THK EFFECT OF SLOPE EXPOSURE. 209 



The same author has also described this influence of slope in 

 greater detail in " The Vegetation of a Desert Mountain Kange as 

 conditioned by Climatic factors."* Bobbins (quoted by Harsh- 

 herger'), in outlining the native vegetation and climate of Colorado 

 in their relation to agriculture, emphasises the fact that : 



■ the intensity of sunlight is distinctly affected by exposure and also by 

 degree of slope. If we assume the intensity of sunlight to be 100. when it 

 strikes a surface at right angles, its intensity when striking that surface at 

 an angle of 70° will be approximately 98"5, at an angle of 60° 96"5. and at 

 an angle of 10° 33'4. In the lower mountain valleys of the north and central 

 parts of Colorado the southern exposures support a semi-arid type of 

 vegetation, composed of cedars, scattered yellow pines, yuccas, etc., while the 

 cooler and moister northern exposures are frequently clothed with Douglas 

 fir, aspen. Rocky Mountain maple, etc' "^ 



Prom a study of the conditions in Eastern Pennsylvania 

 Ilarshberger concludes 



'" that slope and exposure both have been fundamentally influential in 

 producing divergences from an original, fairly uniform virgin soil and by 

 interaction with the higher plants, animals and other soil organisms have 

 produced profound differences in the animal and plant population of the 

 two slopes."^ 



The Presext Investigation. 



The intention of the present paper is to give an account of a 

 few results obtained in the course of a preliminar}- investigation 

 into the precise differences in environmental conditions on northern 

 and southern slopes in Natal. While admittedly giving a very 

 incomplete idea of the total differences, they are presented with 

 the view of illustrating the nature and interest of the results likely 

 to be obtained by more extensive work and of the physiological and 

 ecological problems likely to arise therefrom. 



From what has already been said it is clear that the influence 

 of slope exposure upon vegetation may be traced to: (I) The 

 degree and duration of insolation ; (2) the direction of the dry 

 winds and of the rain-bearing winds ; (8) differences in soil condi- 

 tions depending partly on geological conditions, but largely the 

 indirect result of climatic factors. As a result of differences in 

 these factors, differences in other secondary factors, such as rate 

 of evaporation, relative humidity of the atmosphere and air 

 temperature will be established. These differences are probably 

 responsible in the first instance for the marked differences in the 

 character of the early stages of the succession on the two slopes. 

 The vegetation itself then takes part in further modification of 

 its environment, and differences in such factors as soil temperature 

 and soil water content are probably due, partly to the climatic 

 factors already mentioned, and partly to the influence of the cover- 

 ing vegetation. For example, the thick growth of tall shrubs in 

 the early stages of bush must protect the underlying soil from the 

 direct effects of insolation to a much greater degree than do the 

 isolated pioneers of Tree Veld. All these factors take part in the 

 still further modification of the subsequent stages of the 

 succession, and eventually lead to the profound difference in the 

 ultimate vegetation on the two slopes. The ultiinate vegetation 



