GOLD COAST REPORT ON FORESTS. 173 



commoner transitions are between the savannah forests and 

 Savannahs, the latter being a grassland type very poor in trees, 

 and between thorn forest and open bush or thorny brushwood 

 formations, Avhich latter, in its turn, leads on the Tropical Desert 

 formation. 



As a general rule, really successful forests only occur where 

 the annual rainfall is at least 40 inches, and rich luxuriant 

 growth is limited to zones where the annual rainfall is much 

 greater. Where the atmospheric humidity is greater than that 

 of the kind of climate associated with thorn forest, and the 

 amount of rain precipitated during the vegetative period of the 

 grasses fluctuates within moderate limits between adjacent locali- 

 ties, pure savannahs alternate with savannah forest. Xero- 

 philous woodland becomes established where greater heat and 

 more prolonged rainless periods prevail during the vegetative 

 period, whilst grassland succeeds when a milder temperature and 

 a more even distribution of rainfall prevail during the vegetative 

 season, or a windy dry season is a constant feature of the climate. 



In general, with at least 70 inches of rainfall during the year, 

 evergreen high forest prevails ; with 58 to 70 inches, mixed 

 deciduous forests; with 35 to 58 inches there is a struggle 

 between xerophilous woodland and grassland. These limits are 

 however, not infrequently departed from, and where the con- 

 ditions (climatic) are .not markedly in favour of any particular 

 type, edaphic influences depending on the character of the soil 

 often decide the issue. The extensive high forests of Centra) 

 Ashanti and of parts of the Ondo and Illesha Districts of 

 Southern Nigeria, which are not by any means so xerophilous in 

 character as to be classed with the typical monsoon or mixed 

 deciduous forests, and which inhabit regions with an annual 

 rainfall of only 55 to 65 inches, are cases in point. Perhaps 

 the greater number of rainy days in the year, a condition charac- 

 teristic of these extensive well-wooded areas, and a certain 

 tendency in the soil towards swampiness, are responsible for the 

 greener type of forest to be met with there. These regions are, 

 moreover, very hilly, and, in consequence, the air has a higher 

 relative humidity than it would have in the case of forests 

 situated on a lower level. The luxuriant growth is probably 

 accounted for by a combination of^ all these factors. 



With a rainfall of 35 inches and less, xerophilous scrub and, 

 in particular, thorn forest and^ thorn bush prevail. If the 

 precipitations are decidedly less than the above, both of these 

 types pass into open scrub. Further reductions still in the 

 annual rainfall lead to the Desert type. 



Describing these types somewhat in more detail, especially with 

 respect to their Tropical West African representatives, we have 

 as characteristics of 



A. The Rain or Moist Tropical Ererrjreen Forests 

 (1.) A poor development of bark on most of the tree stems. 



(2.) A less developed branch system as compared with that of 

 trees from temperate zones. 



