[unu UM MODI tU TET 
130 TROPICAL VEGETATION AND CROPS, MILLER 
Not all tropical forests, however, are so difficult to deal with, and wherever site 
conditions become more restrictive on the kinds and numbers of plants which can inhabit 
them simpler associations and consociations may be found. Mangrove forests provide the 
most striking examples of these conditions with the result that many species of man- 
groves can be recognised directly even on small-scale air photos: their pattern of distri- 
bution is, however, usually so intricate that medium or large-scale prints may be needed 
to pick them out. In the peat swamps of Sarawak and in British Guiana [18] there are 
edaphic types, often with a strong tendency to single species dominance, which can be 
easily detected on aerial photographs. Similarly montane conditions give rise to vege- 
tation types of distinct structure and composition which present no difficulty to the 
photo-interpreter. An outstanding instance of gregariousness is the Alpine Bamboo of the 
mountains of East Africa, while characteristic in structure although variable in compo- 
sition is the moss or elfin forest of high peaks throughout the tropics. In less extreme 
climatie condition but on rough terrain in Ceylon, de Rosayro [6] has shown that aerial 
photographs backed by ground sampling can enable ecologically valid types to be delin- 
eated; some of these types were mixed associations and recognition was based on a com- 
plex of characters. 
It has been suggested by Richards in [15] that little of the forests of West Africa 
are true-climax formations, but owe their present character largely to past disturbance 
by man. As a result most of these forests are a patchwork consisting of a mosaic of seral 
stages, and it is difficult to recognise their ecological status. Little good-quality aerial 
photography has been available for these areas, and no detailed work from them has 
yet been published. On the other hand in Uganda, where many of the existing areas of 
closed forest are believed to have developed in recent centuries (some even in decades) 
the seral stages outlined, for example, by Eggeling [7] can mostly be recognised on aerial 
photographs by their structure. 
The woodlands and savannas of the tropics are generally less heterogeneous in com- 
position than the closed forest associations, but they are still likely to contain many 
species. More of their structure is visible in the aerial view, and the details of the 
spatial distribution of the individual plants can often be seen. A few forms, such as 
Palms or Pines, can be identified as single individuals (sometimes more readily by their 
shadows), and there are some other distinct consociations which can be recognised on 
aerial photographs. In general, however, typing by photo interpretation must be based 
on the general aspect rather than on the characteristics of single species, and the occur- 
rence of any one species can only be inferred from the type in which it is likely to be 
found. Howard has shown how such an approach can be used for the woodlands of Tan- 
ganyika [11]. The fact that the aerial photographs of such regions are often taken when 
the vegetation is dormant is possibly a more serious handicap to the photo-interpreter 
than has yet been realised. 
Among the open savanna and grassland formations of the tropics aerial photo- 
graphs have revealed some interesting features of vegetation distribution. The most 
striking of such patterns are those in British Somaliland which Macfadyen [13] first 
spotted and described as “vegetation arcs” and “water lanes”. Very similar but less well- 
formed arc patterns have since been observed on photographs or arid country in Tan- 
ganyika, Kenya, Ethiopia and the Sudan, but I know of no record from elsewhere of the 
so-called “water lanes”. Greenwood [9] and Worrall [19] have given further notes about 
the character of arcs, but I am doubtful if their origin has yet been fully explained. A 
linear arrangement of termite mounds is to be seen on aerial photographs of various 
parts of Africa. Aubreville [1] first detected from the air and then studied on air photos 
a linear distribution of thickets on the coastal plains near Accra in Ghana; he relates 
this pattern to the direction of the prevailing wind, bush-fires and termite mounds. I 
have seen many other examples in savanna conditions of slight vegetation patterns, in