evapotranspiration may be limited by the build-up of a saturated layer of vapour above individual leaves and the forest canopy as a whole; on windy days evapotranspiration increases because the saturated layer is removed.
Rainforests effectively regulate their own humidity. When it is very high, less water can be transpired because the air is already so moist. So in turn the plants draw up less water from the soil. When humidity is low, water demand and transpiration both increase, but this serves to increase the localised humidity again.
5.5.4 MICRO-CLIMATE: NUTRIENTS
Most nutrients in the tropical rainforest are bound up in the active biomass. The most nutritious level of the soil lies in the very narrow surface horizons that contain the detritus and humus, and it is here that plants have the best chance of taking up soil nutrients before they leach any further down.
Thus the plants of the lowland tropical rainforest have evolved to concentrate shallow feeder roots near the soil surface. They are so efficient at doing this that the natural rain forest ecosystem as a whole loses very small amounts of nutrients. Where deeper taproots are put out, they serve mainly as physical anchors and to access water from lower soil horizons.
The major nutritional elements are Nitrogen, Phosphorus and Potassium (the chemical symbols for these elements are N, P and K respectively). In tropical rainforests, 50 to 60% of N and P is found in the leaves, stems and roots of plants, and nearly 90% of the K. By the time the C horizon of the soil is reached, no N is found, only about 11% of the P and only minute amounts of K. Nitrogen is not found in mineral deposits, but must enter the system by being 'fixed' from the air into organic molecules.
Legumes: Nitrogen (N) is essential to plant growth, largely because it is needed to make proteins. When N levels are too low, plants tend to look yellow and unhealthy. The ammonia given out as a waste product by bacteria digesting soil detritus contains N, and this is the major source for most plants.
The legume family of plants have developed a symbiotic relationship with a type of bacteria, known as Rhizobia, which live in nodules on their roots. Rhizobia are able to use the free molecular N in the air and 'fix' it into organic compounds. They release ammonia in turn, so legumes always have a ready supply of N.
The legume family includes peas and beans; they are often cultivated specifically to enrich the soil.
Symbiosis: Organisms living together for their mutual benefit, such a Rhizobia and Legumes.
Trees of the Legume family Papilionaceae, Casalpiniaceae and Mimosaceae) form a major part of the lowland tropical rainforest canopy, and provide the ecosystem with much of its nitrogen. However not all legume trees always carry nodules, and nitrogen also comes from bacteria, blue-green oligophotic algae and certain fungi living on the detritus.
5.6 PHYSICAL CHARACTERISTICS OF THE LEM RAINFOREST
Again remembering that the West African LEM no longer exists as a natural ecosystem, we may still define some of its physical characteristics. It was crossed by slow moving
