202 SCIENCE IN AFRICA 



have spread among the indigenous trees. But on the whole intro- 

 ductions have been successful, and exotic trees will probably be 

 spread through man's agency over wide areas, to the exclusion of 

 many indigenous species. 



If it is possible to generalize on this subject, it may be suggested 

 that the tendency is* to push the planting of exotics too far, with 

 the result that native hardwoods, often of much value, are disap- 

 pearing from many areas. Exotics are in most cases quick-growing, 

 very useful when a crop has to be raised speedily owing to scarcity 

 of fuel or timber and there is difficulty in raising indigenous 

 species. They are not in general of better commercial value than 

 many of the indigenous species, and in areas where there is no 

 object in producing a quick crop, it would very often be preferable 

 to regenerate the natural bush, and retain the natural balance. 



In Nyasaland exhaustive local trials have shown that exotic 

 trees cannot be grown successfully on the poorer soils; they demand 

 soils of agricultural value which cannot usually be spared for 

 forestry. This is also the case with the majority of the more 

 valuable indigenous trees. In view of the dense population of this 

 territory, forestry has to be confined mainly to the less valuable 

 indigenous species. These supply native needs and have a com- 

 mercial value when situated near markets. 



For the conservation of Vv^ater-supplies and prevention of erosion, 

 the preservation of the natural bush may be more effective than 

 the planting of exotics. A committee of the recent Empire Forestry 

 Conference recommended that this question should be thoroughly 

 investigated on scientific lines. The exotics, being fast-growing, 

 appear to have particularly high transpiration rates, and therefore 

 may suck up soil moisture and soluble salts more quickly than 

 indigenous forms of vegetation. For this reason farmers and others, 

 especially in South Africa, are warned against planting such trees 

 in large numbers near sources of springs, streams, and vleis. It is 

 clear that the success of large aflforestation schemes with exotics 

 depends on careful investigation of their eflfects on water-supplies 

 and soil generally. 



It has been suggested from Australia that conifers, particularly 

 Pinus insignis, are wasters of rainfall, in that their foliage prevents 

 light showers from reaching the ground, and the absorptive mat 



