Key Words:
Tropical rain forest - biodiversity - layering - tundra - alpine - deserts - global patterns

Main points

• Tropical rain forests have high NPP and the highest biodiversities of any terrestrial ecosystems. The physical environment favours vast photosynthetic output and high growth rate.
• In rain forests, plants have constant high levels of water and light (at canopy level) and the nutrient supply is as high as possible, due to rapid decomposition. Because the light is strong, it is able to sustain several layers of plant growth. High levels of water allow plants to maintain a constant flow from their roots, bringing up more nutrients. They can keep stomata open all day to optimise CO₂ uptake without becoming short of water. They can use water to cool down if required. High and constant temperatures allow rapid enzyme activity without seasonal dormancy.
• Tundra and alpine environments do not permit plants to grow very high, since the desiccation caused by wind and bending of the stem to induce ice damage. This means there can be less layers of vegetation and less efficient light capture. Cold weather limits NPP by reducing the rate at which enzymes can catalyse reactions. When water is frozen, no biochemistry can occur. Even when the air temperatures rise sufficiently to allow photosynthesis, the soil nutrients may be locked up permanently in frozen soil (permafrost).
• Deserts are areas of low available water. In extreme desert, be it hot or cold, the absence of water will prevent most plants surviving and, hence, productivity is negligible. Plants may show similar adaptations to conserve water in both environments (xerophytism).
• The table (right) shows the estimated NPP for major world biomes. Notice how environments with one or more limiting factors (lack of rainfall or lack of temperature) have very low NPP.

Imagery

The multiple layers of vegetation and the superb constant growing conditions lead to very high NPP. Biodiversity is vast - including fascinating species such as the quetzel - its chicks have claws on their wings

ecosystem Net primary production - dry (g m-2year-1) Area (106km) Tropical forest 2,000 20 Temperate forest 1,300 18 Boreal forest 800 12 Savannah 700 15 Temperate grassland 500 9 Tundra & alpine 140 8 Desert scrub 70 18 Extreme desert (sand/rock/ice) 3 24 Agricultural land 650 14 Open ocean 125 332 Continental shelf 350 27 Coral reef & estuaries 2,000 2 Overall for planet earth 320 510 Table of estimated Net Primary Production using data presented by Whittaker 1971, from Colinvaux 1973