UT EN A Ww 
  
  
TROPICAL VEGETATION AND CROPS, MILLER 129 
ledge and experience, some of which has been directly acquired but much of which may 
be derived from the results of earlier workers in the same or allied fields. The greater 
the fund of information on which he is able to draw, the sooner he will be able to gain 
an understanding of each task he undertakes. Although rapid advances are now being 
made, the student of vegetation and crops in the tropics is still less well endowed in that 
respect than his counterpart in most other parts of the globe. On the other hand he is 
presented with more opportunities for discovery, all the more so because in many parts 
of the tropics commanding views are denied to the ground investigator either by the 
mature and gentle topography or by the dense vegetation which clothes even the sum- 
mits of hills. There are thus probably morc instances in the tropics than elsewhere of 
aerial photographs disclosing features in the distribution of vegetation which were pre- 
viously unknown and which might otherwise have remained in that state for a long time. 
These discoveries may not be of such immediate importance as that of the first rocket- 
launching sites at Penemiinde, but they can bring no less a thrill to the interpreter and 
may be even more difficult to explain. The effects of human activities in the form of 
cultivation patterns may be equally inexplicable at first sight, but are usually more easily 
resolved than purely natural features. 
Between the extremes of nearly bare desert and luxuriant forest the tropics contain 
a very wide range of vegetation forms. These variations in structure provide a first 
broad basis for the classification of vegetation by photo-interpretation. At this level 
there are few special problems beyond that of assessing the exact limits of the more in- 
determinate mixtures of grass and trees. In any part of the world the shorter types of 
vegetation such as grassland and scrub can only be dealt with on their appearance in 
mass, and attention has never been focussed on single plants. I have no experience of 
grassland studies but have found it interesting to note how in both Africa and the Pacific 
the shorter and finer montane grassland is clearly distinguishable from the taller and 
coarser grasses of the lowlands. Even more remarkable is the similarity in photo appear- 
ance of ericaceous communities in Europe and on the mountains of Africa. 
With the taller forms of vegetation dominated by woody plants a greater change of 
attitude is required by the photo-interpreter embarking on work in the tropics. The main 
developments in the use of aerial photographs in forestry took place in parts of the 
northern hemisphere where the recent glacial history had produced for each locality a 
somewhat limited tree flora. In these conditions the forest communities were dominated 
by only one or two species by which they could readily be recognised and labelled. In the 
application of aerial photographs it was natural and helpful to attempt to identify these 
key species, often as individual trees. In the tropics, however, gregariousness or single 
species dominance are the exceptions rather than the rule, and most of the forests, wood- 
lands and even the savannas contain a great variety of tree species. 
Richards [14] has pointed out that the great bulk of the tropical rain forest can 
only be described as à mixed association in which there are local but apparently random 
fluctuations in the distribution of the numerous tree species. Presumably this is the result 
of uniformly favourable topography and climate and long periods free from marked 
geological or climatic changes. These conditions, which are particularly difficult for the 
field worker as well as for the photo-interpreter, appear to reach a climax in the low- 
land Dipterocarp forests of south-east Asia with their great abundance of closely-related 
species. Where utilisation of such forests is fairly intensive, as in parts of Malaya, there 
may not in fact be any wide variation in the total stocking of economically valuable 
trees, but where only a limited range of timbers can be marketed it would certainly be 
valuable if more help could be extracted from aerial photographs. In North Borneo a 
classification based on the number and size of emergent crowns is reckoned to give an 
indication of economic stocking [8], and it seems probable that other types of correlation 
will be found as photographic work is intensified in other places.