Forest Carbon Pools: Where are They?

Introduction

Forests contain the largest store of land-based carbon. Forest management changes the structure and function of forests, which can influence where and how carbon is stored. This article describes where forest carbon pools occur and how carbon is transported through natural processes and as a result of forest management.

Carbon Pools in the Forest Ecosystem

Forests are largely made of carbon, but only a quarter of the carbon is in the living above ground biomass. The carbon remains in a somewhat stable form until disturbed through decomposition or combustion. When oxygen is added, the carbon turns into carbon dioxide which acts as a greenhouse gas in the atmosphere. Forest carbon tends to accumulate into different pools. Five major categories of pools are listed here (Figure 1): 

Aboveground biomass

This major pool includes living plants like trees, shrubs, forbs, grasses, and sedges. Carbon occurs in all parts of the plant including the leaves, stems, branches and limbs, and fruits. Carbon pools in woody plants are sometimes separated into additional components depending on the type of wood and bark.

Belowground biomass

This pool includes the roots of living vegetation, as well as rhizomes, stolons, and other underground fruiting bodies.

Dead wood

Standing dead trees or snags and downed dead wood lying on the ground are another important carbon pool. Dead woody vegetation is often referred to as coarse woody debris (CWD) or fine woody debris (FWD). Dead roots underground can also be considered part of the deadwood pool.

Forest floor litter

Leaves, needles, and twigs on the ground are considered a separate carbon pool, because of how quickly carbon cycling occurs due to rapid decomposition.

Soils: Mineral soils

Mineral soils (sand, silt, and clay particles) and organic soils both contain large amounts of carbon, however in organic soils the carbon primarily comes from decomposing plant matter.

Carbon pools can also be arranged into more general categories, which can be helpful for informing management decisions. For example, the five pools above can be reshuffled into an above ground category (both living and dead) versus below ground category (both living and dead), or the living versus non-living (dead) carbon pools.

Carbon Transport Across Pools

Natural systems are dynamic. Continuous carbon transport can make it difficult to draw strict lines between the pools. For example, how much of the carbon in Figure 2 belongs to the decomposing log and how much to the forest litter or soils? Given this challenge, it is sometimes difficult to quantify how much carbon is in any pool at a specific point in time.

One of the biological processes that move forest carbon between the atmosphere and forest pools is respiration. Respiration involves using the sugars produced during photosynthesis plus oxygen to produce energy for plant growth. Decomposition is another critical process, where microbes which help break down organic matter back into the original elements.

These biological processes often occur at different time scales across pools. For example, a living tree can take up carbon dioxide from the atmosphere on a continuous basis through the process of photosynthesis. The leaf litter on the forest floor, however, decomposes in just a few months, releasing the carbon back into the atmosphere. When the tree eventually dies it becomes part of the dead wood carbon pool. Over the next several decades fungi and microbes break down the wood which allows some of the carbon to be slowly released back into the atmosphere. Some of this carbon will also enter into the soil and when undisturbed it can remain there for up to a millennium. When new trees replace the ones that were lost, they sequester more carbon from atmosphere, offsetting the carbon released from older trees that are now decomposing. This cycle help forests be carbon neutral over time.

Figure 3. The forest sector carbon cycle includes forest carbon stocks and carbon transfer between stocks. Adapted from Heath et al. (2003) and USDA (2011). Published in Janowiak et al, 2018.

Managing live vegetation alters respiration and decomposition processes and can push forests towards being either a carbon source or carbon sink. The arrows in Figure 3 show the direction of carbon movement between carbon pools and in and out of the atmosphere. Carbon releases into the atmosphere can be delayed by using management practices, such as site preparation and fertilization treatments, which help trees grow larger and live longer. Herbicide treatments help reduce competition and water stress on trees which also allows trees to grow larger and healthier. Methods that help control pests like the southern pine beetle (Dendroctonus frontalis), emerald ash borer (Agrilus planipennis), or Asian longhorned beetle (Anoplophora glabripennis) are important for preventing large numbers of tree deaths. Prescribed fire can also help reduce the risk of large wildfires, since extreme wildfires can destroy large numbers of trees, burn soil, and prevent future forest growth.

Timber harvests are another common management activity that impacts how much carbon is in a forest at any given time. Single tree or group selection harvests can reduce the amount of carbon in a stand for a little while until new growth occurs. Another way carbon is released to the atmosphere is through increased soil erosion due to large equipment disturbing the topsoil during harvesting or the loss of surface vegetation in areas prone to rain or landslides. This is why some practitioners prefer to conduct harvests either in the wintertime, when cold temperatures help stabilize the soils or in the summertime when soils are drier to avoid rutting. Trees removed during a commercial harvest help relocate forest carbon to a new pool, the wood product pool.

Carbon Pools in Harvested Wood Products (HWP)

The carbon stored in wood products is often referred to as the harvested wood products (HWPs) carbon pool. Some wood products, such as high-quality wood furniture and wood framed buildings, can hold onto carbon longer than if the tree had been left in the forest. New research from Forest Service scientists and partners indicates the that the wood used to build and maintain houses will continue to account for nearly half to over three quarters of the carbon stored in wood products annually. Wood products are important for helping bank existing forest carbon while harvesting helps give space for replacement trees to grow.

The carbon footprint of a harvested wood product can be calculated by subtracting the amount of carbon in the wood from the amount of carbon released due to related activities such as harvesting, transporting, and processing the wood into a product. A "cradle to grave" analysis is a comprehensive footprint analysis that does carbon accounting throughout the wood products' existence as well as the footprint of substitute products (e.g., metal, concrete) that are offset by the wood product.

When harvested wood products are eventually disposed of, they often enter into landfills or the solid waste disposal site (SWDS) carbon pool. At this point, the carbon in harvested wood products returns to the atmosphere due to the decomposition of the wood product. Thus, in summary, there are actually seven pools of forest carbon: five in the forest ecosystem and two in the wood product pool (HWPs and SWDS).

Quantifying Carbon Pools

Understanding how much carbon is in the different forest pools is important for developing management plans that help maximize carbon storage or prevent the unnecessary release of carbon. A report by the Environmental Protection Agency in 2019 describes the total amount of forest carbon and percent of carbon in major pools across the United States (See Table 1). Just over half of carbon in forests is stored in the soil, followed by aboveground biomass (live trees and plants). Less than 5% of forest carbon is stored in harvested wood products.

The numbers in Table 1 are an average estimate across multiple types of forests in different regions. The US Forest service recently published a report containing "carbon lookup tables" drawing from FIA data to provide estimates of forest carbon attributes based on region, forest type, and stand age. While regional estimates are not as accurate as site specific data, they provide a reasonable estimate of expected carbon stocks when site-specific data are not available. For example, in the southeast Loblolly pine trees (age 30) contains an average of 55.5 metric tons of carbon per hectare (22.4 metric tons per acre). In the northeast, oak and hickory trees (age 100) have an average of 108 metric tons of carbon per hectare (43.7 metric tons per acre). The deadwood, leaf litter and organic soils in oak-hickory stands contain an average of 83.4 metric tons of carbon per hectare (33.7 metric tons per acre).

To understand carbon stocks at the stand level, practitioners often use field inventory data, such as tree diameter and height in conjunction with the allometric equations developed using destructively sampled trees. As a general rule, the amount of carbon in a tree is 50% of the dry weight of the tree, but this ratio can be higher for hardwood species which have less water. US Forest Service researchers recently found wood carbon ranged from 18.4% to 75.1% across 3,600 wood carbon observations and the mean wood carbon fraction was 47.4 percent.

The development of remote sensing and satellite technologies has allowed users to measure changes in living biomass, determine forest stocks and make vegetation maps without being physically in the forest. These maps can be combined with carbon density values collected using ground data to quantify carbon stocks across large areas. This type of quantification tends to focus on living above ground carbon and is not well suited for quantifying carbon in soils, dead biomass, and leaf litter. Quantifying carbon in soils can also be complicated because of the variation in carbon concentrations across space and at different soil depths. Continued research helps provide more precise estimates of carbon within major pools to better inform local management decisions.

This article was produced by the Forest Owner Carbon and Climate Education Program. What do you think about this article? Please take this short survey.

Related FOCCE Articles and Resources

• How to Manage Forests for Carbon
• Conversions Commonly Used When Comparing Timber and Carbon Values
• The Economic Value of Private Forests and Climate Change Mitigation

Article Information Sources

• Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2018 (U.S. EPA 2020 – EPA 430-R-20-002).  Table 6-12, pg 6-30.
• Forests: A Carbon Cycle Checking Account
• US Forest Service. Standard estimates of forest ecosystem carbon for forest types of the United States
• US Forest Service. How Much Carbon is in Tree Biomass?
• US Forest Service. Housing starts and the associated wood products carbon storage by county by Shared Socioeconomic Pathway in the United States