Forests as carbon sinks

Joel Abrams

Abstract

The climate of Earth is changing. Many of these changes are due to greenhouse gases. One of the gases that is of concern is carbon dioxide. CO2 concentrations have been increasing rapidly in the last few decades. Carbon dioxide has many sources and sinks. Among those sources are deforestation and burning of fossil fuels. Some of the sinks are forests and oceans. Forests are a very important part of our ecosystem and provide many benefits. One of these benefits is functioning as a carbon sink. A carbon sink is a storage reservoir for carbon. Because of deforestation, one of the major carbon sinks is decreasing in size. There is much research right now in the field of forestry concerning how forests can better be used as carbon sinks. Forests can serve as carbon sinks through an increase in biomass, and through management and establishment of forests. Many countries, including New Zealand, are finding ways that their forests can better serve as carbon sinks. Carbon sinks will not solve the entire CO2 problem, but will be a step in the right direction and complement other action taken against the problem.

Introduction

The climate of Earth is changing, and changes of large magnitude are projected to take place by the end of the next century. The increase in global average temperature is expected to have increased by 1.5-4.5 degrees C (2.7-8.1 degrees F), according to the Intergovernmental Panel on Climate Change. (USDA Forest Service 1). The increase in greenhouse gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs) are causing temperatures to rise. The level of CO2 currently in the atmosphere is at a level that has never been reached before. The current level of atmospheric CO2 is 358 parts per million, and is increasing at about 1.8 ppm per year (Cannell 92). Values of CO2 concentrations have varied from 200 ppm to several thousand ppm (Shands and Hoffman 1). From 1973 to 1985, the concentration of carbon dioxide increased from 320 ppm to 350 ppm. This amount was about the same in Northern and Southern Hemispheres. In the late 1700's, approximately the time of the Industrial Revolution, the concentration was approximately 270 ppm. Concentrations grew very slowly until the 20th century but since then, and especially in the last 50 years, CO2 levels have grown rapidly (Takle).

Because CO2 is a greenhouse gas, it absorbs infrared radiation, and thus makes global temperatures warmer. CO2 is a gas that is causing many changes to the global climate.

Carbon dioxide sources and sinks

Because CO2 is increasing rapidly and causing many changes, a sensible question to ask would be, "Where does this carbon dioxide come from, and how can it be removed from the atmosphere?".

The main sources of CO2 are industrial activity and land-use change. The CO2 from industrial activity mainly comes from fossil fuel burning, and the CO2 from land use change comes mostly from deforestation.

These sources put more CO2 into the atmosphere than is removed by ocean and land sinks. Currently, between 39% and 53% of the emitted CO2 remains in the atmosphere, but this percentage may not remain constant (Cannell 92).

A carbon sink is a storage reservoir that increases in size, and the carbon sink size or strength is the rate that the storage reservoir grows. Oceans provide a carbon sink, but it takes centuries for the CO2 that is emitted from fossil fuel combustion to dissolve into the oceans and become locked in carbonate rocks at the bottom of the sea. Because it takes so long to dissolve in the ocean, atmospheric CO2 concentrations are rapidly increasing.

Importance of forests

Forests are very important and are one of the many factors that affect what the concentration of atmospheric CO2 is. Unfortunately, massive deforestation has been taking place in recent years and removing some of these carbon sinks. Since 1950, a fifth of the worlds forest cover has been removed, and 55% of the worlds 30 to 40 million hectares of rare but incredibly productive temperate rainforest has been logged or otherwise cleared (Global Futures Foundation, "Deforestation").

An unintended impact of deforestation is the loss of carbon storage capacity. Trees take in carbon dioxide through photosynthesis, and release oxygen to the atmosphere. Increased carbon dioxide in the atmosphere is moderated by plant growth. Carbon dioxide is released back to the atmosphere through decay processes. This increased carbon dioxide concentration in the atmosphere is due to release of carbon dioxide through burning fossil fuels. This carbon was removed from the atmospheric cycle during the Carboniferous period, over 300 million years ago, when large amounts of plant material were buried. This carbon dioxide is being anthropogenically reintroduced to the atmosphere and is leading to global climate change.

There are two ways that carbon can be removed from the atmospheric cycle. One way is through the growth of coral reefs. Coral reefs are a type of long-term carbon sink. The process of the ocean absorbing CO2 can take centuries. The second way, which this paper will focus on, is the growth of woody plant material. This process is much faster than oceans absorbing CO2. Wood is a carbon sink that can keep carbon out of circulation for up to several centuries. Because of this, forests can act as a very large carbon storage mechanism. Massive deforestation is taking place and is having a double effect. Deforestation reduces storage capacity for carbon and also releases additional carbon into the atmosphere through decay and burning. Fifteen percent of the carbon dioxide released into the atmosphere during the 1980's can be attributed to the destruction of tropical forests alone (Global Futures Foundation, "Deforestation").

One method currently being researched is the use of forests as carbon sinks to mitigate the greenhouse effect.

Carbon budget

Before carbon the use of carbon sinks is discussed, the carbon budget needs to be explained. The carbon budget is the balance of exchanges of carbon between carbon reservoirs. Carbon budgets indicate whether a reservoir is emitting or storing carbon dioxide. Table 1 summarizes the carbon budget. The total of carbon sources comes to 7.1 gigatons of carbon per year. This figure is an annual average from the 1980's. Of this, 5.5 gigatons comes from industrial emissions, and the remainder, 1.6 gigatons per year comes from deforestation or other land use changes (Cannell 93). This total from deforestation and other land use changes comes from South and Southeast Asia, Latin America, and Africa. In non-tropical regions, the release was between 0 and 0.1 gigatons of carbon, with a greater likelihood of being closer to zero (Detwiler and Hall).

To balance this budget, there must be sinks that also absorb 7.1 gigatons of carbon per year. The atmosphere absorbs 3.4 gigatons of carbon, and the oceans absorb 2.0 gigatons of carbon per year. This leaves 1.7 gigatons for the balancing sink. This balancing sink is no longer an unknown.

Table 1
Annual average perturbation of the global carbon budget in the 1980's (GtC/yr)

Sources
Industrial emissions = 5.5 (+ or - 0.5)
Deforestation and other land use sources = 1.6* (+ or - 1.0)
Total = 7.1
Sinks
Atmosphere = 3.4 (+ or - 0.2)
Oceans = 2.0 (+ or - 0.6)
Balancing sink = 1.7 (+ or - 1.5)
Total = 7.1

* Made up of: South & Southeast Asia 0.7; Latin America 0.6; Africa 0.3.

(From "Forests as carbon sinks mitigating the greenhouse effect", Commonwealth Forestry Review 75 (1), 1996.)

Using forests as carbon sinks

There are two possible sinks that balance the carbon budget. One sink is the stimulation of vegetation growth in response to increasing CO2 concentrations, N-deposition and temperature (positive global change feedback processes). Another possible sink is through increase in forest biomass and management of forests to maximize carbon storage, especially at high and mid-latitudes. The focus of this paper will be on the sinks involving increase in forest biomass and management of forests to maximize storage of carbon.

Increasing forest biomass

To assist in reducing the atmospheric CO2, forests in high and mid-latitude regions are being used as carbon sinks. Forest biomass is the total quantity of living organisms of one or more species per unit area. According to Dixon et al. (1994), in the late 1980's, forests in the high-latitudes and mid-latitudes served as net sinks of carbon. The total carbon was 0.74 GtC/yr. This is a large part of the balancing sink from Table 1.

In the high-latitude regions, about one-third of the world's forests exist and approximately one-quarter of the world's forest biomass. Dixon et al. (1994) states that there was a net loss in forest area in the late 1980's. However, there was a gain in carbon, particularly in Russia, where large areas of forests have gone unmanaged. This lack of management allows standing and lying dead wood to accumulate. This increases the biomass carbon density, which is 83 tons of carbon per hectare. In Canada, the density is only 28 tons of carbon per hectare. This accumulation of carbon in Russia has provided a substantial carbon sink.

In the mid-latitude regions, about one-fourth of the forest area in the world and about 16% of world's forest biomass is contained. In the mid-latitudes, it is the trees that are the major carbon sink. One reason for the sink is the increasing forest area. The more forest area there is, the more carbon is able to be absorbed. Also, many of the forests are young. This enables the trees to grow and accumulate carbon. Additionally, the forests are being underharvested, thus carbon density in the forests is increasing. Underharvesting means that the growth of trees is exceeding the cutting of trees.

Forest management to maximize carbon storage

Effective management of forests can also increase the amount of carbon that is stored. There are four forestry practices that can help to maximize carbon storage. The first is conserving native forests or replacing them with plantations. Another practice is choosing species and forest type. An additional practice is the intensity and frequency of logging or clearcutting. A final practice is the conservation of soil carbon.

The first choice that will be discussed is the choice to either conserve native forests or replace them with plantations. In the previous section, there was an example of an unmanaged forest. In Russia, where large areas of forest are being unmanaged, carbon is accumulating in the land and will have a high carbon density. When these forests are cleared and replaced with plantations of new forests, this will cause a loss of carbon because of the lower age of the trees. Obviously, with this lower age, there will be less biomass.

A method that can make up for this loss of carbon by clearing trees and replacing them with plantations is the choice of species. If fast-growing species are chosen, these plants can accumulate carbon at a faster rate and make up for the clearing of trees. These trees can go through a rapid one-time period of carbon storage. However, before the tree species is chosen, the decision must be made whether carbon emissions must be offset for a time period of a few decades or a longer time period than that. If a longer interval is chosen, then slower-growing species should be selected. These slower growing species, when harvested, have a larger maximum biomass. Currently, the better option may be to select the slow-growing trees. With CO2 concentrations predicted to continually rise, it is more sensible to think long-term. The benefits may not be as easily seen in a few years or decades, but in the long-term, the payoff will eventually be seen if the slow-growing species are used.

Another practice to be looked at in forestry when trying to maximize carbon storage is the intensity and frequency of logging or clearcutting. There are two important factors when forming an optimum harvesting strategy for maximum carbon storage. One factor is the lifetime of the wood products that come from the trees. These can be products that are long-lasting and recyclable, or they can also be products that are substitutes for metal or concrete. The processes which produce metal and concrete take large amounts of fossil fuels to produce. Substitution for these two materials would reduce emissions from fossil fuels. The second factor is the time that the forest takes to reach the maximum mean annual increment. (Cannell 97). If the lifetime of wood products from the forest is equal to or longer than the time needed to reach the maximum mean annual increment, then the wood should be harvested and made into the wood product. However, if the lifetime of the wood products form the forest is shorter than the time for maximum mean annual increment, then the wood should not be harvested. Instead, the wood should be left in the forest and harvested at the point in time when the wood lifetime becomes equal to or greater than the maximum mean annual increment. To summarize, when wood products have a long lifetime, two objectives have been fulfilled. One objective is to maximize carbon storage, and the second objective is to maximize wood yield.

Another item that forest managers need to take into consideration is carbon content of the soil. Soil contains a substantial amount of carbon, and this amount often exceeds the amount found in trees. When the soil is disturbed, carbon content can change. When forests are cut, and the soils are cultivated and converted to agricultural land, this causes a rapid loss of soil carbon. However, during site preparation and forest harvesting, Johnson (1992) states there are few significant trends of carbon loss from the soil.

Countries' use of forest sinks to reduce CO2

Several countries have taken steps to use their forest resources to reduce CO2 concentration in the atmosphere. New Zealand is creating afforestation models to reduce CO2, and there are several other countries also using various methods.

New Zealand's use of afforestation to reduce CO2

New Zealand is one country that is experimenting with afforestation to reduce atmospheric CO2. Afforestation is the planting of forests on land that has not been previously forested.

Two models have been used by New Zealand scientists to calculate carbon sequestration by plantation forests (Maclaren 100).

The first model is a stand-based model (STANDPAK) and simulates the stem-volume growth of Pinus radiata, a predominant species of tree, under typical conditions. Growth prediction is based on data from some 10,000 permanent sample plots.

The second model uses the forest estate model FOLPI and calculates the annual national change in biomass of existing plantation forests over a period of 100 years. This model is the more sophisticated of the two because it recognizes that a national forest estate is considerably more complex than a simple multiple of typical stands.

New Zealand has approximately 5.5 million hectares of unsustainable pasture that have the potential to be forested. A typical radiata pine forest on a 30-year rotation contains approximately 112 tons of carbon per hectare (Maclaren 101) on average. If one assumes that the 5.5 million hectares of plantable land currently has little carbon, conversion of this land to forest will remove approximately 616 million tons of carbon from the air.

This carbon that is removed from the air through plantation of forests will stay out of the atmosphere for as long as there is a continuous forest cover on that land. This takes into consideration continuous harvesting of mature individual stand and replanting them.

In 1990, plantation forests in New Zealand absorbed approximately 5.6 million tons of carbon. Carbon dioxide emissions were estimated to be approximately 7.6 million tons of carbon. The CO2 emissions are increasing at about 1.6% per year.

The average rate of new-land planting is 44,000 hectares per year. Given this rate, it is projected to take 125 years to completely afforestate of this unsustainable pasture in New Zealand. It will take 16 more years before this plantation estate is considered "carbon-neutral".

In the winter of 1994, planting rates were between 90,000 and 100,000 hectares per year. If this rate is maintained, these forests will remove more CO2 than is emitted from fossil fuel combustion. This will continue for several decades of the next century, but only for about 70 years. After this time, land to plant new forests becomes scarce (Maclaren 101).

Besides CO2, New Zealand also emits large amounts of other greenhouse gases. Methane and nitrous oxide are two of the most prevalent greenhouse gases. Through afforestation of pasture land, methane concentrations will be reduced. Livestock are a major source of methane, and with livestock being displaced in favor of forests, this will reduce methane concentrations.

Other countries' use of carbon sinks

There are several other countries besides New Zealand that are looking at their forests to be used for carbon sinks. In China, a forestation program is being developed. Also, in European countries, forestation is also increasing. Between 1950 and 1990, the forest area increased by 5 million hectares, and also in that same time period, the amount of underharvesting has increased. This has resulted in an increase in forest age, total biomass, and an 43% increase in stem volume (Cannell 97).

A strategy that is being used in Norway consists of increasing biomass in managed forests. In the last century, biomass has doubled. Dynamic models are being created and determining sustainability better than static models. These dynamic models use optimal control theory from electrical engineering to estimate optimal harvest practices. These models show that cutting trees at a mature stage and selectively harvesting them optimizes ground biomass and also resource income (Bradley 1995).

Conclusion

The problem of high CO2 concentrations in the atmosphere is a major problem with many consequences. CO2 is contributing to higher temperatures. The reasons for increased CO2 are many. Deforestation and fossil fuel emissions are just a couple of the reasons.

Forests are one of our most valuable resources. They provide non-timber products, watershed protection, recreational use, tourism, carbon storage, spiritual and cultural significance, genetic resources, plants to be used for medicine, wildlife habitat, and many more.

Carbon storage is one of the very important functions of forests. If forest area is reduced, a very valuable part of the ecosystem will be lost.

It is evident that planting forests as carbon sinks will help to reduce the atmospheric CO2 concentrations. However, it will take much more than forests to solve the atmospheric CO2 problem. After the land runs out to plant new forests, additional methods of CO2 reduction will need to be discovered. Humans will have to do their share to reduce CO2 emissions. More research will need to be done on ways to reduce emissions. The ecosystem is very complex and it will take much research and many changes in what we do to combat the increasing CO2 concentrations.

References


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