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In recent years, CDMX has begun to decorate streets and buildings with walls, columns and green roofs. This is a measure focused on improving air quality and the aesthetics of the busy megalopolis, especially since between 9 and 16 square meters of green areas are required per person - and in the city there are only 5.3 square meters per person - , and between 2012 and 2015 alone, more than 10,114 trees were felled that “obstructed” parts of buildings and infrastructure such as roads and public transport corridors.
Under the landscape of sustainable pollution mitigation, it is important to make a difference between climate change and air quality. According to an article published in Animal Político by Juan Manuel Berdeja, the Greenway “ensures to improve the air quality of Mexico City by filtering more than 27,000 tons of gases, capturing more than 5,000 kg of dust, and processing more 10,000 kg of heavy metals "; However, the plants used for this project are responsible for generating a greater amount of CO2, but not for regulating phytoremediation - the decontamination of soil, water and air through plants - to accumulate, metabolize or volatilize the ozone precursor pollutants - NOx, SOx and Volatile Organic Compounds–. For their part, trees in general are responsible for carrying out all the necessary phytoremediation in urban areas, mainly those with excess population and pollutants, as well as oxygenation for 18 people for a year. So, under this scenario, what is more beneficial for urban areas: trees or green walls?
Faced with the demands of urban living, the trend leans towards green walls. Taking up the CDMX example, the Vía Verde project aims to cover the 27 kilometers of the second floor of the Peripheral with columns to "reduce pollutants, dust, heavy metals and noise." The plants used for this project, which will have a cost of 360 million pesos contributed by the private initiative, will be cultivated with a hydroponic technique and will be irrigated with treated water through a rainwater harvesting system - which was designed by the Institute of Ecology of the National Autonomous University of Mexico, in order to measure how many pollutants will be reduced with vertical gardens as well as their benefits–. In addition, these walls represent a total of 35 thousand green areas in hospitals, schools, offices, public buildings and some commercial squares.
However, according to the calculations of José Antonio Lina Mina, Director of Development and Environmental Research and Environmental Teacher from the Polytechnic University of Madrid, “a square meter of vertical garden captures 2.33 kg annually has a cost of 8,500 pesos, while that an oak costs only 1,200 pesos and catches 22 kg annually. " In other words, with the 8,500 pesos you can buy seven trees that have the capacity of 66 times more CO2 capture - greater than a square meter of vertical garden.
In addition, given that the supply of drinking water in CDMX requires the operation of the Cutzamala and Lerma system as well as the Valley of Mexico aquifer, the maintenance of the green walls will require a great effort to bring the liquid from the most overexploited aquifers in the region. region. This brings, consequently, an aquifer deficit: “for every liter of rainwater that infiltrates the set of seven aquifers, three are extracted, which is causing phenomena of soil fracturing, subsidence, areas of flooding due to deformation of the drains and sinkholes that endanger the integrity of the population. "
So the green walls are intended to be maintained with water from aquifer systems that are characterized by water scarcity. In other words, all the water that is absorbed by the walls will not have any infiltration into the already overexploited aquifers –generating a deficit greater than the benefit of the city–. Although the project mentions that the water used for its maintenance is through a rain catchment system, in the event that a good planning for the implementation is not made, it can fail and have high costs; and it does not solve the problem that currently exists with the runoff from the second to the first floor - being a risk factor for road safety.
And finally, another point that casts doubt on the effectiveness of green walls is the visual pollution that publicity would generate from private sponsors. This consideration of the project is contrasted with the idea of improving the urban image, since the green space will actually be occupied by "foreign elements" harmful to the balance and sustainability of the urban landscape. In other words, advertising saturation has become an agent of distortion and contamination, both visual and environmental for the urban ecosystem.
Therefore, in the face of all these arguments, it seems that an environmental and aesthetic solution for the city continues to be reforestation. In the words of urban planner David Salinas, "Sustainability is misunderstood because it is believed that being sustainable implies putting plants into a project and not thinking about it in an integral way." Therefore, the search for a project that helps to better understand the environmental well-being of a city requires an integrative perspective - rather than placing plants by placing them.