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Published by the Ecological Society of America

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A Synthesis of the Science onForests and Carbon

for U.S. ForestsMichael G. Ryan, Mark E. Harmon, Richard A. Birdsey, Christian P. Giardina,

Linda S. Heath, Richard A. Houghton, Robert B. Jackson, Duncan C. McKinley,

James F. Morrison, Brian C. Murray, Diane E. Pataki, and Kenneth E. Skog

Spring 2010 Report Number 13

A Synthesis of the Science onForests and Carbon

for U.S. Forests

©© The Ecological Society of America • [email protected] esa 1

ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010

A Synthesis of the Science on Forests and Carbonfor U.S. Forests

SUMMARY

Forests play an important role in the U.S. and global carbon cycle, and carbon sequestered by U.S. forest growth andharvested wood products currently offsets 12-19% of U.S. fossil fuel emissions. The cycle of forest growth, death, and

regeneration and the use of wood removed from the forest complicate efforts to understand and measure forest carbonpools and flows. Our report explains these processes and examines the science behind mechanisms proposed for increasingthe amount of carbon stored in forests and using wood to offset fossil fuel use. We also examine the tradeoffs, costs, andbenefits associated with each mechanism and explain how forest carbon is measured.

Current forests are recovering from past land use as agriculture, pasture, or harvest, and because this period of recoverywill eventually end, the resulting forest carbon sink will not continue indefinitely. Increased fertilization from atmos-pheric nitrogen deposition and increased atmospheric carbon dioxide may also be contributing to forest growth. Both themagnitude of this growth and the future of the carbon sink over the next hundred years are uncertain. Several strategiescan increase forest carbon storage, prevent its loss, and reduce fossil fuel consumption (listed in order of increasing uncer-tainty or risk):

� Avoiding deforestation retains forest carbon and has many co-benefits and few risks.

� Afforestation increases forest carbon and has many co-benefits. Afforesting ecosystems that do not natu-rally support forests can decrease streamflow and biodiversity.

� Decreasing harvests can increase species and structural diversity, with the risk of products being harvestedelsewhere and carbon loss in disturbance.

� Increasing the growth rate of existing forests through intensive silviculture can increase both forest carbonstorage and wood production, but may reduce stream flow and biodiversity.

� Use of biomass energy from forests can reduce carbon emissions but will require expansion of forest man-agement and will likely reduce carbon stored in forests.

� Using wood products for construction in place of concrete or steel releases less fossil fuel in manufacturing.Expansion of this use mostly lies in the non-residential building sector and expansion may reduce forestcarbon stores.

� Urban forestry has a small role in sequestering carbon but may improve energy efficiency of structures.

� Fuel treatments trade current carbon storage for the potential of avoiding larger carbon losses in wildfire.The carbon savings are highly uncertain.

Each strategy has risks, uncertainties, and, importantly, tradeoffs. For example, avoiding deforestation or decreasing har-vests in the U.S. may increase wood imports and lower forest carbon elsewhere. Increasing the use of wood or forest bio-mass energy will likely reduce carbon stores in the forest and require expansion of the area of active forest management.Recognizing these tradeoffs will be vital to any effort to promote forest carbon storage. Climate change may increase dis-turbance and forest carbon loss, potentially reducing the effectiveness of management intended to increase forest carbonstocks. Finally, most of these strategies currently do not pay enough to make them viable. Forests offer many benefitsbesides carbon, and these benefits should be considered along with carbon storage potential.

Cover photo credit: Old-growth forest in the Valley of the Giants in Oregon.Photo by Mark E. Harmon, Oregon State University. Inset: Logs harvested at Manitou Experimental Forest in Colorado. Photo by Richard Oakes, USDA Forest Service.

©© The Ecological Society of America • [email protected] esa

Introduction

The movement of carbon between the earthand its atmosphere controls the concentrationof carbon dioxide (CO2) in the air. CO2 isimportant because it is a greenhouse gas andtraps heat radiation given off when the sunwarms the earth. Higher concentrations ofgreenhouse gases in the atmosphere cause theearth to warm. Before the IndustrialRevolution, the concentration of CO2 in theatmosphere was less than 280 parts per million.The burning of fossil fuel for energy and theclearing of forests for agriculture, buildingmaterial, and fuel has led to an increase in theconcentration of atmospheric CO2 to its cur-rent (2010) level of 388 parts per million. Thiscurrent level far exceeds the 180-300 parts permillion found over the last 650,000 years.

As a result of rising CO2 and other green-house gases in the atmosphere, global surfacetemperatures have increased by 0.74˚C (1.3˚F)since the late 1800s, with the rate of warmingincreasing substantially. As more CO2 is addedto the air, temperatures will continue to

increase and the warmer earth will have animpact on the earth’s climate, climate variabil-ity, and ecosystems. Rain and snowfall patternswill shift, and extreme weather events maybecome more common. Some regions that cur-rently support forests will no longer do so, andother regions that currently do not supportforests may become suitable for forest growth.

Forests store large amounts of carbon intheir live and dead wood and soil and play anactive role in controlling the concentration ofCO2 in the atmosphere (Figure 1). In the U.S.in 2003, carbon removed from the atmosphereby forest growth or stored in harvested woodproducts offset 12-19% of U.S. fossil fuel emis-sions (the 19% includes a very uncertain esti-mate of carbon storage rate in forest soil). U.S.forest growth rates are thought to be higherthan those before European settlementbecause of recovery from past land use and dis-turbance, but the current growth rate will notcontinue indefinitely.

Given the role that U.S. forests play in offset-ting CO2 emissions, our report asks: 1) Whichhuman actions influence forest carbon sinks

(storage rates) and can thesesinks be enhanced for ameaningful period of timethrough management anduse of forest products? and 2)What are some of the majorrisks, uncertainties, tradeoffs,and co-benefits of usingforests and forest products inproposed carbon emissionmitigation strategies?

The purpose of our reportis to answer these ques-tions, or, if answers are notyet available, to present thebest current information.We present the state ofknowledge on the role of

A Synthesis of the Science on Forestsand Carbon for U.S. Forests

Michael G. Ryan, Mark E. Harmon, Richard A. Birdsey, Christian P. Giardina, Linda S. Heath,

Richard A. Houghton, Robert B. Jackson, Duncan C. McKinley, James F. Morrison,

Brian C. Murray, Diane E. Pataki, and Kenneth E. Skog

Figure 1. Plants and soil play alarge role in the global carbon

cycle as shown by global stocks(boxes) and flows (arrows) of

carbon in petagrams (1000teragrams). Numbers in light

blue and green are the historicalfluxes between the oceans andthe atmosphere and plants and

soil and the atmosphere thatwould have occurred without

human influence. The number indark blue is the additional ocean

absorption of CO2, resulting fromincreased CO2 in the atmosphere

since the Industrial Revolution.The numbers in black are the

fluxes to the atmosphere fromfossil fuel combustion or

deforestation. The number inbrown is the flux from the

atmosphere to the land, mostlyfrom forest regrowth. The

measured atmospheric increaseof 4.1 petagrams per year is not

equal to the sum of the additionsand withdrawals because they

are estimated separately andwith associated uncertainties.


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Global Stocks and Flows of Carbon

ATMOSPHERE816 (+4.1/year)

OCEANS37,000

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100100

PLANTS & SOIL2,000

SEDIMENTS AND SEDIMENTARY ROCKS66,000,000 – 100,000,000

COAL, OIL &NATURAL GAS

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forests in the carbon cycle in a straightforwardmanner so that it can be understood by forestmanagers, policymakers, educators, and theinterested public. We begin with a descriptionof the forest carbon cycle and biophysicaleffects. We then present details on the strate-gies that have been proposed for using foreststo slow the amount of CO2 entering the air.

These strategies include:

• Avoiding deforestation – Keeping forests intact.• Afforestation – The restoration of forest on

land that has been without forest cover forsome time, and the establishment of forest onland that has not previously been forested.

• Forest management: decreasing carbon loss –Increasing the harvest interval and/ordecreasing harvest intensity.

• Forest management: increasing forest growth –Use of improved silvicultural practices,genetic improvement, and rapid regeneration.

• Forest management: thinning to reduce fire threat.• Urban forestry – Planting trees in urban

areas for carbon storage and shading forenergy savings.

• Biomass energy – Using fuel from wood andbiomass in place of fossil fuel.

• Carbon storage in forest products and substitu-tion – Storing carbon in long-lived forestproducts (such as lumber) and substitutingforest products for products (such as steel andconcrete) whose manufacture releases muchmore CO2 than does the processing of wood.

We then discuss carbon offsets and credits,how forest carbon could be monitored to deter-mine whether changes result in the desiredoutcomes, and what the costs would need to befor carbon to encourage changes. We also dis-cuss some of the uncertainties inherent in theuse of forests for carbon storage, becausechanges in climate, population, and land usemay lower projected carbon storage. We espe-cially note the potential loss of carbon thatmight occur with increased disturbance in awarmer climate. Finally, we provide conclu-sions and recommendations.

Forests and carbon

Carbon in the forest

Forest carbon storage differs from many othermechanisms that control atmospheric CO2

because forests have a life cycle during whichcarbon stocks, gains, and losses vary with for-est age. Carbon enters a forest through photo-

synthesis, where leaves capture the energy insunlight and convert CO2 from the atmos-phere and water into sugars that are used tobuild new leaves, wood, and roots as treesgrow (Figure 2). About half of the CO2 that isconverted to sugars is respired by living treesto maintain their metabolism, and the otherhalf produces new leaves, wood, and roots. Asthey grow, trees shed dead branches, leaves,and roots and some of the trees die.Microorganisms decompose this dead material,releasing CO2 back to the atmosphere, butsome of the carbon remains in the soil. Liveand dead trees contain about 60% of the car-bon in a mature forest, and soil and forest lit-ter contain about 40%. The carbon in live anddead trees (50% of their biomass) varies themost with forest age.

Carbon can leave the forest in several waysbesides tree and microorganism respiration.Forest fires release stored carbon into theatmosphere from the combustion of leaves andsmall twigs, the litter layer, and some deadtrees and logs, leaving behind a great deal ofstored carbon in dead trees and soil. Stormsand insect outbreaks also kill trees and increasethe amount of material available for decompo-sition. Harvesting removes carbon from theforest, although some of it is stored in woodproducts (preventing its immediate release tothe atmosphere) and some is available for useas biomass energy (displacing fossil fuel use).In addition, water can remove carbon from aforest either by transporting soil and litteraway in streams (especially from erosion afterfire) or by transporting soluble carbon mole-cules created during decomposition. After fire,other disturbance, or harvest, regeneratedforests will eventually recover all of the car-

Figure 2. Flows of carbon fromthe atmosphere to the forestand back. Carbon is storedmostly in live and dead wood asforests grow.

RecentCO2

CO2 Photosynthesis

Recent andOlder CO2

Dead WoodMicrobeRespirationLitter

Carbonin leaves,wood,roots Microbes

Dead RootsOld, StableSoil Carbon

New, LabileSoil Carbon

PlantRespiration


bon lost so that a complete cycle is carbonneutral regarding storage if the recovery islong enough (Figure 3). But if disturbancesincrease, as is projected with climate change,a fire, storm, or insect outbreak may occurbefore the ecosystem recovers the carbon ithad prior to the disturbance. In that case, theamount of carbon stored on the landscape willdecrease.

Forests are biological systems that continu-ally gain and lose carbon via processes such asphotosynthesis, respiration, and combustion;whether forests show a net gain or loss of car-bon depends on the balance of these processes.The observation that carbon is lost from forestshas led to the notion that carbon cannot bepermanently stored in forests. However, thisview ignores the inevitable increase and even-tual recovery of carbon that follows most dis-turbances. Thus over time, a single forest willvary dramatically in its ability to store carbon;however, when considering many differentforests over a large area or landscape, such

“boom and bust” cycles may not be appar-ent because the landscape is composed offorest stands that are in different stages ofrecovery from disturbance or harvesting(Figure 4).

To determine how quickly carbonincreases in a forest system, it is impor-tant to know the starting point or “base-line.” A forest that already stores a sub-stantial amount of carbon is likely to losecarbon when converted to somethingelse, and a system with the potential tostore carbon but that does not currentlystore much is easier to convert to onethat stores more carbon (Figure 5). A for-

est’s timeline for increasing carbon storage isimportant because carbon must be removedquickly to reduce CO2 in the atmosphere andthereby slow global warming.

While the biological processes of photosyn-thesis, respiration, and decomposition aresimilar for all forests, their relative impor-tance differs by forest type and location. Someforests grow more rapidly, but dead trees infast-growing forests also decompose morerapidly. In addition, disturbances vary region-ally: for example, fire disturbance is morecommon in the western U.S. and hurricanesmore common in the East. Forests are man-aged in different ways with varying harvestintervals and regeneration practices that willinfluence the optimum strategy for storingmore carbon. Each forest has a differentpotential to store carbon. For example, thispotential is particularly high in the PacificNorthwest where forests are relatively produc-tive, trees live a long time, decomposition isrelatively slow, and fires are infrequent. The

differences between forests musttherefore be taken into consider-ation when determining howthey should be managed to storecarbon.

Carbon from the forest

All forest products eventuallydecompose, but before they do,they store carbon. Some prod-ucts have a short lifespan (suchas fence posts) and some a longerlifespan (for example, houses) –the longer the lifespan, the morecarbon is stored. Disposed forestproducts in landfills can have avery long lifespan; however, thedecomposition in landfills

Figure 3. If a forest regeneratesafter a fire, and the recovery is

long enough, the forest willrecover the carbon lost in the

fire and in the decomposition oftrees killed by the fire. This

figure illustrates this concept byshowing carbon stored in

forests as live trees, dead wood,and soil and how these pools

change after fire. (Adapted fromKashian and others 2006.

BioScience 56(7):598-606.)

Figure 4. Management actionsshould be examined for large

areas and over long timeperiods. This figure illustrates

how the behavior of carbonstores changes as the area

becomes larger and more standsare included in the analysis. As

the number of stands increases,the gains in one stand tend to be

offset by losses in another andhence the flatter the carbonstores curve becomes. The

average carbon store of a largenumber of stands is controlledby the interval and severity of

disturbances, as shown in Figure7. That is, the more frequent and

severe the disturbances, thelower the average becomes.

(Courtesy of Mark E. Harmon,Oregon State University, 2009.)

Total CarbonFire

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generates methane, which is a much morepotent greenhouse gas than CO2, reducingthe carbon storage benefit. In addition,wood and bark that are burned to run a millor heat houses, or made into liquid biofuel,lower emissions from fossil fuel use. Oncethe carbon leaves the forest, it becomesmore difficult to track and measure thancarbon in the forest, particularly becauseimports and exports must then be tracked.

Biophysical effects may cause

warming or cooling

Forests have other influences on climatebesides that of carbon; these are known as bio-physical effects (Figure 6) and include thereflection of solar radiation and transpirationof water vapor. Trees are dark and absorb moreradiation than other types of land cover, suchas crops or snow-covered tundra. Therefore,converting non-forested land to forest canwarm the land and air. Evergreen trees absorbmuch more energy than deciduous trees in thewinter and burned forests absorb more thanunburned forests, so species and disturbancecan also alter the energy absorbed by forests.In addition, transpiration from forests mayhave a cooling effect by contributing to theformation of clouds that reflect sunlight.

Biophysical effects sometimes act in a direc-tion opposite to that of the effects of storing orreleasing CO2. For instance, whereas convert-ing cropland to forest will sequester more CO2,which reduces global warming, it will alsoincrease solar absorption, which increaseswarming. Generally, biophysical effects on cli-mate are not as strong as the effects of green-house gases. Biophysical effects will be mostimportant in evaluating the benefits ofafforestation because the land use change willcause large differences. Unfortunately, currentestimates of biophysical effects are uncertainbecause few studies have been done.

Strategies for increasing carbon

stores in forests

1. Avoiding deforestation

Deforestation, or the conversion of forest landto other uses, has a significant impact onglobal CO2 emissions. Globally, deforestationconverts approximately 90,000 km2 (about thesize of Indiana) of forests per year (0.2% of allforests) to other land uses. Deforestationannually releases 1,400-2,000 teragrams of car-

bon (1012 grams; see Box 1 for units) to theatmosphere, and two-thirds of this releaseoccurs in tropical forests. The amount of car-bon released by deforestation equals 17-25%of global fossil fuel emissions every year and isroughly the amount of U.S. annual fossil fuelemissions. If current deforestation rates con-tinue, more than 30,000 teragrams of carboncould be released to the atmosphere fromdeforestation in the Amazon alone by theyear 2050.

In the U.S., forested area increased 0.1% peryear from 2000-2005, and this gain in forestedarea is partially responsible for the current for-est sink of 162 teragrams of carbon per year.The net growth in forested area results fromboth deforestation and afforestation: About6,000 km2 are deforested annually, but morethan 10,000 km2 of non-forest are afforested.The net increase in forestlands results fromchanges in land use and possibly from reduceddemand for U.S. timber.

Although the U.S. forest carbon sink bene-fits from increased forest area, these carbonbenefits need to be weighed against the globalconsequences of land use change within theU.S. If afforestation or avoided deforestationin the U.S. pushes crop and cattle productionto other countries, it can lead to deforestation

Figure 5. Projections of carbonstorage and fossil fueldisplacement if all biomass isused shows considerablestorage and offsets for (A) aproject that reestablishes forestswith periodic harvests.Harvesting a high-biomass oldgrowth forest (B) shows carbonlosses, even under the bestpossible scenario, for severalharvests. At each harvest, forestbiomass (and thus carbon stock)is removed for use in long- andshort-lived wood products(‘Products-L’ and ‘Products-S’,respectively) substituted formore carbon-intensive products,and for biomass energy todisplace emissions from fossilfuel use. Because substitutiongenerates more fossil fuelsavings than the carbon itcontains, substitution wouldyield a greater carbon benefitafter harvest than that which isstored in the biomass. Thebiomass energy and substitutionfossil fuel savings accumulatebut represent only hypotheticalcarbon benefits, as currentlylittle biomass energy use andsubstitution occurs in the U.S.(Adapted from IPCC 2007.)

SoilLitterTreesProducts–LProducts–S

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and loss of forest carbon elsewhere to createpasture and cropland. Carbon loss associatedwith such deforestation – especially in thetropics – is greater than carbon gain associatedwith tree growth from afforestation in theU.S.

Forest retention in the western U.S. may beeven more important in the future as climatechanges. Our warming climate is very likelycausing, at least in part, the current increase inforest fire size and intensity, insect outbreaks,and storm intensity. If forest regeneration failsbecause the disturbances or regeneration con-ditions are outside of the ecological norms, dis-turbances can convert forests to meadows orshrublands. When this type of deforestation

occurs, substantial carbon is lost to the atmos-phere and not recovered by the ecosystem.Tree planting would help recover forest carbonwhere natural regeneration fails.

There are not many risks associated withavoidance of deforestation. Three to note,however, would be risks related to highly fire-prone ecosystems near human settlement, eco-nomic consequences for not developing agri-cultural or pasture land, and an increase inforest products harvested elsewhere. On theother hand, avoiding deforestation has manyof the co-benefits identified in Box 2.

2. Afforestation

We define afforestation as both reestablishingforests on land that has been without forestcover for some time and the establishment offorest on land that has not previously beenforested (note that some entities involved incarbon markets and reporting use differentdefinitions for this term). Afforestation canremove substantial CO2 from the atmosphere.Between 1850 and 2000, global land-usechange resulted in the release of 156,000 tera-grams of carbon to the atmosphere, mostlyfrom deforestation. This amount is equivalentto 21.9 years of global fossil fuel CO2 emis-sions at the 2003 level.

The rate of carbon storage in tree growthvaries with species, climate, and management,ranging widely from about 3-20 megagrams(Mg, 106 grams) per hectare per year. In thecontinental U.S., the highest potential growthrates are found in the Pacific Northwest, theSoutheast, and the South Central U.S. Muchland currently in pasture and agricultural usein the eastern U.S. and in the Lake States willnaturally revert to forests if left fallow, whilereestablishing forests in many western forestsrequires tree planting.

The benefits of afforestation (outlined inBox 2) are enhanced where forests include asubstantial proportion of native species.Planting native species or allowing natural suc-cession to recreate the forest that historicallyoccupied the site will yield the greatest benefitsfor species diversity and wildlife habitat andthe lowest risk for unintended consequences.Because native species often grow more slowlythan exotics or trees selected for improvedgrowth, restoration of the historical ecosystemmay yield lower carbon accumulation ratesthan other forest reestablishment practices.Planting monocultures of non-native or nativeimproved-growth species on historical forest

Box 1. UNITS FOR CARBON

When discussing regional, national, or global carbon stores and fluxes, the num-bers get large quickly. We report carbon in teragrams (1012 grams). Otherreports may use other units, so we provide a conversion table below. For stand-or forest-level stores and fluxes, we use megagrams (Mg) per hectare (106

grams). Our report uses carbon mass, not CO2 mass, because carbon is a stan-dard “currency” and can easily be converted to any other unit. Many reportsgive stocks and fluxes of the mass of CO2, not carbon. To convert carbon massto CO2 mass, multiply by 3.67 to account for the mass of the O2.

1000 teragrams (Tg) 1 petagram (Pg)1000 teragrams 1 billion metric tonnes 1000 teragrams 1 gigatonne1 teragram 1 million metric tonnes1 teragram 1 megatonne1 megagram (Mg) 1 metric tonne1 metric tonne 0.98 U.S. long ton1 metric tonne per hectare 0.4 U.S. long tons per acrecarbon (C) mass * 3.67 carbon dioxide (CO2) mass

Figure 6. Biophysical effects ofdifferent land use can haveimportant impacts on climate.Cropland reflects more sunlightthan forest, produces less watervapor, and transmits less heat.(From Jackson et al. 2008.Environmental Research Letters3:article 044006.)

Reflected sunlightEvaporationTransmitted heat



land will likely yield greater carbon accumulationrates but fewer benefits in terms of biodiversity.

Afforestation can have negative conse-quences, too. Planting forests where they werenot present historically can have drawbackssuch as lower species diversity (if trees areplanted in native grassland), changes in watertable, and a higher energy absorption com-pared to the native ecosystem. In addition,afforestation generally reduces streamflowregardless of the ecosystem type because treesuse more water than grass or crops.Conversion of agricultural or grazing lands toforest reduces revenue from agricultural prod-ucts. If afforestation efforts include the addi-tion of nitrogen fertilizer, emissions of nitrousoxide (a greenhouse gas roughly 300 times aspowerful as CO2) will increase.

3. Forest management: decreasing

carbon loss

Lengthening the harvest interval or reducingthe amount removed in a harvest will storemore carbon in the forest. The greater theincrease in harvest interval over the currentlevel, the higher the increase in carbon stor-age. For example, a five-year increase in theharvest interval would lead to a 15% increasein carbon storage if the harvest interval waschanged from 25 to 30 years, but only a 4%increase if the interval was changed from 55 to60 years (Figure 7). A 50-year increase from25 to 75 years would increase carbon storage92% (Figure 7).

The carbon impact of reducing the amountof trees removed in a harvest also varies withthe harvest interval. For example, reducing theharvest from 100% to 20% of the live treeswould increase the average forest carbon stockby 97% for a 25-year harvest interval, but onlyby 30% for a 100-year harvest interval (Figure7). Some natural forests are dominated bysmall disturbances that kill a few trees at atime. Reducing harvest amounts in these sys-tems from complete removal of trees to simplya percentage, for example, could mimic thenatural disturbance regime common to thenortheastern and midwestern United States. Inaddition, reducing harvests could be desirablein public forests that are managed for multiplepurposes, such as recreation, biodiversity, andwater.

These strategies would be most suitable inforest regions with active management and ahigh potential to store carbon, such as thosewith long-lived species and slowly decompos-

ing dead plant matter, which are common inthe Pacific Northwest. The carbon benefit ofeither of these practices will depend on thetemporal and spatial scales at which they areadministered – applying these practices overlonger timeframes and larger landscapes leadsto greater carbon benefits.

In addition to an increase in carbon storage,benefits of decreased harvesting also include anincrease in structural and species diversity. Onthe other hand, the costs are an increased riskof carbon loss due to disturbance and thepotential for increased harvesting elsewhere tocompensate for the reduction in forest productsgenerated.

4. Forest management: increasing

forest growth

In addition to afforestation, another strategyfor increasing carbon storage is to increase thegrowth rate of existing or new forests.Management practices that can increase forestgrowth include: regenerating harvested ordamaged forests, controlling competing vege-tation, fertilizing, plantinggenetically improved trees,and selecting species forsuperior productivity. Yieldgains from these practicescan be impressive. In pineforests in the southernU.S., tree breeding hasimproved wood growth(and carbon storage rate)by 10-30%, and fertilizationcan show 100% gains forwood growth. For southern

Box 2. CO-BENEFITS OF FORESTS

Our report focuses on forests seen through the lens of carbon, and only carbon.However, forests are managed for many purposes, and carbon storage and thegrowth of wood for products and fuel to offset fossil fuel use are far from the onlyreasons forests are valuable. Forests also provide many other ecosystem ser-vices that are important to the well-being of the U.S. and its inhabitants: protec-tion of watersheds from erosion, nutrient retention, good water quality, reductionof peak streamflow and an increase in base streamflow, wildlife habitat anddiversity, recreational opportunities and aesthetic and spiritual fulfillment, andbiodiversity conservation. Americans are strongly attached to their forests. Insome cases, managing strictly for carbon would conflict with other co-benefitsof forests. The option of avoided deforestation retains the co-benefits of forestsand the carbon in forest ecosystems, while afforestation adds these co-benefitsin addition to increasing carbon storage. Even simple forests, such as planta-tions, generally reduce erosion, regulate streamflow, and increase wildlife habitatand biodiversity compared to crops or livestock pasture because the frequencyof harvest or stand–replacing disturbance is much less for forests.

Figure 7. Average carbonstored on a landscape will varywith the time between harvests(harvest interval) and how muchbiomass is removed eachharvest. Lengthening theharvest interval will have agreater effect for harvests whereremovals are high (blue arrowsshow an increase in harvestinterval from 25 to 75 years).Decreasing harvest intensityfrom 100% of trees to 20% oftrees (black arrows) will have agreater effect for shorter harvestintervals. (Courtesy of Mark E.Harmon, Oregon StateUniversity, 2009.)

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U.S. pines, operational plantations usingimproved seedlings, control of competing veg-etation, and fertilization grow wood four timesfaster than naturally regenerated second-growth pine forests without competition con-trol. The potential to increase forest growthvaries by climate, soil, tree species, and man-agement.

Increases in carbon stocks will generally beproportional to increases in growth rates. Thatis, a 10% increase in growth will result in a10% increase in carbon stocks, assuming thatthe harvest interval and amount harvested donot change. As shown in Figure 3, the rate offorest growth will naturally slow down as theforest ages. Management decisions for increas-ing carbon stocks should take into account for-est growth over time, the amount of timberthat would end up in wood products if the for-est were harvested, and how long the harvestedcarbon would remain sequestered in the woodproducts. Knowledge of these variables willhelp determine when or whether to harvest.

The area of forestland in the U.S. that couldbe managed to increase forest growth includes

more than 500 million acres and consists ofalmost all U.S. public and private forestland,excluding remote and reserved areas such asnational parks. However, even reserved areascould potentially be managed to restore dam-aged ecosystems, which could also lead toincreased forest growth.

Increasing forest growth through manage-ment has benefits and costs. The benefitsinclude increased wood production and thepotential for planting species and genotypesadapted to future climates. The costs includereducing the carbon benefit by emissions ofnitrous oxide from forest fertilization, reducedwater yield (faster growth uses more water),and a loss of biodiversity if faster growth isaccomplished by replacing multi-species forestswith monocultures.

5. Forest management: fuel manage-

ment to reduce fire threat

Fuel management uses thinning (Box 3) tolower foliage biomass to reduce the risk ofcrown fire because crown fires are difficult, ifnot impossible, to control. Fuel managementoccurs in forests with a variety of historical fireregimes – from forests where historical forestdensity was lower and the natural fires weremostly surface fires, to forests with stand-replacement fire regimes in which crown firesnaturally occurred. Fuel management tem-porarily lowers the carbon stored in forest bio-mass and dead wood because the thinned treesare typically piled and burned or mulched andthen decompose.

If a crown fire burns through a forest thatwas thinned to a low density, the fire maychange from a crown to a surface fire in whichmany of the trees can often survive the fire. Incontrast, many or all of the trees in anunthinned stand will be killed by a crown fire.This contrast in survival has led to the notionthat fuel treatments offer a carbon benefit:removing some carbon from the forest mayprotect the remaining carbon.

There are two views regarding the scienceon carbon savings through fuel treatments.Some studies have shown that thinned standshave much higher tree survival and lower car-bon losses in a crown fire, or have used mod-eling to estimate lower carbon losses fromthinned stands if they were to burn. However,other stand-level studies have not shown acarbon benefit from fuels treatments, and evi-dence from landscape-level modeling suggeststhat fuel treatments in most forests will

Box 3. THINNING AND CARBON

Thinning is an effective forest management technique used to produce largerstems more quickly, reduce fire risk, and increase tree resistance to insectsand disease. Thinning increases the growth of the remaining individual trees,but generally decreases overall forest wood growth until the remaining treesgrow enough to re-occupy the site. The carbon stock in a thinned stand is gen-erally lower than that in an unthinned stand. If the harvested trees are used forbiomass energy or long-lived forest products, these carbon benefits may com-pensate for the lower biomass and the wood growth of the thinned stand.Because of lower overall growth of a thinned stand, even 100% use of the har-vested trees for products or biomass energy may not produce a total carbonbenefit greater than that of the higher storage and storage rate in an unthinnedstand. The net carbon consequences of thinning will depend the most onwhether the harvested trees are used for long-lasting wood products or bio-mass energy, but also on the change in risk of a crown fire relative to the prob-ability of fire occurring, the species, the site, the thinning regime, and thelength of the harvest interval.

Figure 8. A hydro-axe is used togrind up trees to reduce canopy

fuel loads and lower the risk ofcrown fire. Photo by Dan Binkley,

Colorado State University.



decrease carbon, even if the thinnedtrees are used for biomass energy.More research is urgently needed toresolve these different conclusionsbecause thinning to reduce fuel is awidespread forest treatment in theU.S. We recommend that suchresearch focus on the landscape scalebecause carbon loss in thinningneeds to be placed in the context ofthe expected fire frequency andextent, and the potential for regener-ation after fire. Regardless of the out-come of such research, the carbonbenefits of fuel treatments can beimproved by using the harvestedtrees for wood or biomass energy.

6. Urban forestry

Urban forestry offers very limited potential tostore carbon, but we address urban forestshere because of the large interest in usingthem to offset carbon emissions and becauseurban trees provide many co-benefits, includ-ing aesthetic benefits and environmentaladvantages in addition to carbon sequestra-tion. The potential for carbon offsets ofgreenhouse gas emissions through urbanforestry is very limited for two reasons: 1)urban areas make up only a small fraction ofthe U.S. landscape and 2) urban forests areintensively managed and may require largeenergy, water, and fertilizer inputs for plantingand maintenance.

Urban forests can have important biophysi-cal effects on climate. Trees have a coolingeffect on local temperatures due both to shad-ing effects and to evaporative cooling in tran-spiration. Shading intercepts incoming radia-tion in the daytime, which can reduce bothday and night surface temperatures. Whentrees are planted very close to buildings, theycool building temperatures and reduce the fos-sil fuel emissions associated with air condi-tioning. When urban forests are planted oververy large regions, the climate effects are lesscertain, as trees can have both warming(absorption) and cooling effects.

The higher the maintenance required forurban trees, the less likely they will help miti-gate climate change. In some regions, citiesare located in what would naturally beforested areas; thus, urban forests serve torestore forests to land that was previouslydeforested. In such regions, trees may have rel-atively low maintenance requirements. In

cities located in grasslands and deserts, urbanforests require large amounts of irrigationwater for maintenance.

Because of these many tradeoffs, the fol-lowing factors must be taken into account todetermine the net climate impact of urbantrees: 1) the carbon storage rate of the trees,2) fossil fuel emissions from energy associ-ated with planting and maintenance, 3) fos-sil fuel emissions resulting from the irriga-tion process, 4) nitrous and nitric oxideemissions from fertilizer use, and 5) the neteffect of trees on local air temperature andits impact on building energy use. These fac-tors are likely to be highly variable by regionand by species.

7. Biomass energy, carbon storage in

products, and substitution

Biomass energy

The use of forest biomass energy prevents car-bon emissions from fossil fuel use. In 2003,biomass energy was 28% of the U.S. renew-able energy supply and 2% of the total U.S.energy use. Biomass energy is used primarilyfor electric power in the forest products indus-try and for residential heating. In the future,biomass may become an important feedstockfor liquid biofuels.

If cost were not a constraint and the publicsupported this use of forests, U.S. forests couldpotentially provide energy production offset-ting 190 teragrams of fossil fuel carbon emis-sions per year, or the equivalent of 12% ofU.S. fossil fuel emissions in 2003 (as discussedfurther in Environmental costs below). It hasbeen estimated that by 2022, forest biomassfeedstocks could produce 4 billion gallons ofliquid biofuel per year (offsetting 2.6 teragramsof fossil fuel carbon emissions).

Figure 9. Sycamores liningSycamore Street in Los Angeles,California. Photo by Diane E.Pataki, University of California,Irvine.



Carbon storage in wood and paperproducts

In the U.S., forest products are stored in twomajor “pools”: those that are in use, and thoseheld in landfills. Current additions of carbon tothese pools from trees harvested in the U.S. aregreater than decomposition losses from thesepools, so carbon stored in these pools is increas-ing. In 2007, the net increase in carbon storedas products in use and in landfills was 30 tera-grams of carbon (offsetting 1.7% of 2003 U.S.fossil fuel emissions), with about two thirds ofthe 30 teragrams being net carbon additions tolandfills. Recently, additions have been declin-ing due to decreases in U.S. timber harvests.

Carbon is also accumulating in “products inuse”, primarily in buildings. The total carbonheld in single and multifamily homes in 2001was about 700 teragrams of carbon. Annual netcarbon accumulation in landfills is larger thanthat for products “in use” because about 80% ofwood and 40% of paper decays very slowlyunder the anaerobic conditions in landfills.However, these same anaerobic conditions thatslow decomposition also produce methane, agreenhouse gas with greater than 25 times thewarming potential of CO2. Because only 50% ofmethane is captured or oxidized before release,methane release reduces the carbon storage ben-efits in landfills. If we were to use the 30 tera-grams per year of forest products currently goinginto landfills as biomass energy, we would offset1.2% of U.S. fossil fuel use, lower emissions ofmethane, and extend the life of landfills.

Substitution

Carbon emissions can be offset by substitut-ing wood products for products such as steeland concrete, which generate more green-house gas emissions in their production. A

review of studies suggests that ifwood products containing one unitof carbon were used in buildings as asubstitute for steel or concrete, fossilfuel emissions from manufacturingwould be reduced by two units ormore. Opportunities for increasedsubstitution in the U.S. will mostlyneed to be found outside of thehousing industry because most hous-ing is already built using wood.

Environmental costs of biomassenergy and forest products use

The carbon benefits of increasing the use ofwood for biomass energy and for product substi-tution would require more intensive forest man-agement over a much broader area than cur-rently occurs. For example, to obtain theaforementioned 190 teragrams per year of bio-mass energy would involve harvesting all of thecurrent annual net forest growth in the U.S. Todo that would require intensive managementon much of the U.S. forest estate and wouldreduce the carbon stored in the forest. Ifbranches and foliage were to be removed forbiomass energy, fertilization would likely beneeded to replace the nutrients removed tomaintain productivity. Additionally, dead woodwill decrease and soil carbon may decreaseunder harvesting, creating a carbon debt thatwill require time to pay off.

Links between strategies

Strategies can be combined to increase the car-bon benefit. For example, Figure 5 shows thatthe maximum potential benefit from a projectthat reestablished forest increases if the stand isperiodically harvested and the wood is used forsubstitution and the biomass used for fuel.Increased wood use for forest products and bio-mass energy would be compatible with afforesta-tion, increasing forest growth, and fuel manage-ment to reduce fire threat. However, increasedwood use may conflict with increasing carbonstores on the landscape from reducing harvestsand avoiding deforestation. Increased forestgrowth would be compatible with reducing har-vests and avoiding deforestation if the increasedgrowth frees land for these other uses.

Carbon offsets and credits

A carbon offset is a reduction in greenhousegas emissions (or an increase in carbon seques-

Figure 10. Logs harvested atManitou Experimental Forest in

Colorado. Photo by RichardOakes, USDA Forest Service.



tration) by one entity, which cancompensate for – or “offset” – emis-sions by another entity. The lattercan thus continue with business asusual and avoid directly reducing itsown emissions. Offsets are typicallytraded (bought and sold) as “carboncredits.” Typically, offset projects arecertified, which instills confidencethat the offsets are real and enablesthe associated carbon credits to besold or traded to those who voluntar-ily wish to reduce their reductions orare regulated to do so. In the U.S.,carbon credits are traded as part of avoluntary market, and the certifica-tion process varies widely. Europe, which rati-fied the Kyoto Protocol, has a regulated car-bon market. Some of the forest managementstrategies discussed in this paper could “earn”carbon credits, such as afforestation, decreas-ing harvest intensity, increasing forest growth,use of biomass energy, and substitution.

Carbon offsets require additionality, meaningthat the carbon benefits occur directly as aresult of an action deliberately taken toincrease carbon sequestration. Additionality isrequired because reducing greenhouse gas emis-sions over business as usual is a goal andbecause no one wishes to pay for somethingthat would happen anyway. Demonstratingadditionality for forest activities requires thatthe activity be compared against a baseline sce-nario without activity. Demonstrating addition-ality is relatively straightforward for afforesta-tion, urban forestry, and biomass energy usebecause the “starting point” can be quantified.It is much more complex for management thatreduces carbon outputs or increases forestgrowth because larger areas need to be moni-tored for a longer time to validate increasedcarbon storage. It is also difficult to show addi-tionality for the strategy of avoiding deforesta-tion because carbon storage does not necessar-ily increase if forests are simply retained.

Many traders of forest carbon credits are alsoconcerned with permanence, because carboncredits associated with the offset are soldbefore the management is fully implemented.Some forest carbon can be temporarily lost ina disturbance or harvest. It can also be lostwith land use changes, some of which can pre-serve the option of forest reestablishment(such as change to agriculture or pasture) andsome of which do not (urban development).For land maintained as forest, forest carbonstorage can be considered permanent as long

as the climate remains suitable because thelandscape will maintain a level of carbondetermined by the disturbance or harvestinterval.

The most serious concern in any effortwhere forest management is changed for car-bon benefits is leakage – changes outside ofthe project boundary that reduce or eliminatethe carbon benefit. For example, afforestingagricultural land in the U.S. may increasedeforestation elsewhere to meet the demandfor food. Or, subsidizing forest carbon in theU.S. could decrease harvests, increase importsof wood and wood products, and lead toincreased forest harvest – and thus reducedforest carbon – elsewhere. Leakage occurs, butis very difficult to measure because of its globalnature and the difficulty of identifying causeand effect.

Although carbon offsets and credits featureprominently in comprehensive climate-and-energy legislation and may be critical to asociety-wide effort to address climate change,other systems for increasing forest carbonsequestration may be simpler than carbon off-sets. For example, direct payments tolandowners for a particular land use (as in thecurrent Conservation Reserve Program) couldensure desired management, and could rewardavoided deforestation. Land-use regulationcould also be used to force behavior thatsequesters carbon (for example, minimumharvest intervals or requirements to planttrees on agricultural lands).

Measuring, monitoring and

verifying carbon offsets

As the U.S. does not have a regulated carbonmarket, this discussion of monitoring andverifying carbon offsets is based on processes

Figure 11. Tree harvesting atManitou Experimental Forest inColorado. Photo by RichardOakes, USDA Forest Service.



outlined for voluntary markets. Carbon man-agement begins with a project design that hasbeen validated by scientific study to increasecarbon storage rates compared to baseline rates.Once additional carbon accumulates, credibleand accepted measurement and monitoringmethods must be used to document carbongains. Next, many offset projects and activitiesdemonstrate that they do not cause leakage, butnot all voluntary markets require this importantbut difficult step. Finally, an independent verifi-cation confirms that the project was installedcorrectly, is performing as projected, and thatthe carbon reporting is valid.

Measurement of carbon at variousscales

At the scale of individual forest stands, ade-quate measurements (accurate to about 20%)can be made to estimate the carbon stored intrees, plants, dead wood, and in litter on theforest floor using standard inventory methods.Improvements to these methods would likelyinvolve increased monitoring costs. Stand-level measurements of belowground stocks aremore difficult because of the large cost of sam-pling soil carbon and fewer equations for esti-mating belowground biomass. Soil and below-ground carbon monitoring should receiveattention in accounting for forest carbonbecause forest harvest may cause an averageloss of 8% of soil carbon stocks and 30% of theorganic layer (forest floor) carbon.

At the landscape level, projects can be mon-itored and verified using remote sensing.Remote-sensing methods enable direct moni-toring of forest age, cover types, and distur-bance. Changes in carbon stores can be esti-mated with this information using ecosystemor accounting models. Monitoring at theregional level assesses the large-scale impact of

carbon management. The Forest Inventoryand Analysis National Program conducts anational-level strategic forest inventory basedon a combination of on-the-ground measure-ments of all forest carbon pools and remotely-sensed observations. The inventory producesestimates of forest age, cover types, and distur-bance and uses modeling for components thatare difficult to measure.

Carbon stored in wood products is more dif-ficult to monitor than carbon in the forest.Carbon in solid wood products in structurescould be estimated using current census datawith deductions for the fraction of productsthat are imported. Rates of accumulation forall forest products could also be monitoredusing data on production rates, recycling rates,and discard rates (to landfills). Biomass energyuse could be tracked through surveys of bio-mass energy facilities.

How should carbon stores bemeasured?

Since carbon-storage projects take place acrossmany different scales (stand, landscape,regional, and national) and jurisdictions, mul-tiple methods of measurement are needed. Alist of approved methods for measuring carbonpools should include the minimum number ofpools to be measured with methods havingminimal bias (that do not lead to frequentover- or under-counts of carbon) as well as theminimum frequency of measurements. Thereis an inherent level of uncertainty associatedwith any method for measuring carbon, andthere is a practical need to decide how to treatthis uncertainty in decision-making. If we usehigh-end estimates for forest carbon storage,we may over-promise what forests can do andobscure the need for mitigation actions inother sectors. Given the urgent need to meetclimate change mitigation objectives and thehigh risks to society associated with failing tomeet them, we recommend discounting car-bon estimates where they are uncertain. Assampling frequency and specificity increase,uncertainty should decrease, but costs will alsorise. Individual groups or entities can decidewhich approved method should be used foreach project based on a cost-benefit ratio,weighing cost against gaining potential carbonbenefit. The potential for leakage andaccounting for and underestimating distur-bance losses can be reduced by implementinga national-level accounting system that vali-dates the carbon storage at a national scale.

Figure 12. Regeneration inYellowstone National Park

19 years after the 1988 fires,with Dan Kashian

(Wayne State University).Photo by Mike Ryan,

USDA Forest Service.



Economics of forest carbon

Most of the strategies for increasing forest car-bon storage or the use of forest products wouldrequire carbon to have a substantial valuethrough credits for offsets or through someother mechanism to compensate those thathave an economic interest for additional costsor foregone profit. To sequester an additional200-330 teragrams of carbon in forests (theequivalent of offsetting 13-21% of 2003 U.S.fossil fuel emissions) would require paymentsof between $110-$183 per metric tonne of car-bon, or 23-60 billion dollars per year. The per-tonne payment requirement reflects the eco-nomic value of the current use. For example,for afforestation, landowners would expectcompensation for both their lost agriculturalrevenues and for the cost of planting trees. Forlengthening the harvest interval, landownerswould need compensation for the reducedproduct flow.

Although the total costs of such an under-taking are large, the costs of implementingthese forest activities to sequester carbon areoften far less than the cost of reducing thesame amount of greenhouse gas emissions byother means, such as through the transporta-tion or electric power sectors. Therefore,forests can play a key role in reducing theoverall cost of achieving greenhouse gas emis-sion reduction targets. Economic modeling ofU.S. climate policy proposals consistentlyshows that forest carbon sequestration andother “offset” activities can significantly lowerthe cost of complying with the proposed regu-lations.

Climate change and other risks

to forest carbon storage

The potential to increase carbon storage inforests needs to be weighed against the pro-jected increases in disturbances promoted by achanging climate that will lower carbon stor-age. Climate change may also make regenera-tion after disturbance more difficult or renderthe current tree populations geneticallyunsuitable. Finally, population increase andexurban development will decrease the gen-eral amount of forested area. Because distur-bances are likely to increase in the future, werecommend conservative estimates of poten-tial gains from forest carbon management.

A potential negative effect of forest manage-ment strategies to enhance carbon storage isthat, as forest carbon storage increases, there is

a potential for greater loss of carbon storesfrom forest fires, insect outbreaks, hurricanes,windstorms, and ice storms. Climate changethreatens to amplify these risks by increasingthe frequency of these disturbances. If climatechange increases the frequency of disturbance,as observational and modeling studies for theU.S. suggest, many forests could release signif-icant amounts of carbon to the atmosphereover the next 50-100 years – simultaneouswith efforts to harness CO2 emissions. It isimportant to remember that, at the landscapelevel over the long term, disturbance does notcause a net loss of forest carbon…as long asthe forest regenerates. But if the frequencyand/or severity of disturbance increase sub-stantially, long-term carbon storage at thelandscape scale will be reduced because thefraction of the landscape with large, oldertrees (that have high carbon stores) willdecline. Climate change could also increasesoil decomposition, leading to carbon lossesfrom a part of the ecosystem that we considerto be relatively stable and that contains about40% of the total carbon in U.S. forests.

The largest risk to carbon storage from dis-turbance is that the forest may not regenerateand instead be replaced by a meadow or shrub-land ecosystem, losing much carbon in theprocess. As a result of past fire suppression, wesee this happening currently in the westernU.S. as high-severity fires occur in ecosystemsthat are adapted to low-severity fire regimes.Although actions are being taken to reducethe fire risk, the carbon-related effects are cur-rently unknown. Climate change may alsoincrease the likelihood that forests will notregenerate sufficiently since highly adaptedspecies and genotypes may have a difficult timegrowing under altered climatic conditions.

Conclusions and

Recommendations

U.S. forests and forest products currently offset12-19% of U.S. fossil fuel emissions, largelyowing to recovery from past deforestation andextensive harvesting. Increased nitrogen depo-sition and atmospheric CO2 compared to his-torical levels may also be contributing toincreased forest growth, but the science sup-porting their contribution is uncertain becauseof a limited number of experiments and thedifficulty in assessing change over the diverseforests of the U.S.

How long will U.S. forests remain a carbonsink? Since 1940, forest regrowth in the U.S.



has recovered about a third of the carbon lostto the atmosphere through the deforestationand harvesting that occurred from 1700-1935(Figure 13). To recover the remaining two-thirds of the carbon that was lost wouldrequire reestablishing forests in a significantportion of what is now agriculture and pastureland. However, reforesting this part of theU.S. (almost all land east of the Mississippi) isnot feasible from an economic and food-secu-rity perspective. Today’s recovery from the for-est clearing and wood-based economy of the1800s and early 1900s will likely sustain car-bon storage rates at the current rate fordecades, but not indefinitely.

But, forest carbon storage only gets us part ofthe way. Even under the best scenarios, theamount of carbon storage potential is finite.Strategies that combine increased use of forestproducts to offset fossil fuel use (such as use ofbiomass energy and substitution), in conjunc-tion with increasing carbon storage onforested landscapes, are likely to produce themost sustainable forest carbon benefits.

Every strategy we examined has tradeoffs.Avoiding deforestation and increasing the har-vest interval in the U.S. may move timber pro-duction elsewhere, resulting in no net benefitfor carbon in the atmosphere. Reestablishingforests has great potential but will also displacecurrent land uses such as farming and pasture.Increasing forest product use and forest bio-mass energy will require more active forestmanagement over larger areas than currentlyoccurs and may lower forest carbon stores.Intensive silviculture can increase growth, butdecrease streamflow and biodiversity. Forestproducts in landfills increase carbon storage,but the resulting methane emissions pose aproblem. A better use for waste material, there-fore, is energy production. Recognizing thesetradeoffs will be vital to any effort to promoteforest carbon.

Because forest carbon loss poses a significantclimate risk and because climate change mayimpede regeneration following disturbance,avoiding forest loss and promoting regenera-tion after disturbance should receive high pri-ority as policy considerations. Forest lossmoves a large portion of the carbonsequestered in forests into the atmosphere,particularly where the loss includes not onlytrees but also the decomposition of soil car-bon. Because of climate change, increasingthreats from disturbance, and continued popu-lation growth and resulting exurban develop-ment, we cannot assume that all existingforests will remain. Because there is a highlikelihood that climatic patterns will shift andthe frequency of disturbances will increase –potentially making existing tree species lesssuited to their environment – it would be pru-dent to focus on regeneration after disturbanceto help ensure maintenance of forests.

The various strategies for storing carbon inforests have different associated risks and levelsof uncertainty. Retaining forests (which alsoincludes regenerating after disturbance) andafforestation both involve low levels of uncer-tainty regarding carbon consequences andtherefore low risk to carbon storage – asidefrom the risks of carbon loss in disturbance orthat the deforestation will simply happen else-where. The carbon benefits of using biomassenergy and long-lived forest products are alsofairly certain, as long as forests regenerate.Lengthening harvest intervals involves a bitmore risk because disturbance would occur inforests with higher carbon stores and becausedecision-makers can change harvest intensityquickly relative to forest growth.

Regardless of the risks and uncertainties,any policy to encourage forest carbon storageshould: 1) promote the retention of existingforests; 2) account for other greenhouse gaseffects, such as methane and nitrous oxide

emissions and biophysical changes; 3)account for harvest moving elsewhereindirectly caused by changes in manage-ment with the project boundary; 4) rec-ognize other environmental benefits offorests, such as biodiversity, nutrientmanagement, and watershed protection;5) focus on the most robust and certaincarbon storage benefits in any compen-sation scheme; 6) recognize the difficultyand expense of tracking forest carbon,the cyclical nature of forest growth andregrowth, and the extensive movementof forest products globally; 7) recognize

Figure 13. The carbon balanceof the U.S. forest sector shows

that clearing for agriculture,pasture, development, and

wood use released ~42,000 Tgof carbon from 1700 to 1935,

and recovered about 15,000 Tgof carbon from 1935-2010.

(Used with permission, fromJournal of Environmental Quality

35:1461-1469 (2006))

1700 1800 1900 2000

Year

800600

400

200

0

–200

U.S

. Fo

rest

Car

bo

n B

alan

ce(T

g C

arb

on/

year

)

Carbon Release

Carbon Storage



that the value of any carbon credit willdepend on how well the carbon can be mea-sured and verified; 8) acknowledge that cli-mate change and population growth willincrease the potential for forest loss and maykeep large-scale projects from reaching theirfull potential; 9) recognize the tradeoffs; and10) understand that the success of any car-bon mitigation strategy depends on humanbehavior and technological advances inaddition to forest biology. Finally, becauseCO2 remains in the atmosphere for morethan 100 years, any action to avoid furtheremissions should be undertaken as soon aspossible.

Few forests are managed solely for carbon– rather, carbon storage serves as a co-bene-fit that accompanies or perhaps helps payfor other ecosystem services provided byforests (Box 2). As we have discussedabove, elevating carbon storage to the pri-mary focus of management could poten-tially impede the other co-benefits offorests. A focus on carbon storage to thedetriment of other ecosystem services wouldbe short-sighted.

For Further Reading

Birdsey, R.A., K.S. Pregitzer, and A. Lucier.2006. Forest carbon management in theUnited States: 1600-2100. Journal ofEnvironmental Quality. 35: 1461-1469.

CCSP. 2007. The First State of the CarbonCycle Report (SOCCR): The NorthAmerican Carbon Budget and Implica-tions for the Global Carbon Cycle. AReport by the U.S. Climate ChangeScience Program and the Subcommittee onGlobal Change Research [King, A.W., L.Dilling, G.P. Zimmerman, D.M. Fairman,R.A. Houghton, G. Marland, A.Z. Rose,and T.J. Wilbanks (eds.)]. NationalOceanic and Atmospheric Administration,National Climatic Data Center, Asheville,NC, USA, 242 pp.

Harmon, M.E., A. Moreno, and J.B. Domingo.2009. Effects of partial harvest on the car-bon stores in Douglas-fir/Western hemlockforests: A simulation study. Ecosystems. 12:777-791.

Hurteau, M.D., G.W. Koch, and B.A.Hungate. 2008. Carbon protection and firerisk reduction: toward a full accounting offorest carbon offsets. Frontiers in Ecology andthe Environment. 6: 493-498.

Jackson, R.B., E.G. Jobbagy, R. Avissar, S.B.

Roy, D.J. Barrett, C.W. Cook, K.A. Farley,D.C. le Maitre, B.A. McCarl, and B.C.Murray. 2005. Trading water for carbonwith biological sequestration. Science. 310:1944-1947.

Mitchell, S.R., M.E. Harmon, and K.E.B.O'Connell. 2009. Forest fuel reductionalters fire severity and long-term carbonstorage in three Pacific Northwest ecosys-tems. Ecological Applications. 9: 643-655.

Murray, B.C., B.L. Sohngen, A.J. Sommer,B.M. Depro, K.M. Jones, B.A. McCarl, D.Gillig, B. DeAngelo, and K. Andrasko.2005. Greenhouse gas mitigation potentialin U.S. forestry and agriculture, EPA-R-05-006. U.S. Environmental ProtectionAgency, Office of Atmospheric Programs,Washington, D.C.

Nabuurs, G.J., O. Masera, K. Andrasko, P.Benitez-Ponce, R. Boer, M. Dutschke, E.Elsiddig, J. Ford-Robertson, P. Frumhoff, T.Karjalainen, O. Krankina, W.A. Kurz, M.Matsumoto, W. Oyhantcabal, N.H. Ravin-dranath, M.J. Sanz Sanchez, and X. Zhang.2007: Forestry. In Climate Change 2007.Mitigation. Contribution of WorkingGroup III to the Fourth AssessmentReport of the Intergovernmental Panel onClimate Change [B. Metz, O.R. Davidson,P.R. Bosch, R. Dave, L.A. Meyer (eds)],Cambridge University Press, Cambridge,United Kingdom and New York, NY,USA.

Nowak, D.J. and D.E. Crane. 2002. Carbonstorage and sequestration by urban trees inthe USA. Environmental Pollution. 116:381-389.

Skog, K.E. 2008. Carbon storage in forestproducts for the United States. ForestProducts Journal. 58: 56-72.

U.S. Environmental Protection Agency. 2008.Inventory of U.S. Greenhouse Gas Emis-sions and Sinks: 1990-2006. 394 pp.

Woodbury, P.B., J.E. Smith, and L.S. Heath.2007. Carbon sequestration in the US for-est sector from 1990 to 2010. Forest EcologyManagement. 241: 14-27.

Acknowledgements

We thank Noel Gurwick, special editor, for hismany helpful suggestions. Funding for thisproject was provided by Joint VentureAgreement 08-JV-11221633-248 between theUSDA Forest Service Rocky MountainResearch Station and the Ecological Societyof America.

16 esa ©© The Ecological Society of America • [email protected]


expressed by the authors in ESA publicationsis assumed by the editors or the publisher.

JJiillll SS.. BBaarroonn, Editor-in-Chief,US Geological Survey and Colorado StateUniversity, [email protected].

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Issues in Ecology uses commonly-understood lan-guage to report the consensus of a panel of sci-entific experts on issues related to the environ-ment. The text for Issues in Ecology is reviewedfor technical content by external expert review-ers, and all reports must be approved by theEditor-in-Chief before publication. This reportis a publication of the Ecological Society ofAmerica. No responsibility for the view

a synthesis of the science on forests and carbon for u.s ... · pdf fileother hand, avoiding...

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