Tropical rainforests lie in the equatorial belt, where high temperatures and almost constant rainfall produce ideal growing conditions. The high temperatures - around 80°F - vary by less than 3° throughout the year. Most tropical rainforest vegetation is evergreen: because there are no seasons, there is no need for leaves to be shed during a winter period.
Temperate rainforests are different: they contain deciduous trees which do shed leaves each fall. These rainforests show a seasonal variation in rainfall patterns, although the levels of rain is still very high.Rain falls because the heat from the sun evaporates water from vegetation and from the oceans, rivers and lakes. The gathering moisture in the atmosphere forms clouds which fall back on the forests as recycled rain.
Because the sun's heat in the tropics is so strong, the evaporation and rainfall cycle is much more localized and pronounced in these regions than elsewhere. High rainfall levels in temperate rainforests are produced by other factors: for example the US rainforests are in the 'rain shadow' of the Cascades mountain range.
Photosynthesis
Photo comes from the Greek word 'phos' which means light and synthesis which means change. Every living creature on earth depends on green plants in order to survive. They provide us with food and the oxygen in the air we breathe. When human beings and animals exhale they release carbon dioxide into the air. Carbon dioxide is a poisonous gas if we inhale it. Plants and trees remove this carbon dioxide from the atmosphere and transform it back into oxygen by a process known as photosynthesis.
Soil
Most of the soil in tropical rainforests is ancient and infertile. In temperate forests the soil consists of a deep, plant-material rich layer, called humus. Plants can bury their roots deep into the ground maximising their ability to absorb available nutrients. In wet tropical regions the humus layer is much thinner because torrential downpours constantly wash minerals and organic material from the soil. Efficient decomposition of forest debris and a shallow root system allows plants to take up nutrients immediately from the forest floor.
Saprophytes are organisms that act as rainforest decomposers. Some fungi, called mycorrhizals, carry out this function also. Decomposers, together with warmth and wetness which helps accelerate decomposition, can often break down dead animals and vegetation within 24 hours. Leaf litter is often not more than 1 or 2 inches thick on the forest floor. Decomposition in montane forests, which are colder and less humid, however, can sometimes take upto six weeks.
Many saprotrophs are so small, called microbes, that they cannot be seen with the naked eye. Other decomposers, which include insects, grubs, snails, slugs, beetles and ants, aid in the recycling of valuable nutrients from dead organic matter. When released back into the soil it is reabsorbed rapidly by plants and trees. Decayed matter contains essential nutrients like iron, calcium, potassium and phosphorous all of which are necessary to promote healthy rainforest growth. Thus decomposers must work continuously to release these and other elements into the soil.
Gases and the greenhouse effect
Our planet's air is a mix of oxygen and nitrogen plus a few trace gases such as methane, carbon dioxide, and nitrous oxide. These gases capture some of the heat from the sun as it reflects off the earth's surface and retain it in the atmosphere. This is known as 'the greenhouse effect'. As the amount of trace gas increases, so the earth heats up.
Currently carbon dioxide contributes to 54% of global warming, a third of which is directly as a result of burning tropical rainforests. Rainforest trees need to absorb carbon dioxide from the air for food which is made during a process called photosynthesis. They act as carbon dioxide storehouses.
It is predicted that the rise in atmospheric carbon dioxide will lead to global temperatures increasing by as much as 8°F by the year 2050. Already the earth's temperature has risen by 1°F this century. Oceans will expand as polar ice caps melt causing coastal flooding all over the world. As approximately half of the world's population live in coastal areas the consequences would be disastrous.
In June 1992 world leaders gathered together in Rio de Janeiro to discuss environmental issues such as the cause and effect of 'greenhouse' gas emissions and included the part played by rainforest destruction. The meeting was the largest of its kind and was successful in raising public and governmental awareness worldwide to the host of environmental pressures facing our planet today. A treaty was passed in which industrial nations agreed to reduce carbon dioxide emissions to 1990 levels by the year 2000.
Biodiversity
The world's rainforests are extremely biodiverse. The rainforests are thought to be home to nearly half of the estimated 80 million living species on this planet, yet less than 5% have been recorded by scientists. There are many millions, mostly plants and insects, waiting to be discovered. In a typical single acre of primary rainforest you could expect to find as many as 100 different types of trees.
Compare this to a single acre in the richest temperate forests where you might find up to 25 different types, and probably considerably less. However, it has been estimated that as many as one million living plant or animal species has disappeared forever in just the past 20 years. Currently nearly 50 species are lost every single day.
