Urban Scale Timber Carbon Sinks
In the forests of Brandenburg and Berlin, one cubic metre of wood grows every three seconds. This means that enough wood grows within 1.5 minutes to build a single family house – in theory.
Forests as infrastructure: How much can – and should – they afford us in the future?
Part II (Coming Soon)
Today we know that timber not only scores as one of the best building materials in terms of life-cycle assessments and environmental cost indicators, but that its storage capacity for CO₂ will also be a decisive factor in bringing about sustainable building and climate neutrality in our cities. Pilot projects such as the Schumacher Quartier rely almost entirely on renewable raw materials and, above all, on local production and manufacturing methods.
However, in order to anchor urban timber construction sustainably in the city and its (re)development, it is necessary to pursue a systemic approach and to focus on our forests and natural ecosystems. In addition to linking urban and rural ecosystems, forest conversion will play an important role in meeting the increasing demand. Only closer networking and balance between demand, capacity of the ecosystems and production can pave the way.
It is vital that we achieve a broader understanding of systemic interrelationships between building with wood, urban CO₂ storage, natural ecosystems and digitalisation. The goal is to highlight the potentials for the environment and the city which lie in urban timber construction and the infrastructures of sustainable forestry. Because transformation towards urban timber, and climate-friendly, construction also means a transformation of natural forest systems and all systemic elements that connect city and forest.
Once established, urban timber construction would place entirely different demands on the forest compared to the status quo. Brandenburg’s forests, however, have the potential to keep pace with such a transformation. The decisive parameters are the construction industry and the demand for the raw material wood, which are mainly influenced by the regulatory framework created by the federal, state, and local governments.
With his university research project Wood to City, City to Wood, Daniel Dieren developed various planning scenarios that consider the entanglement between the construction industry, architecture, and CO₂ cycles. In addition to global cycles, the examples presented in the publication also deal with potentials for urban CO₂ storage and timber construction applications for the Berlin housing market.
The renewable building material wood is a carbon sink. Sustainable, bio-based materials need carbon for their growth, which is taken from the atmosphere in the form of carbon dioxide (CO₂) – a process which actively binds carbon and releases oxygen. Provided our forests bind more carbon than is released by the timber harvest, the forestry and wood products sector will serve as a global carbon sink.
Wood and wood-based materials are substitute materials. By using more wood, we also avoid carbon dioxide emissions caused by using mineral-based building materials. The sustainable use of wood as a mitigation and substitute building material thus becomes part of a global climate strategy. The effect of timber products as a CO₂ sink is shown here at four levels of observation – building, material, forest, and planet.
In our cities, building with wood and wood-based materials creates a permanent, biogenic carbon reservoir. In a multi-storey timber apartment building, depending on the type and method of its construction, 100 to 200kg of renewable building materials are integrated per m² of floor area (see below Hafner et al.). As long as the building stands, the release of its stored carbon into the atmosphere will be delayed. Only when the wood products are disposed of and burned at the end of their material life-cycle is the carbon reservoir dissolved and returned to the atmosphere in the form of CO₂.
At the same time, when using wood products in the construction sector, a balance must be drawn between the carbon storage effect of renewable raw materials and the efficient and material-effective use of wood.
Renewable materials such as wood describe a biotic material cycle. Their life cycle leads from plant growth to extraction of building materials, to component production and assembly, and finally to dismantling and biological decomposition. Reuse and recycling can extend the life of the material as a cascade material. In all the multiple uses of wood as building material, the stored carbon remains bound for its entire useful life.
Around 50% of the dry mass of wood is made up of carbon (C).
Carbon is used to structure the living biomass in the form of roots, trunks and branches. To build up 1kg of carbon in biomass, 3.67kgs of carbon dioxide are removed from the atmosphere. For the production of 1kg of wood product mass, a raw wood requirement of approx. 2kg is necessary. The harvest of 2kg of raw wood ultimately results from a total biomass of approx. 3.8kg in the forest (see below Vogtländer et. al.).
Softwood and hardwood require rotation times of over 80 years. Timber buildings for the next 30 years will therefore emerge from today's tree population. Sustainably managed forests are an inexhaustible source of raw wood and continuously “produce” wood products for both material and energetic use. The forests of Brandenburg have a raw wood potential of 5.6 m³/year/hectare. It is therefore of particular importance to train supply chains in conjunction with the resource forest as a building material supplier. For the sustainable use of building materials in local forests, the demand for wood must be estimated in the long term and the forest converted with a view to the challenges of climate change.
Deforestation and forest degradation, as well as the associated decrease in the global carbon store of the forest, contribute significantly to climate change today. We cannot yet draw any conclusions about the effect of using wood and wood-based materials alone on the reduction of greenhouse gases and global warming. The origin of the wood and the development of wood consumption patterns are important parameters when assessing the mitigation potential of this building material. Only the increased productivity of forests and the resulting increased use of wood products in the building industry will generate a global increase in carbon sequestration – and thus a reduction in greenhouse gases through timber construction.
In the context of the international climate protection agreement (Kyoto Protocol), forests can be counted as carbon sinks when accounting for emissions. The CO₂ sink effect of forests is secured through sustainable management. Proof that the wood used has not been illegally cleared is provided by a forest certification e.g. PEFC or FSC.
In light of an increasing population, the decision to respond to demand for new buildings with urban timber construction would have a directly proportional effect on global CO₂ storage.
In Berlin and Brandenburg, widespread pine forests planted in the post-war period are now ready for harvest. However, they can only offer a limited range of wood types. In addition to economic factors, climate influences and drought add intensifying stress factors that greatly increase the amount of damaged wood. Meanwhile, many wood stocks in Germany are exported in order to achieve better market prices, while the supply in Germany struggles with bottlenecks.
Thus, the question arises: What can the forest afford us?
The increasing need for climate-friendly building in the context of our continuously growing cities can probably be answered to a large extent with urban timber construction. However, the associated demand for the raw material wood will put local forest ecosystems under further pressure. On the other hand, this change bears great potential for holistic forest conversion towards resilient mixed forests: The path from monoculture to holistic biodiversity would enable a balanced supply of raw materials and help to resolve tensions in the competition for different wood-based materials (e.g. also for energy supply). For a sustainable future, all stakeholders – not only in business but also on a planetary level – need to be taken into account.
Daniel Dieren, Civil Engineer
Complex Team (Living Systems)
Leonard Schrage, Partner
Martin Bittmann, Partner
Julia Dorn, Research Assistant
For more information
Complex is Living Systems’ research journal on the Digital City and Planetary Architecture. It features contributors from various academic and applied disciplines, such as data scientists, robotic engineers, forestry experts, sociologists and behavioural psychologists.