section  3: Regional examples

Vegetation and bioclimate in Liberia

M. A-AS-Saqui

Central Agricultural Research Institute, Ministry of Agriculture
P. O. Box 3929, Monrovia, Liberia

Abstract

The tropical rainforest comprises in Liberia most of the country except a very narrow strip along the coast where mangrove vegetation alternates with coastal savanna. The meteorological condition in the whole country allows the vegetation to develop into a tropical high forest, and most probably the entire land area was once covered with it. Nowadays the total area of tropical high forest and secondary forest consists of one-third of the country, while the remaining 65% is composed af forested areas such as young secondary forest and intermediate forest and further non-forested areas such as farm lands, savannas, towns, swamps etc. Forestry plays a vital role in national economic development. Liberia has a network of several meteorological stations for agroforestry development, planning and management. Still the meteorological division needs to be strengthened and upgraded for effective services. The present status of meteorological service to agroforestry systems planning and management in Liberia is discussed and maps showing vegetation, average annual rainfall and temperature are presented. Future lines of work for improving the meteorological service are indicated.

Vegetation

The tropical rain forest belt in West Africa extends from Sierra Leone to Ghana and comprises in Liberia most of the country except a very narrow strip along the coast where mangrove vegetation alternates with coastal savannah. The climatic conditions in the whole country allow the vegetation to develop into a tropical high forest which most probably covered the entire land area. Today, tropical high forest and old secondary forest cover only one-third of the country; while the remaining 65% is composed of young secondary forest, intermediate forest and grass-woodlands. Although the general climatic conditions are nearly uniform throughout the country, rainfall and humidity decrease toward the interior. Consequently, vegetation is banded into zones more-or-less parallel to the coastline (Figure 1).

These zones are closely linked, but various kinds of transition occur, which are mostly influenced by topography and soil. The influence of shifting cultivation on the vegetation has been immense, and most of the high forests have at times been converted into farmlands. After farms are abandoned, natural vegetation recaptures the land and high forest gradually reappears. Several development stages can be recognized, such as 'recent farmland,' 'old farmland' and 'intermediate forest.'

Figure 1 Vegetation map of Liberia.

Bioclimate

In 1951, the Division of Meteorology was established within the Department of Public Works. It supervises a number of weather station throughout the country which measure daily rainfall and record maximum and minimum temperatures.

The equatorial position and the distribution of high and low pressure belts over the African continent and the Atlantic Ocean determine the climate of Liberia and, more generally, West Africa. Thus the climate of Liberia is marked by an even and fairly warm temperature throughout the year and above all by the very high humidity, above 90% much of the time. Unlike the temperate zones, the seasons are not determined by changing temperatures but by the prevailing precipitation. Therefore, a rainy and a dry season with transitional periods can be differentiated. Rainfall is heavy and intense. There is little year-to-year variability in the climate.

Rainfall

The Liberian coastline runs approximately from south-east to north- west and at right angles to the prevailing south-westerly rain- bearing winds. As the maritime air reaches the coast it is forced to rise, it cools, and condensation takes place which causes the extremely heavy precipitation. Consequently the rainfall is heavier than on any other part of the west coast of Africa except the Freetown Peninsula in Sierra Leone and a small isolated location near Buea in Cameroon. The annual rainfall near the coast is nearly 4550 mm (180 inches) but the amount has exceeded this in some years. Towards the interior, rainfall decreases except for the higher areas where the air is forced to rise, causing orographic precipitation. Because of its high altitude, the area around Mount Nimba receives much more rain (3225 mm = 127 inches) than would be expected from its geographical position. In the north-west there is a similar situation.

Figure 2 indicates a pronounced rainfall pattern. Generally a fairly distinct dry season extends from about November to April and a wet season the remainder of the year. The months of heaviest rainfall vary somewhat in different parts of the country, but normally are June, July and September.

In August most stations show a marked decrease of precipitation, and a short dry season known in Liberia as the 'middries' sets in for a fortnight or so. In September the rain starts with new force. The extreme south-east corner of Liberia is even noted for two distinct rainy seasons, with maxima in May-June and September-October, and with a well-defined short dry season in between.

The distribution of rain throughout the year and particularly the length of the dry periods are at least as important as the amount of rainfall. Since Liberian soils have usually low moisture-storage capacities, the amount and frequency of rainfall during the dry season becomes a limiting factor for the cultivation of many crops.

In spite of the heavy precipitation, it does not rain continuously during the rainy season. It is common to have sunny days even during the months when the rain is heaviest. In the interior heavy showers may occur, but only very rarely will it rain for the whole day. Naturally, the rain may last for a day or two without a stop. At such times the rain pours down steadily and water usually covers the ground for several inches.

Temperature and sunshine

Because of the equatorial position of Liberia the sun is almost overhead at noon throughout the year, and insolation is very intense. Therefore, high temperatures with little monthly variation should be expected. However, since temperatures are not solely dependent on insolation, but are affected by the degree of cloud cover, air humidity, rainfall and vegetational cover, temperatures in fact are much lower than anticipated.

Along the coast, the Atlantic Ocean has a moderating effect on the temperatures, and annual and daily variations are evenly balanced; while toward the interior the continental influence becomes more dominant, the range of temperatures widens, and the variation of minimum and maximum increases markedly. It is difficult to quote figures for temperature, since they vary considerably with location and altitude, but 27 to 32 °C (80 to 90 °F) during the day and 21 to 24 °C (70 to 75 °F) at night seem to be fairly representative. Because of the uniform and high humidity — the relative humidity ranges generally between 85 and 95% within the coastal area — temperatures seem much higher than they actually are. Only in the higher regions near the Guinea border, where the average altitudes are between 550 and 610 m (1800 and 2000 ft), is the climate more pleasant, since temperatures are much lower during the entire year. At times the inhabitants of these regions even suffer from the cold nights.

Figure 2 Average annual rainfall.

Figure 3 shows the average annual temperatures of Liberia. They range in the coastal area from 24 to 27 °C (75 to 80 °F) while in the interior they are usually several degrees higher except for the mountain regions. Although the country is situated north of the equator, the highest temperatures occur between January and April and the owest ones are usually recorded during the months of August and September. These low temperatures are mainly caused by the large amount of cloud cover common over much of coastal West Africa during these months.

Figure 3 Average annual temperature.

Sunshine is at a minimum during the wet months, particularly June to September, with a definite minimum in August. Days of long and bright sunshine occur between December and March, and an average of 6.3 or even 7 hours per day is not exceptional. During June and September only a few hours sunshine are recorded, and during August sunshine is particularly low, rarely exceeding the average of 1.3 hours per day. Sunshine and cloudiness therefore do not necessarily correlate with the rainfall; especially since August, when 'middries' occur, is the time of highest cloud cover.

The equatorial maritime air masses brought on the south- westerlies cause the very high humidity of Liberia. A relative humidity of 90 -100% during the rainy season is common, while during the dry season the average humidity is somewhat lower and usually in the range of 85 - 95%. However, regardless of season, the relative humidity during the night and early morning hours is almost always in the range of 90 - 100%. Since this is also the time of lowest diurnal temperatures, it is evident that before sunrise the air is practically saturated with moisture. Data from other stations show very similar results, leading to the compelling conclusion that the percentage and periodicity of humidity is valid within the whole sphere of constant monsoonal air flow. Only the region north of the Inter-Tropical Convergence Zone, where continental air masses prevail from mid-December to the end of January, has arid conditions. Because of the extreme dryness of the Harmattan, the humidity may drop below 50% at times.