Aspen Global Change Institute Elements of Change 1995

Natural Disturbance and Change in the
Brazilian Amazon

Bruce Nelson
INPA - Instituto Nacional de Pesquisas da Amazonia
Manous, Brazil

Nelson discussed documented and postulated effects of recent "modern" deforestation in the Brazilian Amazon region on rainfall downwind of deforested areas, on local nutrient stocks and on global greenhouse gas emissions. He also discussed naturally disturbed and naturally changing vegetation types which may create spectral background noise which impedes the detection and/or quantification of increased deciduousness caused by anthropogenically decreased rainfall and/or fire penetration as a result of logging activities.

Modern Deforestation

The Brazilian "Legal" Amazon is a collection of political units covering about five million square kilometers (km2). About 75-80% of this area was originally covered by Amazon forest and by closed-canopy woodland, the remainder being natural Amazon savannas or natural woodlands with discontinuous tree canopies ("cerrado").

American and Brazilian scientists have measured modern deforestation using wall-to -wall Landsat Thematic Mapper (TM) images. Estimates of the total area deforested as of 1991 range from about 320,000 to 426,000 km2. This includes all anthropogenic secondary forests spectrally recognizable as such (usually less than 25 years old) as well as dry bare soil, charred grasses or slash, and dry non-photosynthetic vegetation (NPV). These three materials dominate agricultural and pastoral landscapes in the dry season, when the images are acquired. Most of the modern deforestation is concentrated along the eastern and southern flanks of the Brazilian Amazon; very little has taken place in the central and western portions of the region.

These are at least three deleterious effects of modern deforestation:

  1. Reduction in rainfall over forests in the central Amazon;

  2. Loss of nutrients originally stored in biomass of the primary forest;

  3. Amazonian contribution to the green house effect.

Much of the rain which falls on the central and western Amazon is derived from water vapor recycled back to the atmosphere by forests in the eastern Amazon. Therefore, rainfall in the central Amazon could be reduced by deforestation in the eastern Amazon.

Reduction in rainfall

All water vapor which condenses over the Amazon basin is brought in from the Atlantic Ocean by easterly trade winds. Over half of the rain which falls on the forest is recycled back to the atmosphere, either by evaporation from wetted leaves and wood, or by root uptake followed by leaf transpiration of water which reached the soil. This means that much of the rain which falls on the central and western Amazon is derived from water vapor which was recycled back to the atmosphere by forests in the eastern Amazon. It is therefore conceivable that rainfall in the central Amazon would be reduced if deforestation of the eastern Amazon is complete.

The effect of just a 15% decrease in rainfall near Manaus with proportionate lengthening of the dry season could be disastrous. A natural experiment exists to indicate what could happen. Rainfall near Manaus is about 2100 mm/year. Dense forest dominates the landscape, which is spectrally homogeneous on TM images, even over sandy soils. A single hectare of dense forest near Manaus supports about 220 different tree species. Species-abundance curves probably exceed 1000 tree species before leveling off.

Further east, near to Santarem or Obidos, rainfall is about 1750 mm/year. TM images of this region are spectrally more heterogeneous than the Manaus area, due to the extensive non-forest vegetation and semi-deciduous forests, particularly on sandy soils. On the clay soils, a homogeneous dense forest predominates but the diversity is much reduced: species-abundance curves flatten at about 200 tree species. Just six dominant species account for more than half of the trees in one study area.

Whether or not this unsettling scenario comes to pass depends largely on the type of land cover which will predominate in the deforested eastern Amazon. Four to five year old secondary forests attain a leaf area index (LAI) of about 5.0, close to the 5.7 measured for dense forests near Manaus. These secondary forests will presumably be good at recycling rainfall by both evaporation and transpiration. Since the fallow stage occupies at least 65% of a swidden/fallow time cycle (i. e. just 2-3 years of crops followed by a minimum of 4-5 years fallow), most of the agricultural landscape will be in fallow stage with high LAI at any one point in time.

If on the other hand, pastoral landscapes dominate the eastern Amazon, one can expect decreased transpiration, decreased rainfall interception and resultant decreases in rainfall downwind. Though ungrazed grasses may obtain a respectable LAI, pastures in the Amazon are often overgrazed, there is much bare soil exposed by the trampling of cattle and the grasses largely die back during the dry season in some areas.

Loss of nutrients stored in biomass

Several studies have shown that most of the upland (terra firme) Amazon is covered by sandy or clay soils with very low nutrient stores, high acidity and low cation exchange capacity. (Cation exchange capacity is a measure of the capacity of soil to hold charged positive ions against negatively charged soil molecules or crystalline surfaces. Positively charged nutrient ions are available in the soil solution, and the soil holds them tight by electrostatic force and "exchanges" them with plant roots. The plant gives up a positively charged hydrogen ion [a proton] in exchange for positively charged nutrient ions, such as those of Mg or Ca. Organic material in soils and certain types of clay, such as montmorillonite, have a high cation exchange capacity and can therefore hold onto some dissolved mineral nutrients, keeping them from being leached away and making them available to plant roots for exchange. Weathered Amazon soils generally have a low cation exchange capacity, as they are composed largely of silica, kaolin clay, iron oxides and aluminum oxides, with little or no organic material.)

Near Manaus, about 90% of the calcium and 90% of the potassium stocks are held in the living biomass of the forest. Most of the remaining 10% is found in a rapidly decaying thin layer of litter. These nutrients are made available to crops by cutting and burning the forest. Ashes from the burn also increase the pH of the soil, making phosphorous available to corn and manioc.

Within 2-3 years, the pH drops and the plot is abandoned, to be quickly overgrown by secondary forests species which can access phosphorous at low pH. By the time of abandonment, about 50% of the soluble nutrients have been lost from the system. The secondary forest is allowed to grow and a new cut and burn cycle can be started after five or more years. But with each cycle, more of the original nutrient capital is lost to leaching. Thus the system eventually becomes depauperate with successively diminishing crop yields, the fallow cycles must be much longer, and the growth rate of the secondary forests is much diminished. Some sites east of Belem have undergone about ten cycles of slash and burn, but these are located over calcium carbonate deposits. Nonetheless, they have very slow secondary forest growth rates during the fallow stage.

Amazonian contribution to the global increase in the greenhouse effect

According to a review conducted by Philip Fearnside, Brazil contributes about 5% of the total global anthropogenic greenhouse gas emissions. Most of this (4 of the 5%) is due to deforestation of the Amazon, the remainder being caused by burning of fossil fuels in that country. Another way of putting the Amazonian contribution in perspective is to state that about one decade of fossil fuel burning (1975-1985) worldwide is equivalent to complete combustion of the entire Amazon forest, insofar as the production of carbon dioxide is concerned.

Carbon dioxide is removed from the atmosphere by secondary forests in the "deforested" parts of the Amazon. According to recent measurements by the University of New Hampshire, young secondary forests cover fully 30% of the area deforested by non -indigenous people in the Brazilian Amazon. No quantification has yet been done of the role of these secondary forests in partially reversing the effects of deforestation, by absorbing carbon dioxide.

With each cut and burn cycle, more of the original nutrient capital is lost to leaching. Thus the system eventually becomes depauperate with successively diminishing crop yields, the fallow cycles must be much longer, and the growth rate of the secondary forests is much diminished.

Naturally Disturbed And Naturally Changing Vegetation Types

Natural changes in the Amazon are far more subtle than the deforestation caused by the modern human activities of non-indigenous people. (Included among these "natural" changes are the deleterious effects of some land-use practices of indigenous peoples.) These natural changes have also occurred over a much longer time period. Their environmental consequences are less drastic than the deforestation by modern man, but they are nonetheless very extensive in area, and are certainly very interesting.

These natural changes and disturbances were detected in the same satellite images used for a study by INPE (Brazil's Space Institute) of Amazon deforestation caused by ranchers, agriculturalists and loggers. The study utilized Landsat Thematic Mapper (TM) images which are color composites of three "digital photographs," one sensitive to the red portion of visible light and the other two sensitive to different infrared wavelength bands. The images were studied at a scale of 1:250,000. About 140 different images were studied, each measuring 185 km on a side. The images cover the 4,000,000 km 2 of landscape originally covered by forest in the Bra zilian Legal Amazon.

Natural changes in the Amazon are far more subtle than the deforestation caused by the modern human activities of non-indigenous people.

The natural change features detected are:

  1. Forests which have undergone fire penetration (whether of anthropogenic or natural origin). Most Amazon forests are not expected to burn unless they are first cut and allowed to dry out, because of the high water content of the biomass and dampness of the litter layer. Tens of thousands of km2 of standing forest were burned on periodically flooded forests over sandy soil during the 1925 drought. At least 1,500 km 2 of terra firme forest on clay soils near Santarem burned during the 1983 drought, though in that case the burning was catalyzed by mechanized selective logging, which leads to accumulation of dry combustible slash.

  2. Fields of parabolic dunes reactivated at different times in the past, probably in conjunction with fires. These are found along the lower course of the Rio Branco in the states of Roraima and Amazonas. The total area affected by aeolian reactivation in the last few thousand years is very large, but the two dune fields most recently reactivated (tens to hundreds of years ago?) cover just 1,500 km2.

  3. Sandstone table mountains, also susceptible to fires during very dry years. These cover a few hundreds of km 2 in the northern part of the Brazilian Amazon.

  4. Granitic and laterite (canga) hilltops, also susceptible to fire penetration during dry years. The most extensive example is occupied by a fern-savanna fire climax (southern braken fern) and is burned frequently by Yanomami Indians. The fern savannas cover 600 km2.

  5. Mortality of "terra firme " forest caused by ponding and waterlogging of very small depressions, during a year of unusually high rainfall. This affected 2 to 2.5% of the forest in one area north of Manaus in 1989.

  6. Synchronous mortality of forests dominated by arborescent bamboos. These sites may be susceptible to fire penetration if mortality coincides with a very strong dry season. Bamboo dominated forests which undergo synchronous mortality cover 92,000 km 2 in the SW Brazilian Amazon, and over 122,000 km2 when Peruvian and Bolivian forests are included.

  7. Expansion of bamboo forest patches either by successful competition or by allelopathic means. This is indicated by the rounded edges of the bamboo/mixed-forest contact line as seen on satellite images.

  8. Vine forests, which are a mix of sparse tall forest trees interspersed with dense mats of heavy woody vines; vines topple the trees and keep the forest in a late successional stage floristically. Vine forests cover about 100,000 km2 in the eastern Amazon, and additional areas in the southwestern state of Acre, as well as Bolivia.

  9. Extensive patches of forest dominated by babassu palm. The patches have a spatial pattern suggesting that they occupy sites of pre-modern swidden agriculture (slash and burn). The babassu palm is know to increase its numbers when sites are repeatedly burned, but can also maintain these high densities once the fires are eliminated. Spectrally recognizable dark palm forests cover at least 20,000 km2 in the SE Brazilian Amazon.

  10. Dozens of circular patches 2-13 km in diameter, in the upper Xingu Basin, which are apparent fire scars and cover over 1,000 km2.

  11. Blowdowns caused by convective winds. These cover a total of 900 km2, based on summing the areas of 330 features over 30 hectares each in size. The largest blowdowns exceed 2,000 hectares each in size. Blowdowns are concentrated along a zone traversing the Amazon from southern Venezuela to northern Bolivia.

Discussion On Modern Deforestation

As of 1985/86, according to the latest figures of Tucker and colleagues, about 320-330,000 km2 of the Amazon forest had been deforested, that is, converted to urban areas, pastures, crops, or young secondary forests. This is less than 10% of the original forest present in the region. Though 10% seems small, this has occurred over a very short time. Furthermore, wherever good roads provide access, the local deforestation percentage is much higher than 10%. Most of the untouched parts of the Amazon are economically inaccessible to loggers, ranchers and farmers. As such, Nelson remarked, for a rancher or logger, most of Amazonia might as well be lunar real estate. So the 10% figure is misleading. It does not reflect a national conservation policy, but rather the difficulty, so far, of opening up remote areas with good roads. It is true, however, that some government policies which encouraged deforestation, including the building of roads into remote areas, are no longer being implemented. This is due to both domestic and international pressure during the late 1980s and early 1990s.

Discussion on Natural Disturbances

Natural disturbances such as fires, drought and mortality from waterlogging can be very important catastrophic setbacks to forest succession. They may cause most primary forests to be in a state of carbon absorption for long periods, intercalated with brief periods of net carbon release. Thus, eddy correlation measures may artificially suggest that the Amazon forest is a carbon sink, since these measurements are made over short time periods.

Even studies which have long time lines of data are subject to misinterpretation if they ignore the role of catastrophic drought. A study by Philips and Gentry published in Science, 18 Feb. 1994, showed an increase in turnover rates of tree species in forests throughout the world's tropics, occurring in the 1980s. This may simply be the effect of increased mortality and increased subsequent recruitment after the drought of 1982/83. Data from Borneo and from Barro Colorado Island in Panama support this view. Philips and Gentry suggested that the increase in turnover was the result of carbon fertilization from increased concentrations of atmospheric carbon dioxide.

Some government policies which encouraged deforestation, including the building of roads into remote areas, are no longer being implemented. This is due to both domestic and international pressure during the late 1980s and early 1990s.

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