Productivity is one of the primary features of any ecosystem. Most of the matter and energy within forests are contained in the “biomass” – the matter of the organisms which comprise the forest. Green plants, which photosynthesize and convert solar energy into chemical energy (i.e., carbohydrates) are known as primary producers (autotrophs), and their photosynthetic capacity, primary production. Secondary producers (heterotrophs) eat the primary producers and convert some of this energy into their own biomass. Decomposers are organisms which decompose organic matter and regulate nutrient cycling within the forest ecosystem.
Net primary productivity (NPP) is the energy converted, for the most part by green plants, from solar energy to chemical energy (primarily in the form of sugars) minus the energy lost through these converters by their respiration (i.e., their life processes). Net primary productivity is affected by temperature, by the availability of water, carbon dioxide (CO2), and nutrients, and by the efficiency of conversion of light energy to the chemical energy of carbohydrates. Since in the tropics where rainforests are found, water, light, and high temperatures are readily available, and there is a dense concentration of green plants at all levels from the lower stories to the canopy, it is no wonder that these forests have very high levels of productivity.
Net primary productivity appears to be greater in forests containing a variety of plant species than in a homogeneous system such as an agricultural monoculture. This is understandable if we consider that organisms sharing similar habitats also share certain characteristics. For example, if they are emergent canopy trees, the tallest trees in the forest, they will necessarily be light and heat tolerant, able to reduce moisture loss through their leaves, and so forth. Each species of canopy tree will, however, have evolved its own specific mechanisms to deal with these difficult conditions and to photosynthesize efficiently under them. For instance, many canopy trees have thick or waxy leaves to prevent water loss. Trees in the understory or lower canopy must live in partial shade, and at higher humidity and lower temperatures than canopy trees, and they have evolved features which allow them to cope with these conditions. Ground-level vegetation is in almost continual shade and very high humidity, and must be adapted to these conditions. Different species are also adapted to utilize resources in varied ways. Some may be more productive in acidic soils, others in neutral soils; others may flourish at low or high levels of some nutrient or water. Thus, where environmental conditions are detrimental to one species, individuals of other species, with different requirements or tolerances, may be able to survive. Having many species in an ecosystem is therefore a mechanism for fully exploiting the resources available in an area and for buffering the system against unstable conditions or catastrophes.