Urban trees face a double challenge caused by the climate crisis. On the one hand, cities pursue the objective of increasing the number of trees due to their capacity for thermal buffering and carbon capture; But on the other hand, cities where there are prolonged droughts due to climate change are registering the death of hundreds of trees due to lack of water. In Barcelona, last year at least 500 specimens were lost for this reason, according to data from the City Council.
Joan Pino, director of the Center for Ecological Research and Forest Applications (CREAF), explains that urban green grows in difficult conditions and is highly confined. Inma Gascón, vice president of the Spanish Arboriculture Association (AEA), defends the same opinion.
Urban trees usually have soil problems (which is usually not ideal) and are exposed to pollution and daily aggression in cities, which weakens them. “It's like with people; When we are sick, we are more vulnerable. In the case of trees, it will be easier for them to show difficulties in surviving when temperatures rise or lack of water,” Gascón argues.
Water scarcity is, therefore, a mortality factor that should not be understood in isolation, but added to others. “Trees usually die due to a combination of factors and the lack of water is an additional component in this sense,” explains Joan Pino, adding that “for this reason, in cities there may be trees that dry out as happens in forests with those who have previous conditions or who are in very thin soils.”
To understand how a tree can die due to drought, Pino asks us to imagine these beings as “pipes”, which capture water from the ground and evaporate it through the leaves continuously. What causes extreme water scarcity is this column breaking down.
“When a specimen experiences water availability problems, it reacts by closing the openings of its leaves (stomata) to reduce evaporation and, therefore, water loss,” describes Pino. However, these same openings are what trees use to carry out the gas exchange they need for photosynthesis.
By closing the stomata, the trees lose their ability to feed; and although they could survive for a time with carbon reserves (their source of energy), they would end up dying of “starvation” if the situation continues. It may also happen that, in the face of very high water deficits, they die of "thirst", since the lack of water can produce air bubbles in the conductive vessels (embolism) that prevent the transport of this resource to the highest branches. .
Consequently, the main symptom that Parcs i Jardins identifies in trees that suffer from this situation is a reduction in their crowns or a dry appearance of the specimens. “When you identify the fall of leaves on a tree that is not prepared to do so, you can already know that it is an individual that sooner or later will die,” Pino presents.
The Barcelona City Council explains that, when they detect a tree that is suffering from water stress, technicians are in charge of evaluating its condition. If the specimen can be saved, apply support irrigation; If it is already dead, its risk of fall or fracture is evaluated and its removal is scheduled according to priority for future replacement. However, due to the drought decree, the latter is currently not carried out.
Barcelona has become a clear example of how drought affects urban trees. Drought not only reduces the amount of rainwater that urban greenery receives, but also affects the availability of water for irrigation. Thus, the use of drinking water for irrigation was reduced by 60% during the exceptional phase in Barcelona. It was only used to guarantee the survival of the trees in cases where it was not possible to irrigate with groundwater (groundwater), and always with drop-by-drop irrigation.
Likewise, the City Council states that "it has worked to expand the use of ground water and thus maximize the saving of drinking water, according to the indications of the Catalan Water Agency (ACA)". The parks of Ciutadella, Clariana-Glòries, Estació del Nord or some of Montjuïc are some examples of green areas whose water networks have connections to the water table.
In other spaces where this option did not exist, the City Council launched the supply through tanker trucks. “These vehicles can supply smaller ones with groundwater, acting as mother vehicles. In total, there are 25 tanker trucks distributed throughout the city that make it possible to use groundwater for irrigation and cleaning tasks.”
“The current situation is complex and critical for greenery and trees. The city is going through a situation of persistent drought, which has a strong impact in this area,” concludes the City Council, an organization that asked the ACA to be able to maintain irrigation with groundwater in the event of changing to the emergency phase, although in the The initial drought plan of the ACA only allowed the risk of survival of “trees of a singular or monumental nature in public spaces.”
This problem is even more aggravated by the prospect that we will have increasingly less water availability, according to Joan Pino, from CREAF, and the expert warns that “it is time to be imaginative” in proposing measures. Among the possible strategies, a change in the planted species is proposed. “We will have to consider external, exotic species, from warmer and drier territories,” argues Pino, who considers that subtropical species would be a good option in this sense.
Likewise, a transformation in planting methods is also proposed, with priority given to those that focus precisely on increasing the survival of trees when it is known that they will suffer from drought. An example, studied by CREAF, is the Cocoon system; who buries some cardboard “donuts” (biodegradable) filled with water around the tree, which hydrate and protect it during the first year of life (the most sensitive) and then decompose in the soil.
For its part, the Spanish Arboriculture Association proposes that irrigation be maintained as an essential measure. To achieve this, it calls for an adaptation of cities so that they are able to collect rainwater that is currently lost on sidewalks or in the sewers.
Likewise, the Association aspires for cities to be able to collect and use gray water, which is water that has been used in the domestic sphere (for example, to brush our teeth) and does not include fecal water.
“We know that adapting already built cities is difficult, just as it is difficult to put an elevator in an old building. But in the same way that buildings are provided with subsidies, they should also direct efforts here. We have to assume that we are Mediterranean cities and we will have a shortage of water,” defends Gascón.
Beyond the loss of investment that the death of trees entails and their demonstrated benefits in areas such as mental health, these living beings are a powerful tool in the adaptation of cities to the climate crisis, because they act as thermal shelters. “We need more urban nature to withstand the heat, to better regulate the water cycle, to protect us from downpours and to increase the capacity to retain carbon from the atmosphere,” defends CREAF.
The organization adds that in this context, urban forests would be key and would allow cities to also contribute to all these challenges and benefit from the services of trees within a city: shade, ability to absorb pollutants, to refresh the air and to regulate flooding, among others.
