14] VEGETATIONAL TYPES OF TROPICAL LANDS 467 



soil, tend to be much more marked in the dry districts of the tropics 

 than where rain forest prevails. Outstanding are the laterite soils, 

 typical!} reddish in colour owing to ferric compounds, which are 

 extremely poor in alkalis and nutritiye salts as well as in w'ater- 

 retaining capacity. They are consequently unfayourable to most 

 plants and support relatively poor vegetation — examples being the 

 forests in Burma dominated by Eng (Ira), Dipterocarpus tuberculatus, 

 which often forms almost pure consociations and alone grows up well, 

 the other trees being stunted and gnarled. Similar poor forests 

 may also be found on light sands, ' bare ' limestone, and dry ridges 

 of acid-weathering rocks, though these areas often support no more 

 than thorn-w^oodland or even scrub. Such vegetational poverty is 

 usually in part engendered by the relatively little humus-accumula- 

 tion, due to rapid breakdown in the tropics. In other cases porous 

 siliceous soils may be occupied by forests having a particular char- 

 acter owing to dominance by particular plants, such as Sal-tree 

 {Shorea robusta) or various Bamboos. Communities of these last 

 are often virtually pure, containing no associates apart from small 

 cryptogams, and in many cases apparently owe their origin to 

 cultivation. 



Altitudinal Effects 



The vegetation-types of high altitudes above the tree-limit in 

 tropical as well as other regions were covered in a general way in 

 the last chapter, and the communities of fresh and salt waters are 

 treated in Chapters XV and XVI, respectively. But here we must 

 consider briefly the upland types occurring below the timber-line 

 in the tropics and subtropics. 



The basal zone of a range of mountains has in general a greater 

 rainfall than the neighbouring lowlands, being consequently often 

 occupied by communities resembling the relatively moisture-loving 

 ones of the lowlands. This is true in tropical regions where, 

 accordingly, rain forest is very widespread and frequently very 

 luxuriant on the lower slopes of mountains. Above comes the 

 montane zone, where the precipitation is often phenomenally high, 

 and which in the equatorial region is still tropical in its lower levels ; 

 but at higher levels here, and throughout its altitudinal range to 

 the north and south, the montane zone is rather temperate in type, 

 with vegetation corresponding more to temperate rain forest. Thus 

 the dominants are often of particularly massive growth and rich 

 branching, but devoid of plank-buttresses ; they are evergreen but 



