PART IX — TERRESTRIAL ECOSYSTEMS 



lost by this slash-and-burn method, 

 but the ashes contain the other min- 

 erals that had been held by the trees 

 and thereby provide enough fertility 

 for one or more years of primitive 

 crop agriculture. After several years, 

 so much of the mineral has been 

 leached away that the land becomes 

 too poor to crop and new land 

 must be cleared. In some slash-and- 

 burn techniques, additional trees are 

 brought in (especially from the 

 slopes) and burned on the cut-over 

 areas along with the normal slash; 

 crop production can then continue 

 somewhat longer than usual because 

 of the increased amount of fertilizing 

 ash provided. In some areas, it is 

 necessary to maintain a cover on 

 the soil at all times if the lateritic 

 soil is not to be destroyed by the 

 combined effect of direct rain and 

 sunlight. 



Shifting cultivation (slash-and- 

 burn) techniques can work well 

 enough to support low populations, 

 but they require a great deal of land. 

 Five to fifteen years of forest fallow 

 are needed to allow rebuilding of 

 the trees' mineral supply; this means 

 that, at best, several times the area 

 cropped must be available in order 

 to provide continuous support of a 

 farmer. 



Modern Techniques — In general 

 terms, the methods of increasing 

 yields are the same in the tropics 

 as in temperate areas. Nevertheless, 

 attempts to transfer temperate-zone 

 ecology and agricultural practices di- 

 rectly to the tropics have usually 

 met with failure, occurrences which 

 emphasize how much we have yet 

 to learn. Techniques of replacing 

 slash-and-burn methods with care- 

 fully designed crop rotation (fre- 

 quently including care to maintain 

 continuity of a canopy) are now 

 being developed. They show some 

 promise, but a great deal more re- 

 search is needed. Fertilizers can be 

 very effective, but poor understand- 

 ing of soils and of plant requirements 

 means that the kinds and combina- 

 tions of materials that would be most 



useful on each are unknown. Re- 

 quirements and effective methods of 

 application are generally not known 

 for major nutrients or for trace ele- 

 ments. 



Water-conservation (and erosion- 

 inhibition) devices such as bunds, 

 terraces, mulches, tie ridges, and so 

 on are useful. Irrigation can be very 

 effective, but there are usually un- 

 considered costs associated with the 

 use of dams and reservoirs. Perhaps 

 the most important of the latter is 

 the effect that dam-associated flood 

 control will have on downstream 

 lowland alluvial soil such as those 

 along the lower Nile. Fertility of 

 these soils has remained high, some- 

 times in spite of millenia of intensive 

 farming without fertilizers. This is 

 because of the annual natural mineral 

 input of the deposited flood silt. 

 Comparative costs of maintaining fer- 

 tility of these alluvial soils in the 

 absence of flooding are unknown. 



Rebuilding Degraded Soils — If 

 much produce (lumber, crops, etc.) 

 is to be removed from an area, this 

 will remove the nutrients incorpo- 

 rated in the produce. Where the 

 nutrient cycling is tight, as in tropical 

 forest, crop removal can result in 

 spectacular fertility-loss rates. Even 

 if all the unused parts of the plant 

 are returned to the mineral cycling, 

 the fertility of the soil will decrease. 

 Techniques need to be developed to 

 replace these losses from crop re- 

 moval and mineral leaching and to 

 learn how to provide additional min- 

 erals so that production can be in- 

 creased. For example, many tropical 

 soils have properties that are espe- 

 cially effective in the making of added 

 phosphorus and some trace min- 

 erals. 



The extent of forest in many parts 

 of the tropics, especially in the dryer 

 areas, is being reduced by fire and 

 by overgrazing by domestic stock. 

 Grasslands are replacing forests and, 

 partly because grasslands have less 

 close minerals cycling than forests, 

 the quality of these soils is being 



degraded. (See Figure IX-10) Activi- 

 ties of man and his stock have already 

 produced large areas of white-sand 

 savannah which have very low pro- 

 ductivity. Even if the influence of 

 man and stock were removed, re- 

 forestation would be slow because the 

 soils have been so greatly damaged. 

 Efforts must be made to learn how to 

 counter the soil-degradation processes 

 that have produced these areas and 

 how to rebuild the soils that have 

 already been degraded. 



Forest Management for Production 



Timber — Although tropical areas 

 contain almost half of the world's 

 forests, they produce only about one- 

 tenth of the world's timber. Only 

 about one-sixth of the tropical forest 

 is being exploited for timber, in 

 part because of the inaccessibility of 

 about 60 percent of the forested acres. 

 There are several reasons for this: 

 lack of road networks and vehicles; 

 poor markets, which means that 

 building roads and buying vehicles 

 for timber harvest is uneconomical; 

 low levels of available technology; 

 the fact that many tropical hardwoods 

 are so heavy that they will not float, 

 thus precluding use of rivers to float 

 logs as is common in the higher 

 latitudes; and the diversity of tree 

 species usual in tropical forests. This 

 diversity means either that many 

 kinds of timber must be cut and 

 marketed or that a great deal of 

 effort must be expended to extract 

 only the few species desired from 

 an extensive and highly varied forest. 



Much research needs to be done 

 on these problems. If clear cutting is 

 practiced, or even enough trees are 

 taken that the integrity of the canopy 

 is destroyed, soil destruction can oc- 

 cur and erosion may be severe. In 

 some parts of the tropics, soil will, 

 in effect, turn to stone when so ex- 

 posed. In other areas, siltation from 

 erosion following timber extraction 

 of forest clearing has produced se- 

 rious problems. For example, silta- 



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