forest floor is limited, so that there is little undergrowth and passage is easy. (The isolated and relatively undisturbed forest on the low mountains of the Nigeria-Cameroon border, North of Calabar, is a good example.)
Where light does enter the forest, the high intensity of the tropical sun causes an explosion of growth: in the interior, regeneration of many plant species only occurs with the collapse of a dead mature tree, cutting a slash of light through the canopy to the forest floor. This is a natural ecotone.
The high light intensities on the edges of the forest—naturally alongside rivers, lakes and swamps, and man-made beside roads—do present a tangle of vines, shrubs and young small pioneer trees that may indeed seem impenetrable until the deep forest is reached. However, this is defined as 'disturbed forest'. Vines, shrubs and pioneer trees similarly fill the forest where it has been disturbed by logging or by the artillery of war.
5.3.2 THE TROPICAL RAINFOREST DOES NOT NECESSARILY HAVE AN EVEN CLIMATE
Rainforest on the savannah fringe experience the seasonal variations of temperature and rainfall one would expect. Similarly, areas closer to the equator which are subject to continental influences show no dramatic rainfall or temperature variations throughout the year.
However, large areas of the tropical rainforest do not fall into these simple subecozones. The upland tropical rainforests in the Northern Okwangwo Hills of Nigeria, for instance, experience a well-defined dry season often exceeding three months. Even the coastal monsoon rainforests of the Southern Niger Delta experience very low rainfalls in January and December (less than 10mm per month in Brass).
Temperature varies, too. Although Akassa (near Brass) has a mean annual temperature variation of 24 to 27°C, the temperature can drop to 22°C on during Harmattan days when the dry, dusty wind blows down from the Sahara. Anyone who has lived in the tropical rainforest knows the discomfort of sudden cold during the rains other animals, and plants feel this stress also. Yet the temperature may rise to 30°C on clear sunny days.
Temperature also rises with hours of sunlight, and within the Niger Delta (lying as it does between the latitudes 4 and 6 degrees N) the days are up to an hour longer during June than in December. And climate also varies over longer periods. Figures collected between 1948 and 1980 in Port Harcourt show annual rainfall rising steadily from 2075mm in 1950 to 2800mm in 1963 and then falling to 2000mm in 1974 before rising again. Over the same period, mean annual temperatures ranged from 25°C to 27°C with a general upward trend since 1952.
5.3.3 TROPICAL RAINFOREST SOILS ARE NOT UNIVERSALLY POOR
Tropical rainforests cover whole landscapes, from mountain plateaux, through valleys of every conceivable gradient, to coastal flood plains, which contain the whole range of humid tropical climates: correspondingly there is a range of topographical and drainage conditions and therefore of soils, rich and poor.
Because of the high rainfall there is a strong tendency for nutrients to leach out, resulting in poor sandy soils. This is balanced to some extent by the high rate of biomass addition to the soil, as vegetation dies and decays, rapidly re-cycling nutrients. But within this tendency may be specific areas of richer soil, especially on flood plains,
