26 



growth may be doubled or trebled by irrigation, and Hartig, experimenting in a beech 

 forest, finds that the amount of wood produced is related to the current of liquid passing 

 up the stems, which itself depends on the quantify of moisture available in the ground. 

 'So that, where forests cannot spread over the whole of a tract of country, they may be 

 found only at the foot of slopes, as it is there that most water flows over the surface, and 

 that the ground is best protected from dessicating influences.* 



Even if in very heavy rains the forests can only keep back the bulk of the mountain 

 waters during a few hours, they yet prevent floods by allowing a large quantity of the 

 valley waters to run down, before the mountain waters, already diminished in proportion 

 to the storage power of the forests, can reach the lower levels. 



ACTION OF GRASS AND BUSH. Grass and bush have in a lesser degree the same 

 controlling effect over the flow of water, but experience has shown that grass alone 

 cannot serve to check completely torrents and floods. Neither are bushes sufficient 

 for the purpose, for the hygroscopic layer which gives suck a value to the natural 

 forests, does not form under. Still both grass and bush consolidate the soil, and, 

 covering often a large area compared with that of the forests, they very powerfully 

 help the action of the latter. And where forests cover only a portion of the mountains, 

 it is only by a judicious preservation of both grassland and forest, effected by regulating 

 the grazing in the one and cutting in the other, that the evils following denudation 

 -can be reduced to a minimum. 



TORRENTIAL ACTION IN OTHER COUNTRIES. This protective power of 

 vegetationwas first clearly pointed out by the French Engineer, Surell, in 

 a remarkable work**. A large technical literature has since resulted from the 

 considerable attention which the subject has perforce attracted in France, Switzer- 

 land, Germany, and North Italy, where the destruction of the forests in some 

 districts has been fraught with disastrous consequences. In the south of France 

 the damage became so great towards the middle of the present century, that since 

 3860 very large sums of money have been spent, to avoid greater loss, in the work 

 of checking torrents and reforesting their basins ; and in 1880, it was estimated that the 



* This great affinity of forests for moisture explains why the Natal forests are distributed on south and 

 south-east slopes of considerable steepness. The amount of water received and retained on slopes with that 

 .aspect is greater than on any other ; because (a) the evaporation is less than on any other aspect, especially 

 the north, facing the sun. (b) Ihe bulk of the rain comes from the south-east and drives at some inclination, 

 so that slopes with that aspect will receive most \vater. For instance over a low hill with sides sloping at 1 in 2, 

 rain driving at an inclination of 2 in 1 from the south would deposit 1'4 times more water on a south than on a 

 north slope. With slopes at 1 in 3, the proportion would reach one and two-thirds. To put it otherwise, 

 with a rainfall of 40 inches, the effect will be equivalent in the first case to a vertical fall of 47 inches on 

 the south slopes, and 33 on the north ; in the second case, the respective numbers would be 50 and 30. 

 (c) A vapour-laden currents of air deflected upward by a mountain chain will part with some of its moisture 

 on the windward flanks, owing to the dynamic cooling resulting from its expansion at a level of diminished 

 pressure, and Natal, sloping rapidly from the Drakensberg to the Indian Ocean, is favoured in this respect. 



In the Knysna region, the distribution of forests also illustrates strikingly the action of the several causes 

 mentioned. The Outeniqua range, the forests and the coast all run east and west, within a narrow zone ; the 

 mountains cause an increased rainfall on the coast flanks on which alone are any forests found ; then the action, 

 of the sun and the westerly rains cause the forests to prevail on south and west slopes, at a level of 500-2,000 

 feet, where the climate is otherwise adapted to the species that are found. 

 **Etudes sur les Torrents des Hautes Alpes. Paris, 1841. 



