Tropical rainforests, of which in spite of decades of deforestation the Amazon basin remains the planet’s single largest tract, ring the world between the Tropics of Cancer and Capricorn, wherever average annual rainfall exceeds 1500 millimeters (around 72 inches) and where average temperatures remain above 17ºC all year.
When visitors enter virgin forest for the first time, often the first thing they notice is how little light filters down to the forest floor. The big trees that form the forest canopy or project from it block out much of the light essential to other plant life, with each species competing for its own place in the sun. As a result, major tree species tend to have straight trunks with no branches or leaves below canopy level. In fact, in their rush to gain access to life-giving sunlight, some species of large Amazon rainforest trees are capable of producing a growth surge when a gap in the canopy appears due, for example, to a rival tree having fallen during high winds.
The motor for the growth of these mighty trees, some of which rise to heights in excess of 45 meters (150 feet), is their leaf cover. Sunlight is the catalyst for a reaction involving chlorophyll, the chemical essential to the entire process of photosynthesis. It is chlorophyll which gives plants their green color and enables them to absorb energy from sunlight to create the starches and sugars that are the plant’s food, as well as the byproduct of this process in the form of oxygen.
In the world’s rainforests, plants get their water from standing sources on or just below the surface of the forest floor, which is composed of a thick layer of nutrient rich decaying plant matter. Rainforest tree cover appears luxuriant, but this is not an indicator of soil fertility. In fact, rainforest soils are among the most nutrient poor soils on the planet. In rainforests like those of the Amazon basin, the nutrients contained in dead plant matter on the forest floor are recycled directly into living plants without ever becoming part of the soil. In this highly efficient system, as much as 95% of a rainforest’s nutrients remain stored in living matter.
Because water is usually abundant, rainforest trees do not need to put out deep taproots. For such massive trees, shallow roots could compromise stability, which is why many rainforest tree species have developed adaptations such as buttresses –like the architectural innovations of Europe’s late Middle Ages- to provide them with additional support.
Fascinatingly, during periods of severe and prolonged drought, the water column system of large trees breaks down, and the clicking sounds produced as a result by some rainforest species have been compared by observers to the sound of an oxygen-starved human gasping for breath.