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Today:Friday, October 24, 2014


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By

J. 0. Adeiuwon

INTRODUCTION

Organic substances are introduced into the environmental system through basic biological production. The latter results directly from the photosynthetic activities of green plants. Part of the substances synthesised in this way is utilised by the plants for self-sustenance. The balance is what is apparent as vegetation and constitutes the biological resources. It is from the vegetation that the other living members of the ecological system derive their food and, whenever the human element is included in the structure of the ecosystem, it also yields the energy for the maintenance of the cultural and the economic circuits generated.

Living biomass is usually encountered as individual plants and individual animals, whether wild or domesticated. These are recognised as members of particular species, genera or other taxonomic units in the field. Each of these groups has attrib- utes that qualify it as specific biotic resources. This survey of the biological resources of Nigeria reveals the net primary biological productiv- ity, the standing biomass or vegetation, and the diversity of species, both plants and animals. Current changes in the stock of these resources are also illustrated and explained.

PRIMARY BIOMASS PRODUCTIVITY IN NIGERIA

By assembling data on climate and productivity throughout the world, Rosenzweig (1968) has been able to derive a formula connecting actual evapo- transpiration (AE) and net primary productivity. Data on AE were derived using the formula suggested by Thornthwaite and Mather (1957). With this, AE is calculated from the latitude of a place, its mean monthly temperature and its mean monthly precipitation.

Both climatic and biological data were then transformed into common logarithms. Using the method of least squares, linear regression of pro- ductivity on AE was performed. The productivity prediction equation, including 95 percent confi- dence intervals for the slope and the intercept, is given as: LogIO NAAP= (1.66+ _O.Z7) logIO AE - (1.66+ 0.07)

AE is the annual actual evapotranspiration in millimetres; and NAAP is the net 'annual above-ground productivity' in grams per square metre Using the equation given above and the data on AE compiled by Thornthwaite et al (1962), the annual productivity for each of 23 climatic stations across Nigeria was calculated. The values of NAAP vary from 778 grams/m2/year at Nguru in the Sahelian ecological zone, to 4335 grams/m2/year at Port Harcourt, in the coastal mangrove.

Based on the suggestions of Rodin and Basilevich (1968), the wood component of primary productivity for the various locations in Nigeria was computed (Adejuwon, 1981). These are depicted in column 3 of Table 3.5.1. In general, the ratio of wood production to litter production in both forests and grasslands increases with decreasing solar energy available during the growing season and also with increasing precipitation.

In Nigeria, precip- itation is highest in the southeastern parts where annual totals are as high as 3000mm and lowest in the northeastern boundary area with about 500mm. On the other hand, the annual reception of solar radiation in the south is much lower than in the north. The annual total solar radiation received in the Delta area amounts to only llOkcal/cm2/year compared with about 191kcals/cm2/year in the Sokoto area (Davies, 1966).

Thus in the southern parts of the country, the abundant rainfall and the relatively low amount of solar radiation reception result in the conversion of a high proportion of the considerable biomass produced into wood. On the other hand, in the northern areas, the little rainfall and the very high levels of solar radiation reception are reflected in the very low wood productivity.

One obvious query, which needs to be an- swered, relates to the scores of zero for some of the stations. There is no part of the country where wood growth is zero. Yet these figures are interesting and theoretically justifiable. What is to be noted with respect to all the stations with zero scores is that the visible wood productivity has been achieved through a special flora adapted, in the case of Jos to the high altitude, and in the other cases to the relatively arid environment.

 


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