A New Idea to Optimize Forestry to Maximize Carbon Removal
How Forestry Optimized for Carbon Drawdown can learn from an Ancient Technology
It is early October. You know you are walking the grounds of a landfill site, though everywhere you look are leaves of a diverse range of tree species, slowly turning their fall colors. Bird nests peek out from the sweet gums as leaves flush red around them; honey locust trees show black pods against yellow leaves, and foresters are walking through, felling photosynthesis-enhanced poplars on their 11-year cycle. Foresters cut each tree to ground level, separate out leaves and small stems from the trunk, and feed them into a fast pyrolysis machine. The rich black charcoal that results is mixed with crushed basalt and spread onto the soil. Across a shallow valley, you can see stems emerging from the trees felled last year; 7 or 8 surround each stump with their yellowing leaves fluttering in the cool wind, and already over 10 feet tall. Each species of tree is planted in a stand of equal age, and every year a new batch is cut so that in 11 years, these trees will be cut again. Every year the appearance of this forest is new, with trees in differing stages of growth and regeneration. The honey locusts are also cut on a cycle, but the interspersed stands of oak and pine are not, as these species do not respond as well to this treatment, known as coppicing.
The poplar stems form a dense stack at the edge of the felled area. A quarter of their mass is carbon (after drying, it would be half in finished wood). However, this is a carbon removal optimized forest, and these trees will not be processed for wood. Instead, they are chipped and fed into the same pyrolysis machine that is processing the leaves and twigs. The basalt charcoal mixture produced is then mixed into the soil in a portion of the forest prone to erosion, improving soil quality, removing CO2 through enhanced weathering of the basalt, and preventing exposure of landfill waste during heavy rains. A few miles away, poplars on an abandoned mine site in the process of remediation are coppiced on a longer cycle of 15 years, and the wood they produce is processed into cross-laminated timber, which has become the main product of a nearby lumber mill, and forms the structural framing of most new construction in the area.
Water quality tests have shown that the placement of this forest on the landfill site reduced leachate contamination into nearby groundwater fivefold, with documentation of four times the number of bird and mammal species residing in the area, including the return of beavers to a small river on the edge of the site, evidenced by the construction of poplar wood dams, which have increased the residence time of water on the landscape and increased the diversity of fish species. All of the practices employed here have increased the carbon removal rate four times per acre of land over a forest under standard management.
Coppicing is a forestry technique that uses nature's capacity for regeneration to continually harvest wood from a living tree. Coppicing dates back to the stone age, when humans discovered that if they felled trees down to the stump, new shoots would grow. These long straight shoots were used for basketry, building, firewood, bows, and more.
While this ancient technology can already add about 50% to the rate of carbon fixation per hectare, we can double this with our photosynthesis-enhanced hybrid poplar, whose increased growth rate adds nearly another 50% to the carbon fixation rate, and further increase sequestration by incorporating enhanced weathering and conversion of fixed carbon into permanent forms such as biochar. Standard management practices generally yield around 100T carbon/hectare per decade for hybrid poplar on a replacement rotation. Living Carbon’s photosynthesis-enhanced hybrid poplar, coupled with a coppicing harvest system, and enhanced carbon storage through biochar or bio-oil production could yield 4X carbon per hectare per decade than standard management.
In this post, we go into the details of how this is possible, dive deeper into the mechanics of coppicing, and show how comparatively simple applications of standard forestry practices, plus attention to effective removal and storage of carbon from biomass, can greatly increase the drawdown potential per acre.
A Brief Introduction to Tree Growth Parameters
Trees vary greatly in their growth rate and adult size, both according to species and their individual growing environment, but there are unifying principles at play. The rate of increase of durable biomass (in the case of trees, this is equivalent to the production of wood, bark, and roots) is equal to the rate of carbon capture through photosynthesis, with the metabolic expenditure of sugar through respiration, and the allocation of carbon to temporary structures (such as deciduous leaves, or in the case of adult trees, flowers pollen and seeds) subtracted. The total mass of carbon contained in a tree is approximately half of its dry biomass (and half its wet mass is water), so 25% of the mass of a living tree is carbon, and this carbon was almost exclusively derived from the fixation of atmospheric CO2.
As trees grow, they adjust the proportion of fixed carbon allocated to these different pools, according to their needs at each stage of their life cycle. Young trees allocate as much carbon as possible to wood, in order to reach the height of the canopy before they are shaded out, and also to generate a structure with many branching points so that there are as many places as possible to bear flowers as adults. The way the growth form of a tree is determined by its context can easily be seen in how these two considerations differ in closely packed versus open-grown trees. In a dense forest stand, the primary pressure is upward growth to reach the light, so trees grow long, straight trunks and allocate much carbon to their main stem. In an open field, light availability is less of an issue, and so wide-spreading branches are generated.
When a tree reaches a certain age and structural size (these factors are far less rigid and more environmentally determined in trees than animals), it will generally reallocate sugar toward generating flowers and later fruits, representing a significant reduction in its growth rate measured in terms of wood production. This process will continue often for decades or centuries, and there is a similar trade-off involved as that between height and branching. Production of nutrient-dense seeds with an effective dispersal mechanism, and in great numbers, is metabolically expensive, and this expense must be measured against the metabolic cost of maintaining and extending the plant body, and protecting it from disease such as fungal attack.
Tree species vary greatly in how they allocate resources between maintenance and reproduction, depending on the selection pressures their ancestors were exposed to throughout evolutionary time. For example, while a bristlecone pine (Pinus longaeva) is extremely long-lived (up to 5000 years), it is unable to regenerate from root buds, and so this duration measures the lifespan of an individual stem, which must allocate sufficient carbon and nutrients toward the maintenance of this stem to enable such an impressive lifespan in a harsh high altitude environment where germination and establishment of seeds is rare. The total proportion of carbon allocated to the seed crop each year is therefore lower than a shorter-lived species such as Monterey pine (Pinus radiata), which is able to rely on a higher seedling germination and establishment rate and therefore regenerate without relying on long term survival of individual trees. This is how two comparatively closely related species end up with lifespans differing by an order of magnitude (Monterey pine rarely lives more than 200 years, even in ideal conditions).
Poplar trees employ a different strategy; in common with their close relatives, the willows, they form a persistent root system capable of reaching arbitrary longevity (likely at least 20,000 years, but without any hard limit), which generates clonal stands of trees, each of which has a comparatively short lifespan of one or two centuries. Unlike Monterey pines, they do not mainly rely on their seed crop for reproduction, as their seeds are not packaged with extensive food reserves and rely on unusually favorable environmental conditions for initial establishment, but their seeds are surrounded with cottony fibers (hence the name cottonwood for several Populus species), and support occasional long-distance dispersal by wind.
The short lifespans of individual stems of a clonal stand of poplar or aspen trees therefore do not represent a short lifespan of the organism, which is more correctly identified with the clonal stand. Rather than distinct individuals, each stem can be thought of as a replaceable light-harvesting and seed-producing organ generated by the persistent root system. The stem’s lifespan is determined by energetic trade-off decisions made by the organism as a whole and reflects a physiological understanding that after a period of about a century, energy is better spent regenerating a new stem than maintaining an existing one against disease and environmental insult. Additionally, this enables the clonal system to respond to changes in water flow and course (most of these trees are originally from riverside environments).
Leveraging Poplar-Specific Physiology to Increase Carbon Capture
Poplars, willows, aspens, and other trees in the family Salicaceae are particularly amenable to an ancient management practice known as coppicing, in which the stems are periodically cut to ground level and harvested without replacement, relying on the persistent root system to rapidly regenerate replacement trees. Coppiced stands of trees can persist indefinitely (there are individual coppiced linden trees 2000 years old in Britain, and evidence of the use of the practice dates back 6500 years.
Many European coppiced trees were likely never intentionally planted, but represent the persistent roots and stem bases of trees from forests that inhabited much of the land before their clearance for agriculture. The carbon contained in these living root systems is, therefore, up to several millennia old.
A stem base coppiced on a harvest cycle is called a stool, and a land area subject to a coppice cycle is divided into coupes, which represent subsections of the acreage coppiced in a given year, such that annual harvesting of a portion of the trees results in each coupe being harvested at an interval known as the coppice cycle time. The entire coppice area is called a copse. The most important parameter in any coppicing operation is the cycle time because this determines the thickness of the regenerated stems and the biomass harvested per cycle. Copses are therefore managed according to species-specific cycle times, and within these also as long or short rotation. For example, poplar and willow are often grown on a short rotation coppice for biofuel production or a long rotation coppice for the production of poles of a given diameter.
Optimizing Coppice Cycle Time
Although coppice cycle time is often determined by the intended use of the wood products, there is reason to believe that an understanding of the physiology and growth parameters of a given tree species will yield an optimal coppice harvest interval for that species where the goal is to optimize the rate of production of biomass and hence fixation of atmospheric carbon.
Anyone who has watched a tree resprout from a stump will have noticed that the growth rate of the regenerant stems far exceeds that of a seedling. This is because they have the entire root system of the previously established tree to draw on for water, sugar, and other nutrients, and are not dependent on their own ongoing photosynthetic production. Coppicing is also known to extend the lifespans of usually short-lived trees practically indefinitely. This is because cutting a tree to ground level and letting it regenerate represents a metabolic reset to an earlier life stage, with a different physiology to a mature tree. This metabolic reset also reallocates investment to increase stem biomass rather than the production of flowers and fruits, in effect returning the tree to the physiological state of a young sapling.
Tree growth, like many other biological processes, follows a logistic or sigmoidal curve. Growth starts in an exponential increase, becomes linear, and then progressively reduces as it approaches a maximum for a given species. This represents the operation of sets of investment tradeoffs between growth, maintenance, and reproduction, and the imposition of limits on resource availability in a specific environment.
For example, the image above represents data collected on teak trees in India, fitted to a logistic model specific to this species, enabling prediction of their production of above and below ground carbon over their average lifespan of just over a century.
Importantly, for a root resprouting species such as poplar or willow, the above ground and below ground curves are often decoupled, both in natural growth situations and through a practice such as coppicing, which only removes above ground carbon at harvest. This enables the progressive accumulation of below ground carbon over many coppice cycles.
Additionally, it is possible to determine the rejuvenation effect of coppicing on a more quantitative basis. Studies of the growth rate of resprouts from a coppiced stool show that they gain biomass at a rate approximating that of a linear increase from zero to the average maximum for a given species in the first year, and then re-enter the exponential phase of the logistic growth curve in subsequent years. In the teak example shown above, the linear growth rate would be approximately 5.5 tons over 110 years, or 50 kg per year. This is similar to the growth rate of a teak tree planted from seed at an age of nearly 30 years, and indeed an additional study has shown that regenerant sprouts of teak averaged 7 per stool and reached a diameter at breast height of 3.1 cm and a total height of 2.91 m after one year, easily matching this predicted biomass.
Because the onset of slowing in growth rate represents disinvestment in adding stem structure in preference to reproduction, or of reallocation of nutrients to younger stems, it is possible to optimize the coppice interval to keep trees always in the exponential phase of the logistic curve. To complete the teak example above, the optimal coppice interval would be to wait about 60 years for the first harvest, and then to coppice on a cycle of approximately 30 years indefinitely.
Poplar is a much faster growing tree than teak, and therefore has shorter optimal coppice rotation cycles. The sigmoid (S-shaped) logistic growth curve of poplar trees levels off by about age 50, at which point a tree such as teak is still midway through its exponential growth phase. Importantly, this growth curve applies to the rate of above-ground biomass accumulation of a single stem, and this leveling off should not be interpreted to imply that the rate of carbon accumulation overall has become negligible. Rather, photosynthetically fixed carbon has been reallocated to increasing below-ground biomass (the root system), and supporting the initial growth of new stems in the stand, along with increased expenditure of sugar (and therefore carbon) in supporting ongoing production of flowers, pollen, and seeds.
If we are interested in maximizing the rate of carbon drawdown, this energy expenditure in flowering and fruit production (known botanically as reproductive growth), represents increased respiration by the tree, and hence return of CO2 to the atmosphere. This can be prevented through coppicing, and use of the coppiced stems for either durable wood products or, for the maximum permanence of carbon removal, pyrolysis and injection of concentrated bio-oil underground into long term geological storage reservoirs.
Using a similar calculation method as in the teak example, a carbon optimized coppice cycle for hybrid poplar would wait 21 years after initial planting for the first harvest, and thereafter coppice on a 14 year rotation time. This will add about 50% to the rate of carbon fixation per hectare of land over a harvest system involving clearcutting and replacement with new seedlings or cuttings. Using Living Carbon’s photosynthesis-enhanced hybrid poplar, which has an increased growth rate, adds nearly another 50% to the carbon fixation rate, shortening the optimal cycle time to 11 years with a first harvest at 16, and giving a system which utilizes both PE poplar and an optimized coppice rotation system almost double that of a forest under standard management practices. Photosynthesis enhancement also affects only the carbon fixation rate, while the biomass decay rate remains constant, further increasing soil carbon.
The majority of the fixed carbon produced by such a system is easily amenable to transformation into bio-oil through pyrolysis. This gives a theoretical carbon absorption rate of a hectare of land with standard planting density of up to 4KT of carbon per hectare per century, or 400T per decade (averaged over a century of this system operating).
Standard management practices generally yield around 100T per decade for hybrid poplar on a replacement rotation. With the important note that the benefits of a coppicing system accrue over time, and become more apparent in successive cycles, as the carbon benefit of the retained below-ground biomass and increased growth rate after each harvest accumulate, optimized coppice rotation coupled with pyrolysis and subsurface injection competes favorably both in terms of carbon removal rate and permanence with other methods of carbon drawdown.
When It Makes Sense to Optimize for Carbon Removal
Carbon removal using forest systems can take multiple approaches. These can be considered to occupy two broad categories. In the first, carbon drawdown forms part of a larger restorative goal, where the establishment of a diverse forest ecology requiring minimal human management is a primary focus, and, in the case of Living Carbon, inherent additionality is ensured by the appropriate placement of trees with enhanced carbon uptake and/or retention capabilities as part of this restored system. This is the dominant approach we take at Living Carbon, and in most cases the best choice, as ecosystem services and ancillary benefits come as integral parts of the system, which cycles energy and nutrients through an interwoven biological community resilient to external shocks and as far as possible self-perpetuating. This avoids the common pitfalls of unrecognized externalities found when humans optimize a system toward a single goal (in this case carbon drawdown), and allows active human intervention and pre-existing ecological relationships to co-exist, and is in this way differentiated from biomass-based drawdown methods such as BECCS that more closely resemble row-crop agriculture, and often employ similar species, such as switchgrass, cultivated as a rapidly growing feedstock for pyrolysis. For this reason, the often raised objection that a land area equal to the size of India would be needed to offset current emissions does not apply to restorative forestry projects on degraded land, which fill a very different energetic and ecological space to a monoculture BECCS project, and do not represent the replacement of forests with simplified systems under intensive management.
These points being understood, there are situations where the optimization of carbon yield per acre as a primary goal of a project is appropriate, for example, as an emissions mitigation system on a small area of land, such as the acreage surrounding a methane and CO2 emitting landfill site, or on the grounds of a data center. In these situations any biomass-based drawdown system offers efficiency advantages over atmospheric DAC, and I would argue that trees (specifically poplars) are a significantly preferable choice to a species such as switchgrass in many cases, as they come with medium term carbon storage “built in” given their production of wood, are effective in extracting and remediating pollutants from the deep subsoil, and can be planted in conjunction with nitrogen fixing species such as honey locust (native to most of the United States), without shade out risk due to their similar canopy height, providing a forest which occupies a middle ground between a carbon farm and a restored ecology, with most of the advantages of both from a perspective of atmospheric carbon removal.
This post demonstrates how such a system would operate and could be optimized, using as an example the biological properties of Living Carbon’s hybrid poplar, and showing how comparatively simple modifications of standard forestry practices, plus attention to effective removal and storage of carbon from biomass, can greatly increase the drawdown potential per acre.
Zabek, L., & Prescott, C. (2006). Biomass equations and carbon content of aboveground leafless biomass of hybrid poplar in Coastal British Columbia. Forest Ecology and Management, 223(1-3), 291-302. https://doi.org/10.1016/j.foreco.2005.11.009
Johansson, T., & Karačić, A. (2011). Increment and biomass in hybrid poplar and some practical implications. Biomass and Bioenergy, 35(5), 1925-1934. https://doi.org/10.1016/j.biombioe.2011.01.040