Can trees really help fight global warming?

For the past three years, as most of the debate on the global-warming issue has focused on the ability of climatologists to predict the future, AFA has coordinated work by a diverse group of scientists to look at forest-related opportunities to address the potential for offsetting the atmospheric buildup of carbon dioxide and other greenhouse gases. "Are there," we asked, "forest improvements that would make economic and environmental sense even if the fears of global warming turn out to be unwarranted?"

The answer, dearly, is yes. There are many ways to expand and improve the role that trees and forests play in people's lives. Most of them are helpful in addressing the root causes of global warming, and many can be justified on a wide variety of economic and environmental bases. The first phase of this work, looking at opportunities to expand tree and forest benefits on land that is not now in forest usage, is the subject of a book that is in the final phases of editorial preparation by AFA. Titled Forests and Global Warming, Volume 1, it will be available for purchase by midyear. The following article is exerpted from its summary chapter.

BIOMASS AND GLOBAL WARMING

The basic premises about the greenhouse effect are fairly straightforward. Trace gases, such as carbon dioxide (C0C[O.SUB.2]), methane, nitrous oxide, ozone, and chlorofluorocarbons, allow light from the sun to pass through our atmosphere to the Earth but have a tendency to reflect longer-wave heat rays that might otherwise escape back into the upper atmosphere. This phenomenon, called the greenhouse effect, is what keeps Earth's temperatures at their present, life-supporting levels. If the concentration of those gases increases, however, there is a strong likelihood that a global-warming trend will be experienced, although scientists do not agree on how fast, or how far, this will proceed.

What is not in doubt are the high stakes involved. A temperature increase greater than a degree or two could have significant impacts on agriculture, forests, and sea levels, so the possibility is a reasonable cause for concern.

Atmospheric C[O.SUB.2], the most important of the greenhouse gases, is inextricably tied to growing plants and thus is directly linked to the growth of the trees and forests of the world. The global C[O.SUB.2] level has fluctuated in the range of 200-280 parts per million (ppm) over the past 160,000 years, according to studies of historic records contained deep in icecaps and ocean sediments. Since the beginning of the industrial revolution, however, atmospheric levels of C[O.SUB.2] have shot up rapidly, moving from about 280 ppm in 1750 to 350 ppm in 1989. With C[O.SUB.2] levels continuing to increase at an estimated rate of 0.5 percent annually, a growing number of scientists are concerned over the potential for significant global warming within the coming century.

An overwhelming proportion of the net annual C[O.SUB.2] addition--some 80 percent--comes from fossil-fuel burning, and the United States contributes about 22 percent of that total. Thus the reduction of fossil-fuel burning, particularly where it can be done without loss of productive output (through higher efficiencies, for example) becomes a first-order goal in the attempt to reduce atmospheric C[O.SUB.2] accumulation. Wood used in place of fossil fuel for energy represents a recently formed source of carbon and one that can readily be replaced by new growth in forests. This means that a recycling carbon source can replace a nonrecycling fossil source, and the net addition of carbon to the atmosphere over time is reduced accordingly.

Trees and forests are also important as a potential storage sink of carbon dioxide. As trees add mass to trunks, limbs, and roots, carbon is stored in relatively long-lived structures instead of being released to the atmosphere. Obviously, all plants do this same conversion, but grasses and crops that are eaten, or that die and decompose within a season, are not as effective in providing long-term storage as trees and the wood products made from them.

CARBON AND TREES

In measuring the actual amount of carbon stored in forests, available data are based on the measurements of trees made by foresters over the years. These data focus on the merchantable portion of the tree--the straight trunk portion (or bole) that can be converted into wood products. It is possible, however, to utilize this .vast array of scientific information for estimating carbon reserves.

Converting merchantable timber to biomass involves some mathematics. We start with the forester's estimate of merchantable wood, in cubic feet per acre. This is multiplied by a factor of 1.91 for softwoods and 2.44 for hardwoods, or about 2.18 for all species, according to researcher Richard A. Birdsey of the Forest Service. The result is the amount of biomass, in cubic feet, contained in the trunks, branches, leaves, and roots of the forest trees.

The biomass is then converted to weight and carbon content. Birdsey estimates an average of 30.6 pounds of dry weight per cubic foot of wood biomass. The carbon content is about 50 percent of the dry weight. Thus, if the estimated annual growth of merchantable timber for an acre of forest, in cubic feet per year, is known, it can be multiplied by 33.4 (30.6 x 2.18/2) to get an estimate of the annual carbon storage in pounds of carbon per acre per year. If the tree species involved is known, Birdsey's work can be used to get a more precise estimate based on the differences between softwood and hardwood species.

For major timber types, Birdsey also calculated the biomass contained in the litter on the forest floor and the soil carbon, as they are related to the stage of forest growth from seedlings to maturity (see Figure 1). Perhaps the least understood, and least appreciated, carbon that is stored by trees and forests as they grow is that which is stored within the soil profile itself. Current research indicates that the soil organic carbon under forest cover is significantly greater than under either cropland or grassland.

CONVERTING MARGINAL CROP AND PASTURE LANDS TO FORESTS

Dr. Peter Parks of Duke University has estimated that there are 116 million acres of privately owned crop and pasture land that are marginal for crop and pasture use, and that are capable of growing trees. This land pool, three times as large as the target of the Conservation Reserve Program contained in the 1985 farm bill, presents a major opportunity for expanding the area of forest land in the United States.

Of the 62.6 million acres adapted primarily to softwoods, Parks estimates that 23.4 million acres would be economically more profitable in trees than in their current marginal crop or pasture use. On the 53.5 million acres best suited to hardwoods, the lack of forest economic data prevents such analysis, but it is logical to assume that part of those lands would be economically competitive in forest as well.

For the softwood regions where economic data exist, annual net timber growth on converted crop and pasture lands would average 64-90 cubic feet/acre/year. Using the basic conversion factors developed for southern pines by Birdsey, that could mean carbon storage of 2,500 to 3,500 pounds/acre/year during the growth cycle of these new forests.

This means that the planting of trees on 23 million acres of marginal crop and pasture land could in time result in an annual production of 1.5 billion cubic feet of softwood timber per year. That would translate into 33 million tons of total carbon storage per year, as shown in Table 1. Carbon emissions from fossil fuels could be reduced by 11 million tons, as wood waste replaces coal and petroleum for energy. At the high end of the range, if planting were done on the entire 116 million acres, carbon storage in trees and replacement of fossil fuels could offset carbon emissions of 180 million tons per year.

WOODY CROP PRODUCTION

Lynn Wright and her colleagues at the Oak Ridge National Laboratory have conducted intensive investigations into the potential for producing short-rotation woody crops as a substitute for fossil energy resources.

Efficient wood-energy production depends on very rapid growth rates, usually in a rotation of four to eight years. The best species have fast juvenile growth, good stress tolerance and disease resistance, and good regrowth after cutting. Young stands are harvested in the winter after leaf fall, and many of the preferred hardwood species then resprout from the stump, making recultivation and replanting of the land unnecessary for several rotations. Current yields run four to seven harvestable dry tons/acre/year in production research trials, although observed maximums range from seven to 20, and goals of seven to 13 tons/acre/year are possible by the year 2010 on moderate-to-good cropland.

Wright and her coworkers have estimated potential land availability for wood-energy production at 35 to 70 million acres.

A major economic variable is the efficiency with which dry biomass can be converted to energy. This is an area where research and development can make significant changes. Currently, with a net dry biomass yield of five tons/acre/year and present technology, using the wood for producing electricity will result in a net carbon offset of 2.24 tons of carbon/acre/year. If that same biomass were used to produce ethanol the result is estimated to be 0.80 tons/acre/year. Increasing net yields and raising conversion efficiencies through research could, it is proposed, raise those carbon-offset estimates to 4.56 tons/acre/year for electrical production and 2.68 tons/acre/year for ethanol.

POTENTIAL TO INCREASE ENERGY-SAVING TREES AND FORESTS

Windbreaks and shelter-belts are associated mainly with U.S. farm and ranchlands, and their effect upon the root causes of global warming lie in the biomass that they create (within both plants and soils) and the energy savings they create. James Brandle and his coworkers at the University of Nebraska have looked at this aspect of forestry closely, and drawn estimates leading to the following conclusions:

* The potential for new shelterbelt plantings is enormous. It would take 1.4 million miles of windbreaks, occupying 3.4 million acres, to protect the 67.4 million acres of croplands that suffered from excessive wind erosion in 1982. Tree-planting needs are about 920 million.

* An additional 35.4 million acres of cropland could benefit from wind protection, with over 486,000 miles of windbreak on 1.2 million acres. Tree-planting needs are about 320 million.

* There are over 1.6 million unprotected farmsteads that could utilize 200,000 new farmstead windbreaks involving almost 300,000 acres, and over 55 million trees.

* Facilities housing beef and dairy cattle in cold-weather regions could utilize over 500 miles of windbreaks covering 3,000-5,000 acres, just to protect 10 percent of the exposed livestock. Total tree-planting needs are about 1-1.4 million trees and shrubs.

* There are significant opportunities for wildlife-habitat plantings, shelter-belts, and streamside plantings, but it is impossible to estimate their magnitude. * The total 1 to 1.5 billion trees that could be planted in a complete windbreak and shelterbelt program could contain in the range of 100 million tons of carbon in vegetation and soil by the time they reached 25 years of age.

* Indirect carbon savings also result from fuel and fertilizer reductions due to removing acres from cultivation (the windbreaks take some land out of farming and raise yields on the remaining acres enough to more than compensate for the acres devoted to trees).

URBAN AND COMMUNITY TREES

In addition to sequestering carbon directly, properly placed urban trees can have a significant impact on atmospheric carbon buildup through energy conservation. Studies show that electricity for air-conditioning can be reduced as much as 50 percent by properly located trees and shrubs. On the other side of the equation, properly placed trees can reduce winter heating costs by 4 to 22 percent.

The impact of large-scale tree plantings in dries can be important in reducing the well-documented urbanheat-island phenomenon. Researchers have demonstrated that tree planting could save 15 to 44 percent of residential cooling demand on a hot summer day. If the heat-island impact, which often runs 3-5 degrees C, can be eliminated, the cost savings will be enormous.

With energy conservation as a major goal, urban tree-planting programs should focus upon trees that provide direct energy benefits to buildings. The next priority should be trees that will provide shade for parking lots, streets, and other dark surfaces. The lowest priority is to plant where trees "fill in" the open spaces to help reduce the urban heat-island effect by modifying wind patterns as part of the total urban forest.

Hashem Akbari and his coworkers at the Lawrence Berkeley Laboratories have estimated that the total impact of planting 100 million trees around homes and small businesses--coupled with a program to convert dark-colored surfaces such as streets, parking lots, and buildings to light colors--could save about 17 million tons of carbon emissions each year. They estimate that half that effect would be due to the trees, for an annual national savings in the range of six to 10 million tons of carbon per year.

About 800,000 acres of new urban land are developed each year, and over half of it comes from crop, pasture, or "vacant" uses. Planting trees as part of environmentally sensitive development could result in almost as large a net forest gain as converting marginal farmland to trees, and the potential energy savings are enormous.

The "low estimate" shown in Table 1 consists of only the annual impact of a good tree-planting program on newly developed lands. If urban development continues for 20 years at the rate experienced between 1982 and 1987, it would require an annual planting of almost 17 million trees on just the new developments. For the low estimate, it was assumed that the existing urban areas would receive only enough attention to maintain current conditions (this would represent a significant urban tree-planting program in itself, and a major increase in today's tree-care activity in many communities). Though this would be better than letting the urban forest continue to decline, it would not result in any significant new energy savings or carbon sequestration in those areas.

The "high estimate" calculates the impact from both afroresting the newly urbanized lands and achieving the goal of increasing canopy cover in existing urban areas by an average of around 10 percent. About 85 percent of the total benefits, in terms of carbon impact, would be realized as a result of energy conservation.

The high estimate represents a huge tree-planting goal. It would require an annual planting of about 42 million flees to in-fill the existing urban forest over a 10-year effort. In addition, if we assume an average 50year lifespan for urban trees (the realization of which will require a significant improvement in the tree-care programs of many communities), about 30 million trees per year will be needed to provide replacement plants for those that die or are removed. In total the annual tree-planting demand for urban forests appears to be in the range of 89 million trees.

The benefits of an improved urban forest are equally large. The carbon impact, for example, of the high-estimate opportunity looks to be about 30 million tons of carbon mitigation per year, either sequestered in the plants and soils, or in reduced emissions due to energy savings. With about 80 percent of that benefit tied directly to cost savings in energy usage, the cost savings to consumers alone may be adequate to encourage the adoption of significantly higher levels of planting and care in the urban forest.

ADDING UP THE TOTAL POTENTIAL

These studies show that there are major opportunities, through the expansion of U.S. trees and forests, to make a significant impact on net carbon emissions. Those opportunities provide no panacea, and they do not negate the need to consider many other forms of action, including aggressive energy conservation and alternate-fuels research. They are, however, clearly a part of the total approach to the green-house-gas challenge.

The forestry opportunities described here, if implemented, could mitigate from 157-424 million tons of U.S. carbon emissions per year. This would offset somewhere between 10 and 30 percent of the total U.S. net carbon emissions from fossil fuels, which are now in the range of 1.3 billion tons per year.

In addition to offsetting carbon emissions, these forest expansions could have many positive impacts on the environment and the economy. Tree planting on marginal crop and pasture lands can reduce soil erosion, prevent water pollution, and improve wildlife habitat. Additional trees in urban areas, as well as shelterbelts and their many uses, means more moderate summer and winter temperatures, less energy used to heat and cool homes and other buildings, and less fuel, fertilizer, and livestock feed needed in production agriculture. There are significant esthetic and positive social impacts as Well.

Economically, planting trees and more intensively managing the resulting forests mean jobs and more wood products to hold down costs to consumers. Where new forms of products emerge, new processing and other facilities may also provide additional economic stimulus. Improving the conservation usage of trees means less money spent on energy, fuel fertilizer, and feed, and more profit in agriculture.

But there are cautions with our findings, as well. The data supporting them are highly variable in quality, as are scientists' understanding of the relationships between tree and forest growth and biomass impact. A great deal of research is needed to improve their reliability,

Increasing the use of biomass as a replacement for fossil fuels can lower net carbon emissions, but it is not without pollution problems of its own. In addition to the need for new technologies that reduce smoke and particulate release into the atmosphere, scientists must also be wary of the potential for other pollutants, such as heavy metals that may be concentrated in biomass grown in polluted regions or on polluted soils.

However, the total potential of these opportunities is huge, in addition to their impact on the global carbon cycle. Perhaps continued attention to this subject will result in the kind of public attention and public policy support that can pave the way for these opportunities to be realized.