BY H. L. SHANTZ
IN "The Disappearance of the Tropical Forests of Africa, " Aubréville1 points in striking terms to the many means by which the forests are gradually shrinking. The loss of forest area to the grasslands is due largely to tine' clearing done by native populations and to destruction of forests by fire.
1 Aubréville, André Marie A., Unasylva, Vol. 1, No. 1 (July-August 1947), 5-11.
It seems possible to estimate with some degree of accuracy the amount of this destruction. In Africa there are three forests which have lost area to the grasslands and in each of them this process 'of destruction is going on at this time. The tropical rain forest is losing to the high grass savanna, the temperate rain forest to the mountain grassland and the dry forest (monsoon forest) to the acacia-tall grass savanna.2
2 Shantz, H. I. and C. F. Marbut, Vegetation and Soils of Africa, Research Series No. 13, New York: American Geographical Society, 1923.
The boundary line or ecotone between the tropical rain forest and the high grass savanna is typical of a retreating forest line. The scattered clumps of trees throughout the savanna are remnants of what seems from an ecological point of view to have been continuous or nearly continuous forest. The fact that climate, soil, and crop potentiality are the same except for the microclimatic effect of light and wind movement also supports this view.
Beyond the high grass savanna, which is probably derived from the tropical rain forest, lies the dry forest. This forest in turn can with great difficulty be separated from the acacia-tall grass savanna. Where the dry forest is broken into clumps or park land areas or single trees, much the same type of ecotone is presented as that between the tropical rain forest and the high grass savanna. Often the trees disappear entirely, leaving tall grass, but generally the more fire-resistant trees are scattered over the grassland. Natives choose land marked by the clumps of trees for cropland, cut and burn the trees, and plant the crop in the soil fertilized by the ash.
At high elevation in a temperate climate, the temperate rain forest is shrinking rapidly. The soil is productive of a great variety of agricultural crops and some of the region is densely populated. Cropland when abandoned and burned over reverts, not to forest, but to bracken and to grassland. Here, again, small areas of forests persist where naturally protected, and there is every indication that forests once occupied much, if not all, of the mountain grassland area.
It is evident that if this process of gradual destruction of the forests of tropical Africa is not halted or turned back by afforestation methods the forests will give way entirely to grassland or savanna.
It seems reasonable to estimate the amount of this shrinkage by comparing the area of the forest with the ecologically equivalent grassland derived from it. By this means we can express numerically what is left and what has probably been destroyed by present practices.
The tropical rain forest is losing land to the high grass savanna and the area of each is as follows:3
3 Shantz, H. L., "Agricultural Regions of Africa," Economic Geography Vol. 16 (1):1-47, Vol. 16 (2):122-161, Vol. 16 (4): 341-389 (1940); Vol. 17 (3):217-249, Vol. 17 (4)353-379 (1941); Vol. 18 (3):229-246, Vol. 18 (4):343-362 (1942) Vol. 19 (1):77-109, Vol. 19 (3):217-269 (1943).
|
Tropical rain forest. |
233,000,000 hectares |
|
High grass savanna |
381,000,000 hectares |
|
Total (potential). |
614,000,000 hectares |
The ratio of actual forest to potential forest is 0.38. In other words, the present tropical rain forest is shown to be only 38 percent of what it may have been originally.
In the highlands the temperate rain forest is losing area to the mountain grassland and the area of each is as follows:
|
Temperate rain forest |
24,900,000 hectares |
|
Mountain grassland. |
83,100,000 hectares |
|
Total (potential) |
108,000,000 hectares |
The ratio of actual forest to potential forest is 0.23. The present temperate rain forest is shown to be only 24 percent of what it may have been originally.
On the drier side of the high grass savanna in an area with a pronounced dry period is the dry forest. This forest is losing land to the acacia-tall grass savanna. The area of each of these types is as follows:
|
Dry forest |
273,000,000 hectares |
|
Acacia-tall grass savanna |
526,500,000 hectares |
|
Total (potential) |
799,500,000 hectares |
The ratio of actual forest to potential forest is 0.34. This indicates that the present dry forest is only 34 percent of what it may have been originally.
These areas can be combined to give an over-all picture of the forest condition in tropical Africa.
|
|
(Area in hectares) |
|
|
Actual |
Potential |
|
|
Tropical rain forest |
233,000,000 |
614,000,000 |
|
Temperate rain forest |
24,900,000 |
108,000,000 |
|
Dry forest |
273,000,000 |
799,500,000 |
|
Total |
530,900,000 |
1,511,500,000 |
It seems reasonable to say that in tropical Africa today the forest area is about a third of what it could have been except for the ravages of fire and the destructive practices of the agriculturists.
These figures are based on planimeter readings from equal area maps and are probably as accurate as can be obtained without actual vegetation surveys. Such surveys would give more accurate details but it is doubtful if they would appreciably modify the general conclusion.
The present forests of tropical Africa occupy about 531 million hectares. The adjacent savanna and grasslands derived largely, if not wholly, from the destruction of the forests now occupy about 991 million hectares, most, if not all, of which is capable of producing forest under proper management.
The general conclusion is that the forests of tropical Africa. which are shrinking rapidly at the present time, have already been reduced to a third of what they probably were originally.
Raft of okoumé logs floating down a river in Gabon, tropical Africa
CHART 1. FOREST REGIONS AND PRINCIPAL FOREST TYPES
