Soils, as complex ecosystems in themselves, are essential to any future use of rainforest lands. Logging and/or burning remove soil-stabilizing elements – the roots of trees and soil microorganisms. Heavy tropical rains then wash away topsoil and litter, removing nutrients and silting rivers and reservoirs. The nutrients eventually wash into the ocean, causing eutrophication of tropical estuaries.
When forests are cut and burned, the ash and decomposed vegetation release nutrients into the soil. The soil, thus enriched, can support two to three years of growth of shrubby and herbaceous plants, after which nutrient levels fall below that necessary for agriculture. Because of this, cropping in tropical areas is transient, necessitating the abandonment of fields within a few years. Why is this so?
a. Alterations in soil nutrients: Tropical forest soils are often (but not always) very poor and thin. There is generally very little external input of nutrients, so soil fertility depends on an extremely complex system of nutrient cycling based on the decomposition of leaves, dead plants and animals due to the activities of soil flora and fauna (see Part I, G5). This provides most of the nutrients in the soil, but they are rapidly taken up again by the plants, leaving little organic material in the soil. This elegant and fragile system, which permits rainforests to exist on poor soils, is seriously disturbed by logging. When the vegetation cover is removed, nutrients are leached out by rainfall, which also washes away the delicate layer of topsoil because of high water flow rates. The worst offenders are the areas (skid trails, log landings) where the vegetation, litter and topsoil have been scraped away. The remaining soil is left with high levels of quite insoluble compounds of iron and aluminum oxides, but little organic matter (carbon compounds). Since most organic matter is in the vegetation, removing trees reduces many nutrients (and potential organic matter), while burning volatilizes carbon, nitrogen and sulphur. Soil fertility does not return for decades, and if logging is done on a short rotation cycle, so many nutrients are lost that the forest may not be able to recover.
Tropical forests contain twenty to fifty times more carbon in their vegetation than do agricultural lands which replace them. Closed forest is estimated to contain at least 116 metric tons of carbon per hectare in the soil and vegetation (Houghton, 1995), whereas carbon levels in the soil after cultivation are reduced by as much as 50% within five years of forest cutting (Matson, 1997), from between 36%-70%, according to Lodge, et al., (1996). In areas of the Amazon rainforest which were converted to sugar cane plantations, soil carbon levels fell 83-93% within 12 years (Lodge, et al., 1996). Other estimates indicate that the conversion of forests to agricultural land can cause a 40% reduction in soil carbon levels, conversion to pastureland, a 20% reduction. Shifting cultivation causes declines in soil carbon of 18-27%, and fallow periods of at least 35 years are required to for soils to regain their original carbon content (Detwiler and Hall, 1988). Erosion, topsoil removal by machinery, and soil oxidation further reduce carbon content and soil fertility. Loss of forest area also generates ancillary damage. A study done in Malaysia found that an 18% decrease in forested land reduced biomass by 28% (Brown, Gillespie & Lugo, 1991). For every ton of carbon released by deforestation, 0.6 tons more of carbon were released by the reduction in biomass of the forest surrounding the cleared area due to degradation, mostly from “selective” logging and other exploitative activities. The 38% of forest area lost in Southeast Asia between 1880 and 1980 has led to a 50% loss in biomass (Houghton, 1995).
b. Irreversible changes in soil structure: The clearing of forests also exposes the soil to sun and oxygen. In some soils, chemical changes occur in the soil under these conditions, resulting in the formation of a rocklike material, laterite. This has occurred in many places, and is not only a modern phenomenon. The temples at Angkor Wat, in Cambodia, were built 600 – 1000 years ago of sandstone and laterite. Some have speculated that the downfall of the Khmer civilization was due to forest clear-cutting with resulting laterization of the soil, rendering it unfit for agriculture or, indeed, any productive use. The same has happened in many places in the Amazon, producing “pavements of rock.” Only certain tropical soils are suitable for agriculture or ranching. In addition, the use of heavy machinery compacts tropic soils, reduces the ability of seedlings to root, and diminishes soil aeration and water absorption.
c. Loss of soil seed banks and seedlings on the forest floor, and damage to the shallow roots of trees remaining after cutting. Seeds in the soil and seedlings on the forest floor are lost during logging procedures. Plants other than trees are very important in forest ecology and are trampled, damaged or removed.
d. Loss of microbial flora and invertebrates in soils: Microbial decomposition of organic matter is the major source of soil carbon. Many fungi and bacteria which are vital in decomposition and nutrient cycling are vulnerable to disruption by the removal of trees because they are sensitive to the moisture fluctuations and increased forest floor temperatures caused by the opening of the canopy by logging. Among these organisms are nitrogen-fixing bacteria without which plants cannot survive, as soil nitrogen stores need to be replenished. Recolonization by bacteria of disturbed areas may not occur because they cannot normally be disseminated over large gaps. Mycorrhizal associations with tree roots are also disrupted.
e. Erosion: Land under forest suffers erosion rates of about 0.2 – 10 tons per year per hectare. The densest man-made forests (plantations) have erosion rates of from 20 – 160 tons per hectare per annum; grassland, about 200 or more tons; and primitive agricultural systems, 1000 tons per year (Jacobs, 1988). Removal of forest cover in Ghana increased the soil erosion rate from one ton per hectare per year to more than 100 tons (Repetto, 1990). In many places, removal of forest cover has led to desertification. In 1882, 9.4% of the land area of the earth was desert and wasteland, but by 1952, 23.3% of land surface had been classified as such. This correlates with the great decrease in global forest cover, from 43.9% of total land surface in 1882 to 21.1% in 1982. The Sahara Desert is largely manmade. The forest cover was removed, the land was overgrazed, and poor irrigation methods were used. Unfortunately, we have not improved our record since, as the Sahara continues to expand southward by several miles per year. The Thar Desert in India was forested 2000 years ago, but is now extremely dry as a consequence of logging and overgrazing. It is spreading outward by five kilometers per decade and has increased in size by 60,000 square miles in the past 100 years. The same is true of the central plain of the Irrawaddy River in Myanmar (formerly Burma), which not so long ago was forested, and is now arid and barren.
f. Reductions in agricultural yields: As deforestation alters climatic conditions, reducing rainfall, and causes erosion and floods, agricultural production has fallen in many areas. Rice production in some places in Southeast Asia has declined by more than a quarter since deforestation has become rampant.