4.11.1 The Wood from the Trees

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towards a culture of respect

AECB GreenMemes No. 4, December 2019
Lenny Antonelli & Andrew Simmonds


It is not uncommon in our sector to hear that timber production is good so long as the wood is “sustainable”. In this essay, we aim to examine what that means, to consider the issues arising from treating buildings as carbon stores, and whether increased demand for timber is a sensible goal.

We look critically at the key concepts and language being used by academics, campaigners, customers, designers, suppliers, manufacturers, and the timber industry.

We also consider the way we see and use forests — and nature more broadly — and the potential consequences for the biosphere flowing from our patterns of thinking. We examine these issues against the backdrop of climate breakdown and ecological crisis.

Finally, we try to offer some practical approaches for architects, designers and other specifiers to try when specifying and building with timber.


11,000 scientist signatories from around the world, clearly and unequivocally declared this month that planet Earth is facing a climate emergency. The fully referenced report is short and to the point, please do read it.

They state:

“The climate crisis has arrived and is accelerating faster than most scientists expected. It is more severe than anticipated, threatening natural ecosystems and the fate of humanity. Especially worrisome are potential irreversible climate tipping points and nature’s reinforcing feedbacks (atmospheric, marine, and terrestrial) that could lead to a catastrophic “hothouse Earth,” well beyond the control of humans. These climate chain reactions could cause significant disruptions to ecosystems, society, and economies, potentially making large areas of Earth uninhabitable.”


“Economic and population growth are among the most important drivers of increases in CO2 emissions from fossil fuel combustion…therefore, we need bold and drastic transformations regarding economic and population policies.”

  • We should leave remaining stocks of fossil fuels in the ground
  • We should carefully pursue effective negative emissions using technology such as carbon extraction from the source and capture from the air and especially by enhancing natural systems
  • We must protect and restore Earth’s ecosystems. Phytoplankton, coral reefs, forests, savannas, grasslands, wetlands, peatlands, soils, mangroves, and sea grasses contribute greatly to sequestration of atmospheric CO2
  • We need to quickly curtail habitat and biodiversity loss
  • We need to protect the remaining primary and intact forests, especially those with high carbon stores and other forests with the capacity to rapidly sequester carbon (proforestation), while increasing reforestation and afforestation where appropriate at enormous scales. Although available land may be limiting in places, up to a third of emissions reductions needed by 2030 for the Paris agreement (less than 2°C) could be obtained with these natural climate solutions
  • Excessive extraction of materials and overexploitation of ecosystems, driven by economic growth, must be quickly curtailed to maintain long-term sustainability of the biosphere
  • We need a carbon-free economy that explicitly addresses human dependence on the biosphere and policies that guide economic decisions accordingly
  • Our goals need to shift from GDP growth and the pursuit of affluence toward sustaining ecosystems and improving human well-being by prioritizing basic needs and reducing inequality.

“Mitigating and adapting to climate change while honouring the diversity of humans entails major transformations in the ways our global society functions and interacts with natural ecosystems…we must change how we live, in ways that improve the [Earth’s] vital signs..”

Global tree cover – increasing or decreasing?

Of the nine graphs published in the Warning to Humanity that act as ‘warning indicators’, two represent tree cover loss. The tree cover figures used here do not take tree cover gain into account: “Thus, net forest loss may be lower than the reported numbers.”
Sources and additional details are provided in supplemental file S2 (notes on tree cover loss is on page 9)

Based on the same Global Forest Watch method and dataset used above, different countries may be analysed – for example in the UK – which here suggests net tree cover losses from 2001-2015.

Researchers from the University of Maryland, the State University of New York and NASA’s Goddard Space Flight Centre report that, rather than global tree cover shrinking , new global tree growth over the past 35 years has more than offset global tree cover losses. Using satellite data to track forest growth and loss, they contend that between 1982 to 2016 global tree cover had offset tree cover loss by approximately 2.24 million square kilometres.

They say that of all land changes, 60% are associated with direct human activities and 40% with indirect drivers such as climate change: most of the new tree cover occurred in places that had previously been barren, such as in deserts, tundra areas, on mountains, in cities and in other non-vegetated land. They further report that much of the new growth came about due to efforts by humans (such as reforestation efforts in China and parts of Africa) and because of global warming—warmer temperatures have raised timberlines in some mountainous regions and allowed forests to creep into tundra areas. They say that “the mapped land changes and the driver attributions reflect a human-dominated Earth system”.

However, as Gianluca Piovesan of the University of Tuscia and Alessandro Chiarucci at the University of Bologna point out, ” Global forest status assessments are biased toward tree cover and do not properly consider ecological properties of forest ecosystems.”

They argue that “a global map of forests with different degrees of naturalness (intact, old-growth, “rewilding” and managed forests) is required for a sustainable future. This map should serve to trace the future trend of intact, old-growth and mature forests, that, thanks to the natural dynamics, represent an efficient solution for the mitigation of climate changes and the conservation of the biodiversity. The biocomplexity of natural forest landscapes cannot be reproduced by plantations of trees or secondary forests periodically disturbed by man. However, wild forests are destined to decline on the human time scale, and new socio-political solutions are required to give more space to nature. ”

This view of the map shows ‘ecologically intact forest landscapes’, other data layers can be added.
“The Intact Forest Landscapes (IFL) data set identifies unbroken expanses of natural ecosystems within the zone of forest extent that show no signs of significant human activity and are large enough that all native biodiversity, including viable populations of wide-ranging species, could be maintained.”

We all love wood

One common trope in sustainable building circles at the moment is that we should use a lot more timber in construction, in order to ‘suck carbon out of the atmosphere and store it in buildings’. And that instead of building with energy- and carbon-intensive materials like fired brick, concrete and steel we can replace those materials with lower carbon, renewable alternatives based on wood and wood fibre.

“Wood products are carbon stores,” says a brochure from the European Confederation of Woodworking Industries, titled ‘Tackle Climate Change: Build With Wood [1]’. “They play an important role in enhancing the effectiveness of forest carbon sinks, both by extending the period that the CO2 captured by forests is kept out of the atmosphere and by encouraging increased forest growth.”

“The government aspires to build 300,000 new homes,” says UK forest industry group Confor [2].  “Building more of these with wood locks up carbon and saves money over the life of a building.”

A recent New York Times op-ed called for the US to “fill our cities with taller, wooden buildings” in order to “pump carbon from the atmosphere and store it both in forests and in cities”.

But just because wood is a wonderful natural material that many in the building sector love to work with, does not mean that we should fail to subject these claims to the same degree of scrutiny or scepticism that we apply to say, the cement or fossil fuel industries.

Forests – even their ecologically poorer relations, plantation forests, are after all, living systems. Any increase in the production of construction wood products will have to come from either new forests, or increased production of existing ones. And while climate change continues to be the front and centre issue for the sustainable building sector, we are still struggling to understand the meaning of the ecological crisis, and how we should respond to it [3].

[1]: https://www.svenskttra.se/siteassets/1-om-tra/standarder/buildwithwood.pdf

[2]: http://www.confor.org.uk/media/246945/future-for-food-farming-and-the-environment-consultation-response-may-2018.pdf

[3]: https://www.aecb.net/wp-content/uploads/2019/05/Green-Memes-3.pdf

What is sustainable forestry?


Right now, the UK construction sector is heavily reliant on timber from abroad: in 2015, for example, just 15% of construction timber used in the UK was home-grown. Most of our imported timber from countries like Sweden, Finland, Latvia and Estonia.

In 2018,  the UK used 49 million tonnes of imported wood but only 11.3 million tonnes of home-grown wood. Of the 6.5 million tonnes of UK-grown wood that was delivered to sawmills, an estimated 2.1 million tonnes were turned into construction timber by sawmills, and other 1.2 million tonnes was used to make timber panel products like chipboard and OSB.

This means roughly 30% of UK-grown wood is used for construction purposes. Most of the rest becomes shorter-lived products like fuel, pulp or paper, and fencing or pallets, which release carbon back to the atmosphere (through decay or burning) much sooner than longer-lived wood products like construction timber.

There is now a concerted push by industry groups including Confor, Wood Knowledge Wales, and the Alliance for Sustainable Building Products to produce more timber in the UK, and to use more of this for construction, with carbon storage cited as one of the key benefits. NGOs and expert groups including the WWF, the Committee on Climate Change and Royal Society have also endorsed this strategy. This would make the UK more self-sufficient in timber, reduce its reliance on imports and cut the carbon emissions associated with long-distance transport. It could bring the forests that produce our timber closer to us and give us more say over how they are managed at national, regional and community level.


Globally, the sustainability of timber supply is an urgent issue. The world lost an area of primary rainforest — old growth undisturbed forest — the size of Belgium in 2018, and almost three times that amount in further forest cover. Deforestation is a major cause of climate change, particularly in forested tropical regions, because it removes a major carbon sink.

The process of deforestation includes unsustainable logging within forests as a major driver for loss of tree cover. Developed European nations are not immune from this either: illegal and unsustainable logging continues to be a major issue in Estonia for example, a major exporter to the UK, and other European countries. However overall, forest cover in Europe has expanded in recent decades – mainly due to rural depopulation. But it will take a long time for regenerating native forest to reach the ecological richness of old growth, and it is also worth noting that natural forests can be replaced by monocultural plantations without any formal ‘deforestation’ being recorded.

Definitions & Certifications

One of the most common globally accepted definitions describes sustainable forestry as the  “stewardship and use of forests and forest lands in a way, and at a rate, that maintains their biodiversity, productivity, regeneration capacity, vitality and their potential to fulfil, now and in the future, relevant ecological, economic and social functions…and that does not cause damage to other ecosystems.”

But in practice, for architects or other professionals specifying timber, sustainability usually comes down to a short checklist, including how far away the timber has come from, and whether it is certified by a sustainability programme such the Forest Stewardship Council (FSC) or Programme for the Endorsement of Forest Certification (PEFC), for example.

These certification programmes have long provided a fall-back for those looking to ensure timber is from well-managed forests. And while certification programmes like these are essential in a world where timber supply chains can be murky and complex, an over-reliance on certificates perhaps also allows us to specify timber without really thinking about where it has come from.

[Note: 06/01/20 we are currently further researching the issues rasied below concerning FSC and PEFC and will update in due course,]

That would be a mistake. Greenpeace and the Rainforest Action Network, for example, call PEFC and spin-off standards like the Sustainable Forestry Initiative, “weak forest certification systems that do not guarantee responsible or sustainable forest management…these forestry certification schemes are largely developed by the forestry industry with insufficient stakeholder and NGO involvement and endorsement… Hence, we believe that any sustainability claims based on these certification schemes is industry ‘greenwash.”

FSC is generally regarded as the more robust standard, because it includes proper representation from environmental NGOs and local communities, though critics say that some unsustainable timber still manages to attain certification, and Greenpeace International has decided not to renew its relationship with FSC, citing a lack of transparency as a key reason (Greenpeace USA and others are still members). Greenpeace International still advises timber-buyers to look for FSC certification, but also to undertake “additional due diligence”.

Forests in the UK

Of course, deforestation is not so much an issue with British timber — because almost all of our primary forest was cut down a long time ago. Ancient woodland covers a paltry 1.2% of the country now – though surprisingly even that is still under threat in places.  It seems morally dubious however, to argue that loss of old growth is less of an issue here simply because it already gone. Surely our duty now should be to increase efforts to restore it.

One clear advantage of specifying UK-grown timber, apart from the smaller carbon footprint, is that you can go and see the forests where your timber is coming from, rather than just rely on a certificate to ensure sustainability across a complicated international supply chain. This represents a healthy opportunity to meaningfully reconnect with the ‘web of life’, a topic covered in a previous Green Memes article: practical outcomes of this for example might include more thoughtful and responsible decision making, innovation and insight from deeper understanding of ‘on the ground’ experience and practice, strengthened supplier relationships, improved workplace wellbeing and so on.

While the UK does not suffer from issues such as large-scale illegal logging, and forests are managed to relatively high standards (Andrew Heald, technical director with UK industry group Confor, pointed out to us that around 80% of UK timber comes from forests certified to FSC – one of the highest percentages in the world) at least within an industrial plantation model of forestry, we still need to consider its environmental impact.

At the end of March 2019, the latest date for which official statistics are available, there were 3.19 million hectares of forest cover across England, Scotland, Wales and Northern Ireland — about 13% of the country’s land area. This represents an increase of about 1% forest cover since 1998.

Most of Europe has greater forest cover than the UK as can be seen clearly from the maps below.

About half of the UK’s forested area is conifer forestry, and half is broadleaf. Sitka spruce is by far the most commonly grown conifer throughout the UK, and the most common tree overall, and it is particularly dominant in Scotland.

Sitka spruce is so popular it grows very well in our wet climate, and on poor soils, and produces a softwood that is relatively easy to cut and process. It has often been grown in large, even-aged monocultural blocks across open countryside.

For anyone who loves the natural world, it is hard to love these forests in the way we might love a native woodland: they often form dark, dense and monotonous blocks, and leave vast unsightly scars on the landscape when they are clear-felled (there is now a spirited pushback in Ireland against these types of plantations, the planting of which has been incentivised by the country’s climate policies).

In the past, conifer blocks were often planted on peaty soils, releasing large amounts of carbon in the process. And this type of plantation forestry has also been implicated in the siltation and acidification of local watersheds.  But clear-fell spruce plantations are a highly efficient way to produce marketable timber, and environmental standards in UK forestry have improved significantly over the years, with more careful attention now paid to local water quality, biodiversity, and protecting local landscapes.

When we spoke to Andrew Heald, he stressed that UK forest management has improved significantly since huge blocks of even-aged conifers were planted on open hillsides in the 1970s and 1980s, with more emphasis now placed on creating natural-looking forests, with smaller blocks of trees, of different ages, and more native species in the mix. But, he said, it just might take a couple of decades before we start to see the fruits of this shift in the landscape (he mentioned the planting plan for the recently announced Doddington North Afforestation Project as an example of a model new plantation).


Of course, the biodiversity and ecology of timber plantations is still only a shadow of that of an old, undisturbed, native woodland. How does the ecology of plantation woodland differ from woodland less intensively managed – or largely unmanaged? For example, according to one source, a sessile oak tree in the UK can support 423 different insect species, and a silver birch can support 229. A spruce, by comparison, can support 37. Even this only begins to hint at what makes a natural[1], long-established and undisturbed forest so ecologically-rich and complex.

[1] We are aware that the word ‘natural’ is flawed when it comes to forests, given the extent to which they have been modified by humans over time, such as through the introduction of non-native species. But we simply use it as a proxy for ‘as natural as possible’.

Talking Trees

For example, we have only in recent decades begun to understand the complexity of how communities of trees use a vast underground network of fungal filaments in an undisturbed, natural forest to exchange water, nutrients, sugar, chemical messages and hormones: to communicate with each other, as forest scientist Prof Suzanne Simmard, who first discovered these connections, eloquently expresses it in her TED Talk. 

“You see, underground there is this other world, a world of infinite biological pathways that connect trees and allow them to communicate and allow the forest to behave as though it’s a single organism,” she says.

Credit: CNN https://edition.cnn.com/2017/02/07/world/secret-life-of-trees/index.html

Trees share their excess resources with other trees that may be stressed or more in need — they even share with other species. Some mature trees are particularly important in this network, Simmard says. “We call those hub trees, or more fondly, mother trees, because it turns out that those hub trees nurture their young, the ones growing in the understory. In a single forest, a mother tree can be connected to hundreds of other trees.”

She continues: “Forests aren’t simply collections of trees, they’re complex systems with hubs and networks that overlap and connect trees and allow them to communicate… But they’re also vulnerable, vulnerable not only to natural disturbances like bark beetles that preferentially attack big old trees but high-grade logging and clear-cut logging. You see, you can take out one or two hub trees, but there comes a tipping point, because hub trees are not unlike rivets in an airplane. You can take out one or two and the plane still flies, but you take out one too many, or maybe that one holding on the wings, and the whole system collapses.”

Simmard says there is much more that we still have to learn about how forests work. “There’s so much more going on in forests than we’re able to actually understand using the traditional scientific techniques.”

When we showed a draft of this article to writer and eco-philosopher Ginny Battson, she commented that Suzanne Simmard’s work, “has certainly thrown up all kinds of ethical questions about our modern relationship with trees and all interconnected species”.

This is not to reduce the discussion on forests to a polarised comparison between favouring unmanaged native forests versus timber plantations. Also, the forestry industry argues that highly efficient plantation production helps to protect native woodlands by removing the economic pressure to exploit them. This sounds like a self-evident truth: yet, our countryside, our ecosystems, and wildlife are still in crisis – from our inability to provide enough ecological space for non-human life and the sheer, and growing scale of our consumption of resources.

Given industrialised human society is the cause of the planet’s sixth global extinction [1], it is critical that we strive to understand the ecological differences between different forms of productive (i.e. extractive) forestry and the vast complexity arising in intact old growth forest ecosystems. We need a well-informed industry, and we need non-commercial platforms for meaningful discussion of the safest routes to sustainable patterns and levels of use of forest products. In a time when we desperately need to stabilise the planet’s climate and ecosystems these discussions represent part of an urgent, and yet, a very long-term repair and restoration project for humanity.

[1] Science Advances  19 Jun 2015: Vol. 1, no. 5, e1400253 DOI: 10.1126/sciadv.1400253

The carbon question

Should we seek to significantly expand forestry in the UK in order to grow much more of our own construction material? Is this a sensible response to the climate crisis: land use has changed significantly over time and with it, the patterns of naturally embodied carbon stored in the landscape, and the way carbon flows into the built environment, have changed also.

If you want to read what the Committee on Climate Change (CCC) propose for what they term “radical improvement” in how we use land in the UK, see here. The way growing and harvesting biomass relates to UK action for climate change mitigation is also covered by the CCC in this report, which considers that “managing biomass stocks is an important component of global climate mitigation strategies…and that sustainably harvested biomass can play a significant role in meeting long-term climate targets, provided it is prioritised for the most valuable end-uses.”

The migration of naturally embodied carbon over time is the focus of a 2013 UK study,[1] titled ‘Mapping ecosystem service and biodiversity changes over 70 years in a rural English county’. The study highlights that changes in where carbon is ‘stored’ e.g. becoming less widely distributed in “unimproved” pasture (i.e. more natural and species-rich grasslands) due to agricultural intensification, and more concentrated into fragmented woodland ‘hotspots’ (an almost equal gain in carbon). It also highlights how natural carbon stocks may become more (or less) vulnerable to changes in the climate, and sensitive to the way we extract and use natural resources. ‘Managing’ productive carbon sinks in the anthropocene  is going to be an onerous responsibility, one perhaps not well suited to our current capitalist free market arrangements.

[1] Jiang, M. , Bullock, J. M. and Hooftman, D. A. (2013), Mapping ecosystem service and biodiversity changes over 70 years in a rural English county. J Appl Ecol, 50: 841-850. doi:10.1111/1365-2664.12093

However, it is very important to be aware that, given the critical importance of long term storage of carbon away from the atmosphere, we have a realistic view of the risks of any large scale ‘carbon store’, for example a recent sobering article pointed out that, due to annual variations in forests fires, extreme weather and pests:

 “Canada’s forests have fluctuated from being a net sink of 115 million tonnes of carbon in 1992 to a net source of 221 million tonnes of emissions in 2015, with a wide range of results in years in between.

“When the effects of natural carbon emissions and events like forest fires are accounted for, Canada’s forests still do not sequester four times more carbon than Canada emits [as the country’s Conservative party leadership candidate Kevin O’Leary claimed], nor does our land store 20 to 30 percent more carbon than we emit. Canada should certainly continue to pursue improved forestry and farming practices and preservation of our natural wetlands as means of increasing carbon storage, but we cannot claim that these carbon sinks eliminate the need to reduce our fossil fuel use and industrial emissions. To have any hope of meeting our Paris climate commitments, we will need to do both.”

With climate change leading to increasing temperatures, drought, fire risk, pest damage plus increasing human demand for land the strain on forests will only grow, adding to the risk of losing carbon stored in ecosystems and human infrastructure such as buildings and external structures.

The Carbon Cycle of Forests:

The original fully referenced text and illustration can be found here (accessed 26.10.19)

Biodiverse, unmanaged old-growth woods store the most carbon of all forest types, but once these forests get old they can’t add significantly new amounts of carbon every year, so they generally reach an equilibrium, losing (through decay) and gaining (through new growth) roughly the same amount of carbon each year.

Generally speaking, young fast-growing plantations remove more carbon from the atmosphere each year than old forests, because they are constantly full of fast-growing young trees. But old natural forests generally store more, because they are full of undisturbed biomass.

Ecologists, and many foresters, may agree that we shouldn’t anyway be harvesting timber from old forest  (even though Europe’s last great old forests are still under threat) and that instead these ecosystems should be allowed to grow old and support biodiversity, with timber only being harvested from plantations and managed semi-natural forests. However, there still remains the question of how much land we should set aside for ecological restoration if we are serious about restoring the Earth’s living systems.

In a radically ambitious proposal, the legendary biologist E.O Wilson suggests we set aside half the planet for this purpose. This would certainly focus our minds on efficient use of what would probably become a more limited land resource for plantation forestry and timber production. Whilst audacious, such a proposal does seem to fit with what we know about the scale and risk of the rapidly unfolding biodiversity and climate change catastrophe. (A recent study, however, found that this proposal would affect over one-billion people, and warned that social and environmental justice must be at the forefront of conservation).

Valuing ecosystem services

Healthy forest ecosystems are ecological life-support systems. Forests, particularly healthy natural forests, perform a range of functions that are beneficial to human society and well-being. These include:

  • Nutrient cycling
  • Climate regulation
  • Raw materials
  • Erosion control
  • Waste treatment
  • Recreation
  • Food production
  • Genetic resources
  • Soil formation
  • Water supply
  • Disturbance regulation
  • Water regulation
  • Biological control
  • Cultural

There is an increasing focus in the conservation movement on emphasising the ‘ecosystem services’ provided habitats such as forests in order to build a case for their protection, as well as on the so-called ‘natural capital’ that provides these services. It is widely acknowledged, for example by the US Forestry Service, that, “many of these ecosystem ‘goods and services’ are traditionally viewed as free benefits to society, or ‘public goods’— for example wildlife habitat and diversity, watershed services, carbon storage, and scenic landscapes. Lacking a formal market, these natural assets are traditionally absent from society’s balance sheet; their critical contributions are often overlooked in public, corporate, and individual decision-making.”

According to this mindset, it is the failure of our economic system to properly value forests that render them susceptible to development pressures and conversion, not necessarily a lack of growth in wood and fibre products markets, and recognizing forest ecosystems as natural assets with economic and social value can help promote conservation and restoration of ecosystems and more responsible decision-making without needing to rely on expanding extractive-intensive market sectors.

Ultimately while we believe this ‘ecosystem services’ mindset is useful for understanding the ways in which the living world benefits human society, it is also limited: ultimately if we are to save it, we need to value and respect the natural world outside of any utilitarian value it provides humanity, perhaps by even prescribing it rights of its own. (others have argued that the ‘natural capital’ argument is actively counter-productive to conservation.

As previously noted, the primary drivers for forest clearance globally (and historically in the UK) are still increases in demand from agricultural products (food, feed and bioenergy). Future industrial scale (and ‘offset-driven’) bioenergy production will mean increasing climate- and biodiversity-damaging effects from its land-use change impacts.  We clearly need better carbon mitigating bioenergy policies as we simply cannot safely pursue such misguided policy at industrial scale.

Carbon storage – the long view

What the timber industry is increasingly arguing is that, by periodically harvesting trees and using them to make long-lived wood products like timber for construction (let’s call this the ‘forestry-wood products system’), forests keep growing — sucking up carbon — while storing it ‘safely’ away in ‘long-lived’ wood products, and that this provides a sustainable basis to promote ever greater use of wood products. These sort of arguments increasingly appear in timber industry marketing.

However, carbon leaks out of the ‘forestry-wood products’ system at various points in ways that differ in nature, timing and scale compared to undisturbed forest ecosystems: from the soil when trees are harvested; from the decay or burning of forestry harvesting and product processing wastes; from of any of the wood that is sold for fuel; and ultimately when wood products themselves decay or are used as fuel at end of life; or the products, building assemblies or even entire buildings catch fire — an increasingly significant risk as the climate warms, particularly in densely built, fire-prone regions like California[1].

What happens to the carbon embedded in harvested wood products – and when?

Whilst there have been plenty of in-depth studies into land-use change, carbon-storage in wood products and their subsequent (probably very slow) decay in landfills, there have been few looking at the fate of cleared wood and its stored carbon following forest clearance, over time. An insightful paper[2] in 2012 used the IPCC ‘production accounting approach’ to temporally describe how much above-ground biomass carbon will be released to the atmosphere or remains stored in forest products and landfills after clearing a hectare of forest”. They carried out this exercise for 169 different countries. The paper, titled ‘Timing of Carbon Emissions from Global Forest Clearance’ modelled carbon storage over 100 years, and the 30-year period generally considered to be relevant for policy making.

[1] Note that the UK has a Woodland Carbon Code that aims to ensure proper carbon accounting in forestry.

[2] Earles, J. & Yeh, Sonia & Skog, Kenneth. (2012). Timing of carbon emissions from global forest clearance. Nature Climate Change. 2. 682-685. 10.1038/nclimate1535.

Figure 1 Image of wood buried in a landfill 46 years ago. Source: Ximenes, F A, Gardner, W D and Cowie, A L (2008), The decomposition of wood products in landfills in Sydney, Australia, Waste Management, 28, 2344-2354.

Environmental campaigners and some scientists are now calling not for a global expansion of plantations to store carbon, but for native and semi-native forests that are simply allowed to grow, expand and  be ‘left alone’ (termed ‘proforestation’). Some leading scientists have also recently criticised reforestation commitments in tropical and subtropical countries for and over-reliance on plantations rather than natural forests.

And while there is still debate in the scientific literature over how different forest management systems, and their products, sequester and store carbon, the “science” remains at risk of being used captured by vested interests for their own gain.

“Wood buildings store carbon that otherwise would have returned to the atmosphere as those trees died and began to decay,” read one statement in a rather breathless article published in the New York Times recently. But this is the kind of breezy claim anyone who understands carbon flows in forests can easily pick apart: it is entirely possible, even probable, that if left to grow old those trees would still be standing long after any wood products created from them had decayed, or been incinerated.

A paper published last year by forest scientists directly challenged this type of thinking. “Pacific temperate forests can store carbon for many hundreds of years, which is much longer than is expected for buildings that are generally assumed to outlive their usefulness or be replaced within several decades,” wrote its authors.

The evidence, as best as we can summarise it seems to show that both old-growth forests and long-lived wood products are both good places to store carbon (so long as no old growth forest or peat ecosystems are lost in the production of the wood, the forest is well managed, and the lifespan of the timber products is long).

The IPCC’s recent ‘Climate Change and Land’ report acknowledges the aspect of thinking longer term, stating: “Using harvested carbon in long-lived products (e.g., for construction) can represent a store that can sometimes be from decades to over a century while the wood can also substitute for intensive building materials.” But the IPCC also emphasises the need to protect and grow natural forests, stating: “Maintaining and increasing forest area, in particular of native forests rather than monoculture and short-rotation plantations, contributes to the maintenance of global forest carbon stocks,” and warns that any conversion of mature native forests to managed plantations can result in “large reduction in carbon stocks”.

Here we emphasise the evidence for just how good old native forests are at storing carbon, and the pressing need to protect and expand them, not to create a false choice between timber plantations and old growth woods, but because you are much less likely to hear about the latter in timber industry marketing, and because as well as storing large amounts of carbon, native undisturbed forests can also contribute significantly to the repair and renewal of the natural world.

Uncertainty, Risk and Time

The science of how we measure the flow of carbon in forests and their products is not well understood outside of expert circles, and we do not yet have widely agreed, standardised ways of comparing different forest and forest-product systems, which are highly complex and variable. Until we do, there is a real risk of vested interests misrepresenting “the science” for commercial, political, academic and/or ideological advantage.

The literature is full of studies that compare how forest management regimes remove and store carbon. And the results of these studies can vary widely depending on a huge range of factors, including: whether or not soil carbon is measured; the variable and often undocumented burning of waste timber; the local climate; tree species; forest type; the type of harvesting regime; specifics of the forest product harvested; and where you draw the boundary of your measurements.

When we asked Michael Jennings, at the Department of Geography at the University of Idaho (who is researching how climate change will affect future landscapes) about uncertainty, models and the hard-to anticipate effects of climate change on forest, he told us:

“The bottom line is that until we have a spatially explicit modelling capability to determine the current and future potential for the probable carbon cycle of an area of interest, we are not going to be able to really answer the question of how quickly carbon is likely to be sequestered by forests, and, critically, how long it is likely to remain sequestered. The “how quickly” part of this is the most critical right now, when we are beyond the worst case scenario for atmospheric carbon emissions. Is it going to make a difference if we have to wait 20 years for sequestration by the trees to be taking place at the scale and rate that is needed? Regarding trade-offs, many of the uncertainties of the above apply. At the same time there is no doubt that the maintenance of ecosystems for biological conservation and, in our own interests, the maintenance of ecosystem services, is vitally important. This is where the wicked part of the problem gets even more wicked. I wish I had a silver bullet solution for you.”[1]

Recently a research paper published by Science made dramatic claims about the amount of carbon that could be sequestered globally through the extensive creation of new tree-based ecosystems, claims quickly repeated in the press and used to justify greater use of forest products. It is healthy to see the robust challenge and counter-challenge to the findings of this paper: we need to allow time for a consolidation of experts’ understanding of such impactful ‘natural climate solutions’ to settle down before rushing ahead, despite our desperation to identify large-scale actions to ‘save ourselves’. Ecosystem repair and creation is a long term, complex project for humanity – fossil-fuelled climate change has a far earlier deadline for action.

At least, relatively speaking, using timber in buildings seems to be a better way to keep carbon out of the atmosphere than burning biomass or using it for pulp, paper, or short-lived timber products – although of course any economically viable plantation needs a range of co-products for timber that isn’t of construction grade. It would seem beneficial for the UK forestry industry to shift its focus towards longer-lived products (designed and used for longer lived building assemblies) certainly if it wants to justify its products as a long-term carbon storage solution (60% of the output of large UK sawmills is still for fencing, packaging and pallets, according to the latest figures.

“It isn’t just the forest industry that needs to change – it is architects and specifiers,” says Andrew Heald. “I regularly see stud walling and non-load bearing timber spec’d at C24 [timber strength class] – there is simply no need for such a high strength grade of timber to be used. If a lower spec was used then more UK timber would be used in construction.” Wood Knowledge Wales is one group that has been pioneering the use of lower-grade UK timber for structural purposes.

Arguably the whole way we now use timber in buildings needs to change. We need to apply both building craft and science and refine and innovate practices with rigour and transparency. Perhaps the most immediate task is to better align design and specification with forest resources and carbon flows, over long time periods: this means aligning the service life of timber products with the source forests’ harvesting periods. A cladding board, sawn from an eighty-year old tree needs to last at least 80 years before re-entering the atmosphere as CO2 and CH4. This brings in issues of inherent/artificial durability, designing components (and the assemblies they are in) for longevity, the risk of fire of course as well as guarding against uses that are susceptible to fashion and whim.

[1] Full email response from Michael Jennings, UOI

Can stored carbon make my building carbon negative?

With sustainable building (or perhaps appearing to build sustainably) becoming more in vogue among the mainstream building sector, there has been tendency for designers to claim their buildings are “carbon neutral” or “carbon negative” by subtracting the carbon stored in the timber from the embodied energy of the build.

The veracity of such claims are questionable, and there is no widely agreed methodology for factoring stored carbon into life cycle embodied carbon. The Inventory of Carbon and Energy (ICE), an ‘open-book’ embodied carbon and energy database for building materials, which measures carbon emissions “from cradle to (factory) gate”, does not count stored carbon. It states: “The inclusion or exclusion of sequestered carbon is a complex discussion. The present authors do not believe that this data should be included in the data for cradle to gate. Without including the end of life stage it is difficult to justify.” 

ICE also states of all the major building materials, the embodied carbon and energy of timber is the most difficult to quantify (this is not to say it is worse, of course). The reasons for this include a lack of detailed studies on timber within the UK, variations in moisture content of trees, variations in the consumption of total energy to manufacture the same product, and variations in the fuels used for timber processing. ICE also says that biomass may only be considered to be “carbon neutral” if it comes from a “sustainably managed forest” .

Meanwhile in their professional standards and guidance document, ‘Whole life carbon assessment for the build environment’[1], RICS state that carbon stored in timber should only be counted if the carbon assessment includes end-of-life stage carbon, and the timber is certified FSC, PEFC or equivalent. (Note: based on reasons we have outlined in the main article, we would question whether PEFC certification particularly is sufficient to guarantee the sustainability of timber).

The end of life stage carbon means that we must also consider the stage where the building is eventually demolished, because this is the point that which the timber may be subject to decay or incineration, and thus at risk of releasing its stored carbon back to the atmosphere. According to RICS, timber panels have an expected lifespan of 30 years and will likely need to be replaced at least once.

When eventually the building does reach the end of it life, RICS say that 25 % should be assumed to be landfilled and 75 %  incinerated with energy recovery. All is therefore returned to the atmosphere according to this assumption, highlighting the pressing need to develop methodical systems for the recovery, reuse and recycling of construction timber if we are to assume that carbon remains stored for long periods of time (Andrew Heald pointed out to us that MDF and chipboard mills are using increase levels of recycled fibre). RICS give the assumed life (the reference life of a domestic building) to be 60 years, however, the current demolition rate of 0.5% suggests this may be closer to 200 years in practice.

Free Markets, Silver Bullets and Offsetting

It should not be the “free market” that determines the balance between different forms of forestry. Ultimately, we must become more informed and think more critically about the kinds of forests we want to create, how we want to use them, and not get sucked into treating the specification of timber as a panacea or techno-fix for removing and storing carbon and while expecting to create ecologically, socially and economically viable ‘sustainable forests’ . 

 “Why should anyone expect energy companies and airlines to reduce their emissions if they anticipate being able to pay to plant trees to offset everything they emit, for the paltry price of less than 50 cents a tonne. The promises of cheap and powerful tech fixes help to side-line thorny issues of politics, economics and culture,” writes Professor Duncan McLaren, an expert in climate engineering and environmental justice, on the risks of over-reliance on trees to sequester carbon.

“But when promises that look great in models and spreadsheets meet the real world, failure is often more likely. This has been seen before in the expectations around carbon capture and storage,” he writes.

New Carbon for Old

Of course, besides the carbon that is physically stored in the wood, there are also the carbon emissions theoretically avoided when lower embodied energy materials such as timber are specified to substitute for more carbon-intensive products like fired brick, concrete or steel. Indeed, this “substitution effect” is generally regarded as being more substantial, in terms of carbon savings, than those provided by sequestration and storage.

How we describe and represent materials’ and products’ role in the carbon cycle is important in order to help us make sensible decisions as designers and specifiers. We strongly emphasise the point here that it is biological processes that sequester CO2 from the atmosphere, and not the act of building or manufacturing wood-based products. Those acts also release greenhouse gases from harvesting through to manufacture, during construction, in use and typically disposal at end of the product service life. Any benefit of reusing products after end-of-life (EOL) is not attributed in the Royal Institute of Chartered Surveyors (RICS) carbon accounting method [1], although a good reusable design strategy is of course a good thing, attractive to designers and customers.

[1]: https://www.rics.org/globalassets/rics-website/media/news/whole-life-carbon-assessment-for-the–built-environment-november-2017.pdf

We disagree that it is reasonable to describe a building or a product as ‘carbon negative’ or as ‘sequestering carbon’ – in a similar vein we also find it confusing to call the built environment a ‘carbon sink’ as there is no process actively drawing down atmospheric carbon. The only exception to the latter is where ongoing carbonisation of lime or cement based products reacts and locks in atmospheric CO2 during its service life – this issue is complex and currently under researched, and for now we do use the term ‘carbon sink’ for the flows of carbon in and out of the human ‘technosphere’.

[edited to correct axis description 15.12.19] Treating the built environment, its buildings and components as carbon ‘stores’, however, makes sound conceptual sense. Hence, we feel that a sensible way to present embodied carbon is as below, where emissions to the atmosphere are shown above the axis, and carbon embodied – or stored – is shown below the axis. The example below shows embodied CO2 from ‘cradle to gate’, not whole life cycle figure:

Carbon dioxide is emitted during the cradle to gate stage of a product’s lifecycle as shown above. Living-biomass based materials, that have sequestered carbon from atmospheric carbon dioxide during their growth, retain some of the carbon in their structure (for example with timber harvested from the forest, about 50% of the tree’s carbon is retained – ’embodied’ or ‘stored’). The graph above expresses emitted and stored carbon per cubic metre of materials.

Comparing kgCO2 emitted and stored per cubic metre (m3) of materials is quite useful, but even better would be to compare constructions where specific volumes of those materials are taken into account according to the way they are used in typical building constructions. This is the approach used in PHribbon in its calculations, volumes of materials are calculated from the U-value tab, as we see later.

The AECB housing stock model, currently being used to explore decarbonisation scenarios, suggests that reducing the ‘carbon burp’ — the up-front carbon emissions from materials and products used in construction work — could avoid the emissions of millions of tonnes of CO2. All materials that involve fossil fuel energy to extract, process, transport and install (and maintain, repair and ultimately dispose of) have associated carbon emissions. Some materials require less energy than others and some, based on living biomass ‘resources’ such as plants or trees, also form part of the world’s carbon storage stocks: these natural-fibre based materials carry some, but not all of their carbon content into the built environment – whether buildings or landfills sites.

Their carbon is stored safely or less safely for different periods of time, before it once again enters the soil or atmosphere, some materials also releasing other greenhouse gases though combustion or decomposition. Building products are rarely ‘pure’ but rather composites of different materials, and as such it is more useful to consider whole building assemblies, rather than individual types of material. Similarly, assemblies are more clearly assessed against others with similar levels of operational performance (e.g. thermal, or robustness and longevity) thus for example a thicker natural fibre-based wall assembly (possibly incurring wider foundations and wider eaves) is compared to a thinner, petroleum-based wall assembly. It is well recognised that many ‘co-benefits’ associated with living biomass-based materials — social, ecological and environmental — are inadequately factored into such assessments.

The graph below, based on the AECB PHribbon tool, illustrates a basic approach to building assembly assessment, and simply compares some of the up-front CO2 emissions (cradle to gate basis[2]) for two types of external wall insulation of the same U-value (example assumes that the wall structure behind, the adhesive and render stay the same) and three types of floating floor (glue for the vinyl system not included).

The amount of CO2 that ends up being stored as carbon in each assembly is clearly reported as is the CO2 emitted to produce that assembly.

[2] There are omissions – for example not including land-use change emissions, emissions from disturbed soils, from waste timber burnt in the forest etc. Also important to consider: in the flooring example, the cork floor combines with a woodfibre layer (e.g. made from younger trees, wood residues and plant fibre waste), as opposed to plywood (requires larger diameter, older trees) with ultimately different ramifications for the ecology of the forest/plantation source required.

Carbon storage efficiency ratio

One way to assess relative benefits of constructions might be to use a ratio of stored:emitted CO2 for each ‘stand-alone’ building assembly. Anything with a ratio better than 1 stores more CO2 than it emits.

For example

External wall insulation

EPS (expanded polystyrene) gives 0/11.47 = 0, no storage.
Cork 106.05/42.66 = 2.5, stores 2.5x as much CO2 as emitted


Vinyl/ply flaoting floor 6.14/2.56 = stores 2.4x as much CO2 as emitted

Cork/woodfibre floating floor 4.08/1.32 = stores 3.1x as much CO2 as emitted

Consuming to save the planet

Clearly using timber for many applications can be beneficial, as is increasing the carbon- storage time of timber products by making them last longer: but is it also beneficial for industries to use much more timber in absolute terms, rather than less of everything?

The nagging problem with over-zealously promoting the use of timber for carbon storage isn’t that it’s fundamentally incorrect, but that it represents an upside-down approach that seems to suggest consuming more of a product to save the planet.

The ratio of efficiency used in the examples above appears useful. In the flooring example the petrochemical based vinyl flooring is typically used requiring a plywood sheet underlayer, bringing in carbon storage i.e. ‘good’ – but is a more polluting assembly overall.

 In contrast the cork and woodfibre board ‘glue-free’ floating floor system has a higher carbon storage ratio i.e. stores carbon more efficiently per square metre. Chasing ‘mass’ carbon storage in building design could encourage inefficient usage – therefore unnecessarily increasing demand of natural living biomass-based resources, which we argue in this essay is an unsustainable approach. Carbon storage may be better enhanced through efficient resource use, plus substituting for petrochemical alternatives across more buildings.

Recently published US research based on “a rigorous approach…and a regionally calibrated life-cycle assessment for calculating cradle-to-grave forest sector emissions and sequestration” incorporated a method that tracked short-term and long-term forest wood products used in buildings, in order to determine the way carbon moved out of the forests, into and back out of this built-environment carbon sink over a hundred year period. The results support the principle of using wood efficiently in order to maximise carbon storage in forests and the built environment (as well as enhancing ecological characteristics of the source forests):

“Western US forests are net sinks because there is a positive net balance of forest carbon uptake exceeding losses due to harvesting, wood product use, and combustion by wildfire. However, over 100 years of wood product usage is reducing the potential annual sink by an average of 21%, suggesting forest carbon storage can become more effective in climate mitigation through reduction in harvest, longer rotations, or more efficient wood product usage.”[1]

We must carefully consider what it is we really need to build, then build as simply and modestly as possible, reusing and repurposing what we can and using virgin timber resources efficiently, designing for long component life, in a way that displaces as many higher embodied energy materials as possible.

We should ask how much timber our landscapes can sustainably provide at same time as providing rich, landscape-scale native forest ecosystems (not to mention other habitats and sustainable food products) — particularly given how rapidly the climate is changing. We should not start with an assumption that increased market demand for wood products is a first prerequisite or a goal.

[1] Tara W Hudiburg et al 2019 Environ. Res. Lett. 14 095005 Published 23 August 2019

Need or Greed?

The UK government has committed to building 300,000 homes each year up to 2025 and timber advocates argue that if we need to build these homes, would it not be better to build them with UK timber, not fired brick and steel, and store carbon in those homes?

It would probably be better in those circumstances, yes. But the goal of building 300,000 homes a year is based is arguably based in the logic of a speculative, market-driving housebuilding industry, rather than a rational assessment of how to meet human need in an era of climate breakdown. We will never build a truly sustainable building sector, or society, if that is our starting point. England, for example, has an estimated 200,000 empty homes,  enough to comfortably house its homeless population of 277,000 .(A GreenMemes article has previously looked at greenhouse gas emissions scenarios arising from different levels of new house building versus retrofit.).

A report published in August by the UK Collaborative Centre for Housing Evidence concluded that the UK’s housing crisis is not a problem of numbers, but one of affordability and quality. There is sufficient supply to meet demand, it found, but people simply cannot afford to buy. Building 300,000 more homes a year will not lead to a significant drop in prices, but it will lead to more empty homes, the report concluded. Is building empty timber homes really the best way to store carbon? Are we using a scarce and energy hungry resource (the UK housing stock) wisely? It is too rarely that such questions are discussed – with some notable exceptions, including a recent article in The Land magazine, which stated:  “The major issue is not the amount of housing relative to the number of households but its distribution.” And similarly economist Ann Pettifor writes that housing need is best described as a “lack of affordability, not a lack of supply”.

The same is true of the argument that we need more plantation forestry because global timber demand is set to quadruple by 2050 — this implies that we should simply accept our current consumption-driven cultural and economic value system, and to respond to it in the least damaging way, when really our first job should be to start the much deeper work of dismantling our current value system and building a new one. While much of this increased demand for timber is expected to come from poorer countries, perhaps developed countries can compensate by using less, in the interest of a just-transition? Using market demand as the principle starting point for how shape our landscapes will only ever, at best, be an exercise in damage limitation, at best, for the natural world.

The connected living self

In her book Doughnut Economics, the radical economist Kate Raworth provides an interesting anecdote that neatly illustrates how we have come to see forests, and all of natural world.

She quotes the Iroquois Onondaga Nation chief Oren Lyons, from an address he gave at the University of Berkley in 2005. “What you call resource we call our relatives,” he said. “If you can think in terms of relationships, you are going to treat them better, aren’t you?”

Thinking & acting radically

Ultimately, the message that our research into this topic has crystallised in our minds is that we need to think more deeply than the purely technical level about such issues, to seek to expand our whole relationship with the biosphere. This starts with considering the kind of deep cultural changes that will enable us to see ourselves as part of the web of life, rather than letting market demand for carbon storage be our primary guiding factor.  A change in our fundamental values combined with a better understanding of key ecological and carbon flux accounting concepts, ever improving modelling tools and design techniques is long overdue.

How might our models of forestry look if they were based around a sense of interconnectedness with, the living world, and a deep desire to facilitate its renewal and regeneration?

There are alternative ‘close to nature’ models of forestry that are based around allowing more natural forest conditions, with more native species and great diversity. These include continuous cover forestry, under which only individual trees are harvested to maintain forest structure, and the US Forest Service approach to harvesting, which is based on natural regeneration and harvesting that mimics natural disturbances.

But these often require more detailed planning, more selective harvesting, and can be more labour intensive, and more expensive. Might we start to re-examine their viability for timber production in the UK if our primarily value towards the natural world was one of connectedness? Or how might the case for maximising timber production from highly efficient plantations — to leave some natural forests undisturbed — fit into such a world view?

Equally, how might we specify and use timber if we adopted this sort of value system instead of one based only on nature-as-resource? Perhaps we might aim to use it as efficiently and with as much care as possible, designing our buildings for long-life and easy disassembly. Could our first call for timber products, before the harvesting of forests, become the careful salvaging, reuse and recycling of existing timber products? (Which, of course, will mean these store carbon for longer). Community Wood Recycling says that 60% of UK wood was recycled as of 2011, but most goes to low-grade uses like chipboard and MDF, wood fuel, animal bedding, or mulches and coverings.

The group also points out: “A big challenge with construction wood waste is that it is very variable. It consists of all sorts of different wood-type waste, including off-cuts of solid wood, broken pallets, bits of laminated chipboard, plywood, MDF and preservative-treated off-cuts.” Perhaps a community led, bottom-up approach is appropriate – for example where managed multi-purpose local storage/repurposing facilities allow for time for uses to be found by local people?

Credit: Community Wood Recycling website


Radical Efficiency

A new report by the World Green Building Council[1], and their general strategic advice as to ‘how do we make buildings green’ has been expanded upon by the architect Lloyd Alter on his green design website Treehugger.com, might provide some practical ways forward for building designers thinking about the questions we have outlined in this essay.

The graph below outlines an approach to building that starts with building nothing as the first port of call, followed by building less. This does not mean denying ourselves essential housing or infrastructure, but it does mean asking what essential is, exploring alternatives, and making the most of our existing resources — including by reusing and recycling them — before building from scratch using new resources.

Credit: World Green Building Council

In this framework, relying on technologies becomes one of the last ports of call, rather than the first. We would like to see designers, builders, manufacturers and academics explore in their work the following concepts:

  • Respect for life. Both human and non-human life: the principle to underpin all of the above (our previous Green Memes article [1] explored the relationship between the built environment and the natural world in detail)
  • Sufficiency. Asking if it is really needed, and if there are any alternatives. Lateral thinking, more modesty and less profligacy.
  • Simplicity. Designing and building as simply as possible — true value engineering or “integrated design”. See here for more.
  • Efficiency. Use natural biomass resources respectfully and efficiently to widely substitute for higher embodied carbon materials and products. Using as few resources/materials as possible to achieve the design. This includes achieving very low operational energy.   
  • Circular economy. Explore circular design approaches. Design realistically for reuse & disassembly, be open about your assumptions for the post end of life stage of buildings and products [2], in order to facilitate wider discussion and development.

[1]: https://www.aecb.net/wp-content/uploads/2019/05/Green-Memes-3.pdf

[2]: https://www.rics.org/globalassets/rics-website/media/news/whole-life-carbon-assessment-for-the–built-environment-november-2017.pdf

One final suggestion: go and spend time in the woods, in forests of all types, from plantation monocultures to undisturbed native woodland. Go and see where the timber you use comes from, to rebuild that direct connection between the materials we use and the planet that provides them, but also just to observe forest processes at work, and experience what the Scottish-American naturalist John Muir said was “the clearest way into the universe” — spending time in a forest. Our native woods are among our most rich, complex and wondrous ecosystems.

The topics covered here are complex and sometimes controversial. If you have constructive criticism you would like to share with the AECB after reading this article (please try to keep it succinct), send it with ‘GreenMemes4’ in the subject field to contactus@aecb.net.

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