It rains less and less and temperatures are higher. What seemed like a weather anomaly has become the norm. A situation that could worsen this summer in the main cities due to the “heat island” effect that causes the temperature to rise between 2 and 8 additional degrees. “Trees are key to lowering the temperature in cities, as well as providing many psychological benefits,” emphasizes Mariano Sánchez, Head of the Gardening and Arboriculture Technical Unit of the Royal Botanical Garden-CSIC and president of the Spanish Association of Arboriculture.
But, to enhance its thermoregulatory activity, it would be necessary to better manage its plantation. There should be, for example, more large trees with enough space to develop, without getting in the way of those next to them. Likewise, it would be necessary to change mentality with autumn pruning, says Sánchez. A recent IS Global study cited by the head gardener confirms that people who live in wooded areas have better mental health, visit the psychologist less and require less medication when they fall ill. As if that were not enough, the trees produce oxygen, sequester Co2 and refresh the environment.
What are the trees telling us?
More than not raining, what the trees are detecting, especially the mature ones, is that, as there are more sunny days, less rain and higher temperatures, spring is stealing their moisture, instead of providing it. And they are saying it with a lower number of leaves and less vitality, especially in the species with broader leaves, whose edges are yellowing. Both in the CSIC’s Royal Botanical Garden and in many streets and gardens in Spanish cities, we are seeing that some species (such as horse chestnut, lime or maple) are losing vitality due to changes in rainfall and humidity that is registering the Iberian Peninsula For this reason, having mature trees of 30 years or more, already adapted, is a privilege that cannot be renounced.
Is it true that plants such as corn, wheat, vines or nettles are emitting sounds due to water stress?
Actually, these sounds have a certain parallel with the noise that the pipes make when the water supply is cut off and they are given again. When there is a climate as dry as the one we are having, there are plants that are not capable of absorbing the water they need, which causes air to enter their vessels, giving rise to a physiological dysfunction called cavitation.
According to the authors of this research, these acoustic signals are not perceived by the human ear, since their frequency is in the ultrasound range, although they can be informative for other species. Could it be a possible communication mechanism between plants and animals?
Indeed. It is very possible for some animals to detect when the water circulates through the vessels of the plant, it is that it is healthy. On the contrary, if it makes a certain amount of noise because its vessels are clogging or because it does not have enough humidity or water, the insect in question may decide that the plant is not interested in laying its eggs, for example.
A study published in Nature recalls that trees can reduce the ambient temperature between 2 and 8 degrees. Are there enough trees in Spanish cities or would more be needed?
In Spain, many more trees are needed, but yes, better located. But we must clarify that there are cities that have many. However, since they are planted close together, they cannot develop. In almost all cities, the planting frames are placed every four or six meters, when a banana has a crown of 30 meters… Since they don’t fit, then they have to be pruned. But, by doing so, you lose a very important part of the advantages that they can offer.
Where are more trees needed?
In the peripheral areas of the cities: in the urban centers, in the industrial estates, on the access roads to the large cities, and in the peri-urban area, that is, in what is neither the countryside nor the city. But always well distributed.
A study carried out in 93 European cities estimates that a third of the deaths attributable to heat islands that are created in cities could be avoided if trees covered 30% of the urban space. What is the percentage of surface they cover in Spain?
In most Spanish cities there are many trees but the area of ​​the city they cover bears no relation to their number. The surfaces covered in our cities are: Madrid 9.5%, Barcelona 8.5%, Valencia 7.5%, Seville 5.5%, Malaga 15.8%, Murcia 10.5%, Palma de Mallorca 8.0 %, Bilbao 9.5% and Alicante 2.1%. Much work remains to be done. Madrid, for example, is the European capital with the greatest number of trees, but it is located in the middle of the table in terms of covered urban area. Architects who work in urban planning should consider whether it is correct to continue putting tree pits every four or five metres. It could be correct, as long as, of course, they did not put the same species, but a large tree and then two medium-sized ones of different species or one medium and one small one. In this way, instead of having a single species in the streets, there would be three, four or five different species. With this number of trees, this measure would actually allow the covered area to double, since each species of tree could develop depending on its size.
The problem, then, is that the trees are planted very close together…
It is about applying the same reasoning that is used with vehicles and their parking lots in the sense that buses cannot park, because they do not fit, in the spaces set aside for cars and motorcycles. It has to be coherent with the knowledge of the biology of the trees and, if the species is large, the separation must be great; if its size is medium, the space must be that of a car; and, if it is small, that of a motorcycle. We speak of equivalences, not of surfaces or distances.
In some Spanish cities old trees are uprooted to replace them with young ones. However, you claim that a large, mature tree brings many more benefits…
A mature and large tree can retain pollutants and provide us with oxygen and cooling up to the equivalent of between 10 and 20 medium and small trees, due to its greater volume of leaves and its projected shadow. What retains pollution is not the number of tree trunks, but the number of leaves.
He affirms that there are ancestral traditions such as cannibalism that fortunately have disappeared but that, on the other hand, autumn pruning continues to be a habit that is impossible to eradicate. How can we afford it this summer?
With more noise and more heat. Normally, the trees on the sidewalks insulate the windows somewhat. It is not that environmental noise decreases much, because the trees only attenuate the acoustic impact between one and three decibels, depending on the species. Much more important is the psychological feeling of not seeing the traffic. The thud that a person hears from a house is perceived with much less intensity when cars, motorcycles and trucks are not seen passing by. On the other hand, the fact that the trees are not as leafy as they could be, increases the sensation of heat, since solar radiation heats the pavement to a greater extent, increasing the heat island effect. In addition, for the tree to be able to transform carbon dioxide into starches, it needs to trap the heat from the air with its leaves, and by evaporating the water, the temperature drops somewhat. The more large trees with many leaves, the greater the benefits.
The main problem, you say, is that they opt for indiscriminate pruning instead of selective pruning
Trees in their natural habitat do not need to be pruned. In cities like Berlin, London, Oslo or northern France, the trees on the streets are not pruned. However, in the Mediterranean region they are pruned every year by virtue of an ancient tradition. In the past, fruit trees were cultivated, trees that do need pruning so that the fruit can be accessed from the ground. The problem is that this tradition has been unnecessarily extrapolated to urban trees. Its only justification is that in Spain trees are planted every four or six meters, which means that, when they grow, they do not fit. Only those trees that cover traffic lights or a window should be pruned, as well as branches or trees with rot and risk, but more as the exception than as the norm. It is important that you can see at least three trees from home because that improves our mental health.
Which native trees are suffering the most from climate change, and which other foreign trees could help?
Pine trees, for example, are a good option because they withstand drought and rusticity well. What is inadvisable is planting them in the meadow (on the lawn) because the roots they form are superficial, which increases the risk of overturning on rainy and windy days. Other trees that adapt well to high temperatures are holm oak, cork oak, araar (a plant that now only exists in the Murcia and Cartagena area) or ash trees. Regarding foreign species that could improve urban biodiversity, we are collecting information on some plants from northern Chile or Mexico (a very hot and rustic area, with low humidity and little water in the soil). In general, it could be studied bringing plants and trees from places with a Mediterranean climate, such as Chile, Australia, the area of ​​California and New Mexico or southern South Africa, in addition to the Mediterranean itself. But it would be necessary to watch carefully, and this is important, that they were not invasive species or that they hybridized with our native trees.
Is it justified that there are so many plane trees in big cities? In Barcelona we have more than 45,000, as is the case in some Spanish cities
Probably yes, because they grow quickly, so a few years after being planted they already provide shade. In addition, no matter how many atrocities are done to them in pruning, they are capable of closing the wounds without rotting. Banana trees are one of the species that absorb the most carbon dioxide, because they have large leaves. In a way, their large presence in cities is justified, despite the allergies they cause. There is another species such as the melia (Melia azedarach) that has an even greater CO2 absorption potential but suffers from pruning and can cause risks.
What do trees do with the polluting particles they trap?
They retain them until the leaves fall to the ground in autumn, but they don’t take advantage of them as nutrients or anything similar. One of the species that retains the most particles is pine, because they remain stuck in the resin, although in their case the needles, the pine leaves, fall every two or three years.
The TreeHugger portal calculates that a house with trees nearby or on the same property increases its economic value between 10% and 23%. What do you think?
Which is completely true. Since I studied arboriculture, more than thirty years ago, there has always been talk of the increased economic value that having trees represents for a property. But the psychological benefits are even greater: there are many studies on the benefits of hospitals that have green areas (or biophilia) and it is known that the recovery of patients is faster, in addition to administering less medication.
