1953] 



VEGETATION OF NY AS ALAND 



165 



the start of the heavy rains, when shade temperature Fahrenheit rises to 105°- 

 112° and even 120° in the sweltering valley of the Lower Shire, to 100° -105° on 

 the plains of Lake Nyasa, and 90°-95° or rarely over 100° on the plateau high- 

 lands. In the coolest months — May, June, and July — standard temperatures drop 

 to lows of about 50° -5 5° on the Lower Shire, and about 40° -50° on the plateau 

 highlands, where the winter climate is cool and bracing. Frosts occur not infre- 

 quently on the more elevated plateau highlands. Heavy falls of snow have been 

 reported at intervals of years on the upper levels of Mlanje Mountain. 



Hornby (1933) recognizes two primary climatic regions for Central Nyasaland, 

 viz., 1) the dry areas of the Shire-Valley-Lake-Nyasa basin up to 2,300 feet, with 

 a rainfall of 20 to 40 inches, and 2) the high plateaux flanking the Rift, with a 

 rainfall of 40 to 60 inches. In terms of tolerance to Furopeans, the climatic divi- 

 sions of Nyasaland as a whole may be considered as 1) the hot, unhealthy low- 

 lands of the Shire and Lake Nyasa, 2) the relatively cool and healthy plateaux, 

 and 3) the higher mountains which from August to November offer a pleasant re- 

 treat from the heat of the two lower regions. 



SOILS 



In addition to extensive areas of residual and alluvial soils fairly uniform in 

 character, there are, within small areas, heterogeneous groups of soils due to dif- 

 ferences in the parent rocks and also the arrangement in catenae of soils trans- 

 ported from the heights of land to the lower levels. Thus, in the prevailing dry 

 woodlands, where soil preferences of the plant communities are more marked 

 than in moist closed forest, extensive tracts are covered with vegetation mono- 

 tonous in its uniformity, while significant and even very striking changes in 

 plant cover often occur within quite small areas. 



An increase in available soil moisture along the edges of streams and water- 

 courses will bring in narrow strips of closed forest of various types. Ravines, 

 rocky slopes, and rock outcrops often support, as isolated communities in the 

 prevailing woodlands, closed forest or related brushy growths which appear to be 

 edaphic, although protection from fire might also be a factor involved. In the 

 Shire Valley and on the lake plains, especially, local soil differences apparently 

 are responsible for abrupt changes from open woodland to dry closed forest. The 

 large termite mounds characteristic of most of the woodlands often support 

 brushy growths of species which are absent from the surrounding vegetation, pre- 

 sumably because of their soil requirements. Certain plants of the termite mounds, 

 such as Sansevieria spp., appear to be restricted to this limited habitat, while 

 others, such as the baobab tree, are not. Large baobabs (Adansonia digitata L.) 

 are frequently found perched on the eroded remains of old termite mounds in the 

 Shire Valley. 



The principal soils of the broad plateaux which flank the Rift Valley are red 

 or reddish loams. Dixey (1928) points out that the more densely populated areas 

 on these plateaux and on the scarps of the Rift coincide with the graphitic gneiss 

 and crystalline limestone series of the crystalline rocks, and he contrasts the 

 red fertile soils of the graphitic gneisses with the pale, very sandy, dry and 

 sterile soils derived from granites, granulites, and acid gneisses. Dixey also 

 states that the poor soils of the sandstones, grits, quartzites, and marls of the 

 older sedimentary formations are almost unpopulated. 



TYPES OF VEGETATION 



The intricacies of Nyasaland' s vegetation pattern are comparable with the di- 

 versities of topography, geology, climate, and soils in the country. On the broad- 



