The importance of plants as energy producers

Energy perspectives are relevant to the ecosystem concept mentioned above. The process of photosynthesis enables a plant to retain, as chemical energy, approximately 1 per cent of the sun's radiant energy falling on the particular leaf's surface. As the plant is consumed by primary consumers, approximately 90 per cent of the leaf energy is lost from the biomass (see p54), either by respiration in the primary consumer, by heat radiation from the primary consumer's body or as dead organic matter excreted by the primary consumer. This organic matter, when incorporated in the soil, remains usefully within the ecosystem.

The relative levels of the total biomass as against the total organic matter in an ecosystem are an important feature. This balance can be markedly affected by physical factors such as soil type, by climatic factors such as temperature, rainfall, and humidity, and also influenced by the management system operating in that ecosystem. For example, a temperate woodland on 'heavy' soil with 750 mm annual rainfall will maintain a relatively large soil organic matter content, permitting good nutrient retention, good water retention and resisting soil erosion even under extreme weather conditions. For these reasons, the ecosystem is seen to be relatively stable. On the other hand, a tropical forest on a sandy soil with 3000 mm rainfall will have a much smaller soil organic matter reserve, with most of its carbon compounds being used in the living plants and animal tissues. As a consequence, nutrient and moisture retention and resistance to soil erosion are usually low; serious habitat loss can result when wind damage or human interference occurs. For temperate horticulturists, the main lesson to keep in mind is that high levels of soil organic matter are usually highly desirable, especially in sandy soils that readily lose organic matter.

Figure 3.3 Mature woodland

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