Researchers are carrying out studies in Alicante with pawlonias, in Pontevedra with Betula pendula, and in Las Palmas de Gran Canaria with Laurus Nobilis. These are a few of the investigations across the region measuring how much carbon certain plant species can absorb. Not all trees have the same capacity to stabilize emissions, and some should be avoided in city settings. While many trees help lower air pollution, certain species contribute to the formation of bad ozone when isoprene released from organic compounds mixes with traffic pollutants and sunlight. The goal is to plan in detail which trees to plant in urban gardens to improve the environment while reducing fossil fuel use or opting for more sustainable mobility options.
Biologist Vicent Calatayud Lorente from the Mediterranean Center for Environmental Studies, CEAM, has spent months gathering data to prepare a roadmap for addressing climate change. Among the most recommended trees for highly polluted areas are jacaranda (Mimosifolia), linden (Tila platyphyllos), and mulberry (Morus alba). In contrast, false acacia (Robinia pseudoacacia), common in many Spanish parks, is a notable emitter of isoprene.
Date better than canary
The canary palm, Phoenix canariensis, is widespread on the islands due to its tolerance of sea proximity and sandy soils, but it is less suitable for residential zones with heavy traffic. If choices are available, the date palm, Phoenix dactylifera, is favored, while Washingtonia is not highly recommended, and poplars, such as those found in many Chinese gardens, are also cautioned. Eucalyptus (Eucalyptus globulus) should be avoided because it contributes to problematic emissions; its leaf composition can render soils poisonous, water-repellent, dry, and compact. In Asturias, new plantings are restricted on lands not already occupied by this aggressive exotic invader from Australia and New Guinea, due to biodiversity concerns.
The plan also aims to limit pollutants from elm trees, ornamental plums, pears, and apples. Some conifers, like pines and cypresses, have intermediate emissions but offer the benefit of pollutant removal in winter when they retain their leaves all year round.
Meanwhile, the University of Alicante is among the latest projects analyzing how much CO2 a pawlonia plantation near the Cemex cement plant can sequester. Pawlonias can store ten times more carbon. The site spans eight hectares with about 3,200 specimens; its large leaves can capture around 21.7 kilograms of CO2 daily and convert it to roughly six kilograms of oxygen. Though praised for its carbon capacity, pawlonia is still considered an invasive exotic, originally from China.
The University of Oviedo has developed mathematical models to determine how much carbon dioxide the most common forest species in Asturias and Galicia can absorb. The study compiles formulas for the ten most common northwest peninsula species, including chestnut (Castanea sativa), Celtiberian birch (Betula pubescens), beech, and three oak species.
Volatile compounds
The device Calatayud uses in various parts of Valencia captures the leaves’ volatile organic compounds in a small tube and later tests their reactions in the lab, revealing how pollutants from transport interact with plant emissions. Pandemic restrictions impeded progress since samples had to be collected in city gardens, but renewed momentum is expected.
Beyond air cleaning, trees provide services by regulating temperature. The European Union’s Forest Strategy envisions planting 3,000 million trees by 2030, with attention to local varieties that are more drought-resistant and better suited for extreme weather. Urban areas are a priority due to higher pollution. If the plan succeeds, Europe could see a significant degree of climate resilience within a decade.
“Brussels wants greener cities, but it needs careful planning.”
Biologist Calatayud seeks trees that minimize bad ozone formation in urban settings. The Mediterranean Center for Environmental Studies has spent twenty-five years researching extreme weather, climate change mitigation, and urban air quality. Calatayud studies how traffic pollutants interact with isoprene from plants.
“Europe’s climate strategy calls for greener cities, and we should also look for trees that emit less,” he notes. “I would not remove existing trees, but I would plan for the future and integrate this knowledge.”
“The services trees provide are clear: they regulate temperature and can reduce urban heat by one to three degrees compared with other city areas,” Calatayud explains. Yet he emphasizes that relying solely on trees for carbon capture is insufficient; vehicle emissions must also be reduced. “Plants and trees in mountain regions are not a problem in themselves; the issue arises when volatile organic compounds meet high nitrogen oxides and form tropospheric ozone,” he adds.
The researcher recalls that plants emit a wide variety of VOCs. Some VOCs protect photosynthesis under heat, others attract pollinators, and fruits’ aromas help animals discern ripeness. VOCs also serve as defense mechanisms against pests: stressed plants release specific VOCs to attract natural enemies of pests. Rafael Calatayud stresses continuing work that could lead to more sustainable cities. For example, shade-providing plane trees are major emitters.