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How to Make Biomass Energy Sustainable Again

From the Neolithic to the beginning of the twentieth century, coppiced woodlands, pollarded trees, and hedgerows provided people with a sustainable supply of energy, materials, and food.

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Image: Pollarded trees in Germany. Image: René Schröder (CC BY-SA 4.0).

How is Cutting Down Trees Sustainable?

Advocating for the use of biomass as a renewable source of energy – replacing fossil fuels – has become controversial among environmentalists. The comments on the previous article, which discussed thermoelectric stoves, illustrate this:

  • “As the recent film Planet of the Humans points out, biomass a.k.a. dead trees is not a renewable resource by any means, even though the EU classifies it as such.”
  • “How is cutting down trees sustainable?”
  • “Article fails to mention that a wood stove produces more CO2 than a coal power plant for every ton of wood/coal that is burned.”
  • “This is pure insanity. Burning trees to reduce our carbon footprint is oxymoronic.”
  • “The carbon footprint alone is just horrifying.”
  • “The biggest problem with burning anything is once it’s burned, it’s gone forever.”
  • “The only silly question I can add to to the silliness of this piece, is where is all the wood coming from?”

In contrast to what the comments suggest, the article does not advocate the expansion of biomass as an energy source. Instead, it argues that already burning biomass fires – used by roughly 40% of today’s global population – could also produce electricity as a by-product, if they are outfitted with thermoelectric modules. Nevertheless, several commenters maintained their criticism after they read the article more carefully. One of them wrote: “We should aim to eliminate the burning of biomass globally, not make it more attractive.”

Apparently, high-tech thinking has permeated the minds of (urban) environmentalists to such an extent that they view biomass as an inherently troublesome energy source – similar to fossil fuels. To be clear, critics are right to call out unsustainable practices in biomass production. However, these are the consequences of a relatively recent, “industrial” approach to forestry. When we look at historical forest management practices, it becomes clear that biomass is potentially one of the most sustainable energy sources on this planet.

Coppicing: Harvesting Wood Without Killing Trees

Nowadays, most wood is harvested by killing trees. Before the Industrial Revolution, a lot of wood was harvested from living trees, which were coppiced. The principle of coppicing is based on the natural ability of many broad-leaved species to regrow from damaged stems or roots – damage caused by fire, wind, snow, animals, pathogens, or (on slopes) falling rocks. Coppice management involves the cutting down of trees close to ground level, after which the base – called the “stool” – develops several new shoots, resulting in a multi-stemmed tree.

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Image: A coppice stool. Credit: Geert Van der Linden.

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Image: A recently coppiced patch of oak forest. Credit: Henk vD. (CC BY-SA 3.0)

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Image: Coppice stools in Surrey, England. Credit: Martinvl (CC BY-SA 4.0)

When we think of a forest or a tree plantation, we imagine it as a landscape stacked with tall trees. However, until the beginning of the twentieth century, at least half of the forests in Europe were coppiced, giving them a more bush-like appearance. 1 The coppicing of trees can be dated back to the stone age, when people built pile dwellings and trackways crossing prehistoric fenlands using thousands of branches of equal size – a feat that can only be accomplished by coppicing. 2

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Maps: The approximate historical range of coppice forests in the Czech Republic (above) and in Spain (below). Source: “Coppice forests in Europe”, see 1

Ever since then, the technique formed the standard approach to wood production – not just in Europe but almost all over the world. Coppicing expanded greatly during the eighteenth and nineteenth centuries, when population growth and the rise of industrial activity (glass, iron, tile and lime manufacturing) put increasing pressure on wood reserves.

Short Rotation Cycles

Because the young shoots of a coppiced tree can exploit an already well-developed root system, a coppiced tree produces wood faster than a tall tree. Or, to be more precise: although its photosynthetic efficiency is the same, a tall tree provides more biomass below ground (in the roots) while a coppiced tree produces more biomass above ground (in the shoots) – which is clearly more practical for harvesting. 3 Partly because of this, coppicing was based on short rotation cycles, often of around two to four years, although both yearly rotations and rotations up to 12 years or longer also occurred.

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Images: Coppice stools with different rotation cycles. Credit: Geert Van der Linden.

Because of the short rotation cycles, a coppice forest was a very quick, regular and reliable supplier of firewood. Often, it was cut up into a number of equal compartments that corresponded to the number of years in the planned rotation. For example, if the shoots were harvested every three years, the forest was divided into three parts, and one of these was coppiced each year. Short rotation cycles also meant that it took only a few years before the carbon released by the burning of the wood was compensated by the carbon that was absorbed by new growth, making a coppice forest truly carbon neutral. In very short rotation cycles, new growth could even be ready for harvest by the time the old growth wood had dried enough to be burned.

In some tree species, the stump sprouting ability decreases with age. After several rotations, these trees were either harvested in their entirety and replaced by new trees, or converted into a coppice with a longer rotation. Other tree species resprout well from stumps of all ages, and can provide shoots for centuries, especially on rich soils with a good water supply. Surviving coppice stools can be more than 1,000 years old.

Biodiversity

A coppice can be called a “coppice forest” or a “coppice plantation”, but in reality it was neither a forest nor a plantation – perhaps something in between. Although managed by humans, coppice forests were not environmentally destructive, on the contrary. Harvesting wood from living trees instead of killing them is beneficial for the life forms that depend on them. Coppice forests can have a richer biodiversity than unmanaged forests, because they always contain areas with different stages of light and growth. None of this is true in industrial wood plantations, which support little or no plant and animal life, and which have longer rotation cycles (of at least twenty years).

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Image: Coppice stools in the Netherlands. Credit: K. Vliet (CC BY-SA 4.0)

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Image: Sweet chestnut coppice at Flexham Park, Sussex, England. Credit: Charlesdrakew, public domain.

Our forebears also cut down tall, standing trees with large-diameter stems – just not for firewood. Large trees were only “killed” when large timber was required, for example for the construction of ships, buildings, bridges, and windmills. 4 Coppice forests could contain tall trees (a “coppice-with-standards”), which were left to grow for decades while the surrounding trees were regularly pruned. However, even these standing trees could be partly coppiced, for example by harvesting their side branches while they were alive (shredding).

Multipurpose Trees

The archetypical wood plantation promoted by the industrial world involves regularly spaced rows of trees in even-aged, monocultural stands, providing a single output – timber for construction, pulpwood for paper production, or fuelwood for power plants. In contrast, trees in pre-industrial coppice forests had multiple purposes. They provided firewood, but also construction materials and animal fodder.

The targeted wood dimensions, determined by the use of the shoots, set the rotation period of the coppice. Because not every type of wood was suited for every type of use, coppiced forests often consisted of a variety of tree species at different ages. Several age classes of stems could even be rotated on the same coppice stool (“selection coppice”), and the rotations could evolve over time according to the needs and priorities of the economic activities.

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Image: A small woodland with a diverse mix of coppiced, pollarded and standard trees. Credit: Geert Van der Linden.

Coppiced wood was used to build almost anything that was needed in a community. 5 For example, young willow shoots, which are very flexible, were braided into baskets and crates, while sweet chestnut prunings, which do not expand or shrink after drying, were used to make all kinds of barrels. Ash and goat willow, which yield straight and sturdy wood, provided the material for making the handles of brooms, axes, shovels, rakes and other tools.

Young hazel shoots were split along the entire length, braided between the wooden beams of buildings, and then sealed with loam and cow manure – the so-called wattle-and-daub construction. Hazel shoots also kept thatched roofs together. Alder and willow, which have almost limitless life expectancy under water, were used as foundation piles and river bank reinforcements. The construction wood that was taken out of a coppice forest did not diminish its energy supply: because the artefacts were often used locally, at the end of their lives they could still be burned as firewood.

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Image: Harvesting leaf fodder in Leikanger kommune, Norway. Credit: Leif Hauge. Source: 19

Coppice forests also supplied food. On the one hand, they provided people with fruits, berries, truffles, nuts, mushrooms, herbs, honey, and game. On the other hand, they were an important source of winter fodder for farm animals. Before the Industrial Revolution, many sheep and goats were fed with so-called “leaf fodder” or “leaf hay” – leaves with or without twigs. 6

Elm and ash were among the most nutritious species, but sheep also got birch, hazel, linden, bird cherry and even oak, while goats were also fed with alder. In mountainous regions, horses, cattle, pigs and silk worms could be given leaf hay too. Leaf fodder was grown in rotations of three to six years, when the branches provided the highest ratio of leaves to wood. When the leaves were eaten by the animals, the wood could still be burned.

Pollards & Hedgerows

Coppice stools are vulnerable to grazing animals, especially when the shoots are young. Therefore, coppice forests were usually protected against animals by building a ditch, fence or hedge around them. In contrast, pollarding allowed animals and trees to be mixed on the same land. Pollarded trees were pruned like coppices, but to a height of at least two metres to keep the young shoots out of reach of grazing animals.

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Illustration: Different ways of lopping trees. Credit: Helen J. Read, see 1

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Image: Pollarded trees in Segovia, Spain. Credit: Ecologistas en Acción.

Wooded meadows and wood pastures – mosaics of pasture and forest – combined the grazing of animals with the production of fodder, firewood and/or construction wood from pollarded trees. “Pannage” or “mast feeding” was the method of sending pigs into pollarded oak forests during autumn, where they could feed on fallen acorns. The system formed the mainstay of pork production in Europe for centuries. 7 The “meadow orchard” or “grazed orchard” combined fruit cultivation and grazing — pollarded fruit trees offered shade to the animals, while the animals could not reach the fruit but fertilised the trees.

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Image: Forest or pasture? Something in between. A “dehesa” (pig forest farm) in Spain. Credit: Basotxerri (CC BY-SA 4.0).

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Image: Cattle grazes among pollarded trees in Huelva, Spain. (CC BY-SA 2.5)

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Image: A meadow orchard surrounded by a living hedge in Rijkhoven, Belgium. Credit: Geert Van der Linden.

While agriculture and forestry are now strictly separated activities, in earlier times the farm was the forest and vice versa. It would make a lot of sense to bring them back together, because agriculture and livestock production – not wood production – are the main drivers of deforestation. If trees provide animal fodder, meat and dairy production should not lead to deforestation. If crops can be grown in fields with trees, agriculture should not lead to deforestation. Forest farms would also improve animal welfare, soil fertility and erosion control.

Line Plantings

Extensive plantations could consist of coppiced or pollarded trees, and were often managed as a commons. However, coppicing and pollarding were not techniques seen only in large-scale forest management. Small woodlands in between fields or next to a rural house and managed by an individual household would be coppiced or pollarded. A lot of wood was also grown as line plantings around farmyards, fields and meadows, near buildings, and along paths, roads and waterways. Here, lopped trees and shrubs could also appear in the form of hedgerows, thickly planted hedges. 8

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Image: Hedge landscape in Normandy, France, around 1940. Credit: W Wolny, public domain.

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Image: Line plantings in Flanders, Belgium. Detail from the Ferraris map, 1771-78.

Although line plantings are usually associated with the use of hedgerows in England, they were common in large parts of Europe. In 1804, English historian Abbé Mann expressed his surprise when he wrote about his trip to Flanders (today part of Belgium): “All fields are enclosed with hedges, and thick set with trees, insomuch that the whole face of the country, seen from a little height, seems one continued wood”. Typical for the region was the large number of pollarded trees. 8

Like coppice forests, line plantings were diverse and provided people with firewood, construction materials and leaf fodder. However, unlike coppice forests, they had extra functions because of their specific location. 9 One of these was plot separation: keeping farm animals in, and keeping wild animals or cattle grazing on common lands out. Various techniques existed to make hedgerows impenetrable, even for small animals such as rabbits. Around meadows, hedgerows or rows of very closely planted pollarded trees (“pollarded tree hedges”) could stop large animals such as cows. If willow wicker was braided between them, such a line planting could also keep small animals out. 8

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Image: Detail of a yew hedge. Credit: Geert Van der Linden.

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Image: A hedgerow. Credit: Geert Van der Linden.

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Image: Pollarded tree hedge in Nieuwekerken, Belgium. Credit: Geert Van der Linden.

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Image: Coppice stools in a pasture. Credit: Jan Bastiaens.

Trees and line plantings also offered protection against the weather. Line plantings protected fields, orchards and vegetable gardens against the wind, which could erode the soil and damage the crops. In warmer climates, trees could shield crops from the sun and fertilize the soil. Pollarded lime trees, which have very dense foliage, were often planted right next to wattle-and-daub buildings in order to protect them from wind, rain and sun. 10

Dunghills were protected by one or more trees, preventing the valuable resource from evaporating due to sun or wind. In the yard of a watermill, the wooden water wheel was shielded by a tree to prevent the wood from shrinking or expanding in times of drought or inactivity. 8

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Image: A pollarded tree protects a water wheel. Credit: Geert Van der Linden.

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Image: Pollarded lime trees protect a farm building in Nederbrakel, Belgium. Credit: Geert Van der Linden.

Location Matters

Along paths, roads and waterways, line plantings had many of the same location-specific functions as on farms. Cattle and pigs were hoarded over dedicated droveways lined with hedgerows, coppices and/or pollards. When the railroads appeared, line plantings prevented collisions with animals. They protected road travellers from the weather, and marked the route so that people and animals would not get off the road in a snowy landscape. They prevented soil erosion at riverbanks and hollow roads.

All functions of line plantings could be managed by dead wood fences, which can be moved more easily than hedgerows, take up less space, don’t compete for light and food with crops, and can be ready in a short time. 11 However, in times and places were wood was scarce a living hedge was often preferred (and sometimes obliged) because it was a continuous wood producer, while a dead wood fence was a continuous wood consumer. A dead wood fence may save space and time on the spot, but it implies that the wood for its construction and maintenance is grown and harvested elsewhere in the surroundings.

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Image: Pollarded tree hedge in Belgium. Credit: Geert Van der Linden.

Local use of wood resources was maximised. For example, the tree that was planted next to the waterwheel, was not just any tree. It was red dogwood or elm, the wood that was best suited for constructing the interior gearwork of the mill. When a new part was needed for repairs, the wood could be harvested right next to the mill. Likewise, line plantings along dirt roads were used for the maintenance of those roads. The shoots were tied together in bundles and used as a foundation or to fill up holes. Because the trees were coppiced or pollarded and not cut down, no function was ever at the expense of another.

Nowadays, when people advocate for the planting of trees, targets are set in terms of forested area or the number of trees, and little attention is given to their location – which could even be on the other side of the world. However, as these examples show, planting trees closeby and in the right location can significantly optimise their potential.

Shaped by Limits

Coppicing has largely disappeared in industrial societies, although pollarded trees can still be found along streets and in parks. Their prunings, which once sustained entire communities, are now considered waste products. If it worked so well, why was coppicing abandoned as a source of energy, materials and food? The answer is short: fossil fuels. Our forebears relied on coppice because they had no access to fossil fuels, and we don’t rely on coppice because we have.

Our forebears relied on coppice because they had no access to fossil fuels, and we don’t rely on coppice because we have

Most obviously, fossil fuels have replaced wood as a source of energy and materials. Coal, gas and oil took the place of firewood for cooking, space heating, water heating and industrial processes based on thermal energy. Metal, concrete and brick – materials that had been around for many centuries – only became widespread alternatives to wood after they could be made with fossil fuels, which also brought us plastics. Artificial fertilizers – products of fossil fuels – boosted the supply and the global trade of animal fodder, making leaf fodder obsolete. The mechanisation of agriculture – driven by fossil fuels – led to farming on much larger plots along with the elimination of trees and line plantings on farms.

Less obvious, but at least as important, is that fossil fuels have transformed forestry itself. Nowadays, the harvesting, processing and transporting of wood is heavily supported by the use of fossil fuels, while in earlier times they were entirely based on human and animal power – which themselves get their fuel from biomass. It was the limitations of these power sources that created and shaped coppice management all over the world.

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Image: Harvesting wood from pollarded trees in Belgium, 1947. Credit : Zeylemaker, Co., Nationaal Archief (CCO)

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Image: Transporting firewood in the Basque Country. Source: Notes on pollards: best practices’ guide for pollarding. Gipuzkoaka Foru Aldundía-Diputación Foral de Giuzkoa, 2014.

Wood was harvested and processed by hand, using simple tools such as knives, machetes, billhooks, axes and (later) saws. Because the labour requirements of harvesting trees by hand increase with stem diameter, it was cheaper and more convenient to harvest many small branches instead of cutting down a few large trees. Furthermore, there was no need to split coppiced wood after it was harvested. Shoots were cut to a length of around one metre, and tied together in “faggots”, which were an easy size to handle manually.

It was the limitations of human and animal power that created and shaped coppice management all over the world

To transport firewood, our forebears relied on animal drawn carts over often very bad roads. This meant that, unless it could be transported over water, firewood had to be harvested within a radius of at most 15-30 km from the place where it was used. 12 Beyond those distances, the animal power required for transporting the firewood was larger than its energy content, and it would have made more sense to grow firewood on the pasture that fed the draft animal. 13 There were some exceptions to this rule. Some industrial activities, like iron and potash production, could be moved to more distant forests – transporting iron or potash was more economical than transporting the firewood required for their production. However, in general, coppice forests (and of course also line plantings) were located in the immediate vicinity of the settlement where the wood was used.

In short, coppicing appeared in a context of limits. Because of its faster growth and versatile use of space, it maximised the local wood supply of a given area. Because of its use of small branches, it made manual harvesting and transporting as economical and convenient as possible.

Can Coppicing be Mechanised?

From the twentieth century onwards, harvesting was done by motor saw, and since the 1980s, wood is increasingly harvested by powerful vehicles that can fell entire trees and cut them on the spot in a matter of minutes. Fossil fuels have also brought better transportation infrastructures, which have unlocked wood reserves that were inaccessible in earlier times. Consequently, firewood can now be grown on one side of the planet and consumed at the other.

The use of fossil fuels adds carbon emissions to what used to be a completely carbon neutral activity, but much more important is that it has pushed wood production to a larger – unsustainable – scale. [14] Fossil fueled transportation has destroyed the connection between supply and demand that governed local forestry. If the wood supply is limited, a community has no other choice than to make sure that the wood harvest rate and the wood renewal rate are in balance. Otherwise, it risks running out of fuelwood, craft wood and animal fodder, and it would be abandoned.

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Image: Mechanically harvested willow coppice plantation. Shortly after coppicing (right), 3-years old growth (left). Credit: Lignovis GmbH (CC BY-SA 4.0).

Likewise, fully mechanised harvesting has pushed forestry to a scale that is incompatible with sustainable forest management. Our forebears did not cut down large trees for firewood, because it was not economical. Today, the forest industry does exactly that because mechanisation makes it the most profitable thing to do. Compared to industrial forestry, where one worker can harvest up to 60 m3 of wood per hour, coppicing is extremely labour-intensive. Consequently, it cannot compete in an economic system that fosters the replacement of human labour with machines powered by fossil fuels.

Coppicing cannot compete in an economic system that fosters the replacement of human labour with machines powered by fossil fuels

Some scientists and engineers have tried to solve this by demonstrating coppice harvesting machines. 15 However, mechanisation is a slippery slope. The machines are only practical and economical on somewhat larger tracts of woodland (>1 ha) which contain coppiced trees of the same species and the same age, with only one purpose (often fuelwood for power generation). As we have seen, this excludes many older forms of coppice management, such as the use of multipurpose trees and line plantings. Add fossil fueled transportation to the mix, and the result is a type of industrial coppice management that brings few improvements.

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Image: Coppiced trees along a brook in ‘s Gravenvoeren, Belgium. Credits: Geert Van der Linden.

Sustainable forest management is essentially local and manual. This doesn’t mean that we need to copy the past to make biomass energy sustainable again. For example, the radius of the wood supply could be increased by low energy transport options, such as cargo bikes and aerial ropeways, which are much more efficient than horse or ox drawn carts over bad roads, and which could be operated without fossil fuels. Hand tools have also improved in terms of efficiency and ergonomics. We could even use motor saws that run on biofuels – a much more realistic application than their use in car engines. 16

The Past Lives On

This article has compared industrial biomass production with historical forms of forest management in Europe, but in fact there was no need to look to the past for inspiration. The 40% of the global population consisting of people in poor societies that still burn wood for cooking and water and/or space heating, are no clients of industrial forestry. Instead, they obtain firewood in much of the same ways that we did in earlier times, although the tree species and the environmental conditions can be very different. 17

A 2017 study calculated that the wood consumption by people in “developing” societies – good for 55% of the global wood harvest and 9-15% of total global energy consumption – only causes 2-8% of anthropogenic climate impacts. 18 Why so little? Because around two-thirds of the wood that is harvested in developing societies is harvested sustainably, write the scientists. People collect mainly dead wood, they grow a lot of wood outside the forest, they coppice and pollard trees, and they prefer the use of multipurpose trees, which are too valuable to cut down. The motives are the same as those of our ancestors: people have no access to fossil fuels and are thus tied to a local wood supply, which needs to be harvested and transported manually.

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Image: African women carrying firewood. (CC BY-SA 4.0)

These numbers confirm that it is not biomass energy that’s unsustainable. If the whole of humanity would live as the 40% that still burns biomass regularly, climate change would not be an issue. What is really unsustainable is a high energy lifestyle. We can obviously not sustain a high-tech industrial society on coppice forests and line plantings alone. But the same is true for any other energy source, including uranium and fossil fuels.

Written by Kris De Decker. Proofread by Alice Essam.

Comments

Rafael Carrascosa

Thanks Kris, amazing work

timo

Thank you! I always thought of pure aesthetical reasons why pruned trees exist. I learned something today :)

Craig Macdonald

I found this piece very instructive and inspiring. I think it is going to change the way I restore a small plot of land we now own in southern France.

I would like to point out what appears to be an error in the text of Note [11]: I believe the word “easier” found in the first sentence ought to be “harder”.

a

[14] is not linked in the article.

Martin

I was really looking forward to your announced new article and I did not get disappointed. You made a fairly good and balanced point.

I would question a bit about your relativization of the 2017 study text section. Their energy consumption is quite high, also the emission if you consider what is done with that energy. It is just used for a basic lifestyle mostly cooking, some heating, no industry. All shipping industry in the world takes 3% human CO2 emission. Also you have unfortunately no source for these interpretations you do why they are so efficient or if they really use wood like you proposed. From my limited education on that field I do not think they use these technics, for example in India at the Himalayans, big areas were cut down by poor people so radically, that erosion take over immensely. https://www.indiatoday.in/magazine/environment/story/19760515-problems-of-deforestation-and-soil-erosion-in-himalayas-819149-2015-04-04 This article suggests for example that the people there also use wood like Brazilians do, they cut it away and use it for livestock and cultivation separately.

But I recognise your intent that is telling that every culture should reconsider their wood sourcing and that pruning trees might be the way to go. Which I would actually want to agree with. I have also read that small growing trees take so much more CO2 out of the atmosphere than grown-up trees.

What you did not consider from my comment under your mentioned last article are several but I pick a few: For climate change it is relevant not only the CO2 but also the dust particles emitted from stoves. And even “clean” stoves are really dirty considering improper usage and not attentious supervision of process. http://www.soliftec.com/PhD_Ricardo_Luis_Teles_de_Carvalho.pdf

Also this article now only justifies burning wood as CO2 balanced with ecological benefits for nature. But it does not weaken my point that an electrical system will be cheaper and offer more economic opportunities than sustainable burning wood. It does not increase total energy efficiency even though you outlined an efficient local economy, the constant burning is so inefficient that you can transport a solar cell from China and have a small battery for 20 years is possibly same efficient but without the labour and the potential with illumination and electric transport. I do not have a scientifiy proof but a practical one: Solar is becoming more and more widespread even in poorest regions. The question is, if batteries will too, after economics of scale is full running.

Daniel

I’m looking forward to seeing a 20 or 30 million people conurbation heated with coppiced wood. Let’s get real. These techniques are suitable for small communities, as in villages, not for bilions of people.

Laszlo F.

I come from an area with great history and pride in sylviculture & forestry and thus I always wanted to believe that forests can only exist detached from agriculture. After all, who would like to associate themselves with smelly farms, dung and peasantry when you can dress in an all green uniform and sing hymns about nature all day? Looks like I was terribly mistaken, those dehesa landscapes are mesmerizing and seem like a bloody smart solution to combine wood production with agricultural activity. Though I imagine with all the invasive species and weeds growing everywhere it can be a pain to manage them, especially in contemporary Europe (full forest cover can mostly eliminate that problem).

@Daniel: Solve a problem today, have one less tomorrow. There are estimates that the ongoing COVID pandemic may set back the course of global urbanization by qutie a few years. Millions of people around the world won’t be moving to cities as forecasted, but remain in villages and townships and keep heating with locally produced wood. So despite the fact that pruning and coppicing scales awfully, it can have an effect in many developing areas.

kris de decker

@ Daniel

The cities we take for granted now are products of fossil fuels.

Martin

@Kris de Decker

I don’t think our cities are necessarily products of fossil fuels. Keep in mind that urbanisation is a process which started even before large scale charcoal production in middle ages began. There is reason to assume the accumulation is human wealth gathering achievement.

@Lazlo

In Germany, there is serious discussion if hedgerows should be reestablished. There is even some funding for it. Also there is a bigger plan to interconnect natural habitats, to reestablish migration, so the animal gene pool is healthy. Also I would count the established bush planting and frequent pruning along the Autobahn and rails for example as pretty much the same system. The wood gets chipped and pelleted and burnt in wood chip plants. Just because urbanisation might slow down it does not mean there is a trend reversal. But that would be needed, if you assumed that burning wood on an end consumer level is the future.

joel LeGrand

I am a big fan of the Rocket Mass Heater: A rocket mass heater is:

cheap to run. About a tenth the cost of natural gas, electric or conventional wood heat. clean. They emit about a hundredth of the smoke of a conventional wood stove. good for the environment. Less than 2% of CO2 emissions of natural gas or electric heat. Can reduce your carbon footprint as much as parking 7 cars. sustainable. It uses a renewable fuel which is easy to find and store. cheap to build. About $200 to $600. quick to build. Often built in a weekend luxuriant. Like the luxury of a heated floor but without all that standing. https://permies.com/forums/search/search/-1?match_type=all&sort_by=time&q=RMH&groupByTopic=true

Bruce Teakle

Thanks Kris for another excellent essay. I congratulate you on persisting with describing the importance of traditional wood energy use, despite some negative feedback. Considering the negative feedback, clearly from people holding genuine concerns about environmental issues, 2 issues appear to me.

The first is theoretical: understanding how the carbon cycle works. Forests are not a carbon “sink”, into which modern humans can dispose their fossil fuel CO2 emissions, they are a carbon store. Forests hold a limited capital of carbon, cycling a little as leaves rot, trees die, and new growth occurs. Every stick and leaf that is formed by plants will return its CO2 to the air, when an animal, microbe or fire harvests its energy. For the entirety of human existence, we have been part of that cycle.

Only rarely is the carbon stored as peat or charcoal for longer periods - our modern growth and climate problems come from having discovered how to burn these long-term stores.

The second is practical: if you don’t work in the forest, it’s hard to understand the practical manifestation of the carbon cycle. If you work in reforestation or in restoring the health of existing forests, the surplus of waste wood can be overwhelming.

In Australia, where most of our forests dry and burn periodically, the Aboriginal people developed sophisticated systems of cool burning that managed fuel loads and protect the forest from hot fires. White people took the traditional owners and their fire practices out of the forest, so fuel loads escalated.

Now, enhanced by warming climate, hot bushfires burn up the carbon capital of the forests, not just the cyclic accumulation, with terrible ecological and human consequences. Fixing these forests requires dealing with a huge surplus of fuel.

Clearly wood can’t provide the energy to run the huge population and rapacious lifestyles of modern humans, but it is a valid and sustainable fuel when used in a balance.

David Bourguignon

Thanks Mr De Decker for this great review of one of the remarkable achievements of the “rural civilisation” of our European forebears. I fully agree, since it is still the lifestyle of so many people on this planet, it is worth considering.

However, either coppiced or pollarded, all these trees are only what is left of gigantic forests and as such, they cannot deliver even a fraction of the ecological functions originally delivered by these ancient forests, which were major contributors to the planetary life-support system before the invention of the “rural civilisation” 10,000 years ago. (Among these ecological functions, water cycle management, for example, which is now also threatened by GHG-induced radiative forcing, putting into jeopardy this very same “rural civilisation”.)

Moreover, as you already know, heavy deforestation was not invented by humans after the discovery of fossil fuels. Europe was covered by forests after the end of the last Ice Age and agricultural practices coming from the Middle East deforested it quickly. In fact, the “industrial civilisation” invented way before the 19th century, during Antiquity, was built on top of the very same extractive principles as the “rural civilisation” (deforestation, loss of top soil and water). The situation was already terrible before the 19th century, fossil fuels simply made this more obvious by accelerating the trend.

Maybe it is time to rethink our situation in a completely different way and consider the past has very little to teach us, as we are now leaving the climate steady-state of the Holocene? I like this quote: “You could not step twice into the same river.” (Heraclitus)

All the best, and thanks again for all these stimulating reflections.

kris de decker

@ David

I am not sure what you want to say. That we should restore the planet in the state it was before humanity flourished? Why? All species leave their footprint on the landscape. The problem is that our footprint has become very destructive.

Preindustrial society was not a paradise of sustainability, and of course deforestation did happen. But fossil fuels have changed everything. They have removed all limits. For example, although there was deforestation in the past, the planet never lost all its forests because many wood reserves were simply inaccessible. Today, worldwide deforestation is a real risk, cause there are no more limits. We now have the technological means to harvest all the wood in the world. We simply could not do that before fossil fuels.

There’s a great book about this topic: Sieferle, Rolf Pieter. The Subterranean Forest: energy systems and the industrial revolution. White Horse Press, 2001.

Gregory Corning

I note that the much-hated exotic invasive Siberian elm (Ulmus pumila), which grows fast and spreads very fast here in New Mexico USA, is excellent for coppicing, based on my experiences. Maybe coppicing can be a way to manage invasive species while getting some benefit from them.

Fin Butler

@David

Also coppicing and the like doesn’t need to be done to every last bit of the landscape, and I see no reason why dense “wild” forests and coppicing practices wouldn’t exist together (in fact if I bothered to do research right now Im sure I could find examples of such).

As well, we have much better information sharing currently (and even transitioning to lower tech life style is one of the things I expect us to keep (even if we switch to other methods)) so avoiding mass deforestation is certainly practicable, especially if you remove the incentives to deforest. Agreed on Kris De Decker’s points.

Martin

@Bruce Teakle

In a healthy climate I would totally agree. But you have not considered that our stable climate is based on a huge reduction of former CO2 from our atmosphere. If we had not risen the CO2 levels by the fossil fuels we could burn almost as much forest we liked, it would return to the trees like you described and stayed in normal cycle.

But now we have the situation of climate change, with too much CO2 in the atmosphere. So one of our only solution to get it out is photosynthesis, because the oceans can only bind so much and their Carbon sink rate is pretty slow. So instead of wasting this machinery that produce potential locked Carbon in plant matter and burn it right away, it is very important to store that plant fuel as long as possible. Either in wooden buildings, or any other product that is suitable.

@Kris de Decker

I like the idea of making it uneconomical to cut trees from everywhere. I always like solutions that give strong incentives instead of permits. The European Union did great with their CO2 emission trading. I am not so sure if therefore we need to abolish our technical capabilities that are not bound to fossil fuels anymore. You say there is a direct connection, I would say this connection is not there anymore. Today we could also cut down all forests without emitting CO2. We have the means of transport, the knowledge (especially about the potential value, the demand, and by satellites where to find it) and rising demand for “green” fuels.

We needed to find another way of making forests more valueable by managing them sustainable and most CO2 effective (and maybe anti fire effective like s.o. said), instead of destroying these plants totally and their bound CO2 (the biomass). Your proposed forest and field management could be a thing to look at as legislative authority.

David Bourguignon

@Kris

What I am trying to explain is simply that looking at the past to guide the future can be sometimes misleading, in particular when dealing with complex systems which are experiencing hysteresis. There is a current theory pretending we could go back to a “rural civilisation” and this would address our issues, but it is probably another illusion.

Low-tech Magazine work is invaluable to ask questions and make us think about the past, since learning about ancient ways can allow to continue using them in specific contexts or to get inspiration from them, but IMHO this practice will not be sufficient to address the immense global challenge we are facing.

There is this strange idea that global ecosystems could support more and more humans and their continuous biomass extraction behaviors, and still function properly. Quick fact: above 20-25% deforestation (caused by rural practices and population growth in Brazil), the Amazon rainforest will turn into a savannah biome. This will happen very soon and will transform the water cycle of the Americas and beyond, putting in jeopardy all kinds of agriculture (organic or not, they need rain).

Therefore, we might have to truly change our ways, that is give up for good: - a 2M years old energy production system based on burning dead carbon - a 10k years old food production system based on deforestation then soil+water abuse by cattle&crops To sum up: we need entirely new ways of thinking and this is urgent.

@Fin

Please consider also these simple facts in your reasoning: in Western Europe, at the end of the 18th century, in a fully “organic”, “pre-industrial”, “low-tech” world, after millennia of “rural civilisation”, the land was heavily deforested and degraded. France, for example, reached its minimum forest cover ratio around 1850. However, the water cycle at the global scale was still operational, since huge intact forest biomes in the Amazon & Congo basins, South-East Asia, etc., were delivering their contributions to the planetary life-support system.

Now that people are deforesting these biomes too (as Europeans did of their own forest biomes to create their “rural civilisation”, so why are we complaining), massive disturbances in the water cycle are now happening, made worse by our GHG emissions. This is not at all a surprise and will simply render all biomass-based strategies for the future completely obsolete within a short time frame.

To sum up: we might simply have to say farewell to this “rural civilisation”, whether we desire it or not.

kris de decker

@ David

So what are these “entirely new ways of thinking”?

Do you advocate to go back to a hunter-gatherer society? Or is sophisticated technology involved?

Bart Vanden Driessche

@Kris de Decker

I think David suggests that we should move to agroforestry and maybe foodforests.

Although I think wood for heating and other uses discussed in previous articles.

It is also important to know that coppiced has other use then being burned as wood or charcoal. It was often used in buildings as construction materials and basketry. as example daisugi in Japan

There was last year a reportage on ARTE named The beauty of pollards.

Hanno Hodgkin

I’m all for deconstructing the high energy society in which we live, and really want to be able to support seemingly sustainable systems like coppice management.

But as a market gardener and small farmer my attitude is that what is grown on the land must stay on the land. My intuition says that if I grow a crop of hay and sell it to my neighbour, hence returning nothing to the soils that grew the hay, my soils will be depleted, and over time my hay crops will decline. Thus we only sell the small amount of biomass we ‘skim’ off the top of the farms nutrient and energy cycles, the rest of the biomass ‘life’ if you will, being kept on the holding.

i.e. the vast majority of the biomass of the holding musty stay on the holding or else the overall productive capacity of the farm will decline.

So, given I really want to believe in low impact forrest management techniques which can give me timber for building and wood for burning, can someone please explain. How does this not in the longer term decrease the ‘life’ of the farm/capacity/health of the soils? If I just burnt my hay every year, even if the ash was returned to the land, over time the capacity/potential of my soils would decrease and yields would fall. Why is forestry different? Can we harvest close to 100% of a coppice crop and burn it without gradually undermining the capacity of the land/soil to create biomass, without undermining the ‘life’ of the land.

David Bourguignon

@Kris

First and foremost, thinking in terms of binary alternatives (low-tech vs. high-tech is my favorite, since no one agrees on what these popular expressions mean), is not really my way of considering the sustainability debate.

Given the constraints I presented briefly previously, a new way of thinking starts with the recognition that the continuation of the rural civilisation invented 10k years ago is not a solution, but the main problem we are now facing. In fact, biomass extraction by the rural civilisation has always slowly degraded the quality of the planetary life-support system, has always fostered population growth and the advance in extractive technology (which is not at all the invention of the industrial civilisation, it has always been part of the rural civilisation, which follows the same paradigm).

To answer your question, there could be several complementary ways forward (I do not pretend to have considered them all):

  • On land, where most photosynthetic plants grow, and where forest biomes in particular manage the water cycle, which is essential for humanity activities outside of agriculture, we must operate land rewilding on a large scale, and maybe practice very-low yield extensive food production in those ecosystems (this has been experienced by humanity already, you could consider this as hunter-gatherer technology if you want).

  • At sea, where most of the animal life exists on Earth, humanity could create the conditions for photosynthetic plants to grow (eg. seaweed farms on artificial structures), defining a new frontier for a new kind of “gardening” (you could also call it hunter-gatherer technology if you want, since this is very extensive too, given the surface available and the uncontrollable growth conditions).

  • In cities, where most humanity will live in a few decades, we must develop an efficient “post-agriculture” focusing on either very high-yield intensive and controlled ecological farming (eg. aquaponics) or new forms of “breeding”, ie. insects, or “brewing” of micro-organisms (bacteria, yeast, fungus) from either byproducts of the former “farming”, or directly from atmospheric carbon dioxide, water and sunlight, using chimiolithotrophic or photosynthetic pathways (there is no sophisticated technology involved, but still many practices to be discovered yet). Of course, there will still be a few traditional farming remaining, but without cattle very probably. If you are interested, there are many studies and reports on all this.

As a conclusion, I would say that, as hunter-gatherer civilisations were overthrown by the rural civilisation 10k years ago, which transformed the planet and humanity forever, now it is probably the time for this rural civilisation to disappear:

  • Its land-based power system (from agriculture to mining) and extractive practices (from ecosystem biomass to metallic ore) can no longer address the issues humanity is facing.

  • The entire sector, from the small farmer to the large industrial agribusiness, will be probably unable to cope with the rapid change in climatic conditions which will occur in the coming decades.

This change in civilisation could allow for the development of more efficient ways of producing food and energy for a larger human population, while regenerating the planetary life-support system in the process.

@Bart

Agroforestry is certainly an interesting approach in specific contexts and on a short time frame but it is only an intermediate step and will never be part of the solution that will truly tackle our most urgent issue: the progressive degradation of the main constituents of the planetary life-support system, which allowed humanity to thrive over the past 15k years.

Regarding “food forests”, I am not sure how you define them, but if they mean “managed wild ecosystems delivering extensive low-yield food production services” I agree it could be part of the vision I briefly presented.

@Hanno

You are very right. This is what most biomass advocates do not want you to think about: no matter how you see it, their approach is extractive, not regenerative. Wild ecosystems do recycle everything in very short-distance loops, without humans involved. The argument of atmospheric circulation recycling through carbon dioxyde is part of what is now known as “the UN Kyoto Protocol carbon neutrality accounting fraud”. There is currently a huge scientific argument against this fake science approach and hope that at least EU policy will take that into consideration soon.

kris de decker

@ David

High-tech and low-tech are not “popular expressions”. They are clearly defined adjectives. I quote from the Cambridge dictionary:

High-tech: Using the most advanced and developed machines and methods.

Low-tech: Not using the most recent equipment or methods.

As for your new ways of thinking, I cannot see what’s new about it, and I am not impressed. It sounds a lot like the Venus Project.

@ Hanno

I agree. But as far as I understand, that was the case. Coppicing was local. Everything a coppice forest produced was eventually returned to the soil. Furthermore, animals were used to fertilize coppice forests. Humans ate the animals, and human waste was also returned to the soil.

Martin

What I like and highly appreciate from low-tech magazine is, that you find clever old techniques and other mind set approaches that can be reconsidered for our modern world. Nobody would really want a world where the newest tech is negated and stigmatised as the source of all the bad things. High-tech helps in many ways to have a better life on earth but these advances are all the time thematized by most media. So the low-tech side gets here a representation. But it is no decision between two poles. Our information age allows us to reconsider forgotten techniques better than ever before, when it was mostly driven by heriting from the direct ancestors or the surrounding population.

If the goal of the article is to make woods cultured sustainable, in energy sourcing matters I will suggest to at least gasify the biomass and then burn it or better use the material first for useful items like cardboard and then burn it in cogeneration. Better of course it is turned into longterm products like houses. That is really sustainable, because in our unsustainable world it is not sufficient to just do no further harm, but make it better.

Goran

Kris,

Thanks for another enlightening article.

I very much would like to recommend the work of Ben Law in England (e.g. his book “The Woodland Way”), mainly focussed on living in the forest and living off the forest. He stewards an area of mainly sweet chestnut coppice that is centuries old.

Regarding lime trees (Tilia cordata) - the fresh leaves are also edible for humans. And delicious. Unfortunately the gastronomical use has been largely forgotten. I maintain a small hedge of lime trees for salad use in our garden.

And of course the global population and energy use will go down considerably. Either in a planned way or unplanned. What is not sustainable will eventually stop…

Thanks again, Goran

David Bourguignon

@Kris

Well, with all due respect to the Cambridge dictionary, I am afraid those definitions are of little use. In fact, if I follow them, a technology of today is high-tech wrt to a technology of yesterday, but will be low-tech wrt a technology of tomorrow. Let us wait then, for what people consider bad/ugly now, to be transformed into something good/beautiful? IMHO we will need to build our sustainability strategies on firmer ground.

Your new ways of thinking” are not mine at all. These ideas and facts come from a review of the scientific literature.

I am preparing a bibliography ranging from the UK Committee on Climate Change reports, to peer-reviewed journals such as Nature and Science, to public white papers from innovative companies such as Solar Foods. I can send you this list when it will be ready, if you are interested.

BTW I had never heard about the Venus Project before. I checked the website and do not really understand your comparison, to be frank, since I am not talking about Utopian fantasies but real, existing solutions.

Thanks for this conversation. I learned a lot, but not necessarily regarding sustainable biomass. I recommend your readers to check what will come out of the coming revision of the EU biomass policies, which I hope will be inspired by the most recent recommandations of the joint declaration of the European academies of science. All the best.

kris de decker

@ Goran

Thanks !

@ David

In fact, if I follow them, a technology of today is high-tech wrt to a technology of yesterday, but will be low-tech wrt a technology of tomorrow.”

This is exactly the point. This is what Low-tech Magazine is about. Our society continues to be focused on technological progress and tech solutions, while we have long invented every technology we need. Technology has become a problem instead of a solution. We need to innovate in different ways. For example: social innovation, institutional innovation, economic innovation.

The direction you suggest doesn’t seem to include any of that. But feel free to send me any information you like.

David Bourguignon

@Kris

Thanks for this complementary info. Now I start to understand a bit better what Low-Tech Magazine is all about. Therefore, I suggest the following definitions for low- and high-tech and they have nothing to do with technologies themselves and their relative oldness/newness, since IMHO this ends up with non-operative definitions. (And your strange statement: “we have long invented every technology we need”… How do you know that, if we do not know what we do not know, as any scientist would say?)

My alternative proposal:

  • High-tech: Using a high proportion of technological innovation and a low proportion of other forms of innovation (social, institutional, economic, etc.) Example: using autonomous drones for reforestation.

  • Low-tech: Using a low proportion of technological innovation and a high proportion of other forms of innovation (social, institutional, economic, etc.) Example: using satellite Internet mobile communication to help local communities develop their own network of local native tree species nurseries, fostered by timebanking transactions.

As you noticed, I avoided on purpose focusing on the age of the technology and rather focused of how much of it is used in developing the entire solution. Maybe better examples could be found… What do you think?

Last but not least: “The direction you suggest doesn’t seem to include any of that [other non-technological forms of innovation].” Well, on the very contrary… They are intensely low-tech, but still relying on tiny pieces of technology that are yet to be developed. Therefore, we will need to invent a few new technologies and methods for reaching transformative objectives.

To make a comment, please send an e-mail to solar (at) lowtechmagazine (dot) com.


  1. Multiple references: Unrau, Alicia, et al. Coppice forests in Europe. University of Freiburg, 2018. // Notes on pollards: best practices’ guide for pollarding. Gipuzkoako Foru Aldundia-Diputación Foral de Gipuzkoa, 2014. // A study of practical pollarding techniques in Northern Europe. Report of a three month study tour August to November 2003, Helen J. Read. // Aarden wallen in Europa, in “Tot hier en niet verder: historische wallen in het Nederlandse landschap”, Henk Baas, Bert Groenewoudt, Pim Jungerius and Hans Renes, Rijksdienst voor het Cultureel Erfgoed, 2012. 

  2. Logan, William Bryant. Sprout lands: tending the endless gift of trees. WW Norton & Company, 2019. 

  3. Holišová, Petra, et al. “Comparison of assimilation parameters of coppiced and non-coppiced sessile oaks”. Forest-Biogeosciences and Forestry 9.4 (2016): 553. 

  4. Perlin, John. A forest journey: the story of wood and civilization. The Countryman Press, 2005. 

  5. Most of this information comes from a Belgian publication (in Dutch language): Handleiding voor het inventariseren van houten beplantingen met erfgoedwaarde. Geert Van der Linden, Nele Vanmaele, Koen Smets en Annelies Schepens, Agentschap Onroerend Erfgoed, 2020. For a good (but concise) reference in English, see Rotherham, Ian. Ancient Woodland: history, industry and crafts. Bloomsbury Publishing, 2013. 

  6. While leaf fodder was used all over Europe, it was especially widespread in mountainous regions, such as Scandinavia, the Alps and the Pyrenees. For example, in Sweden in 1850, 1.3 million sheep and goats consumed a total of 190 million sheaves annually, for which at least 1 million hectares deciduous woodland was exploited, often in the form of pollards. The harvest of leaf fodder predates the use of hay as winter fodder. Branches could be cut with stone tools, while cutting grass requires bronze or iron tools. While most coppicing and pollarding was done in winter, harvesting leaf fodder logically happened in summer. Bundles of leaf fodder were often put in the pollarded trees to dry. References: Logan, William Bryant. Sprout lands: tending the endless gift of trees. WW Norton & Company, 2019. // A study of practical pollarding techniques in Northern Europe. Report of a three month study tour August to November 2003, Helen J. Read. // Slotte H., “Harvesting of leaf hay shaped the Swedish landscape”, Landscape Ecology 16.8 (2001): 691-702. 

  7. Wealleans, Alexandra L. “Such as pigs eat: the rise and fall of the pannage pig in the UK”. Journal of the Science of Food and Agriculture 93.9 (2013): 2076-2083. 

  8. This information is based on several Dutch language publications: Handleiding voor het inventariseren van houten beplantingen met erfgoedwaarde. Geert Van der Linden, Nele Vanmaele, Koen Smets en Annelies Schepens, Agentschap Onroerend Erfgoed, 2020. // Handleiding voor het beheer van hagen en houtkanten met erfgoedwaarde. Thomas Van Driessche, Agentschap Onroerend Erfgoed, 2019 // Knotbomen, knoestige knapen: een praktische gids. Geert Van der Linden, Jos Schenk, Bert Geeraerts, Provincie Vlaams-Brabant, 2017. // Handleiding: Het beheer van historische dreven en wegbeplantingen. Thomas Van Driessche, Paul Van den Bremt and Koen Smets. Agentschap Onroerend Erfgoed, 2017. // Dirkmaat, Jaap. Nederland weer mooi: op weg naar een natuurlijk en idyllisch landschap. ANWB Media-Boeken & Gidsen, 2006. // For a good source in English, see: Müller, Georg. Europe’s Field Boundaries: Hedged banks, hedgerows, field walls (stone walls, dry stone walls), dead brushwood hedges, bent hedges, woven hedges, wattle fences and traditional wooden fences. Neuer Kunstverlag, 2013. // If line plantings were mainly used for wood production, they were planted at some distance from each other, allowing more light and thus a higher wood production. If they were mainly used as plot boundaries, they were planted more closely together. This diminished the wood harvest but allowed for a thicker growth. 

  9. In fact, coppice forests could also have a location-specific function: they could be placed around a city or settlement to form an impenetrable obstacle for attackers, either by foot or by horse. They could not easily be destroyed by shooting, in contrast to a wall. Source: 5 

  10. Lime trees were even used for fire prevention. They were planted right next to the baking house in order to stop the spread of sparks to wood piles, haystacks and thatched roofs. Source: 5 

  11. The fact that living hedges and trees are harder to move than dead wood fences and posts also has practical advantages. In Europe until the French era, there was no land register and boundaries where physically indicated in the landscape. The surveyor’s work was sealed with the planting of a tree, which is much harder to move on the sly than a pole or a fence. Source: 5 

  12. And, if it could be brought in over water from longer distances, the wood had to be harvested within 15-30 km of the river or coast. 

  13. Sieferle, Rolf Pieter. The Subterranean Forest: energy systems and the industrial revolution. White Horse Press, 2001. 

  14. On different scales of wood production, see also: Jalas, Mikko, and Jenny, Rinkinen. “Stacking wood and staying warm: time, temporality and housework around domestic heating systems”, Journal of Consumer Culture 16.1 (2016): 43-60. // Rinkinen, Jenny. “Demanding energy in everyday life: insights from wood heating into theories of social practice.” (2015). 

  15. Vanbeveren, S.P.P., et al. “Operational short rotation woody crop plantations: manual or mechanised harvesting?” Biomass and Bioenergy 72 (2015): 8-18. 

  16. However, chainsaws can have adverse effects on some tree species, such as reduced growth or greater ability to transfer disease. 

  17. Multiple sources that refer to traditional forestry practices in Africa: Leach, Gerald, and Robin Mearns. Beyond the woodfuel crisis: people, land and trees in Africa. Earthscan, 1988. // Leach, Melissa, and Robin Mearns. “The lie of the land: challenging received wisdom on the African environment.” (1998) // Cline-Cole, Reginald A. “Political economy, fuelwood relations, and vegetation conservation: Kasar Kano, Northerm Nigeria, 1850-1915.” Forest & Conservation History 38.2 (1994): 67-78. 

  18. Multiple references: Bailis, Rob, et al. “Getting the number right: revisiting woodfuel sustainability in the developing world.” Environmental Research Letters 12.11 (2017): 115002 // Masera, Omar R., et al. “Environmental burden of traditional bioenergy use.” Annual Review of Environment and Resources 40 (2015): 121-150. // Study downgrades climate impact of wood burning, John Upton, Climate Central, 2015. 

  19. Haustingsskog. Rettleiar for restaurering og skjøtsel, Garnås, Ingvill; Hauge, Leif ; Svalheim, Ellen, NIBIO RAPPORT | VOL. 4 | NR. 150 | 2018. 

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