FOREST ECOSYSTEMS 



ORIGINAL WOODLAND- 

 evapotranspiration continuous 

 greater part of year 



Soil at Field Capacity 



ajsra sggggga 



W ^-?: i-V ,>!> 'X^'t'" - *-,*£,? ^^■'*^^# s '*^^'' ILayerof Saturated Soil 



'lllllllllillilllllllllllllllll/lllllll/llililllllllli/, 

 i Impermeable Layer* 



I 



The illustration shows the differences between a forested area and an unforested 

 one in Rhodesia. In the forested area, the depth of soil and amount of water are 

 sufficient to support the growth of trees. Evapotranspiration (a combination of 

 water loss by evaporation from the soil and transpiration from plants) is in balance 

 with the available water supply. Once the tree cover is removed, evapotranspiration 

 is reduced, thus allowing the water table to rise and reducing the depth of usable 

 soil. The net result inhibits crop growth. 



tion following poor forest-utilization 

 practices has greatly decreased the 

 life expectancy of reservoirs; it some- 

 times causes problems by silting up 

 irrigation channels and often reduces 

 efficiency and causes damage to 

 pumps through clogging and abra- 

 sion. If the silt can be deposited on 

 the cropland, it provides fertilization 

 (but at a high cost). Research needs 

 to be done to find out how best 

 to crop the timber of these forests 

 with minimal damage and promotion 

 of minimal forest-regeneration time. 



The relatively little developmental 

 work that has been done on intensive 

 management techniques to increase 

 the timber yield of tropical forests 

 suggests that it will be possible, with 

 more understanding, to raise produc- 

 tivity to at least three times present 

 natural levels. Under some circum- 

 stances, natural productivity may be 

 surpassed by twenty times — or pos- 



sibly even more. Much research will 

 be required, however, if this goal is 

 to be reached. 



Tree Monocultures — Biological 

 mechanisms that operate to control 

 populations of various plants and 

 animals appear to be more prevalent 

 and more effective in the tropics 

 than in temperate and arctic zones. 

 The high diversity of tropical forests 

 may, in part at least, be a response 

 to this condition. When attempts 

 are made to grow monocultures of 

 various tree species or other crops, 

 therefore, tropical populations may 

 be subject to particularly severe bi- 

 ological attack. For example, it has 

 so far proved impossible to establish 

 successful rubber plantations in 

 South America, the home of the Para, 

 or hevea, rubber tree, because of 

 disease problems. Under natural con- 

 ditions, where rubber trees are widely 

 scattered, disease is transmitted 



poorly because of the distance be- 

 tween trees. 



The biological attack that many 

 tropical monocultures suffer can be 

 blunted or stopped in some instances, 

 but the problems are frequently se- 

 vere and a great deal of good research 

 is needed for their solution. The 

 problem has been partly overcome 

 (or avoided) for some species by 

 transporting them to parts of the 

 tropics in which they are not na- 

 tive — with great care taken not to 

 transport simultaneously their dis- 

 eases or insect pests. The highly 

 productive rubber plantations in Af- 

 rica and Southeast Asia, for example, 

 were started with South American 

 trees. 



This technique can doubtless be 

 useful in the future, but it is of 

 paramount importance that careful, 

 thorough, and appropriate research 

 precede it. Inadequate research could 

 lead to release of species in areas 

 where, in the absence of their natural 

 biological controls, they would spread 

 to become pests of major magnitude. 

 This could produce a catastrophe that 

 would dwarf the disaster that fol- 

 lowed the release of prickly-pear 

 cactus in Australia. By 1900, this 

 cactus covered an estimated 10 mil- 

 lion acres, and by 1925 it had spread 

 to about 60 million acres; in half of 

 this area, the cactus was so dense 

 that neither man nor horse could 

 enter. 



Looking to the Future 



Genetic breeding programs for 

 most tropical plants, with a few out- 

 standing exceptions, are not well 

 developed or are not being carried on 

 at all. With respect to forest trees 

 and their yield, enough has been done 

 with a few temperate species to dem- 

 onstrate that programs of this kind 

 can be of great benefit in increasing 

 yield and decreasing inter-cropping 

 interval. Tropical-tree yield can 

 doubtless be greatly increased as 

 well, and research and breeding pro- 



297 



