Processing of data of a number of observations and model investigations of photosynthesis have led to the conclusion that a net primary productivity (NPP) of natural vegetation can be estimated by the following model (Uchijima and Seino, 1985).
NPP = [0.29 exp (-0.216 RDI2)] Rn (4)
where RDI (= Rn/lr) is radiation dryness to express climatic dryness, in which Rn is net annual radiation (kcal), r is the total annual precipitation (cm) and l is the latent heat of vaporization (kcal/gH2O). The above equation implies that as RDI increases (drier), the efficiency of conversion from radiation energy to dry-matter production drastically decreases, and also that with the RDI value kept constant, the dry-matter production increases proportionally to the net radiation.
From Equation (4), where the climatic factors (RDI and Rn) are assumed to be variables, a model graph to express the NPP values and vegetation types can be created as Figure 3. As observed, the humid climatic zone with RDI < 1.5 is covered by the forest zone from coniferous forests with low Rn values through tropical rain forests with high Rn values. In those areas, the net primary production increases from about 5 ton to some 25 - 30 ton/ (ha・year). On the other hand, in the dry climatic zone with RDI > 1.5, water supply drastically decreases, and so does the net primary production, with its vegetation type changing from forest-steppe, steppe, semi-desert to desert. The net primary production sharply drops to below 5 ton, and, in the area with RDI > 3.0, down to below 1 ton/ (ha・year), which is the biological border.
To estimate the plant production of whole continents of 14,898 million ha under the current climatic conditions, the geographical distribution of the net primary productivity of natural vegetation has been obtained, as shown in Figure 4. From the diagram, the NPP values vary in the wide range from under 1.0t/ (ha・year) in the Arctic, tundra and horse-latitudes zones to 25 - 30t/ (ha・year) in the tropical rain forests in the equatorial zone, reflecting combinations of thermal environments, water supplies, and solar radiation energy.
