Tropical rainforests, of which the Amazon basin remains the largest single tract on Earth, circle the globe between the Tropics of Cancer and Capricorn, wherever rainfall averages at least 1500 mm a year (around 6 feet) and where temperatures do not drop below an average of 17ºC throughout the year.
The first thing most visitors notice when entering primary rainforest is just how little light actually filters down to the forest floor. The enormous trees that form the forest canopy or project from that canopy block out much of the light essential to plant life, with each species competing for their own place in the sun. 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 trees are capable of producing a growth surge when a gap in the canopy appears, due perhaps to a rival tree having fallen in the wind.
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 and, of course, the byproduct of this process in the form of oxygen.
In the rainforest, plants get their water from standing sources on or just below the surface of the forest floor, which itself is composed of a thick carpet of nutrient rich decaying plant matter. While rainforest tree cover may appear luxuriant, 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 the trees of the rainforest do not need to put out deep taproots in search of water, their roots tend to be shallow. For such massive trees, this could create problems of 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 trees have been compared by observers to the sound of an oxygen-starved human gasping for breath.