48 SEEDING AND PLANTING 



9. THE CHOICE OF SPECIES IN REFERENCE TO THEIR 

 EFFECT ON THE SITE 



The effect of forest vegetation on the site is pronounced and far- 

 reaching. Attention should be given to the effect that it has 

 upon the site factors, particularly soil moisture and soil fertility. 

 Species which tend to improve the soil should be selected. Where 

 it is desirable to grow species under which the site deteriorates, 

 they should be mixed with others which have a tendency to im- 

 prove the site. 



10. The Effect of Forest Vegetation on the Climatic Factors 



Forest vegetation influences atmospheric temperature to a per- 

 ceptible degree. The daily extremes are less, the air within the 

 forest being cooler during the day and warmer during the night 

 than in the open. The seasonal variations are also less within the 

 forest, and the mean annual temperature is slightly lower. This 

 effect of forest growth is of importance where tender species are 

 used in artificial regeneration. 



There is an increase in the relative humidity of the air within the 

 forest of from 5 to 10 per cent as compared with that in the open. 

 We have no conclusive evidence that forest growth materially in- 

 fluences the amount of precipitation. It is reasonable to suppose, 

 however, that large bodies of timber increase rainfall to some 

 extent because the air is generally cooler in and near the forest. 

 Because of the higher relative humidity of the air within the forest 

 the upper layers of the soil are moister and the loss through 

 evaporation is correspondingly less. 



11. The Effect of Forest Vegetation on the Soil Factors 



The forest very materially affects the temperature of the soil. 

 Forest soil is warmer in winter and cooler in summer than denuded 

 soil. The cooling effect of the forest on the soil is most marked 

 on hot, southern aspects. 



The forest soil has a more uniform degree of moisture in the 

 surface layers and is more open and porous than soil in the open. 

 It permits the moisture to enter more freely and, consequently, a 

 larger proportion of the precipitation is taken up and less escapes 



