With pawlonias in Alicante, Betula pendulum in Pontevedra, and Laurus nobilis in Las Palmas de Gran Canaria, the region has conducted studies to measure how much carbon certain plant species can absorb. Not all trees share the same capacity to stabilize emissions, and some should be avoided in urban areas. While most trees help reduce air pollution, some species contribute to the formation of ozone when isoprene released from organic compounds mixes with traffic pollutants and solar radiation. In the context of global warming, planning in advance what to plant in city spaces becomes essential to improve environments while cutting fossil fuel use and promoting sustainable mobility.
Biologist Vicent Calatayud Lorente from the Mediterranean Center for Environmental Studies (CEAM) has spent months collecting data to shape a climate action roadmap. Among the trees most recommended for highly polluted areas are jacaranda (Jacaranda mimosifolia), linden (Tilia platyphyllos), and white mulberry. In contrast, false acacia (Robinia pseudoacacia), a common ornamental tree in many Spanish parks, is among the strongest emitters of isoprene.
La Marjal city park in Alicante is an example of urban forestry practice. Canary palm trees (Phoenix canariensis) are widespread on the islands because they tolerate sea proximity and sandy soils, but they are less suitable for residential zones with heavy motor traffic. If a choice is available, Calatayud recommends the date palm (Phoenix dactylifera).
Species to Avoid
Washingtonia palms, like poplars, are not among the most recommended species for crowding urban environments, where they can dominate green spaces without delivering maximal air-cleaning benefits.
Eucalyptus (Eucalyptus globulus) should also be avoided due to emissions that cause significant problems in European forests. The leaves of eucalyptus can make soils more poisonous, drier, and harder for other plants to establish. In some regions, planting eucalyptus on lands not already occupied is restricted to protect biodiversity.
Best Partners with Elm, Plum, Pear, and Apple Trees
Elm trees, ornamental plums, pears, and apples can help reduce the concentration of pollutants. Other conifers, such as pines and cypresses, emit moderate amounts of pollutants but offer the advantage of winter pollution reduction thanks to their evergreen leaves. The University of Alicante is among the latest to study how much CO2 a Pawlonia plantation can sequester, located near a cement factory. Pawlonias are noted for high carbon capture potential, though they are considered an exotic invader in some regions. A plantation near eight hectares holds around 3,200 specimens, with large leaves capable of absorbing carbon dioxide and releasing oxygen—figures that underscore the tradeoffs involved with introducing non-native species.
The Lourizán Gardens in Vigo and other sites have been used to study how the most common species in the northwest of the peninsula absorb carbon dioxide. Mathematical models from the University of Oviedo describe the carbon uptake of ten prevalent species in the forests of Asturias and Galicia, including chestnut, Celtiberian birch, beech, and several oaks. The research relies on gas sampling methods that measure leaf responses to ambient pollutants, allowing researchers to quantify how different trees interact with the traffic-related pollutants found in urban atmospheres. The aim remains clear: build more sustainable cities by selecting species with favorable atmospheric interactions while maintaining biodiversity and aesthetic value.
The Valencia study method uses a portable device to capture the leaves’ volatile organic compounds in a small tube, then analyzes the reactions to traffic pollutants. Although the pandemic initially slowed data collection because specimens had to be gathered in city gardens, ongoing momentum promises deeper insights into how urban trees influence air quality. In addition to air purification, trees regulate temperature and contribute to broader environmental services. The European Union’s new Forest Strategy envisions planting billions of trees by 2030, with emphasis on regional suitability, drought resilience, and urban adaptability. If successful, the plan could improve city microclimates and increase protected landscapes across Europe within a decade.
Biologist Vicent Calatayud emphasizes that while trees provide undeniable benefits, reducing vehicle emissions remains crucial. He notes that forests and urban greenery offer temperature moderation, potentially lowering urban heat by one to three degrees compared with highly built-up areas. He also cautions against relying solely on trees as carbon collectors, urging a comprehensive approach that includes cleaner transportation and smarter urban planning. VOCs emitted by plants serve diverse roles—from protecting photosynthesis against heat to attracting pollinators and signaling fruit ripeness—yet they must be managed to minimize harmful ozone formation when interacting with nitrogen oxides in dense urban atmospheres. A broad, cautious approach to selecting species is essential for sustainable, healthy cities. Acknowledging the impact of high-emission species such as certain plane trees and other fast-growing exotics, researchers continue to study how best to balance ecological benefits with urban safety and resilience.
The work of Calatayud and CEAM highlights the need for a well-planned transition toward greener cities. It is not about removing every existing tree but about making informed choices, leveraging the benefits of green spaces while mitigating potential downsides. The overarching goal remains clear: maximize public health, climate resilience, and biodiversity through thoughtful urban forestry practices.
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Report
Biologist Vicent Calatayud from the Mediterranean Center for Environmental Studies
“Brussels wants to make cities greener, but it needs to be well planned”
The Mediterranean Center for Environmental Studies has spent twenty-five years researching extreme weather events, climate change mitigation, and air pollution across urban, rural, and industrial settings. Calatayud is now studying how traffic pollutants interact with isoprene emitted by flora to understand their combined impact on air quality. The findings inform the push for greener, better-planned cities across Europe. The aim is not to eliminate all trees but to guide future plant selections toward those with lower emissions while preserving the services forests provide.
Calatayud notes that trees regulate temperature and can reduce urban heat effects, offering substantial benefits for dense cities. Yet the core message remains: success depends on balancing green infrastructure with cleaner transport and smarter planning. Plants emit a wide range of VOCs for various ecological reasons, including protection of leaves, attraction of pollinators, and signaling ripeness to animals. These emissions also serve as a defense mechanism against pests, as plants stressed by pests release different VOCs to recruit natural enemies of those pests. The ongoing research underscores the value of continuing studies to advance more sustainable, resilient urban environments. Some species, like plane trees, are highlighted as high emitters and therefore warrant careful consideration in future plantings.