forestry -practices, and carbon dioxid ribs o u m ia : oress, MINI~TRy LIBRARY 1450 QOVERNIt I ST RESEARCH Introduction Forests and th e carbon cycle

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1 /E3CMF F:ES! 1994/N Ni INISTRY OF F OFIESTS. F': I T I SH COLUMBIA F OF; ESTS, FORESTRY F'RAi= T I C_ ES, AND 1 ma Main ST RESEARCH ribs o u m ia : oress, forestry -practices, F and carbon dioxid...e Introduction Human activities are causing increases in the - concentration of greenhouse gases,. such as carbon dioxide and methane, in the earth's atmosphere. Scientists believe that increases in the concentrations of these greenhouse gases, will cause the earth's atmosphere to warm. This global warming could have a serious impact on our environment and our quality of life. Carbon dioxide is a major greenhouse gas. While it composes only 0.03 per cent of the earth's atmosphere, its concentration has increased by 20 per cent since The major sources of carbon dioxide are human. activities, such as the burning of fossil fuels and tropical deforestation. ) Forests play an important-role in the earth's carbon cycle.. Like other green plants, trees take carbon dioxide out of the atmosphere and release oxygen during the process of photosynthesis ;.The carbon taken in by trees during photosynthesis is stored in growing wood, branches, leaves, and roots. In a forest, carbon is also stored in soils and in dead. logs, twigs, and needles. Forests also release carbon. dioxide back to the atmosphere through processes such as respiration and the decay of dead organic material. These processes consume oxygen. This fact sheet provides an overview of the role of forests in the global carbon dioxide budget and the impact of forestry practices on carbon dioxide emissions. Forests and th e carbon cycle On a global basis, the earth's forests and other. land ecosystems release approximately as much carbon dioxide as.theytake up from the atmosphere each. year. This is shown on the next page (Figure 1). 1 Deforestation is-defined as the clearing of forests and the use of the land ;fo other purposes (e.g., agricultural and residential). What is the greenhouse effect? Like the glasss of a greenhouse, certain gases in the earth's atmosphere, such as carbon dioxide and methane; allow sunlight to pass in, but reduce heat loss from the lower atmosphere. It is widely believed that increases in the concentration of greenhouse gases will result in warmer air temperatures and changes in the earth's weather patterns. At present, carbon dioxide accounts for about one half of the world's,total emissions of greenhouse gases. Burning gasoline, coal, and other fossil fuels is responsible for 73 per cent of the total global carbon dioxide emissions. Deforestation accounts for the remaining 27 per cent of carbon dioxide emissions. By the year 2060, the amount of carbon dioxide in the atmosphere is expected to double. A change in average world temperatures of between 1.5 and 4.5 Celsius is predicted to result from this doubling of atmospheric carbon dioxide. At the same time, the quantity of other greenhouse gases in the.. atmosphere is also expected to increase substantially. MINI~TRy LIBRARY 1450 QOVERNIt I Province of 't CTORIA BC VfS,=, ya. British Columbia Ministry of Forests

2 t sf9 FOREST RESEARCH h c0'. B ~ IQgical and cf inical processes 340) Figure 1 : In the global cycle, carbon dioxide is exchanged between the atmosphere and terrestrial and marine ecosystems. Numbers shown give the estimated annual exchange of carbon dioxidein billions of metric tonnes - perr year. Terrestrial ecosystems are thought to take in about as much carbon dioxide as they release each year, while oceans appear to absorb approximately 11 billion tonnes of carbon dioxide more than they release. Burning of fossil fuels and tropical deforestation are releasing approximately 22 billion tonnes of carbon dioxide to the atmosphere each year. Schneider, S The Changing Climate. Scientific American. Sept pages

3 The world's oceans take up slightly more carbon. dioxide than they release, accumulating roughly three billion tonnes of carbon (the equivalent of 11 billion tonnes of carbon dioxide) each year. Figure 2 illustrates the flow of carbon through a forest ecosystem : Carbon dioxide is taken up from the air by the leaves of trees and other plants in the forest. Through the process of photosynthesis, this carbon. dioxide is converted to organic material and oxygen is released. In a forest ecosystem, much of the carbon dioxide that is taken up during photosynthesis each year is returned to the atmosphere through plant respiration and the decay of dead plant material.. This process consumes oxygen. The carbon dioxide that remains in the forest is stored as carbon in trees, decaying logs, and as organic material in the soil. The amount of carbon dioxide taken up or released by a forest each year is influenced by the age of the forest and by climate and soil conditions. Vigorous young forest ecosystems take' in carbon dioxide and store substantial amounts of carbon in new plant material each year (Figure 3). In a young forest, the release of carbon through respiration and decay is relatively small. Consequently, relatively large amounts of carbon are stored each year. As a forest gets older, the amount of carbon dioxide released through respiration and decay increases and growth of new plant material decreases. Eventually the forest ecosystem reaches a "steady state" where it releases about the same amount of carbon dioxide as it takes up each year. Ancient (old-growth) coastal forests are close to this steadyy state. is - Net carbon storage (tonnes/hectare/year) Figure 3 : The rate at which a forest takes up or releases carbon dioxide changes with the age of the forest. This figure illustrates how carbon dioxide exchange changes as a coastal Douglas-fir forest ecosystem grows older. Immediately after a disturbance, such,as timber harvesting, the ecosystem releases carbon dioxide through the decay of organic material left in the forest. As the new forest grows and the amount of decaying organic materiall declines, the net release of carbon dioxide diminishes. After about 15 years ; the forest ecosystem starts to take in more carbon dioxide than is released. The annual rate of carbon accumulation reaches a maximum at about 37 years, in this example, and then declines as the stand ages. Forest harvesting and carbon dioxide Figure 2 In, a forest ecosystem, trees and other plants take carbon dioxide from the atmosphere through a process called photosynthesis. Some of the carbon taken up is used in growing new wood and plant biomass. Some of the carbon dioxide absorbed through photosynthesis is released back to the atmosphere through plant respiration, and some is released by the decay of leaves, branches, and tree trunks when they die. When a forest is harvested, some of the carbon in the forest is removed in the logs taken from the site. A substantial amount of slash, rotting logs, and other organic debris can be. left behind after harvesting.. This organic material decays over a period of several decades, releasing carbon dioxide to the atmosphere. As a new forest grows, it takes up carbon dioxide from the air, accumulating carbon in the growing trees. In some ecosystems, it may take 15.to 20 years before the annual rate of carbon dioxide uptake by the trees exceeds the release of carbon dioxide through decay of the debris (Figure 3).

4 ' 300 Carbon (tonnes/hectare) Year Figure 4: When a- new forest is established after harvesting, the amount of carbon in the ecosystem will recover to preharvest levels if the forest is left undisturbed for sufficient time. This figure illustrates the general pattern of carbon accumulation in living trees, soil, and decaying material for a coastal Douglas-fir ecosystem on a medium-productivity site. As the stand ages, carbon accumulates in trees, in. soil, and in detrital organic material (Figure 4). Eventually the total amount of carbon contained in the ecosystem returns to preharvest levels. Old-growth forests can contain large amounts of carbon in trees, in the soil, and in detrital materials. When a forest is harvested without being replaced,. there is a long-term reduction in forested area. When land is converted from forest to non-forest uses, there is both a release of carbon to the atmosphere and a reduction in the amount of forest that is taking carbon from the atmosphere. It is important, that we avoid this by ensuring the regeneration of a young, vigorously growing forest that will recapture much of the carbon lost from the site. It can take 150 years or longer to recapture all of the. carbon lost from an old-growth forest through harvesting. Consequently, converting slow-growing, old-growth forests to faster-growing young forests will not help in :reducing atmospheric carbon dioxide. Carbon dioxide release by wildfire and prescribed burning Trees and slash burned by wildfires and prescribed slash fires release carbon dioxide to the atmosphere. Forest fires have been releasing carbon dioxide to the atmosphere for thousands of years. The release of carbon dioxide by forest fires in B.C. varies from year to year due to variations in the amount of forest that is burned (Figure 5). During the 10 years ending in 1989, forest fires burned an average of hectares of forest annually, releasing roughly 9.6 million tonnes of carbon dioxide each year. An average of six million tonnes of carbon dioxide were released annually by silvicultural prescribed burning (slash burning) during (Figure 5) : Prescribed burning of slash is done for a number of reasons. It is used to prepare a site for planting - to improve planter access and control competing vegetation - and to reduce fire hazard later inn the life of the plantation. It is worth noting that in 1988 the amount of carbon dioxide released by burning gasoline, oil, and coal in B.C. was three times greater thann that released by slash burning. About 1.4 million tonnes of carbon dioxide are also released annually from burning that is prescribed to improve rangelands for cattle and wildlife habitat. Prescribed slash burns consume branches, twigs, and forest-floor material. After timber harvesting, much of this material would decay naturally, releasing carbon dioxide. Consequently, whether prescribed burning is used after harvesting or not, it should have little long-term effect on the total carbon dioxide. released after harvest Million tonnes of CO2 Use of forest products and carbon dioxide release The total amount of carbon dioxide released after harvesting depends on how the harvested wood is used. For instance, wood used for short-lived products, such as paper or fuelwood, is converted back into carbon dioxide within a few years. However, the carbon in lumber used to build houses or other long-lived structures may not be released to the atmosphere for many decades. Year Figure 5 : The annual release of carbon dioxide by forest fires and prescribed fires varies substantially from year to year. In the absence offire suppression, emissions of carbon dioxide would be roughly 50 trillion tonnes annually.

5 , Carbon dioxide capture through reforestation The prompt establishment of a hew forest is essential for keeping the net release of carbon dioxide to a minimum. Reforesting of harvested lands helps in recapturing carbon removed from the site by harvesting or released from the site after harvesting. Planting forests on non-forested lands (afforestation) or on areas where forests have not been re-established can provide.some small benefits to the global carbon budget. However, assuming that new forests would grow as fast as our best. coastal Douglas-fir stands, more than 465 million hectares of new forest plantations would have to be established worldwide to counter the 11 billion tonnes of carbon dioxide released to the. atmosphere each year from the burning of coal, oil, and gasoline, and from deforestation : This is an area about five times the size of B.C. and is approximately the areaa covered by all of Canada's forests. The cost of such a program and the lack of suitable, non-forested lands on which to establish these new forests means thatsuch a massive afforestation program is not, on its own, the solution to.the problem of carbon dioxide emissions. British Columbia's forests and carbon dioxide Recent estimates indicate that the.province's forests are capturing approximately 50 million tonnes of carbon dioxide each year. At current rates of harvest, the amount of carbon dioxide removed from our forests equals this rate of ::_ uptake. Consequently,' the carbon budget for our forests is approximately balanced. Better information on the accumulation and cycling of carbonn in our forest ecosystems and wood products is being collected. This information will be used to prepare refined carbon budget estimates for our province's forests and to examine strategies which might increase carbon dioxide uptake by our forests. Summary Global warming could have a major impact on our forests and our quality of life. It is essential that we understand the impact of human activities on the global carbon cycle if we are to find effective means for reducing carbon dioxide emissions : Forestry practices in our province appear to have a minimal effect on the global carbon budget. Prompt reforestation is the key to minimizing impacts of forest harvesting on atmospheric, carbon dioxide. Establishing new forests on suitable nonforested lands could provide a small additional benefit to the global carbon budget. ' Reducing our dependence on fossil fuels and managing the earth's forests on a sustainable basis are key elements in controlling carbon dioxide emissions to the atmosphere. The British Columbia Forest Service, Forestry Canada, and other agencies are participatingg in studies relating to this important issue. For more information, contact Dr. Dave Spittlehouse or Dr. Phil Comeau, British Columbia Forest Service; Research Branch, 31 Bastion Square, Victoria, B.C. V8W 3E7. Phone : (604) Notes :