Integrating Urban Tree Carbon Sequestration into Metropolitan Ecosystem Services for Climate-Neutral Cities: A Citizen Science-Based Methodology

Abstract

Urban trees play a critical role in mitigating climate change by capturing atmospheric CO₂ and providing multiple co-benefits, including cooling urban environments, reducing building energy demand, and enhancing citizens’ physical and psychological well-being. This study presents the Co Carbon Trees Measurement project, a citizen science initiative implemented in the city of Viladecans, Spain, involving 658 students, local administration, and academia, three components of the EU mission’s quadruple helix governance model. Over one year, 1274 urban trees were measured for trunk diameter and height to quantify annual CO₂ sequestration using a direct measurement approach combining field data collection with a mobile application for a height assessment and a flexible measuring tape for diameter. Results indicate that carbon fixation increases with tree size, displaying a parabolic function with larger trees sequestering significantly more CO₂. A range between 10 and 20 kg of CO₂ is sequestered by the urban trees in the period 2024–2025. The study also highlights the broader benefits of urban trees, including shading, mitigation of the urban heat island effect, and positive impacts on mental health and social cohesion. While the total CO₂ captured in Viladecans (≈810 tons/year) is small relative to city emissions (≈170,000 tons/year), the methodology demonstrates a scalable, replicable approach for monitoring progress toward climate neutrality and integrating urban trees into planning and climate action strategies. This approach positions green infrastructure as a central component of sustainable and resilient urban development.

1. Introduction

Addressing the climate emergency is one of the most pressing challenges of our time. The impacts of climate change are already evident worldwide, albeit with varying intensity [1,2]. Cities, as densely populated hubs and key units of governance, play a pivotal role in responding to this global crisis. Although they occupy only about 3% of the Earth’s land surface, cities consume more than 70% of global energy and generate nearly 75% of greenhouse gas emissions [3,4].

Decarbonizing cities is therefore essential, with the attainment of carbon neutrality serving as a fundamental step. The European Union, for example, has set the ambitious target of achieving carbon neutrality across the continent by 2050 [5], through a dual strategy of reducing CO₂ emissions and enhancing carbon sequestration, partly by planting billions of trees. Within this framework, the role of cities is central. Indeed, the EU has designated the Mission on Climate-Neutral and Smart Cities as one of its five flagship initiatives, aiming to enable 100 European cities to reach climate neutrality by 2030 [6]. The governance models and local policies developed in these cities are expected to provide replicable frameworks for other urban areas in Europe and globally.

Achieving carbon (or climate) neutrality, however, cannot be accomplished in isolation by public administrations or single levels of government. Instead, it requires a new governance paradigm based on the quadruple helix model [7], which emphasizes shared management, collaboration, and co-creation among four key actors: academia, public administrations (at all levels), the private sector, and organized civil society. Together, these groups form an innovation ecosystem in which cities must develop demonstrative projects capable of being consolidated and scaled to accelerate decarbonization and strengthen climate action. The project shown in this article is a sample of a collaboration of three of the four actors of the quadruple helix: the academia, the administration and the organized civil society.

At its core, the Climate Mission rests on two interdependent pillars: the reduction of anthropogenic CO₂ emissions and the enhancement of atmospheric CO₂ capture mechanisms within the urban environment. Achieving this balance entails that, over a defined temporal window, commonly one calendar year, the total carbon emissions produced by a city’s anthropogenic activities are quantitatively offset by the carbon sequestration capacity of its green and blue infrastructure [8]. This objective requires both a drastic decarbonization of energy, transport, and building systems and a strategic expansion of urban carbon sinks, including urban forests, vegetated roofs, and soil-based sequestration systems ([9,10]).

Urban CO₂ mitigation thus depends on a systems-based integration of emission reduction pathways, such as renewable energy deployment, efficiency improvements, and behavioral changes, with nature-based solutions that enhance carbon uptake and storage ([11,12]). Within this framework, achieving a net-zero urban carbon balance implies that the rate of CO₂ sequestration by local biophysical processes equals or surpasses the rate of emissions within the city’s administrative boundaries, aligning with the global targets outlined by the IPCC [2] and the United Nations Framework Convention on Climate Change (UNFCCC).

Cities are already pursuing emission reduction through various strategies, most notably the electrification of mobility. Mazon [13] categorizes these efforts under the “5E levers” of urban decarbonization. Monitoring CO₂ emissions is relatively well established, with standardized direct and indirect methodologies allowing quantification in line with supra-local authorities. By contrast, the CO₂ sequestration capacity of cities remains far less defined and is generally absent from municipal datasets, likely because the amount of CO₂ absorbed by urban vegetation is orders of magnitude smaller than anthropogenic emissions. Nevertheless, effective monitoring of progress toward climate neutrality requires annual quantification of both emissions and CO₂ captured by urban green infrastructure.

Several studies have assessed vegetation’s sequestration potential using diverse methodologies (e.g., [14,15,16,17,18]). Urban trees, in particular, serve as reservoirs of CO₂. Through photosynthesis, they actively sequester atmospheric carbon, storing it in trunks, branches, leaves, and roots. This process is inherently sustainable and energy-efficient, relying solely on solar energy and requiring only basic maintenance to ensure tree survival.

Urban trees thus directly influence the urban atmosphere: they reduce greenhouse gas concentrations, mitigate the urban heat island effect by cooling air, and indirectly lower building energy demand, a point elaborated later. Consequently, urban forestry and green infrastructure are integral to short- and medium-term carbon neutrality strategies. However, sequestration rates vary considerably across tree species and are strongly conditioned by local meteorology and climate. Reported values range from 2 to 80 tons of CO₂ per hectare of urban forest per year (e.g., [15,16,17,18,19,20,21]). Direct measurements of street trees in Helsinki, for instance, showed annual sequestration rates between 3.5 kg CO₂ and 13.44 kg CO₂ per tree [22].

The CO₂ captured by a tree is highly variable, depending on factors such as local climate, species, annual weather, and soil characteristics. The scientific literature provides only broad reference values, which, due to their variability, are insufficient for precisely balancing carbon sequestration against emissions. To address this gap, this study applies a direct methodology to quantify the annual CO₂ sequestration of urban trees, involving citizens through a citizen science initiative that also engages academia and public administrations, three of the four actors in the European quadruple helix model. The project underscores the transformative role of urban trees in 21st-century urban planning, shifting their function from ornamental elements to essential components of healthy, livable, and climate-resilient cities.

This study also tests whether citizen-collected dendrometric data can produce reliable city-scale estimates of CO₂ sequestration.

2. Materials and Methods

The amount of CO₂ captured by a tree can be estimated using different models. This study adopts the method and equations proposed by [23], which provide a relatively straightforward approach, by using simple and easy measurement methods, which are able to use in a citizen science project like that. The total CO₂ mass stored in a tree (WCO₂) is calculated as 3.67 times the total carbon mass (TC), based on the molecular ratio CO₂/C = 44/12 = 3.67. The carbon content of a tree (TC) is assumed to represent 50% of its total dry weight (TDW), which in turn accounts for 72.5% of the dry biomass (TB). Total biomass (TB) comprises aboveground biomass (AGB) and belowground biomass (BGB), with BGB conventionally estimated as 20% of AGB. AGB itself is derived from tree diameter and height. Figure 1 summarizes the equations used to estimate CO₂ storage following [23].

The total mass of CO₂ stored in the N measured trees (M) can be obtained by summing the mass of carbon stored (m_i) for each individual tree, according to the following expression:

$$M={\sum }_{i=1}^N{m}_i$$

(1)

The average mass of carbon for a given species containing n individuals is given by expression

$$m={\displaystyle \frac{\sum _{i=1}^n{m}_i}{n}}$$

(2)

The mass of carbon fixed by all urban trees in the city can be extrapolated using the following expression:

$$M_{Total}={\sum }_{i=1}^{N°}{n}_i·m$$

(3)

where n_i is the number of trees of species i, mi is the average carbon capture mass of species i, and N is the number of species comprising the city’s urban tree infrastructure.

To directly estimate the annual CO₂ sequestration of a tree, the procedure is as follows. First, each tree is geolocated, and an annual measurement date is established, for instance, March 14 (Pi Day), or any day within that week. In the Northern Hemisphere at mid to high latitudes, this timing is appropriate because trees are just beginning to emerge from winter dormancy, and growth is minimal. Next, two key parameters are measured: diameter at breast height (DBH, 1.37 m above ground level) and total tree height. Two approaches were developed by participating citizen groups:

• Manual method: DBH is measured with a flexible measuring tape, and total tree height is determined either using mobile applications designed for measuring heights or through basic trigonometry based on similarity ratios.
• Mobile application method: The GreenGuard app (https://play.google.com/store/apps/details?id=com.alteraid.males_herbes&hl=es_AR accessed on 2 November 2025), developed by the Technical University of Catalonia (UPC-BarcelonaTECH) for this initiative, geolocates the tree, automatically records DBH and total height, and stores the results together with calculated CO₂ accumulation in an open-access database.

From these measurements, the carbon accumulated in different tree components is estimated, and total stored carbon is derived. One year later, the process is repeated on the same date (±one week). Growth in DBH and height translates into additional CO₂ sequestration. The difference between the two annual measurements represents the amount of CO₂ captured during that year. By repeating this procedure, long-term sequestration trends of urban trees can be directly monitored. It is assumed a homogeneity of climatic conditions across the entire urban area, which is a quite real assumption considering the small-mid size city, less than 4 km².

Both methods present merits and limitations that influence their accuracy, repeatability, and scalability. The manual method, based on flexible tape measurements of DBH and height estimation via trigonometric calculations or mobile applications, allows for direct operator control and low-cost implementation, making it suitable for educational and community-based monitoring programs. Manual data collection also encourages citizen science participation, promoting environmental awareness and enhancing spatial data coverage. The main limitations in measuring height and diameter arise from the high sensitivity to operator skill, which can lead to systematic measurement biases. To mitigate these potential errors, each measurement was repeated three times, and the average value was used in subsequent calculations. Furthermore, a large number of trees were measured to reduce random variability and improve statistical robustness.

By using the above mentioned GreenGuard App, the acquisition of dendrometric data is automated, recorded, and stored digitally, integrating geolocation with built-in algorithms to estimate the total carbon content of each tree. Digital measurement reduces transcription errors and enables large-scale, standardized monitoring across multiple participants and urban locations. However, the accuracy of mobile-based DBH and height estimations depends on sensor calibration, camera resolution, and ambient lighting conditions, all of which can introduce systematic errors. In addition, this method tends to underperform in dense canopies or irregular terrains, where line-of-sight obstructions interfere with photogrammetric calculations.

Overall, both approaches offer valuable tools for quantifying CO₂ sequestration in urban trees, but each involves trade-offs among accuracy, cost, and scalability. Manual methods remain advantageous for small-scale, high-precision studies, whereas mobile application–based techniques provide greater efficiency and data integration for large-scale or participatory monitoring networks. Combining both methods may therefore yield the most robust framework for the long-term assessment of urban tree carbon dynamics. In order to reduce the error, the measurements of the heights and diameters of the trunks were repeated three times, calculating the arithmetic mean as the final result for each tree.

City-level scaling is carried out under the assumption that urban trees across the city experience similar climatic and meteorological conditions. This requires knowledge of the total tree population and measurements of a representative sample of different species.

The Co Carbon Trees Measurement Initiative was launched in 2024 as a demonstrator project within the EU Climate Mission in Viladecans (15 km south of Barcelona, Spain). Figure 2 shows the location of this city. It involved three of the four actors of the EU mission’s quadruple helix governance model: the Universitat Politècnica de Catalunya (Academia), Viladecans City Council (Administration), and organized citizens (student associations and local schools).

The project pursued two main objectives:

• To quantify the CO₂ stored in Viladecans’ urban trees and annually monitor the amount captured, providing essential data for assessing progress toward climate neutrality.
• To generate species-specific and canopy-size-specific sequestration estimates, thereby offering a tool to improve urban planning and prioritize green infrastructure.

The dataset comprised the geolocation of 1274 trees (out of ~21,000 in the city) that corresponds to 51 different species, and the identification of their species. A total of 658 students, aged 14–16, measured DBH (at 1.37 m) and tree height for 1274 trees. Measurements were conducted on 14 March 2024, and 18 March 2025.

3. Results

The total mass of carbon fixed by the measured trees of each species was calculated from the annual difference in accumulated carbon per tree. The Co Carbon Trees Measurement Initiative was launched in Viladecans (Barcelona, Spain) in 2024 as a citizen science initiative. In total, 658 participants measured the diameter at breast height (DBH, 1.37 m above ground level) and total height (H) of 1274 trees on 14 March 2024, and repeated the measurements on 18 March 2025. Figure 3 shows some pictures during the field measurement on 14 March 2024.

Using DBH and height, the carbon content of each tree was estimated following Shadman’s equation [23]. The annual increment in stored carbon was then derived for each species.

Figure 4 presents the mass of carbon fixed over the one-year period as a function of DBH, disaggregated by species. The interannual period 2024–2025 corresponding to both measurements was significantly wetter than usual (714 mm, above the 620 mm average climatic precipitation) and slightly warmer (17.6 °C, about 1 degree above the climate average).

As tree size increases, the sequestration of atmospheric CO₂ (stored as carbon within the various structural components of the tree) also rises. When considering all measured specimens, a clear trend emerges: the amount of carbon fixed increases with trunk diameter, following a parabolic growth pattern (Pearson coefficient of correlation higher than 0.9 in all cases). Figure 5 illustrates the difference in carbon fixed between March 2024 and March 2025 for the 1274 measured trees, plotted against trunk diameter as recorded in 2025.

The total mass of CO₂ stored in the N = 1274 measured trees (M) is obtained by using Equation (1). A total of 1274 trees, corresponding to 45 species, were measured. The calculation of the carbon stored in these trees in 2024 was 340,873.2 kg, and in the measurement one year later, in 2025, it was 389,488.2 kg. The difference between the two measurements represents the carbon fixed in one year by this number of trees and species in the different parts of the plants. This carbon is fixed through the transformation of atmospheric CO₂ in the process of photosynthesis. Therefore, it can be concluded that, over the course of one year, these 1274 trees captured approximately 48,615.1 kg of CO₂ from the atmosphere. The city has approximately 21,000 urban trees. The measurement of 1274 trees corresponds to only about 6% of the total, but it is sufficient to extrapolate to all trees in the city, taking into account the average capture per species and the number of species. The type of species and the spatial distribution across the urban area are representative of the whole city. These data can be extrapolated to the city’s approximately 21,000 trees using Equations (2) and (3), providing a first approximation of the CO₂ fixed in the form of carbon in urban trees. In 2024, the total mass of CO₂ captured was approximately 5,676,714 kg, and in 2025 it was around 6,486,322 kg. The difference between these two masses represents the CO₂ mass fixed in solid form in the city’s urban trees, amounting to approximately 809,608 kg.

Table 1. Values obtained in the 2024 and 2025 experimental measurements.

Concept	Mass of Carbon (kg) 2024	Mass of Carbon (kg) 2025	Carbon Captured (kg)
Estimated in the measurements	340,873.1	389,488.2	48,615.1
Estimated in all urban trees	5,676,714.1	6,486,322.5	809,608.4

presents a summary of the values obtained in the 2024 and 2025 measurement campaigns.

An interesting outcome of this initiative has been its recognition as a European Good Practice project in 2024 within the Urbact EU program (https://urbact.eu/good-practices/co-carbon-tree-measurement accessed on 2 November 2025). It was the winning project among the 2024 demonstrator projects of EU Mission Cities in Spain, and more recently, in 2025, recognized as a Good Practice among exemplary projects in Spain by the Pi i Sunyer Foundation (https://www.bbp.cat/practicas_detalle_imp.php?id_ficha=2167 accessed on 2 November 2025).

4. Discussion

The EU climate mission has a clear and well-defined objective: to achieve climate neutrality and address climate change, arguably the greatest challenge facing humanity. In cities, however, reaching climate neutrality is not only an environmental goal but also a transformative lever that reshapes urban dynamics such as mobility, energy use, public space, urban planning, and construction practices.

In this new climate context, it is evident that cities require a redefined relationship between green infrastructure and citizens, one that grants trees and vegetation a central role. Urban planning must move beyond considering trees as decorative elements and instead recognize them as strategic assets for mitigating climate change. Large urban trees, in particular, have greater CO₂ capture capacity, larger canopy cover, and provide shading that reduces energy demand for cooling. Their contribution to urban naturalization projects highlights the dual role of green infrastructure: sequestering atmospheric carbon and reducing urban heat through shading and albedo effects.

The Co Carbon Trees Measurement Initiative exemplifies this approach by quantifying CO₂ sequestration through citizen science. Beyond its technical objective, the project reframes urban trees as key allies in climate mitigation. Maintaining large trees requires a new management model for urban greenery, one that prioritizes canopy expansion, limits unnecessary pruning, and integrates natural cooling strategies into the design of public space.

The urban heat island effect further emphasizes the importance of vegetation. Research shows that tree shading, reflective pavements, and naturalized surfaces can significantly reduce urban temperatures, thereby lowering energy demand. For example, simulations indicate that a 30% increase in street vegetation can reduce summer cooling needs by 30–100% and cut building heating costs by 10–20% [24,25,26,27]. Additional studies demonstrate that urban trees can reduce household cooling energy consumption by 5–20% [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42], while large canopies provide the greatest savings. Collectively, these findings underscore that urban naturalization is a cost-effective, nature-based solution to urban overheating, energy overuse, and related greenhouse gas emissions.

Equally important are the social and health benefits. Exposure to greenery has been shown to improve mental health, accelerate physical recovery, and reduce stress [39,40,41,42,43]. The “3-30-300” rule [44] (seeing three trees from home or work, achieving 30% canopy cover at the neighborhood scale, and ensuring access to a green space within 300 m) offers a practical guideline that integrates climate mitigation with psychological well-being. Larger trees, in particular, have disproportionate positive effects on emotional comfort and neighborhood livability [45,46].

Psychological comfort also interacts with thermal comfort: shaded neighborhoods not only cool the environment but also encourage outdoor activity, social interaction, and stronger community bonds [47,48,49,50,51,52]. Thus, tree canopy expansion simultaneously supports public health, biodiversity, and climate adaptation.

Despite these multiple benefits, the absolute contribution of trees to climate neutrality remains limited by climatic context. The sequestration rate varies by species, size, and local climate: trees in temperate, humid environments fix more carbon than those in arid, warm regions. In Viladecans, for example, annual CO₂ sequestration was ~810 tons, compared to ~170,000 tons of direct Scope 1 emissions in 2022. Although this illustrates the modest scale of tree sequestration relative to total emissions, it also highlights the need for complementary strategies: drastic emission reductions combined with progressive increases in CO₂ capture through expanded green infrastructure.

In conclusion, the methodology applied in the Co Carbon Trees Measurement Initiative is not only useful for monitoring urban progress toward carbon neutrality, but also provides a compelling framework for re-centering urban trees in climate policy and city planning. By integrating trees into the core of urban design, cities can simultaneously advance carbon mitigation, reduce energy demand, improve health and well-being, and adapt more effectively to the climate realities of the 21st century.

The findings of this study have important implications for urban climate policy and governance, particularly within the framework of the EU Mission for Climate-Neutral and Smart Cities. Quantifying the carbon sequestration capacity of urban trees provides the empirical foundation necessary to incorporate green infrastructure into municipal carbon accounting systems, enabling more transparent and evidence-based decision-making. By making the disparity between urban CO₂ emissions and biological carbon capture visible, the results emphasize that climate neutrality cannot be achieved through sequestration alone, but must rely on an integrated policy mix combining emission reduction, ecosystem restoration, and adaptive urban design. Municipal climate action plans should therefore recognize trees not merely as aesthetic or recreational elements, but as functional climate assets whose management (through canopy preservation, species selection, and spatial planning) can yield measurable mitigation and adaptation benefits. Furthermore, the participatory nature of the Co Carbon Trees Measurement Initiative demonstrates the potential of citizen science as a governance tool, fostering local ownership of climate goals and enhancing the legitimacy and social acceptance of environmental policies. Embedding such participatory monitoring systems into urban policy frameworks could strengthen cross-sectoral collaboration among citizens, scientists, and policymakers, ensuring that urban greening strategies contribute coherently to long-term climate neutrality objectives.

This study provides a significant contribution to understanding how urban green infrastructure can be quantitatively integrated into metropolitan climate strategies. By implementing a citizen science-based methodology to assess CO₂ sequestration in urban trees, the research operationalizes the concept of nature-based solutions as measurable components of urban climate mitigation. The approach not only strengthens the scientific foundation for evaluating ecosystem services but also promotes civic participation and awareness through the involvement of students, local administrations, and academia. Although the total annual CO₂ captured by urban trees in Viladecans represents only a small fraction of the city’s total emissions, the project makes this discrepancy explicit, allowing for a clearer understanding and dissemination of the substantial gap between emissions and carbon capture in urban environments. Recognizing this imbalance is essential for setting realistic expectations regarding the role of urban trees in achieving carbon neutrality and for motivating deeper emission reduction efforts. Importantly, the capacity to quantify CO₂ capture and monitor temporal trends in carbon storage provides critical data to evaluate progress toward climate-neutrality goals and to guide evidence-based policy interventions. The methodology proposed herein offers a scalable and replicable framework that can be applied across cities, supporting improved carbon accounting, informed urban planning, and the positioning of green infrastructure as a key component of resilient, climate-neutral urban development.

5. Conclusions

The Co Carbon Trees Measurement Initiative provides a practical framework for assessing urban CO₂ capture through direct measurement of tree growth, integrating citizen science with academic and municipal collaboration under the quadruple helix model. By measuring trunk diameter at 1.37 m and tree height annually, the methodology allows precise estimation of carbon sequestered by individual trees and, through city-level scaling, the total CO₂ captured by urban forests. In Viladecans, measurements of 1274 trees by 658 students revealed an annual CO₂ capture of approximately 810 tons, highlighting the potential, yet limited, contribution of urban greenery to local carbon neutrality given the city’s Scope 1 emissions of around 170,000 tons. Results confirm that larger trees sequester significantly more CO₂, emphasizing the importance of preserving and expanding high-canopy urban trees. This methodology aligns with the EU Mission on Climate-Neutral and Smart Cities, providing a replicable model for cities to monitor CO₂ capture, track progress toward climate neutrality, and implement evidence-based urban planning. Beyond carbon capture, urban trees provide cooling effects, reduce building energy demand, enhance biodiversity, and improve residents’ psychological well-being. Citizen-engaged urban tree monitoring demonstrates how community participation, combined with scientific methods, can accelerate the transition toward climate-neutral cities, while delivering both environmental and social benefits.