124 Kkpukt S.A.A. Advanckmkxt of .Sciknce. 



sidertible uinouiit of tree planting has also l)eeii cairied out in the 

 ■catchment areas of the resei-voirs on Table Mountain. 



Value of Atf'oresfat'ioit. — 'J'he great practical ronedy for desiccation, 

 Hoods, dongas, itc, is affoi*estation. Besides hjwering tempei-atu)e and 

 reducing evaporation, forests retai-d the melting of snow, covei- the 

 ground with a thick layer of moisture-absorbing humus, and when 

 on a surticiently large scale, induce a greater rainfall. The leaf mould 

 which is forme<l on the surface of the giound has ast<mishing powers 

 of absorption. It will letain twice its own weight of water, and it has 

 been shown that mosses of the genus Hypnura, which grow under the 

 shade of conifers, can absorb up to five times their own weight of water, 

 whilst peat mosses of the genus SphmjnHm absorb up to seven times 

 theii" own weight. The absorbent power of hunuis is about eight times 

 that of sand, and from two and a half to five times that of differeiit 

 argillaceous soils. It is therefore easy to see that a surface^ covering 

 with so developefl a hygroscopic property must be a powerful factor in 

 preventing Hoods, foi- we must assume a succession of very heavy rains 

 to completely saturate a thick bed of humus. An additional and im- 

 portant part is also played by the permeability of the soil, which must 

 not be confounded with the hygroscopicity. The combined action of 

 these two physical properties is that pai-t of the water percolates into 

 the subsoil, to strengthen tlie natural Avater-supply, while anothei- part 

 slowly finds its way into the neighbouring streams, and thus ctmtributes 

 to the permanence and regularity of How. The trees, roots and layers 

 of humus by friction, absorption, reducing the gradients, itc. — foim 

 such obstacles to the movement of water that, under ordinary con- 

 ditions, there is practically no surface travel in a forest, and conse- 

 quently no erosive action. And this protection is afibrded while the 

 commercial value of tlie timber is increasing. 



An extremely convincing e.Kample of tlie moderating influence 

 that forests exercise on floods is to be found in two gorges on the 

 Colorado River near Austin, 'J'exas. The northern gorge is thickly 

 covered with an almost impenetiable growth of civlar and mixed 

 timber, whilst the upi)er slopes of the southein gorge have been 

 almost denuded. The former suflfers from neither floods nor erosion, 

 even after a heavy lainfall ; but in the latter evei'y vestige of organic 

 soil has been eroded, and the floods possess all the chai'acteristics of 

 a mountain torrent.* All the rivers near Geoi-ge, Cape Colony — the 

 •Swart, Kaimans, Silver,! Touw, Ac- now run with brown and «lis- 

 coloui'ed water. The forest has been more or less destroyed, and the 

 water is mostly the surface drainage tliat runs oif freely. Consequently 

 these rivei's rise and fall rapidly, and i>^i<- nearly tlry tluring droughts. 

 On the other ha)id the Glelx' liiver, that (hains the greater portioii of 

 the main Knysna forest, has an unfailing flow of ciystal water. The 



* Thf Tinibir >,f the Eihnirtls I'hitnm „f 'I'r.nisfniil its U,hiti<,„ in Wiitir 

 Su/jjt/i/, (.'/iiiiiifr (tiid Soil, liy W. L. IJiay. • •^- Umeuti of Forestry, liulletin 

 X«). 49. 



t So railed Iioiii the cUiai'iicss of tlu; water Ix'l'nre iletovestatiou l»e;;aii. 



