20 SYLVICULTURE IN THE TROPICS r 



T. I 



to 44*5^ C. (112*1 Fahr.) in June. This does not repre- 

 sent the extremes of temperature in these regions where 

 the thermometer sometimes falls to freezing-point in 

 winter while it may rise to 46*5" C. (115 7" Fahr.). 

 Plants growing in these regions, therefore, require to be 

 able to stand very low as well as very high temperatures. 



The elevation above the sea-level is another climatic 

 factor affecting the vegetation, but within the Tropics 

 its influence is not much felt at below 600 metres 

 (2000 ft.) or more, according to whether a region is 

 nearer to or more distant from the equator ; and it is 

 mostly in mountain ranges that differences of vegeta- 

 tion will make themselves felt with distinctness. The 

 subject will be dealt with, therefore, under the head of 

 " Locality." 



To resume, the climatic factors chiefly influencing 

 forest vegetation are : (1) rainfall, (2) relative 

 humidity of the atmosphere, and (3) range of tempera- 

 ture. As, however, with an increasing rainfall the 

 humidity of the air is also increased, and to a certain 

 degree also the range of temperature, it is convenient 

 to describe the forests of the Tropics according to 

 zones of rainfall which have each their characteristic 

 vegetation. These zones can be classified as follows : 



1. Desert zone with a rainfall of to 100 mm. (0 to 4 inches). 



2. Arid zone 100 to 400 mm. (4 to 16 inches). 



3. Dry zone 400 to 1000 mm. (16 to 40 inches). 



4. Moist zone 1000 to 1900 mm. (40 to 75 inches). 



5. Wet zone over 1900 mm. (over 75 inches). 



The desert zone is not favourable to the existence 

 of forests. The vegetation consists generally of a few 

 scattered herbaceous plants with long taproots or 

 succulent plants, which store up their water ; only in 

 depressions, such as the dry watercourses, can narrow 

 fringes of shrubby trees be seen occasionally. Except, 

 therefore, where water can be supplied from other 

 sources than rainfall, this region is of little interest for 

 the forester. 



