The autotro­phs determine the available energy whereas the heterotrophs affect the turn­over and cycling of matter within ecosystem (Ryszkowski, 1974).

The effi­ciency of both these components is affected by physicochemical and cli­matic factors, etc. The productivity of plant communities or vegetation is measured as the amount of organic dry matter produced per unit time in a given area. The two commonly used units for expressing productivity are as grams of organ­ic dry matter/m2/day or tons/ha/yr.

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The production (P) of a plant community is greater, the higher the net assimilation rate (NAR) of the plant species, the more completely the avail­able light is utilized or trapped by the assimilation surfaces (as estimated by leaf area index, LAI), and longer the time (t) in which the plants maintain a positive gas exchange balance: P = NAR x LAI X t The productivity of stands of maize, for instance, increases linearly with increase in its leaf area index up to around 5 m2 and then becomes more or less constant. In contrast, the net assimilation rate of single maize plants falls with increasing LAI. As regards yield per unit ground area, an open stand of corn is less productive than a closed stand (Larcher, 1975).

The duration of the production period is an important factor in deter­mining the carbon balance of individual plants and also of the annual photo- synthetic yield of plant communities in a given area. This is why long time spans prevailing in warm-temperate, subtropical and humid tropical regions produce greater yields as compared to regions with shorter favourable time spans. The gross productivity P of a plant community is the sum of its net productivity Pn and its respiration R Pg = Pn +R These figures are usually computed as annual values. For a tropical rainforest in Thailand, Yoda (1967) estimated the Pg to be around 125-130 tons dry matter/ha/yr.

The Pn in turn is a sum of increase in phytomass B, loss of phytomass L, and the amount of phytomass grazed by consumers, G Pn = ? B + L + G. The factors L and G constitute important parameters in the annual carbon balance of an ecosystem. These parameters, along with the produc­tivity coefficient kp of the community chiefly determine whether the organ­ic mass of a community will increase, remain constant (unchanged) or decrease under a given combination of species composition, age and stage of succession, extent of stress, etc. The primary producers slowly produce organic mass which at first becomes incorporated into their own bodies and then into the soil, either directly or via the consumer chain. The organic matter that is lost at first accumulates as litter on the ground. Litter is a very important component of an ecosystem. Its amount at any time depends primarily on the total amount deposited during a year and the rate of its decomposition. Much of the litter in a tropical forest becomes decomposed rather quickly.

In deciduous for­ests in temperate areas, on the other hand, such litter decay is a much slower process. In the tropical rain forest, the carbon is bound up mainly in the phytomass; the litter is rapidly decomposed and the organic matter content in the soil is quite low. By contrast, in the subalpine spruce forest, there is much more organic matter in the litter and humus than in the plant cover.

These differences are primarily caused by the slower decay of organic carbon in the soils as compared to the faster decay in the tropical conditions.