In this book an attempt is made to record the
principal features of tree distribution in a dry tropic
country of one million square miles, an area equal to that of western Europe, and to interpret that distribution in terms of soil and water factors. •
Exhaustive lists of species have been omitted in
the belief that it is more important to see the wood
than to see the trees only. •
A tendency has been noticeable in recent years to
give greater attention to the physical factors' of soil and water in the study of distribution. •
Clements, after confessing that the bulk of the
material which had come the way of investigators, including himself, was mesotropic material, says, in discussing his hydroseres and xeroseres -
"With increasing study of desert and tropical
succession it is probable that the direction of the water reaction will assume its basic importance ".
Runkiaer (52 p. 7) writes :-
"The relationship, of plants to water, influences
vegetation to such a degree that it is by far the most important factor in the plant climate ".
Braun- Blanquet (5 p. 214) writes
"In the ecological characterization of plant
communities the water (and air) capacity of the soil, will, in the future, demand more attention". •
More recently in almost every study of African distribution in which the water relation has been emphasized, the admission in made or implied that ; -
"The influence of the rainfall on local conditions
is not proportional to its quantity" (49). (H. W. Moore.) •
In the search for the key to the efficiency of rainfall, T.F. Chipp devised a "degrees of wetness" formula designed to express effective rainfall by incidence rather than by total. C. G. Trevor (43 p. 361) in describing a South African tour made during the Empire Forestry Conference in 1935, said: - "it is a curious fact that usually the soil is poor where the rainfall is high, as in the south-east of the Cape province, while it is comparatively fertile in the area of low rainfall ". •
The "degree of wetness" formula of Chipp and the
precipitation - Evaporation ratio of Transeau and Mayer have helped towards a fuller understanding of site values. In the agricultural sphere, conditions in the cases of some cultivated plants are still so obscure as to be known by such terms as "Gezira conditions" in the irrigated Sudan
plains (68) or as "sudden death" among cloves in Zanzibar (61) or as "winter killing" without frost in America. •
Such problems deserve reexamination, on purely physical lines, of the soils of the site, and it is believed that the site analysis and Rainfall-Texture theories presented here may suggest a new approach at least to some of these problems. •
As Sir Frank Stockdale writes (61 p. 13) of the "sudden death" of cloves: "The progressive spread may also be due, to a change in
the moisture balance of the soil as result of its exposure to the sun". •
Before judging what water can do for a species in a given site it is necessary to note what water has done, or is doing, to the soil of that site.
Along its longitudinal axis the Sudan stretches for 1000 miles from the rainless desert in the north to the Congo border and closed forest in upwards of 1400 mms of rainfall. •
The great length and small rainfall span of this axis make it a particularly suitable one on which to study the changes in species which, given uniform soil conditions, accompany even small charges in rainfall, changes which are more easily identified here than where the rainfall axis is fore-shortened. •
Part I gives those basic facts of geography, soils and climate, existing in the Sudan, which have strong influence on distribution. •
Part II deals with the forest geography, summarized after twenty two years of service and travel throughout the area, dealing first with the major ecological formations and
second with type species and their occurrences as species.
A system of distribution analysis by transects cut through rainfalls and through site types is offered as likely to be of value in the Sudan and elsewhere, and the relative moisture values of different site types is assessed. •
Part III discusses the influence, on forest geography, of the paramount soil and water factors, offers, for the fuller interpretation of the facts of distribution, the concepts of the climatic climax soil, of the datum soil, and of the clay-water line or rainfall-soil texture ratio, and
concludes with instances of the practical application of these. •
Many, if not most, of the tree species for which an evaluation of moisture requirements is made are species common also in French Equatorial Africa Northern Nigeria and adjacent territories.