Is planting trees in cities useful against the climate crisis?

Is planting trees in cities useful against the climate crisis?

Since the consequences of the effects of global warming have passed from scientific news to news reports, a single solution seems to have conquered the scene: planting trees. As proof of the relevance of this "magic wand" for the climate crisis, last September 21, on the occasion of National Tree Day, the Minister of the Environment and Energy Security Gilberto Pichetto Fratin said that "trees have a great positive impact on the environment of our cities: they are a value to be defended and strengthened. The National Recovery and Resilience Plan envisages that 6.6 million plants will be distributed in the 14 metropolitan cities over the next three years. They are being defined precisely in these days the agreements with eleven administrations for the achievement of the annual goal of over 1.6 million trees planted in 2022".

An important effort, five times greater than what has been done recently given that always in line with the Pnrr, as part of the so-called Climate Decree, the same ministry had started the planting of about 300,000 trees in the two-year period 2020-2021, always in the areas of in the 14 metropolitan cities. Finally, a large public investment for environmental purposes that aims at a quantifiable result in hundreds of thousands of new plants that should mitigate the impact of global warming in our cities. And that will soon be reality. However, a doubt begins to emerge when these actions are announced:

But is planting trees really a useful tool to curb urban overheating?

The cause of global warming is in cities

More than half of the world's population (about 55%) lives in cities, which, even though they cover only 3% of the global area in size, are the centers responsible for 70% of global greenhouse gas emissions. The process of urbanization of humanity is unstoppable and continuous: it is estimated that in 2050 seven people out of ten will live in cities. As is now also evident in Italy, heat waves in the Mediterranean are increasingly frequent, especially in urban areas, where we speak of "heat islands": a phenomenon, as Greenpeace explains, according to which cities as a result of land consumption they have both daytime and nighttime air temperatures even 12°C higher than the surrounding natural areas. This is why we start from interventions in urban agglomerations to deal with the effects of the climate crisis.

Trees are used to refresh cities

According to data reported by Lina Fusaro, researcher at the Bioeconomy Institute of the National Research Council (Cnr-Ibe), in a dedicated study to the theme from Greenpeace, planting trees in the city serves . First of all because the crowns of the trees guarantee thermal comfort which derives both from the direct shading of the artificial surfaces and from the transpiration process.

More and more trees are being burned in Europe against the energy crisis With the skyrocketing energy prices, many are turning to wood for heating, a trend that EU regulations help to encourage Through the The water evaporation from the foliage lowers the air temperature, thus partially replacing the use of air conditioners. This is why in the last 15 years the availability of urban green spaces in the big cities of the world has increased by more than 4% as has the accessibility to urban green spaces (+7% in the same period of time). Yet we are still far from the minimum standard of presence of greenery in cities: according to the World Health Organization it should be 9 square meters of green spaces per inhabitant, with the ambition in the best of all possible worlds of being able to reach around 50 m 2 per you understand. From this point of view, Rome boasts a ratio of 39m 2 per capita, Madrid stands at 21m 2 . But the big cities of the world have ratios far from even the minimum, such as Mexico City (6 m 2 per capita) or Bogota (5 m 2 per capita). For this reason, WHO's ideal 50m2 is truly a mirage, or rather a distant prospect. This is why there is still a need to populate our cities with trees (and above all with greenery).

Planting trees is not enough

A tree is not a solution, but a living being . Planting trees matters little compared to taking care of their development and taking care of their location. Planting hundreds of thousands of trees could prove to be not only an ineffective but even harmful choice if this does not take place within a strategic planning of urban regeneration. This is because given the speed with which environmental changes occur, especially within cities, the difficulty of species to adapt to the rapidly changing environment could decrease their mitigating functionality, however requiring important maintenance. As Fusaro summarizes:

" Proceeding only in the direction of planting millions of trees does not seem a complete and effective strategy if all the other development dynamics remain unchanged ”

Several recent reports , also at the Italian level, have underlined the importance of integrating information on the state of conservation of the "Natural Capital" into the decision-making processes of urban development and of the country in general. With this expression we refer to an overall vision that frames the trees within an overall strategy to make cities greener through different actions, as explained to by Federica Ferrario, responsible for the agriculture campaign and special projects of Greenpeace Italy: “On the one hand it is important to plant and increase the green surface in the city: we know very well what the difference is between a paved road and one shaded by trees. Having said that, what is missing in general is adequate upstream planning of green spaces, and not placing them after where there is leftover space ”. The urban strategy also includes the protection of the available space, which must no longer be taken away for purely speculative reasons. " The other issue to keep an eye on is overbuilding: before identifying spaces to plant new trees, we stop soil consumption, which is a significant problem ". And finally, Ferrario emphasizes the added value of a forest compared to planting many trees: “These are two different worlds. Recreating forests has an added biodiversity value, for example, compared to planting a large number of trees” which are not a synergistic living system. And even to recreate a forest, a precise strategy shared on a general level with respect to the needs of the territory is required.

It is more important to create ecological services

Be careful though: trees are not the the only green solution that a city must find. In fact, urban forestation ensures various benefits, in addition to the removal of CO2, when it is planned in a synergistic way with the area in which it is intervened. But as Fusaro explains again, the numerous ecological services provided by the vegetation depend on the full functionality of the species that are planted. A tree in itself is not functional for the mitigation of the urban climate: this is instead the effect of urban planning where to create, or regenerate, an ecosystem of which the tree is only one of the protagonists. Furthermore, the researcher continues, the right tree must be chosen in the right place: 

“ The right tree must be chosen not only by favoring native species to safeguard biodiversity, but also by identifying the one most tolerant to various types of present stresses, and with respect to future scenarios that predict a change in the pattern of rainfall, which will tend to decrease as temperatures rise ”

Trees against global warming? An unsustainable solution

In the public sphere, announcing the planting of thousands or millions of trees follows the slippery slope of compensation projects in the private sphere. The most polluting companies in the world continue to plant trees. In 2020, Chevron said it had planted 30,000 trees in an abandoned area in British Columbia (Canada) and Gazprom more than 60,000 trees in Russia.

Buying and selling permits to emit CO 2 always pays off In 2050 they could generate one trillion dollars a year. For this reason, at Cop27, the United Nations climate conference in Egypt, the states aim to find an agreement to regulate international markets. The challenges open to negotiations In 2021, Total in collaboration with Forêt Resources Management announced its intention to plant acacias in a 40,000-hectare forest on the Bateké plateaus in Congo. And the world's largest emitter of all oil companies, Saudi Aramco, has presented 5.3 million mangroves along the coast of the Persian Gulf as reforestation. These are often choices dictated by brand reputation reasons or even by a greenwashing intention that have little to do with climate mitigation intervention. Among others, Oxfam explains it in a report: to offset their emissions, the four sisters of oil and gas (BP, Eni, Shell and Total Energies) alone would have to consume an area twice the size of the United Kingdom to plant trees and achieve zero emissions by 2050. And if the entire energy sector – whose emissions continue to grow – were to set targets close to zero, a region the size of the entire Amazon rainforest would be needed, the equivalent of one third of all the arable land on the planet. At Cop26, one of the most significant agreements was the halt to deforestation by 2030. For this reason, before replanting lost trees, we need to curb the causes for which they were destroyed.

Reforest? Let's stop deforestation first

If we consider planting as a suitable measure to offset our emissions, this effort brings little results. Given the amount of CO 2 emitted by major polluters and given the fact that reforestation averages 1,000 plants per hectare, it will take decades to offset just a fraction of global emissions. Also according to the Oxfam study, 1.6 billion hectares would need to be reforested, equivalent to 5 times the size of India to absorb all the carbon that the major polluters continue to emit. And in any case, the planting in place so far does not in the least compensate for what has been lost.

Forests on the earth's surface exceed 4 billion hectares: in the last twenty years, 386 million hectares of forests have been lost in the world, while in the same period, more have been recovered through reforestation and spontaneous regeneration only 59 million . And what is felled cannot be easily replaced because quantity is not quality: just think that in the last 250 years the surface of woods and forests has increased by 10% in Europe, yet it absorbs 3 billion tons of CO 2 less. Because the coniferous forests that have replaced the deciduous ones make it better for economic purposes, but absorb less carbon dioxide.

" The speech must be 360 ​​degrees - resumes Ferrario - if I plant beautiful trees, but I don't act on the models production and consumption, this reforestation will be of little use. 90% of deforestation worldwide today takes place to make room for livestock and feed crops: this impasse must be overcome".

A national plan, not individual local initiatives

When it comes to climate, there are no borders: local actions can be ineffective, expensive or even counterproductive if the context is not considered. The same goes for urban tree planting. Instead, it is necessary "to redesign urban and peri-urban areas in order to achieve widespread naturalness in which biodiversity at all scales (local and landscape) is encouraged and protected - Fusaro recalls - A reforestation plan should start from these networks and strengthen them by investing in planning that involves the entire national territory, acting more incisively in critical areas such as metropolitan cities, but aiming to restore widespread environmental quality, the only one that can guarantee long-term resilience. it must permeate the entire city fabric from the center to the suburbs".

Just think, for example, that according to a study conducted in the 14 Italian metropolitan cities affected by the Minister's opening statement, one hectare of urban forest can remove an average of 17 kg/year of PM10, and 35.7 kg/year of tropospheric ozone. Here, the trees of a peri-urban forest can absorb up to 1005 kg/year, per hectare, of carbon. This means that planting trees in the center of a city is not always the best choice from a climate mitigation perspective. Cities should be designed and planned taking into account the benefits of the presence of natural areas and green spaces: this commitment implies placing trees in the city only as a result of an overall strategy, which goes beyond the urban boundaries of any Italian city.

Advice for an effective strategy

Beyond the necessary overview of a plan that distributes millions of trees throughout the country, in trying to set up an effective urban planting strategy, the rule of 3-30-3005 . Let's start with number 3: anyone who lives in the city must be able to see at least three trees from their home. A possibility that does good for various reasons to our psychophysical health. So 30: this is the percentage of tree crowns to be demanded in each individual district. This is precisely the percentage that cities such as Barcelona, ​​Bristol, Canberra, Seattle and Vancouver are aiming for. And finally 300: it is the maximum distance in meters that according to the WHO there should be between every citizen and the closest green space. First fix the plan and the objectives, then take care of choosing the - living - tools that can support the effectiveness of the plan. Otherwise, the risk is to think that the announcement of planting trees regardless is just a beautiful (and ephemeral) slogan.

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