one of the few large trees that could thrive in the heavily polluted air. Before the Clean Air Acts in the 1950s coal-burning caused London’s air to be really foul, with days of appalling smog well captured by some of Monet’s famous London paintings. That the plane trees survived at all is remarkable; without them conditions would have been even worse. But actual planting of London planes seems to have stopped a long time ago, as if the problem of air pollution would now go away.

In future, as summer temperatures rise, a second benefit from London’s plane trees may become even more important – providing shade and helping cool the city from the ‘heat island’ effect that increasingly blights urban areas everywhere. In particularly hot conditions the temperature difference between shade and sun may be 10 -15 degrees Fahrenheit; maximising the urban tree-canopy is likely to become more important year by year. In Darwin, northern Australia, the streets were laid out on a north-west/south-east axis to take advantage of prevailing winds both in the dry and wet seasons, and many streets were lined with fine saman trees, providing shade across the city. Then, astonishingly, they were mostly cut down; I understand Darwin is now dangerously torrid for much of the time. As in many other cities, misguided municipal programmes have resulted in large trees being removed, often in the teeth of local opposition. While it is true that tree roots can damage existing buildings, far too often local authorities seem ready to remove trees rather than find ways to accommodate them. Even now several local authorities in London (including Lewisham) are engaged in battles over tree removal with residents who object to the loss of much-loved local specimens and appreciate their importance.

In fact, the tide is turning and public authorities in cities around the world are starting to shift from tree-cutting to tree planting. At Darwin the local government has had a rethink and now identified 542 vacant sites where it believes trees should be planted, while Bangkok’s new Governor Chadchart Sittipunt has invited city-dwellers to improve their own city by planting a million new trees to replace the ones that were lost during anti-tree drives in past decades. Increasingly, city authorities recognise the need to find natural ways of cooling urban environments while cutting the effects of pollution; some, such as Belfast and Paris plan ambitious tree-planting schemes. In Vietnam, Vo Trong Nghia Architects have pioneered the use of elevated tree-based gardens, and forest trees planted in tall boxes, to bring the cooling advantage of forest tree shade even in the centre of urban areas. In French Colonial times Ho Chi Minh City (or Saigon as it then was) benefitted from the inspirational planting of giant dipterocarp trees in the city, which now, eighty to a hundred years later, enhance major streets and turn public parks into airy and cool cathedral-like spaces.

So how much new tree-planting is taking place in London? Not much that I have seen; also there is a tendency to plant small often non-native trees, while it is larger trees offering more shade that will increasingly be needed. Retro-fitting existing cities where greenery has long been discouraged is now a major challenge, not least because the involvement of local communities is essential if the new ideas are to be accepted. Mini community forests, first developed in the 1970s by Japanese botanist Akira Miyawaki, ‘bursting with biodiversity’ are now taking root in some cities around the world, with plots as small as a single tennis court contributing to a new green approach based on local citizen participation. The idea is yet to catch on in London, but it should become a priority, together with the possibility of local seed collection for community tree-nurseries as well as both tree-planting, protection and aftercare for young saplings.  

The choice of appropriate tree species also needs new thinking; London planes may offer little in terms of biodiversity but so far they have been largely free of disease. However, since the millennium a fungal infection known as Massaria has been recorded, which can cause large branches to die and fall, with obvious danger to the public. As with Dutch Elm Disease this vulnerability to disease arises from the limited genetic variability; in this case because it is a single hybrid species. New pests such as the Emerald Ash-borer may be more difficult to counter (although many ash trees are wild sown and therefore genetically diverse) but in principle a diversity of tree species and of seed provenance both for single sites and urban forests should be encouraged. Some species not generally used could also become more important if taller and shade-producing trees are to be encouraged. Native lime, hornbeam, goat willow, wild cherry, alder and wild service are all trees that offer significant shade and could make a valuable contribution to cities like London – boosting urban biodiversity at the same time.

The benefits of trees for psychological as well as climate-related benefits are well known, and direct involvement in what might be termed arboricultural care and maintenance is increasingly popular, especially among younger citizens. Can I urge everyone to put pressure on local politicians to support urban tree-planting and community involvement? Many politicians may still be behind the curve, but the multiplicity of tree welfare and tree-appreciation organisations, from tree huggers to seed collectors, tree-planters to tree growers suggests there is enormous public support for making cities greener and healthier right now, and not just as targets for arbitrary dates like 2030 and 2050. It’s never too soon to start planting.