Saturday, October 20, 2007

CLIMATE AND VEGETATION

CLIMATE AND VEGETATION
Nigeria has a tropical climate with sharp regional variances depending
on rainfall. Nigerian seasons are governed by the movement of the
intertropical discontinuity, a zone where warm, moist air from the Atlantic
converges with hot, dry, and often dust-laden air from the Sahara known
locally as the harmattan. During the summer, the zone of intertropical
discontinuity follows the Sun northward. As a result, more and more of
the country comes under the influence of moisture-laden tropical
maritime air. Thus, much of the country experiences a rainy season during
summer. As summer wanes, the zone shifts southward, bringing an end to
the rainy season. Temperatures are high throughout the year, averaging
from 25° to 28°C (77° to 82°F). In the higher elevations of the Jos
Plateau, temperatures average 22°C (72°F). Northern Nigeria typically
experiences greater temperature extremes than the south.

Rainfall varies widely over short distances and from year to year.
Parts of the coast along the Niger Delta, where the rainy season is
year-round, receive more than 4,000 mm (160 in) of rain each year. Most of the
country’s middle belt, where the rainy season starts in April or May
and runs through September or October, receives from 1,000 to 1,500 mm
(40 to 60 in). Within this region, the Jos Plateau receives somewhat
more rain, due to its higher elevation. In the dry savanna regions,
rainfall is especially variable. The region along Nigeria’s northeastern
border receives less than 500 mm (20 in) of rain per year, and the rainy
season lasts barely three months.

Vegetation also varies dramatically at both the national and local
level in relation to climate, soil, elevation, and human impact on the
environment. In the low-lying coastal region, mangroves line the brackish
lagoons and creeks, while swamp forest grows where the water is fresh.
Farther inland, this vegetation gives way to tropical forest, with its
many species of tropical hardwoods, including mahogany, iroko, and
obeche. However, only in a few reserves—protected from the chainsaw and the
farmer—is the forest’s full botanic diversity intact. Elsewhere, forest
is largely secondary growth, primarily of species like the oil palm
that are preserved for their economic value. Forests cover only about 15
percent (2000) of the country’s total land area.

Immediately north of the forest is the first wave of savanna: the
Guinea, or moist, savanna, a region of tall grasses and trees. The southern
margins of the Guinea savanna—which has been so altered by humans that
it is also called the derived savanna—were created by repeated burning
of forest until only open forest and grassland were left. The burnings
destroyed important fire-sensitive plant species and contributed to
erosion by removing ground cover. Tropical forest is giving way to the
Guinea savanna at such a rate that the only forests expected to survive
the next generation are in reserves. Beyond the Guinea savanna lies the
drier Sudan savanna, a region of shorter grasses and more scattered,
drought-resistant trees such as the baobab, tamarind, and acacia. In
Nigeria’s very dry northeastern corner, the semidesert Sahel savanna
persists. Throughout these drier savannas, drought and overgrazing have led to
desertification—the degradation of vegetation and soil resources

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