58 SYLVICULTURE IN THE TROPICS K . i 



the north during the hottest months, the northerly 

 aspects, especially the north - westerly, will be the 

 hottest, and in the southern Tropics the south-westerly. 



In localities where frosts occur it is on easterly 

 aspects that plants are most liable to suffer from frost- 

 bite, owing to the sudden heating by the sun's rays 

 immediately after sunrise. This is particularly the 

 case at high elevations, where the atmosphere, being 

 thin, lets the full force of the rays strike the plants. 



There is not very much to add about the effect of 

 configuration of the ground on forest vegetation. The 

 angle of the slope affects the drainage of the country, 

 and it is clear that where this is too slight the "flow of 

 the water is liable to be retarded unless the soil is of 

 sufficient porosity and depth to allow it to sink. 

 Where it is not so constituted it will have a tendency 

 to become marshy, and only sw T amp forest can grow on 

 it, or even only reeds and sedges, such as the Papyrus 

 of Africa. On the other hand, steep slopes are quickly 

 drained, especially where vegetation is scanty. On steep 

 slopes trees must either have a strong root-apparatus 

 or they must be able to grow in a sufficiently dense 

 mass to resist atmospheric disturbances, landslips, etc. 

 The soil is generally less deep on the slopes than in the 

 valleys, and this, combined with the quicker drainage, 

 makes the vegetation assume a more open character. 

 Fires therefore are more to be feared on them, and for 

 this reason, in many places, as has already been seen in 

 the case of the central African plateau, the forest is 

 more or less confined to the low grounds. Similar 

 changes may be observed in the " campos " of South 

 America and elsewhere. 



