Trees play a vital role in the global carbon cycle, absorbing carbon dioxide (CO2) from the atmosphere through photosynthesis and storing it in their biomass. They are one of nature’s most effective carbon capture and storage systems and a critical component in mitigating climate change. Accurately measuring the amount of carbon stored in trees is essential for understanding the overall carbon balance of ecosystems and informing climate change mitigation strategies.

Carbon projects focused on nature-based solutions (NBS) like afforestation, reforestation, wetland restoration or conservation of existing forests are the most prominent type of carbon projects around the world today. In 2023, there were over 175 NBS projects registered on the Gold Standard and over 1,000 on the Verified Carbon Standard (VCS). The number is growing rapidly and reflects the increasing recognition of NBS in climate change mitigation and adaptation. With such potential, it is important to understand how exactly trees store carbon and how this carbon can be measured. Accurate quantification of carbon stocks in forest biomass is imperative in determining the sequestration potential from NBS projects and the resulting generation of carbon credits over the project’s lifetime. The two main types of project structures are ARR (Afforestation, Reforestation, and Revegetation) and REDD+ (Reducing emissions from deforestation and forest degradation):

• ARR: establishing new forests or restoring degraded land by planting trees and implementing soil conservation practices.

• REDD+: is a framework to encourage developing countries to reduce emissions and enhance removals of greenhouse gases through a variety of forest management options, and to provide technical and financial support for these efforts. It is a voluntary mitigation framework developed by the UNFCCC (3).

Besides being designed as carbon projects and generating carbon credits through certification standards, forestry projects are also implemented as Insetting projects, where corporates devise NBS solutions in their own supply chain rather than offsetting elsewhere, in an effort to reduce their own environmental impact and generate emission reduction for their carbon accounting.

How do trees grow and store carbon?

Over the course of their lifecycle, trees maintain the capacity of storing carbon in their biomass, but the rate at which this sequestered carbon accumulates will depend on the tree’s growth stage. Tree growth typically follows a parabolic or S-shaped curve, and this pattern can be attributed to various factors influencing tree growth and different metabolic processes taking place.

Several factors influence the specific pattern of carbon uptake over a tree's life (2):

• Tree species: different species have varying growth rates and lifespans, leading to differing patterns of carbon sequestration. Fast-growing species may exhibit a more pronounced parabolic curve, while slower-growing species will show a more gradual increase in carbon storage.

• Environmental conditions: factors such as soil fertility, water availability, sunlight exposure, and temperature can significantly impact tree growth and carbon uptake. Optimal conditions generally promote faster growth and higher carbon sequestration.

• Disturbances: natural or human-induced disturbances like fires, pests, or diseases can interrupt the typical pattern of carbon accumulation. These disturbances can lead to temporary declines or permanent changes in carbon sequestration.

However, the growth of any single tree within a forest ecosystem can only be understood by examining the successional dynamics that are influencing the development of the forest stand at a particular point in time. The change in forest ecosystems over time can be broadly referred to as forest succession.

Forest succession can be broadly grouped into four seral stages:

1. Pioneer: in the early stages of life, trees exhibit rapid growth as they invest energy in developing roots, stems, and foliage to establish themselves in the environment. During this period, the rate of carbon uptake is relatively high as trees rapidly accumulate biomass.

2. Young/Seral: as trees mature, they compete for finite resources and undergo a process of self-thinning referred to as the stem-exclusion phase. As the density of the stand decreases, the growth of the surviving trees accelerates due to increased availability of sunlight and soil nutrients, resulting in a rapid increase in the net carbon accumulation across the forest stand.

3. Maturing: as trees mature, their growth rate slows down, but carbon sequestration continues. This transition occurs as trees shift their energy allocation from rapid growth to maintaining existing structures and initiating reproductive processes. While the rate of carbon uptake may decrease, the net amount of stored carbon continues to increase due to the overall increase in size and biomass of the tree.

4. Steady State: in the later stages of their life, trees may experience a stagnation in growth. The concept of steady state is important for understanding the long-term potential of forest carbon storage In steady state climax, forests continue to sequester large amounts of carbon in their now significant biomass, preventing its release into the atmosphere. Even in death, large snags (dead trees) and decaying woody debris on the forest floor continue to play a vital role in supporting biodiversity and maintaining the hydrological function of forest soils, in turn facilitating significant accumulations of organic carbon in belowground ecosystems.

Understanding the dynamics of carbon uptake over a tree's life is crucial for accurate carbon accounting and evaluating the role of forests to mitigate climate change. Knowing when trees reach their peak carbon storage potential helps prioritize forest management practices for long-term carbon sequestration benefits.

The role of trees in carbon storage

Understanding the contribution of trees in carbon storage requires evaluating both aboveground (AGB) and below-ground biomass (BGB). AGB refers to the carbon stored in the visible components of trees, such as the stem, bark, branches, and foliage. BGB refers to the root network of trees below the soil surface and includes both large structural roots as well as fine root networks. AGB represents a significant portion of a tree's total carbon storage capacity but the ratio of AGB to BGB will differ with the age (2). type of tree species and stocking density, highlighting the importance of species-specific allometric models to determine forest biomass. Other forest carbon pools include soil organic carbon (SOC), dead woody debris, and litter (yet un-decomposed foliage on the forest floor). SOC is accumulated through the microbial decomposition and transformation of litter and deadwood from the forest floor and belowground roots (3). SOC is also transferred directly from the roots into the soil via root exudates (excretions from root tips). In some ecosystems, such as the boreal forest, the soil carbon pool far outweighs carbon accumulation in AGB and BGB. Only by considering all forest carbon pools including AGB, BGB, SOC, litter, and woody debris (see image below [3]) can a more comprehensive estimate of carbon storage potential in forest ecosystems be obtained.

Various techniques are employed to measure aboveground biomass, including field surveys that apply allometric equations, geospatial tools, LiDAR and Infrared. In this blog piece, we will explore these different techniques and how they work.

What is Allometry and how is it used on the ground?

Allometry refers to the study of the relationship between the size or shape of an organism and its various physiological aspects. In the context of carbon storage, allometric equations play a crucial role in estimating the carbon storage potential of trees. These equations use measurable tree dimensions, like diameter at breast height (DBH) or tree height, to estimate AGB (4).

As standard procedure, DBH is measured from 1.3m above ground height (1). This is a simple procedure for a relatively straight tree with one trunk, but with trees of different sizes, growing at varying angles, on slopes or with exposed roots (such as mangroves), DBH measuring techniques are adapted accordingly – this can be seen in the images below.

By applying allometric equations, researchers can quickly assess the carbon storage potential of large, forested areas and guide carbon project planning and implementation. Within allometry, wood density is an important parameter as it is a measure of the dry wood biomass or wood per unit volume, and it varies among species and within trees. These equations typically include wood density as a coefficient, which reflects the fact that denser wood has more biomass per unit volume(1). As an example, the average wood density of Maple trees is 0.547g per cm3 whilst the average wood density of a lighter wood like Spruce is 0.398g per cm3..

Techniques for measuring below ground biomass in carbon projects

When it comes to below ground biomass, additional techniques alongside allometric equations like soil coring and radar can provide further estimations of carbon stored below the surface. Soil coring is a method which involves extracting and soil samples that contain roots. It is a direct method, meaning that it involves physically measuring the amount of root biomass present in the soil. Root-to-shoot ratios are parameters that can also be used to estimate BGB in the trees’ root systems, by converting the total aboveground biomass calculated from allometric equations for the tree species. Standardised figures for the ratios can be found under the IPCC guidelines in the 2019 refinement of the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (7). They are based on the regionally specific climactic conditions and associated forest biome classifications.

Destructive sampling involves the felling of several individual trees, of the same species, within different age classes. These trees are then separated according to components (stem, branches, bark, foliage, and roots) and weighed to give you fresh wood biomass. Each component is then dried and re-weighed to ascertain the dry wood biomass (i.e. wood density in g/cm3 or kg/m3). By adding up the total dry weight biomass of each tree component, forest biometricians are able to derive an allometric equation that relates DBH and tree height measurements to the expected AGB. The more destructive samples you have (with a range of diameter classes), the greater your accuracy in estimating AGB when applying the appropriate allometric equation.

Species-specific allometric equations and wood density parameters are indispensable in estimating the AGB of forest ecosystems using field-based surveys. By establishing permanent, fixed sample plots, within a project area, surveyors are able to identify and measure all the trees within smaller plots to estimate the total AGB across the landscape using the relevant allometric equations. Field surveys are also important in evaluating forest health , disturbances and growth rate that can subsequently inform forest management.

Relevant tools and technologies

For ground monitoring and field surveys, allometric equations remain a reliable form of analysis on tree carbon pools. To strengthen understanding of project sites and carbon sequestration however, it is possible to make use of other tools and technology to build on the ground measurements and assess trees from a different vantage point; for example, tools can measure carbon stock by analysing tree canopy cover. Some of these technologies include Infrared Imaging, LiDAR (Light detection and ranging) and SAR (Synthetic aperture radar). With time these are becoming more and more sophisticated and can support assessments of tree canopy cover, forest loss and growth and biomass accumulation.

Conclusion

Forest ecosystems across the world have a remarkable capacity to store carbon in their biomass. The management and conservation of existing forests and the reforestation of degraded lands is a critical component of climate change mitigation. Yet this focus on carbon must not overshadow the indispensable role of forests in maintaining healthy living ecosystems, preserving the ever-threatened biodiversity of flora and fauna which they harbour, and providing humanity with the ecosystem services such as freshwater and clean air, that we often take for granted, and which are not as easily quantified as a tonne of CO2e.

Nonetheless, forest carbon accounting remains an important mechanism that, along with continually improving focus on biodiversity conservation and socio-economic development from the principal carbon certification standards on the voluntary carbon market (VCM), represents a substantial opportunity to mitigate past, current, and future anthropogenic CO2e emissions. By understanding how carbon is stored in forests, and the methods employed to quantify it , we can maximize the effectiveness of reforestation, afforestation and conservation efforts. Accurate measurement of aboveground and below ground biomass, utilizing remote sensing, field surveys, and allometric equations, is vital for estimating carbon storage and guiding future carbon projects. As one of nature’s most effective systems of carbon sequestration and long-term storage, there is immense opportunity, and necessity, to channel much-needed finance to regenerate and manage the conservation of forest ecosystems in a rapidly changing climate. However, to ensure the credibility and accuracy of any forest carbon project, and thus encourage the growth of nature-based projects on the VCM, it is critical for project proponents to develop sophisticated methodologies to quantify the change in forest biomass via ground-based and remote sensing analyses throughout the lifecycle of a project. These must be transparent and reliable.

At HAMERKOP, our work has spanned a multifaceted array of NBS projects that look at REDD+ and ARR efforts that aim to restore ecosystems or create new sources of food and income for local communities. Projects have been in collaboration with local governments of implementing countries as well as the private sector, supporting design, implementation and carbon certification of projects with the relevant standards and methodologies. The team has also been involved in providing field training for project developers implementing ARR projects globally, ensuring accurate measurement techniques and site analyses are being conducted on the ground, and the correct allometric equations and calculations are made from gathered data. More information about our ongoing projects and forestry-related work can be found on our LinkedIn page and the team can also be contacted directly for further insights.

References:

1. Wood density, phytomass variations within and among trees, and allometric equations in a tropical rainforest of Africa (Henry et al., 2010) https://www.sciencedirect.com/science/article/abs/pii/S037811271000424X

2. How trees capture and store carbon: https://carbonneutral.com.au/carbon-jargon-how-trees-capture-and-store-carbon/

3. What is REDD+? https://unfccc.int/topics/land-use/workstreams/redd/what-is-redd?gclid=EAIaIQobChMI9KfX-pDpgwMVtZBQBh3eBw8JEAAYAiAAEgInCPD_BwE

4. Wood density, phytomass variations within and among trees, and allometric equations in a tropical rainforest of Africa (Réjou-Méchain et al., 2014) Link to article

5. International Centre for Research is Agroforestry Methods for sampling carbon stocks above and below ground

6. Carbon sequestration in forests: https://bwsr.state.mn.us/carbon-sequestration-forests

7. IPCC, 2019 refinement of the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. (2019). Available at: CHAPTER 1 (iges.or.jp)