Harvest wood products, sourced from sustainably managed forests, have proven to be a renewable alternative with a lower environmental impact compared to materials that are intensive in fossil fuels and contribute to climate change with higher greenhouse gas (GHG) emissions.
Additionally, their potential to capture carbon dioxide from the atmosphere during photosynthesis and tree growth presents a unique opportunity as a material in which wood product manufacturing can store biogenic carbon over its lifespan.
In simplified terms, for every two kilograms of anhydrous wood, approximately one kilogram of biogenic carbon can be estimated, around 50 percent of its mass, which is equivalent to storing approximately 3.67 kg of carbon dioxide from the atmosphere when considering the mass of carbon and oxygen involved.
This condition remains in time, unless the carbon is released through combustion and oxidation or degraded by biological agents, mainly being released as carbon dioxide and methane.
This potential for atmospheric carbon dioxide storage over time depends directly on ensuring that any wood product remains in the environment for as long as possible and prevents the release of carbon, whether accidental or deliberate.
This can be achieved through the manufacture of long-lasting products, reuse, and, to a lesser extent, recycling or modification for other uses, considering that any recycling or modification process tends to lose some material and release stored carbon to some extent.
Consequently, wood products used in construction, especially in structural applications, offer buildings the potential to store substantial amounts of biogenic carbon for decades, or even centuries.
Moreover, the lower GHG emissions and reduced environmental impact associated with wood construction offer a substantial potential for decreasing the environmental impact of this sector.
Currently, the construction industry is responsible for approximately 37 percent of atmospheric GHG emissions.
Additionally, traditional construction materials such as concrete and steel currently account for approximately 11 percent of global GHG emissions due to their production processes, in which it is difficult to reduce emissions, and their global production volumes.
In fact, approximately 70 percent of the planet’s anthropogenic mass currently consists of materials such as concrete and aggregates, and it is expected that by 2040, concrete by itself would exceed the entire earth’s biomass.
Moreover, these materials are increasingly scarce, requiring transportation over long distances for processing, resulting in emissions and environmental impacts associated with their extraction.
Importance Of Forest Resources
Forest resources are abundant in many regions worldwide and have the potential to grow even further through reforestation and afforestation strategies, offering substantial potential for combating climate change.
This is especially important in countries that consider forests and wood products as strategic resources for achieving GHG emission reduction targets and meeting raw material demands.
Moreover, durable wood products harvested from sustainably managed forests can effectively help prevent CO2 emissions, even when sourced from younger forests.
For example, newly planted forests, when managed with a focus on denser planting schemes and rapid growth strategies, can capture CO2 more effectively than older forests.
And although older forests capture less CO2 overall, individual mature trees capture more CO2 than several young trees; more importantly, they may have stored CO2 for a longer time, potentially even predating the industrial revolution.
Therefore, cutting down old trees in mature forests can have a higher impact on CO2 release compared to cutting young trees in young forests, where reforestation plans can help capture new CO2.
Likewise, if the destination of wood products involves a life in construction elements, where the materials’ duration in the built environment could span several replacement tree growth cycles in the environment, any release of carbon dioxide could eventually be compensated.
Thus, it is desirable that the useful life of any wood product exceeds the time it took the tree to capture the stored biogenic carbon.
Furthermore, forests with rapid growth and carbon dioxide capture, such as those with a regeneration cycle of just a couple of decades, are more appealing from this perspective than forests that are centuries old, where the slower regeneration of carbon capture may outweigh the benefits.
In this context, wooden constructions within the urban environment of a specific city act as a biogenic carbon stock that endures over time and varies depending on the longevity of wooden construction elements in the built environment.
Moreover, the integration of new wooden buildings can potentially offset carbon released into the atmosphere or increase urban carbon storage.
Furthermore, if wood surpasses the replacement rate of equivalent carbon harvested from the original tree through the planting of a new tree, it could provide a long-term benefit to the global carbon balance. This not only represents a transfer of biogenic carbon from the forest to the city but also creates a carbon reservoir independent of the original forest stock.
Methodologies & Aim
In this regard, previous research has attempted to quantify the amount of wood in urban environments using various methods but has encountered challenges that have constrained the results.
Most of these challenges are associated with limitations in the availability of public data on the materials used in buildings, making it difficult to represent material quantities on a large scale and track their evolution over time; the restriction of results to a specific range of building types within particular timeframes and spatial constraints; or the use of material quantity indicators that need to be extrapolated from other contexts.
These limitations highlight the need for methodologies that address gaps in the quantification of urban wood use and reduce uncertainty, particularly in developing countries with limited data on construction materials and their environmental impacts.
This study focuses on Santiago City as a case study due to its unique urban characteristics, including a growing use of timber in construction and its role as the capital of a developing nation with ambitious carbon neutrality targets.
By examining Santiago, this research provides insights applicable to other rapidly urbanising regions with similar constraints, offering a replicable framework for understanding the environmental potential of urban timber.
The concept of Santiago as a ‘Second Forest’ reflects the premise that urban timber structures can replicate the carbon storage capacity of natural ecosystems.
This concept suggests that urban environments, when designed with an emphasis on wood-based construction, have the potential to become significant carbon sinks, simultaneously reducing emissions while enhancing carbon storage.
By envisioning cities as anthropogenic extensions of natural carbon storage systems, this study highlights the need for a paradigm shift in urban planning where built environments actively contribute to climate change mitigation by retaining part of the original forest’s carbon through harvested wood products while allowing the development of low-emission buildings in emerging nations.
Furthermore, this perspective emphasises the broader potential of urban wood use in reducing reliance on carbon-intensive materials such as concrete and steel.
Additionally, as a forest-rich country, Chile depends on the maintenance and expansion of its forests to offset at least 50 percent of its growing emissions and achieve carbon neutrality, underscoring the need for an industry that promotes long-lasting and renewable harvested wood products that foster sustainably managed forests.
By integrating these principles into policy frameworks, nations like Chile could develop effective strategies to offset emissions while enhancing carbon storage. This approach positions timber construction not only as a sustainable alternative but also as a critical element in achieving carbon neutrality by 2050 and aligning with the nation’s commitments under the Paris Agreement.
The primary purpose of this work is to present a methodology for quantifying the biogenic carbon stored in wooden buildings within the urban built environment of a city, particularly in contexts with limited or no available data on wood material use and carbon storage.
Additionally, a secondary aim of this research is to provide an initial understanding of how biogenic carbon is distributed in a large developing city, such as Santiago, Chile.
Considering its significant expansion in recent decades and its pressing need for new infrastructure, this study examines the temporal behaviour of biogenic carbon and identifies future research directions to improve projections of wood usage and biogenic carbon storage in the coming years.
Reason Of Choosing Santiago
Chile has positioned itself as a regional leader in Latin America in terms of sustainability and development in the construction industry. Despite being a developing country, Chile has adopted policies that promote energy-efficient buildings through the implementation of sustainable certifications, alongside tools designed to encourage a more circular construction industry.
These initiatives aim to reduce the environmental footprint of the rapid urban growth the country has experienced in recent decades—a characteristic common to emerging nations that face the need to quickly expand their infrastructure.
Furthermore, Chile has gradually begun to adopt more complex models for assessing environmental impact, laying the groundwork for the development of life cycle assessments in construction and the quantification of embodied emissions.
Chile is a strong advocate of international climate commitments, being a signatory to the Paris Agreement, which reflects its determination to achieve carbon neutrality by 2050. This goal is supported by a comprehensive strategy that includes significant progress in emission reductions, accounting for 50 percent of the projected reduction, and the sustainable management of its extensive forests, which cover 17.9 million hectares.
These forests are expected to act as natural carbon sinks, offsetting the remaining 50 percent of national emissions. Within this strategy, the construction sector accounts for approximately 31 percent of national emissions and is projected to contribute, through sustainable building strategies, at least 17 percent to the national emission reduction plan.
Over the past two decades, Chile has emerged as an international leader in the adoption of renewable energy, particularly solar and wind energy, with more than 50 percent of the installed capacity in its electrical system coming from renewable sources.
These energy sources have transformed the country’s energy landscape, significantly reducing greenhouse gas emissions associated with electricity generation and, consequently, the operational emissions of the built environment.
While the country is expected to reach 100 percent renewable energy by 2050, and national industries like lithium and green hydrogen enable energy storage, the significant reduction in operational emissions from the built environment alone would not be sufficient.
This is due to the construction sector’s heavy reliance on fossil fuel-intensive materials such as concrete. Within this Chilean context, it is essential to understand the carbon-storing benefits of materials like wood in the construction sector, alongside their advantages in promoting a forestry sector that produces sustainable wood products capable of storing carbon and reducing embodied emissions.
The city of Santiago, represented in this study by the 32 municipalities of the Province of Santiago, with a total area of approximately 2,110 km2 and a population exceeding five million inhabitants, serves as a representative example of national building and public policy trends in Chile.
Like many cities worldwide, Santiago faces significant challenges related to urban densification, transportation, and resource consumption. Nonetheless, the city has implemented pioneering initiatives in green infrastructure, urban planning, and efficient public transportation, positioning itself as a model for other cities in the Latin American region. However, the impact of these policies on wood-based buildings and the urban biogenic carbon stock remains unclear.
More critically, it is uncertain whether these policies are contributing to biogenic carbon storage or facilitating its release into the atmosphere.
Quantifying Wood In The Built Environment
Accurately quantifying the volume of wood present in buildings within the built environment is the key to understanding its extent and distribution patterns. This requires considering various factors, such as construction systems, building types, and wood species, among others.
Moreover, the potential benefits of wood construction at the urban level can only be analysed if its scope and variation in space and time are well understood.
However, determining how much wood exists in the built environment is not a straightforward task, and this information is not readily available in all countries. Various approaches can be used to obtain these data, including (i) data from national records, such as building permits or real estate property records; (ii) estimates made through sample surveys, such as censuses or sampling methodologies; (iii) approaches based on aerial imagery and model estimations; and (iv) commercial records of forest production and sales of wood products.
This study is based on the use of national real estate records to quantify wood in the built environment. The case study of Santiago in Chile benefits from tax records maintained by the country’s Internal Revenue Service, which detail the primary materials used in building structures for the purpose of a property tax assessment.
This database includes multiple records that allow for the identification of built square metres according to the materials used in structural walls, the intended use of the building (e.g., residential, commercial, educational, or other), the year of construction, records of modifications over time, the quality of construction, and others that are not relevant to this research.
Chilean records have been available since 2018, with updates being made every six months, and they include public access data on demolitions, modifications, and new constructions. These records are gathered using detailed manuals and assessments carried out by professional property appraisers.
The eight-year measurement period is due to the system’s digitisation in 2018; earlier records exist but lack the consistent structure found in the current dataset.
Furthermore, while the Chilean National Statistics Institute (INE) keeps a record of building permit requests, which identifies the main materials used and conducts a national housing census that records the materials of surveyed buildings, these sources are less reliable than the real estate registry.
This is because the former does not confirm whether the permitted building was constructed or modified, and the latter faces issues related to survey-based data collection, where residents may provide incorrect or incomplete information about the building systems they inhabit.
The data provided by the real estate record and their uses in this work consist of eight files (from 2017 to 2024) that address urban areas and focus on parcels and building information.
The Chilean real estate record also provides detailed information at the municipality, block, and property levels, with over eight million entries nationwide. In this case study, 32 municipalities that make up the province of Santiago or city of Santiago are analysed (the country’s most densely populated urban area).
For the purposes of this study, analyses are conducted at both the city of Santiago and municipality levels, summing the square metres accordingly, to understand how wood and its associated biogenic carbon are dis- tributed within the urban environment.
For the same reason, the results and analyses are limited exclusively to buildings that list wood as their primary construction system regardless of whether it represents only a small part of a building complex that may include a greater proportion of square metres constructed with other materials, either before or after.
It is important to note that while the real estate record is the most reliable source of this type of information for this case study, it contains some inconsistencies and lacks de- tail.
An example of this is that it might include properties that have not been properly regularised or have undergone undeclared modifications not identified by inspectors.
Additionally, the registry does not specify the construction methods used for wood, making it impossible to distinguish between frame systems or mass timber, which could affect the quantification of the material.
Historical, Traditional, & Contemporary Construction Systems
Since the record of wooden square metres in the Chilean real estate database refers to the gross built surface area and not the volume or specific mass of the material, a methodology is developed to quantify the material intensity (MI) of wood per square metre.
The MI is an indicator that represents the mass of various materials per surface area or volume, typically expressed in kg/m2 or kg/m3. It is calculated by dividing the mass of a given construction material by the gross surface area or internal volume of the project that is being measured, which subsequently generates a record for the set of materials used.
In this way, by calculating the material in a project (whether based on design specifications, the material used during construction, or the material registered during demolition), it is possible to establish the MI for a specific material. In the case of wood, the mass depends on the equilibrium moisture content, so the MI of wood can be calculated either at the equilibrium moisture point or in a completely dry state.
The MI is commonly used by researchers to track the flow of construction materials within buildings, assess their lifespan, create inventories, study material recovery potential, analyse temporal–spatial identification, and evaluate environmental impacts, among other applications.
Materials associated with products that are prone to renovation, such as interior finishes or coverings, are more difficult to assess over time and to keep a consistent record of. Conversely, structural materials tend to exhibit fewer changes over time and remain throughout the entire lifespan of a building.
Since it is impractical to conduct an MI analysis of wood for every building in the case study, an assumed MI is proposed for different types of wooden construction systems. This is achieved by defining typical wooden walls and roofs linked to historical periods of construction in the country.
Maximum and minimum MI indicators are proposed for the wood used in these structural elements. Considering that structural elements are the least likely to change over time and that the Chilean real estate registry primarily records load-bearing wall structures and their modifications, the focus remains on these aspects.
This study centres exclusively on typical construction systems based on sawn wood, acknowledging that this is the primary material used for the construction of such buildings in Chile.
Engineered wood products, such as laminated timber, only gained significant market presence during the last decade of the 20th century—the first laminated structure is presumed to date from 1969, OSB products emerged in the early 2000s, and mass timber products like CLT only began to be used in the 2020s.
While these products and construction systems may have higher MI values than frame-based systems, they are much less common and not representative of the historical wood construction practices in the country.
Thus, three historical periods are defined with the objective of evaluating the MI of wood in the case study: pre-1900 systems, those from the 20th century, and those from after the year 2000.
It is important to note that while the real estate registry includes heritage structures dating back to the 17th century, most of the recorded built surface area is concentrated in the second half of the 20th century and the first decades of the 21st century.
Additionally, an unknown number of historical wooden constructions in Santiago have been affected by fires, earthquakes, or have simply been replaced by masonry and reinforced concrete structures.
To facilitate the analysis of a large number of wooden architectures represented in the real estate record and to generate representative wood volumes for construction systems and historical periods, two typical wooden walls and two typical roofs are defined for analysis.
These typologies assume building heights not exceeding three stories, in line with data collected by the INE in its building permit records. This approach is also consistent with research conducted in Finland and other Northern European countries, which attribute nearly the entirety of wood MI in low-rise framed buildings to the walls and roof structures.
For the walls, a small wall of 2,400 mm in length and a larger one of 7,200 mm is defined, with a height set at 2,400 mm. Although historical architecture often features taller ceilings than those found in constructions from the second half of the 20th century and the 21st century, due to variability depending on the project, a uniform and more conservative height of 2,400 mm is used for all wall types regardless of the historical period.
Furthermore, to determine the volume of wood in walls per square metre of built structure, the wall density data from the floor plan are utilised. According to the literature, wooden structures should have a wall density in the floor plan between three percent and six percent, values that are higher than those found in equivalent concrete structures.
Regarding the defined roof types, both are gable roofs with a 40 percent pitch. One case considers a smaller volume of wood covering a span of 3,000 mm and a length of 2,400 mm, while the second case involves a higher volume of wood, spanning 6,000 mm with a length of 4800 mm.
Horisontal structural elements, such as ventilated floors or intermediate floors, are not considered in this analysis because they cannot be identified in the real estate database. It is assumed that these elements are partially represented within the wood volume addressed by the roof cases.
Quantification Of Biogenic Carbon
The methodology used to determine the biogenic carbon content in wood follows the guidelines established in the standard ‘UNE-EN 16449: Wood and wood-based products. Calculation of the biogenic carbon content of wood and conversion to carbon dioxide’.
The calculated volume of wood for each building and year recorded in the real estate database is used to estimate the biogenic carbon.
Additionally, since wood density can vary by species, the analysis considers two species that establish representative minimum and maximum values. From the second half of the 20th century onward, the period with the highest number of wooden constructions according to the real estate record, Pinus radiata (radiata pine) has been the most widely used wood species.
Meanwhile, Nothofagus obliqua (Chilean oak) is one of the most common woods in the central–southern region of the country and represents the oldest constructions in the case study. Radiata pine, with a density of 450 kg/m3 (oven-dry), is one of the lightest local woods used in construction, while Chilean oak, with a density of 558 kg/m3 (oven-dry), is among the densest common construction woods.
The moisture content for both species is considered to be around 14 percent, in line with the equilibrium moisture content established for the climate in the case study, allowing for the determination of the carbon content per unit mass of wood.
While there is some debate regarding the carbon fraction associated with the dry mass of wood, this analysis follows the recommendations outlined by the UNE-EN 16449 standard. Therefore, the carbon fraction in the oven-dry state is established as 0.5, consistent with other similar studies.
This enables the quantification of both the biogenic carbon present in the mass of wood used in construction in the case study and its equivalent in carbon dioxide captured from the atmosphere.
Results & Conclusions
The results are presented by first quantifying the wood usage per square metre in buildings from the case study and generating wood MI data, categorised by historical period.
Second, these results are cross-referenced with the real estate database to identify average values for the entire case study and to determine their variation by the studied municipality.
Finally, the results for biogenic carbon storage and the equivalent CO2 amount are presented, both aggregated for the entire city and by municipality.
Regarding the wood MI results, which range from 54.69 kg/m2 to 13.07 kg/m2 with an average of 33.88 kg/m2, previous studies have established values for housing, considering variations in wall and roof structures, between 15 kg/m2 and 50 kg/m2.
Also, these studies show similar wall/roof wood usage ratios. Meanwhile, studies addressing the full set of wood-based materials have indicated a range of around 50 kg/m2 of wood being used for framed buildings, while maintaining the roof-to-wall ratio identified in the Chilean case.
In this sense, the upward variation in some case studies is due to the fact that these analyses do not only include structural elements.
However, an average wood MI of 37 kg/m2 has been identified in the context of European and Asian studies in accordance with the results obtained in this work.
On the other hand, it has been identified that historical dwellings based on other construction methods with high wood usage, such as stacked log systems, could have a wood intensity of up to 135 kg/m2 on average for Swedish homes and 88 kg/m2 on average for traditional Japanese homes.
However, these types of buildings are rare in the Chilean context as they were not recorded during the data collection period and should not represent a significant correction factor for the results presented.
It is recognised, however, that in the case of Chilean historical buildings, wood usage may be slightly under- estimated due to the height used in the quantification of material in the defined wall types.
This is because a review of projects from that era suggests that the heights of historical buildings are greater than those of contemporary buildings.
Nevertheless, due to the high variability in this aspect, a conservative height was chosen in alignment with modern structures.
Regarding the quantification of wood buildings in urban environments, as expressed in the methodology, several assumptions had to be made to obtain the presented results.
This may lead to underestimations or overestimations, which are addressed by the sensitivity analysis that considers minimum and maximum ranges for wood mass estimation.
In this case, the representative structural elements and the wall density factor present an alternative to previous research, which used representative building types or random samples.
However, since the methodology considers only basic structural elements and not the entire wood construction, and it also underestimates the wood usage in historical structures, overall, these results tend to be conservative in relation to the total wood carbon storage.
With regard to the specific results of the case study, it is also noteworthy that there was a decline in timber construction in 2021, following continuous growth in previous years. This effect has been seen in other countries across the entire construction sector and can be easily explained by the Covid-19 pandemic and associated restrictions.
Furthermore, although a recovery in wood construction growth is evident between 2022 and 2024, this recovery is quite modest due to a local building sector crisis that the country has been experiencing for the past four years, related to investment issues and bureaucratic delays in permits.
This clearly demonstrates how global economic events and local conditions can affect wood construction growth and its capacity to store carbon.
To understand the potential of wood buildings for carbon and CO2 storage in the built environment, it is important to compare them with other carbon sinks, such as forests. In this regard, Chilean forest land covers 17.9 million hectares, of which 14.8 million are native forests and 3.1 million are plantations.
According to the Chilean national carbon inventory, based on the IPCC methodology, native forests have sequestered ap- proximately 67,474 kt of CO2 per year, while plantations have captured around 74,935 kt of CO2 per year, averaged over the most recent eight-year reporting period from 2013 to 2021.
This means that, on average, native forests have captured 5077 kg CO2/ha per year, and plantations 21,440 kg CO2/ha per year. However, most of the carbon captured by plantations is assumed to be released later during harvest. In contrast, over the eight years of study, the wooden surface area in the case study has stored 1002 kg CO2/ha per year.
However, this figure could increase in some peripheral municipalities to up to 7607 kg CO2/ha per year, while municipalities undergoing a densification process could release up to 3794 kg CO2/ha per year due to the replacement of low-height wood buildings by taller concrete structures.
This indicates that wood constructions in the built environment have the potential to retain annual CO2 amounts comparable to a native forest in the case study scenario, although this potential may be significantly influenced by building trends.
That said, through its wooden constructions and the timber products used in its structures, the city of Santiago has managed to retain a significant part of the biogenic carbon originating from forests.
The results have shown that this carbon may have been stored for a few decades in some younger municipalities and for up to hundreds of years in the case of downtown Santiago.
Furthermore, the rate of wood construction in some expanding municipalities indicates that the annual CO2 storage from new buildings may even surpass the per-hectare capture of native forests in the Chilean case.
This highlights a temporal and quantitative potential for annual CO2 capture, suggesting, to some extent, the city’s potential as a “Second Forest” in relation to its biogenic carbon storage capacity.
This study presents a methodology for quantifying biogenic carbon and stored CO2 in timber structures within the built environment.
It has effectively assessed the city of Santiago’s potential to function as a wood carbon sink or ‘Second Forest’.
Through the analysis of real estate tax databases, construction permits, timber building typologies, and carbon accounting models, this study quantifies the carbon stored in the city’s timber structures, revealing storage volumes comparable to those of native forest ecosystems in some urban areas.
Additionally, the analysis shows that urban expansion zones tend to increase the use of timber in their constructions, while areas undergoing vertical densification tend to reduce the presence of timber.
This finding is significant in terms of CO2 storage in the built environment and the impacts of vertical densification using traditional materials other than wood.
Moreover, maintaining a record of CO2 stored in the built environment not only improves the understanding of buildings’ carbon mitigation potential but also lays the groundwork for developing tools and public policies aimed at promoting low-emission, CO2-storing construction practices.
Data generated by studies like this are essential for creating regulatory frameworks that encourage timber construction, especially in taller buildings, thereby strengthening urban carbon storage strategies.
Such methodologies also allow for extrapolation to other urban cases in countries lacking comprehensive material intensity records or timber use data within the built environment.
Finally, it is recommended that future studies focus on developing more accurate methodologies to quantify and characterise biogenic carbon storage in the built environment, particularly in regions where this methodology may not be suitable due to the stated limitations.
Additionally, it would be valuable for future research to explore how urban planning trends influence the permanence of biogenic carbon in timber within the built environment, as well as to examine future scenarios for high-rise timber buildings and their potential impacts on urban biogenic carbon stock.
