Man's Influence on Climate. 129 



a dense re-growth to follow, the result naturally is that the former 

 forest atmosphere dries out, the surface herbage becomes parched, and a 

 forest fire sooner or later finds its way in and destroys the remaining 

 vestiges of forest growth. 



And where trees are naturally sparse and xerophytic, as in South 

 African thorn scrubs, evolution and the survival of the fittest gradually 

 produce in them an ability to withstand drought or even fire, or to 

 suffer less from it, while undergrowth is annuallv burned clean off". 

 In such case the effect upon the atmosphere is only what is produced 

 by the foliage itself, and usually that foliage is of the small and 

 gradually disappearing nature characteristic of such localities. 



The effect of cover is, then, more pronounced under trees which 

 form heavy canopy and produce much humus, than under trees of 

 lighter canopy, or where humus is scarce . 



In South African forestry the trees of light canopy and small 

 humus are usually the Australian Eucalypts and Acacias, trees which 

 often have leaves or phyllodes more or less vertical and conse- 

 quently of slight protection against insolation, and whose leaves decay 

 slowly and consequently form poor humus. These trees are also 

 of exceedingly rapid growth, and transpire the enormous amount of 

 moisture necessary to allow the fixation of the carbon forming the 

 large amount of leaves and stem. To do so they must draw it from 

 the soil, and it is found, in actual practice, that though these trees 

 do cause a considerable amount of precipitation, their powers of 

 transpiration and evaporation are even greater, and they actually 

 pass off m.ore moisture than they draw. To keep that up a natural 

 water-supply in the ground is required, and it is taking advantage 

 of this fact that has led to the use of certain trees for the purpose 

 of drying swamps. They actually draw the moisture from the ground 

 into the atmosphere and sometimes form a valuable natural drainage 

 where absence of fall prevents artificial drainage l^eing adopted. 



The moral of all this is that in South Africa wherever forest 

 is formed or maintained in a sufficiently dense condition and of kinds 

 not too absorptive of moisture, but still sufficientlv humus-producing, 

 then the result in regard to the local climate is a reduction of 

 temperature and an increased amount of rainfall or other precipita- 

 tion, with a slight tendency for these results to be felt to some 

 extent in the surrounding district. 



The effect produced bv the canopy and humus of trees in dense 

 forest is also produced, though in a lesser degree, by the close 

 vegetation and natural decay of good grass-veld. 



That "every blade of grass has its own drop of dew " is 

 absolutely true wherever moisture-laden atmosphere comes in contact 

 with foliage colder than itself, and where there is a dense green 

 sward with a moist undermulch of decayed foliage, the herbage is 

 cooler than the atmosphere during the night, with the result that 

 i\n immense precipitation takes place. The beneficial effect of this 

 dew has always been recognised, but its volume is often under- 

 estimated, especially that from driving mist, which has been shown 



