Trees and the city

A drawing by the French cartoonist Laurent Fabre, known under the pseudonym of Konk, depicts an urban landscape in which a canopy of tall skyscrapers surrounds a ring of car-lined streets. In the centre is a clearing and a tree standing alone. The only people shown in the drawing are all gathered around this tree.

This image sums up better than words the centrality of trees to the human quality of urban environments.

Can an urban space do without trees? The cities we live in say it can. But a place without trees, be it a street or a small garden, lacks the atmosphere that only trees can create, through the variations in colours and shadows, in space and over time.

Trees and urban spaces

Using trees well means above all choosing the right species: you need to know what overall shape they’ll have when they’re fully grown, the speed of growth, the extent of the root system, the soil requirements, resistance to cold and dry urban air, and the desired colour effects (the colour of the leaves, flowers and fruit).

The most common mistake that is made is to choose species that don’t fit their allocated space next to houses – people’s houses. Trees are bought when they’re only a few years old and are not yet fully grown. As we can all imagine, the result is trees that are hideously mutilated through “containment pruning”, which leads to a wide variety of diseases (perhaps resulting in expensive remedial treatments) and the untimely death of the plant after just a few years.  Another common mistake is the belief that pruning is always necessary – this is perhaps a throwback to ancient agricultural strategies for obtaining leaf fodder to feed livestock. Our esteemed ancestors would, I’m sure, be stumped as to why we insist on stimulating leaf production in trees that we now need for other purposes. The story goes that Moses delivered a commandment, perhaps along with other commandments, on his descent from Mount Sinai, which can be summed up as follows: eleventh, thou shalt not prune! In layman’s terms, this rule can be amended by noting that very limited pruning of the dead parts of the plant may be necessary, but it is still a minor operation that should not disfigure or weaken the plant. 

The shape of trees is a crucial element in the urban use of these plants, not so much as “furniture” (a concept that would have us believe that a tree and a lamppost are the same thing), but as a living element that plays a part in creating and maintaining the urban ecosystem, in our garden or along the streets. Trees can be globose, cone-shaped, dome-shaped, tulip-shaped, conical, antler-shaped or come in irregular and bizarre shapes. The arrangement of species according to their shape is a central theme in garden architecture (if space permits): trees with flowing shapes, such as olive, cedar or birch trees, are best placed in the foreground, while plants with imposing, compact shapes, such as plane trees, oaks, maples and firs, should feature in the background.

The effects on the urban climate and beyond...

The choice and layout of trees according to their shape is not only a matter of aesthetics, but it also has significant effects on the temperature, the moisture content of the air and its local circulation – in short, on the urban climate. The shading of streets and buildings is combined with the release of water vapour into the air as a result of transpiration – a phenomenon linked to photosynthesis – which takes place in leaves. To transform liquid water, which flows in the conduction systems of plants, into water vapour, the leaves absorb a lot of energy from the atmosphere, thus helping to lower the temperature. The difference in air temperature between the tree canopy and the surrounding areas gives rise to convective motion, which, in turn, lowers the temperature in these areas. 

Moreover, rows of trees should not be overlooked as an acoustic barrier and as a screen against atmospheric currents in the wide-open spaces surrounding, for example, urban outskirts and motorways. The latter effect reduces wind speed, without the formation of vortices that comes with barriers made of inert materials. This effect was used for centuries in the Po Valley in the form of large rows of cypress poplars (Populus nigra var. pyramidalis), to such an extent that eighteenth-century English travellers named the tree the “Lombard” poplar (Lombardy meaning, in the medieval sense of the world, all of northern Italy). I believe it would be really useful to resume this practice in the large open spaces around motorways, in arid and sun-scorched service stations, and in urban suburbs, perhaps alleviating the suffering of the huge numbers of allergy sufferers through the use of the white poplar (Populus alba v. pyramidalis or Populus bolleana).

But the greatest beneficial effect of trees in the city is on people’s spirits: wherever there are lots of trees, such as in city parks, children play and everyone can meet and even talk quietly. Everyone can see that people live better in places like these. One of the lessons from modern urban planning is that we should measure quality of life by the needs of children. That’s why houses should always have enough space next to them for trees: if there’s room for them, that means there really is room for a garden and for our children’s games. 

A row of trees is all it takes to change the look and feel of an anonymous suburban street and, above all, to improve the lives of the people who live there. This is demonstrated by people’s protests about the felling of roadside trees, which is seen as a distressing measure that (I would add) should always require essential justification. If only to determine that it’s not actually essential after all. I don’t know about you, but when I’m driving along the wide streets leading into the city centre, I cope better with the queues and the noise if the street is flanked and shaded by large trees. I dream of there being a rule stating that a row of trees must be planted on either side of every city street wider than 10 metres. 

Unfortunately, as for humans, the urban environment (and other similar environments with vast expanses of asphalt and concrete) can be seriously harmful to trees’ health. Lime trees, hornbeams and the Douglas fir (Pseudotsuga menziesii) are particularly sensitive to many airborne pollutants. Horse chestnuts (Aesculus hippocastanum) suffer when the air and soil are dry. 

The choice of species is crucial to avoid futile expenditure.