Tropical Deforestation And Climate

ROGER A. SEDJO

1 INTRODUCTION

Tropical forests cover a large portion of the globe's land surface running along the equator, roughly between the Tropic of Cancer and the Tropic of Capricorn. The largest expanse of tropical forest is found in the South American equatorial region, predominantly in the Amazon Basin, but extending up into Central America and down into northern Argentina. Large tropical forests are also found in the equatorial regions of Africa and West Africa and in Southeast Asia, running from India to Malaysia, north into China, and continuing to the islands of the East Indian Archipelago and extending into northeast Australia.

Tropical forests take many forms, largely controlled by variation in rainfall, temperature, and season, but also are affected by soil conditions. The climate of the region between the Tropics of Cancer and Capricorn is uniform in that there is a steady year-round temperature. However, annual rainfall may vary from less than 10 mm along the Peruvian coast to more than 10 m along the Colombian coast only a few hundred kilometers to the north (Terborgh, 1992).

Although tropical forests are often rainforest or wet tropical forest, large areas of dry tropical forests exist in almost all of the regions discussed, covering large areas in South and Central America as well as Africa and, to a lesser extent, Southeast Asia. Tropical forests range from open savannas where rainfall is limited to dense tropical rainforests where rainfall is most abundant. Obviously, the type of tropical forest that occurs in an area depends critically upon the availability of precipitation and moisture. The annual cycle of seasonal change is also an important feature of

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tropical climates, but the seasons are characterized by variation in rainfall rather than temperature. Evergreen forests occur where there is little or no dry season. Where dry and wet seasons are of approximately equal duration, deciduous forests are the norm.

A unique feature of tropical forests is the broad representation of biodiversity in a small area. Tropical forests contain much, if not most, of the world's biodiversity in the trees and plants that comprise the vegetative system and in the animals, especially anthropoids, that exist in the forest soils, floor, and canopy. Tropical forests, especially wet tropical forests, typically contain far more species of trees, plants, birds, butterflies, and so forth than their temperate counterparts.

In general, the net primary productivity (NPP), a measure of plant growth given by the net amount of carbon fixed by plants in a given time period, increases the closer the forest is to the equator, although it is moderated by rainfall patterns.

As Table 1 shows, the NPP of tropical forests is substantially greater than that of other types of forest ecosystems, thus indicating higher levels of overall biological growth. This is the case even though tropical soils are often poor in nutrients and minerals as a result of exposure to torrential rainfalls over millennia, which have washed away most of the soluble materials. The high productivity found in these sterile soils is due to the ability of these forests to store the materials in the forest itself—either in living plants or in the litter of dead plants (Terborgh, 1992).

Without human intervention, tropical forest ecosystems are a mixture of living, growing, and dying systems that are periodically impacted by natural disturbances. Natural disturbances, as, for example, when an old tree falls, contribute to this biodiversity by creating numerous unique niches that create a host of varying environments thereby promoting the wide range of biodiversity. The disturbances vary from local disturbances that may open a small hole in the forest canopy that allows sunlight to reach the forest floor thereby stimulating small seedlings, to much broader disturbances. Examples of broader disturbances include those caused by land slides and floods, as well as those caused by forest fires, which occur not only in dry tropical forests but also in wet forests that experience distinct dry seasons, such as those found in some parts of Borneo and Southeast Asia.

TABLE 1 Predicted Net Primary Productivity (NPP) by Forest Ecosystem Types

Ecosystem Type

Boreal forest Temperate conifcr Temperate deciduous Temperate broadleaf evergreen

224.20 459.25 361.17 590.29

forest

Subtropical and tropical

892.74

2 HUMAN INFLUENCES IN THE TROPICS

Humans introduce additional complexity to this system by introducing another possible source of disturbance through the process of utilizing forests for their own needs. Few tropical forests are free from the effects of humans. Most tropical forests have been inhabited by humans, many for thousands or tens of thousands of years. Human impacts typically involve the collection of various forest items for food and fiber. Very often humans provide management to increase the future supply of desired outputs. In a subsistence context such management may include modifying the forest to promote certain desired nontimber forest products, e.g., promoting the growth of certain fruit and nut-bearing trees or promoting the growth of young edible bamboo shoots.

In addition, shifting cultivation has been practiced in some tropical forests for millennium. Under this system a small area is cleared and usually burned, in part to provide a nutrient-rich ash. Crops are planted on the site for several years until it loses fertility or the weeds become untenable. The site is left fallow for a period of years, often more than 20, at which time the site is again cleared, burned, and replanted in crops. This cycle appears to be sustainable indefinitely. Thus, even in subsistence and relatively primitive societies, humans have influenced the forest ecosystem. When considering the sustainability of forests, human behavioral influences must be considered as well as natural adjustments.

Forests have also been utilized for millennia by people for building materials and timber. Although there has always been local use and some international trade in tropical timbers, the importance of trade has increased in the post-World War II global economy. Timbers from the West Coast of Africa have been supplemented by much larger volumes entering international trade from the East Indian countries of Malaysia, the Philippines, and Indonesia. This trade has provided these countries with substantial volumes of foreign currencies, which have provided some of the capital that financed their economic development.

3 VALUES OF FORESTS

Tropical forests, indeed all forests, generate a host of values to humans. Human benefits often involve the collection of various forest items for food and fiber such as the various timber and nontimber forest outputs discussed above. In addition, tropical forests provide local and regional ecological services in the form of watershed protection, mitigation of soil erosion, and reduction of downstream flooding. Additionally, tropical forests provide the residence for much of the world's biodiversity, with the majority of species believed to be found in the tropical forest habitat. Finally, tropical forest, together with the rest of the world's forests, provide the majority of the world's biomass that provides a "sink" for carbon, thus mitigating the buildup of carbon in the atmosphere, which is believed to contribute to global warming.

TABLE 2 Recent Estimates of Carbon Flux, Pg C yr from the Tropical Landscape for 1980 and 1990

Source

Year

Range

Average

Detwiler and Hall (1988)

1980

0.4-1.6

1.0

Hall and Uhlig (1991)

1980

0.52-0.64

0.58

Houghton et al. (1987)

1980

0.9-2.5

1.7

Houghton (1992)

1990

1.2-2.2

1.7

Forests are well recognized as having the potential to affect the forestlands' microclimate. Additionally, forests have an impact on the global climate through their capacity to sequester carbon. Although other terrestrial systems also sequester carbon, forests by far constitute the largest terrestrial carbon pool; tropical forests account for about one-half of all forest area and perhaps a large portion of the total forest biomass (Brown et al, 1993). By holding carbon captive in pools of forest biomass and soils, the amount of carbon in the atmosphere is reduced, thereby mitigating the greenhouse warming provided by the atmosphere.

While it is widely agreed that the preponderance of human-generated releases of carbon into the atmosphere in recent decades has been due to the use of fossil fuels and that the global warming "problem" is largely a fossil fuel problem, it is also recognized that land-use changes play a role. It is generally believed that some of the buildup of carbon dioxide in the atmosphere experienced over the past century or so is due to land-use changes associated with land clearing and deforestation. In recent decades, probably all of the net carbon releases from forests have come from tropical deforestation (since temperate and boreal forests are in approximate carbon balance). An estimate of the carbon releases in recent years is provided in Table 2.

The range of carbon releases from tropical forests is from 0.4 to 2.5 Pg C yr~ This compares with an estimate of about 6.0 Pg C yr" ' total human-generated releases of carbon. Thus, although tropical forest carbon releases are significant, they are well below 50% of total annual releases and probably in the range of 10 to 25% of the total.

4 DEFORESTATION IN THE TROPICS

In 1990 the tropical forests of the world were estimated to cover an area of about 1,756.3 million ha (Table 3), or about 13.4% of the globe's land area, excluding Antarctica and Greenland. This is down from an estimated 1,910.4 million ha in 1981 (Table 3). The annual deforestation rate for this period averaged 0.1540 million km2 or about 0.8% of the area of tropical forest (FAQ, 1993). The rate of deforestation, however, varied substantially throughout the tropics. Perhaps somewhat surprisingly, the tropical rainforest, the forest type over which the international community

TABLE 3 Estimates of Forest Area and Rate of Deforestation by Geographical Subregions"

Total

Forest

Forest

Annual

Rate of

Land

Area

Area

Deforestation

Change

Area0

1980°

1990°

1981-1990

1981-1990

Number of

(percent per

Continent

Countries

million hectares

annum)

Africa

40

2236.1

568.6

527.6

4.1

-0.7

West Sahelian Africa

9

528.0

43.7

40.8

0.3

-0.7

East Sahelian Africa

6

489.7

71.4

65.3

0.6

-0.8

West Africa

8

203.8

61.5

55.6

0.6

-1.0

Central Africa

6

398.3

215.5

204.1

1.1

-0.5

Tropical southern Africa

10

558.1

159.3

145.9

1.3

-0.8

Insular Africa

1

58.2

17.1

15.8

0.1

-0.8

Asia

17

892.1

349.6

310.6

3.9

-1.1

South Asia

6

412.2

69.4

63.9

0.6

-0.8

Continental South East Asia

5

190.2

88.4

75.2

1.3

-1.5

Insular South East Asia

5

244.4

154.7

135.4

1.9

-1.2

Pacific Islands

1

45.3

37.1

36.0

0.1

-0.3

Latin America

32

1650.1

992.2

918.1

7.4

-0.7

Central America

7

239.6

79.2

68.1

1.1

-1.4

Mexico

Caribbean

19

69.0

48.3

47.1

0.1

-0.3

Tropical South America

7

1341.6

864.6

802.9

6.2

-0.7

Total

90

47,783

1910.4

1756.3

15.4

-0.8

" Totals may not tally due to rounding. Source: FAO (1993).

" Totals may not tally due to rounding. Source: FAO (1993).

seems to have the greatest concern, experienced the slowest rate of overall deforestation at 0.6% annually. The highest rates of deforestation were experienced in the upland forests. Both moist and dry upland forests experienced a 1.1% annual rate of deforestation (Table 4). By major region, Central America, including Mexico, experienced the highest rate of deforestation at about 1.4% annually while the Caribbean had the lowest rate at 0.3% annually. Of the large forest formations, continental Southeast Asia had the most rapid rate of deforestation at 1.5% annually while Central Africa had the lowest rate at 0.5% annually.

It is estimated that 90% of tropical deforestation has occurred since 1970 (Skole et al., 1994). If this estimate is correct, the tropical forest of the world at its apex would have covered about 22 million km2 or about 16.8% of the globe's land surface. Although reduced in size the world's tropical forests still constitute an area equal to that of the whole of South America. Even at the current rate of tropical deforestation, the world's tropical forests would continue to exist through the entire twenty-first century and well into the twenty-second century. Of course, the rate of tropical deforestation will almost surely be changing over time.

TABLE 4 Estimates of Forest Cover Area and Rate of Deforestation by Main Forest"

Population Population Annually

Density Growth Forest Deforested Area

1990 (1981-1990) Area 1990 (1981-1990)

Population Population Annually

Density Growth Forest Deforested Area

1990 (1981-1990) Area 1990 (1981-1990)

Forest Formations

Land Area (million hectares)

(inh./km)

(% per year)

(million hectares)

(% of land area)

(million hectares)

(%)

Forest Zone

4186.4

57

2.6

1748.2

42

15.3

0.8

Lowland formations

3485.6

57

2.5

1543.9

44

12.8

0.8

Tropical rainforest

947.2

41

2.5

718.3

76

4.6

0.6

Moist deciduous forests

1289.2

55

2.7

587.3

46

6.1

0.9

Dry deciduous forests

706.2

106

2.4

178.6

25

1.8

0.9

Very dry zone

543.0

24

3.2

59.7

11

0.3

0.5

Upland formations

700.9

56

2.9

204.3

29

2.5

1.1

Moist forests

528.0

52

2.7

178.1

34

2.2

1.1

Dry forests

172.8

70

3.2

26.2

15

0.3

1.1

Nonforest Zone

591.9

15

3.5

8.1

1

0.1

0.9

(hot and cold deserts)

Total Tropics

4778.3

52

2.7

1756.3

37

15.4

0.8

" Totals may not tally due to rounding. Source: FAO (1993).

" Totals may not tally due to rounding. Source: FAO (1993).

The causes of tropical deforestation are complex and not well understood. The term deforestation refers to situations in which the land is more or less permanently converted from forest cover to other cover and/or uses. A common but simplistic view, now largely rejected by analysts familiar with tropical forests, is that tropical deforestation is due to commercial timber logging. Commercial logging in the tropics rarely results in significant direct land conversion; however, as discussed below, it does make indirect contributions to the process of deforestation.

Another common explanation of tropical deforestation is to attribute it to population growth. As populations rise, one would expect that the pressures on the forests might increase. It is also argued that population pressures force a reduction in the fallow period in slash-and-burn regimes increasing pressures to bring more forest land into agricultural use. However, it is difficult to directly link population and economics growth to tropical deforestation, for example, Skole et al. (1994) conclude that population growth alone does not explain tropical deforestation.

Most analysts now believe that most tropical deforestation is driven by human desire for land-use changes, primarily the replacement of forests by agricultural activities. In fact, many governments have currently, or have had in the past, explicit policies to promote the conversion of forests to agricultural uses. In Central and South America, for example, large areas of forest clearing reflect government policy to open forest areas to agricultural settlement. In Brazil, for example, one rationale for promoting migration into the Amazon region, with its attendant deforestation, was to solidity Brazil's sovereignty claims to the region. Other clearing for use as pasture and other agriculture results from spontaneous actions of individuals. In Southeast Asia, forest land was gradually converted to paddy lands in river bottomlands over many years, and more recently conversion has occurred where water development projects, which allow irrigation, have been implemented. Additionally, in Southeast Asia, substantial native forests have been replaced over the years by the introduction of tree crops including rubber, palm oil, coconut, and so forth.

As noted, although commercial logging rarely plays a direct role in deforestation, it often plays an indirect role by providing access to previously inaccessible forest areas. Logging penetrates the dense forest with roads, which then provide access for spontaneous migration. The forest, which is accessible due to the logging roads and is not less dense and impenetrable due to the logging, is now more vulnerable to alternative land-use activities and land-use changes. The improved access makes investments in land clearing, spontaneous and otherwise, more feasible and attractive.

Although governments are often involved in explicitly promoting forest conversion, they are also often involved by the absence of their management of the forests that fall under their jurisdiction. In much of the tropical world today the forests are under the control of the central government, even though in earlier periods they had tended to be under local control. Although the central government has responsibility for the forests, often it is unable or unwilling to exercise effective management control. Thus the forests often become degraded or destroyed by uncontrolled use. In effect, the forests become a type of open access resource over which the responsible authority does not exercise effective control and the users have only illegal or attenuated use rights that provide little or no incentive for long-term management. More generally, careless use of tropical forest land often signals the absence of clear well-defined property rights and/or effective management, either private or public. With unsecured rights, the incentives are for "cut-and-run" behavior.

5 SIMILARITIES WITH EARLIER DEFORESTATIONS

In many respects tropical deforestation today is not dramatically different from temperate deforestation that occurred one and two centuries earlier. During that period, pressures for land-use change, primarily the demand for new lands for agriculture, resulted in large-scale deforestation of areas of Europe and North America. In the United States much of the forestlands of the eastern seaboard, the south and the Great Lakes states, were converted to croplands and pastures. This same phenomenon had begun earlier in Europe but continued in places well into the early part of the twentieth century. The denuding of the forest landscape was often the result of spontaneous actions but also often reflected governmental policies. In the United States, for example, the Homestead Act required land clearing as a prerequisite of obtaining land title. For much of North America and Europe the early land clearing has been offset by the renewal of the forest, largely through natural processes. Today, as the work of Kuusela (1994) of the European Forestry Institute has shown, the European forest has reclaimed large areas once deforested. Similarly, in America the forest has reclaimed much of the area deforested in New England (Barrett, 1988), the Great Lakes states, and the south as abandoned agricultural lands regenerated naturally into forest and, more recently, planted forest cover many former tobacco, cotton and other crop lands.

6 TIMBER HARVESTS IN THE TROPICS

Unlike much of the commercial logging in the temperate forest, even today tropical commercial logging almost never involves clear-cutting of the forest. Rather, the usual approach is to select only the trees that are suitable for commercial uses and log those trees, leaving large numbers of live trees in the forest. In past periods, relatively few trees were removed, and those that were felled by hand were commonly transported out of the forest by animals. The forest would be periodically relogged as trees reached desired sizes.

In recent decades logging has involved chainsaws, roads, and equipment. Additionally, larger areas have been logged, reflecting expanded demand. Nevertheless, selective cutting is still almost universally practiced, with only the larger trees of desired species harvested. This practice does not indicate benign motives by loggers but rather reflects the fact that, due to the high diversity of tree species, only a relatively few of the total trees of the tropical forest are suitable for commercial markets. Additionally, unlike many temperate forests that are even-aged (i.e., most all the mature trees are the same age and species), tropical forests typically are uneven-aged. The diverse mix of tree species and ages makes clear-cutting an unsuitable and uneconomic approach for commercial logging in most of the tropics.

This selective logging approach is also generally conducive to forest regeneration and regrowth. During the period immediately following the logging, the sunlight now reaching the forest floor stimulates the growth of seeds and seedlings, especially of the so-called pioneer species, which include many of the more important timber species such teak, mahogany, and many of the dipterocarp species. Typically the stock of seed and seedlings is adequate; however, this can be supplemented by human activities if required.

The idealized tropical forest management regime regarding logging varies with forest type. In the timber-rich forests of Southeast Asia it is common to follow the logging with a period of 30 to 70 years during which the forest recovers and has time to grow new trees of the desired size. Additionally, existing saplings and medium size trees will continue to grow and, in some cases, accelerate their growth now that the dominant trees are gone. When such an approach is followed, a viable and sustainable forest system can be achieved.

7 RENEWABILITY

Although it is sometimes claimed that tropical forests have difficulty renewing themselves, the evidence is to the contrary. For example, large areas of the American tropics had been in terraces, irrigated agriculture, and agroforestry in the pre-Columbian period but reverted to tropical forests as local populations were decimated by disruptions and disease. For example, Turner and Butzer (1992) argue that "the scale of deforestation, or forest modification, in the American tropics has only recently begun to rival that undertaken prior to the Columbian encounter." Similarly, the great temples of Angkor Watt in Cambodia, Borobodor in Java, and other similar large structures in Southeast Asia, once located in the mist of a high level of human activity, were lost for centuries due to the incursion of tropical forest when human activity declined. Also, the banks of the Panama Canal, which were almost wholly defoliated during the canal's construction in the early 1900, are now covered with lush tropical forests.

8 CONCLUSIONS

A major difference between the prior and current view of tropical deforestation is that the global community is now aware of and concerned for both the preservation of biodiversity and the sequestration of carbon, two global ecological functions that were largely unknown and generated little concern one and one half centuries ago. Another consideration being faced by the global community is increasing awareness that, although deforestation is reversible, species loss is not. Thus, the tropical forests that could be regenerated in the future may not have all of the constituent parts of the original forest. Although there is no guarantee that large amounts of additional deforestation will be prevented nor that reforestation will follow the pattern of the temperate industrial countries, recent events including the Earth Summit in Rio in 1992 have demonstrated the growing concern over deforestation by the global community and the emerging of a commitment to moderate, if not preclude, its continuance.

REFERENCES

Barrett, J. W., ed; The northeastern region, in Regional Siliviculture of the United States, Wiley,

New York, pp. 25-65, 1988. Brown, S., C. Hall, W. Knabe, J. Raich, M. Trexler, and P. Woomer, Tropical forests: Their past, present, and potential future role in the terrestrial carbon budget, in terrestrial biospheric carbon fluxes, in J. Wisniewski and R. N. Sampson (Eds.), Quantification of Sinks and Sources of C02, Kluwer Academic, Dordreceht, 1993, pp. 71-94. Detwiler, R. P., and C. A. S. Hall, Tropical forests and the global carbon cycle, Science, 239, 42^*7, 1988.

Food and Agricultural Organization of the United Nations (FAO) FAO Yearbook: Forest

Products, 1981-1992, FAO Forestry Series No. 27, FAO, Rome, 1992. Food and Agricultural Organization of the United Nations, (FAO), Forest Resources Assessment 1990: Tropical Countries, FAO Forestry Paper 112, FAO, Rome, 1993. Hall, C. A. S., and J. Uhlig, Refining estimates of carbon released from tropical land-use change, Canadian Journal of Forest Research, 21, 118-131, 1991. Houghton, R. A., R. D. Boone, J. R. Fruci, J. E. Hobbie, J. M. Melillo, C. A. Palm, B. J. Peterson, G. R. Shaver, G. M. Woodwell, B. Moore, D. L. Skole, and N. Meyers, The flux of carbon from terrestrial ecosystems to the atmosphere in 1980 due to changes in land use: geographic distribution of the global flux, Tellus, 39B, 122-139, 1987. Houghton, R. A., Tropical forests and climate, paper presented at the International Workshop Ecology, Conservation, and Management of Southeast Asian Rainforests, October 12-14, Kuching, Sarawak, 1992. Kuusela, K., Forest Resources in Europe, Cambridge University Press, Cambridge, 1994. Skole, D. L., W. H. Chomentowski, W. A. Salas, and A. D. Nobre, Physical and human dimensions of deforestation in Amazonia, Bioscience, 44(5), 314-322, 1994. Terborgh, J., Diversity and the Tropical Rain Forest, Scientific American Library, New York 1992.

Turner II, B. L., and K. I. Butzer, The Columbian encounter and land-use change, Environment, 34(8), 16-20, 37^*4, 1992.

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