Forest Services

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Temps de lecture : 4 minutes  

We are all familiar with the goods that forests supply, as just described. We are less familiar with the services they provide. Prominent among these are climate regulation, flood control, soil conservation, water cycling, nutrient storage and recycling, and recreation—all of which are a basic part of any economy’s support systems.

In a landmark article in Nature in May 1997, Robert Costanza and 12 collaborators estimated that the earth’s ecosystems provide $33 trillion worth of services per year—only slightly less than the $43 trillion worth of goods and services provided by the global economy. Of this total, Costanza and his coauthers estimated that the earth’s forestland provides $4.7 trillion worth of services, or $969 of services per hectare per year. (See Table 8–2.) This can be compared with roughly $800 worth of corn produced per hectare a year in the U.S. Corn Belt, one of the world’s most productive farming regions.

Table 8-2. Principal Services Provided by Forests

Service

Annual Value per Hectare (dollars)

Climate regulation

141

Erosion control

96

Nutrient storage and recycling

361

Recreation

66

Other

305

Total

969

Impressive though the Costanza team’s analysis is, it omits one of the most valuable services provided by forests—namely, their role in the recycling of rainfall inland that makes the interior of continents productive and habitable. If we continue to destroy coastal forests, the interior deserts of continents will continue expanding, squeezing humanity into an ever smaller area. We often discover the services that forests provide when it is too late, after the trees have been cut. This is perhaps most true of flood control, as China, Thailand, and Mozambique have belatedly discovered. Forests also store nutrients. This is particularly important in the tropics, where almost all nutrients in forest ecosystems are stored in the vegetation itself. Many tropical soils have little organic matter and almost no nutrient storage capacity. If a forest is burned off to plant grass for cattle ranching or crops, whatever is planted can do relatively well in the first few years because of the nutrients remaining in the ashes. But once the ash washes away, as it soon does, the nutrients are gone. This is why much of the land cleared in the tropics quickly becomes wasteland and is abandoned.

Tropical rainforests are highly productive ecosystems, efficiently converting sunlight into plant material. But they can do this only as long as they are intact. Once they are destroyed, they can take centuries to regenerate. And some may never recover—simply because the conditions that existed at the time of their original formation may no longer exist.

Forests help control soil erosion by adding organic matter to the soil and by slowing the flow of water runoff. Leaf litter on the floor protects the soil from being loosened by raindrops, creating a tight link between the vegetation and the soils. The forest vegetation permits soil to accumulate and keeps it from washing away. The accumulated soil in turn provides a healthy medium for the forest to develop. In this symbiotic relationship, losing the forests sometimes means losing the soil, which may in turn prevent the return of the forest.

The ability of forests to slow rainfall runoff and let it percolate downward also means forests play a central role in the hydrological cycle. They recharge aquifers, the underground rivers that supply water for the wells downstream. The more water that runs off when it is raining, the less there is to recharge aquifers. Thus the loss of forest cover leads to a double loss—more damage from flooding and a reduced recharge of aquifers. Forests can purify drinking water as well. Walt Reid, who works with the Millennium Ecosystem Assessment, notes that “within the United States more than 60 million people in 3,400 communities rely on National Forest lands for their drinking water, a service estimated to be worth $3.7 billion per year.” He then notes that this single service, one among many provided by national forests, is worth more than the annual value of timber harvested from these lands.

New York City, with its population of nearly 17 million, recently discovered just how valuable nature’s services are. Faced with the residential and industrial development of the Catskill forest region, the basin that is the source of its water, the city was told it needed a water purification plant that would cost $8 billion to build and $300 million a year to operate. The bill for this would reach $11 billion over 10 years. After analyzing the situation, city officials realized that they could restore the watershed to its natural condition for only $2 billion, thus avoiding the need for the purification plant and saving taxpayers $9 billion.

As mentioned in Chapter 3, forests also help carry water to the interior of continents. Reduced recycling of rainfall inland is already evident in China. Deforestation in southern and eastern parts of the country is reducing the moisture transported inland from the Bay of Bengal, the South China Sea, the East China Sea, and the Yellow Sea, notes Wang Hongchang, a Fellow at the Chinese Academy of Social Sciences. Rainfall in the northwestern interior is declining, contributing to the dust bowl conditions that are developing there. […] A similar phenomenon is evident in Africa […] As deforestation in Africa’s high rainfall coastal regions and in the southern Sahel itself progresses, the capacity of the land to recycle water to the continent’s interior is diminishing. Forests also have a stabilizing effect on local climate, modulating the more extreme day-to-night temperature fluctuations, such as those found in deserts. They store huge amounts of carbon that otherwise would be in the atmosphere in the form of carbon dioxide, contributing to climate change. When forests are cleared, this carbon storage capacity is lost not only in the vegetation above ground but also in the organic matter in the soil from roots and the leaf litter on the forest floor. Another service provided by forests is protection of streams and rivers from silting. In the U.S. Northwest, for example, the clearcutting of forests has destroyed nearby salmon fisheries because of increased muddy runoff. Mismanagement of one natural asset is decimating another. Silting also affects the productivity of dams, whether they are built for power generation or for irrigation. As they silt up, they lose their storage capacity and hence their ability to generate electricity and provide water for irrigation. In extreme cases, reservoirs fill with silt and the investment in the dam is lost.

Lester Russell BROWN

Eco-Economy: Building an Economy for the Earth

(W. W. Norton & Co., NY: 2001)

© Earth Policy Institute

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