S t r u c t u r e   o f   t h e   F o r e s t

Emergent trees
Rainforests have a distinctive structure. Above a sea of green, which is called the "upper canopy", tower just a few “emergent trees” per acre, sometimes as tall as 40 meters (over 130 feet.) In all the forest they alone enjoy unfettered access to the sun and sky. The ways in which they disperse their seeds—by wind—and the creatures who inhabit them—birds like the harpy eagle, and the toucan—are very different from those found below.

Most photosynthesis occurs in the canopy—that vast carpet of green which Alexander von Humboldt called a “forest above a forest”—which absorbs as much as 90% of the sunlight falling on the forest, darkening the lower regions. Here, 20-30 meters up (65-100 feet), live butterflies, and mammals like the three-toed sloth, moving slowly but efficiently in search of vegetation, descending only once a week to the ground to defecate. (See ECOsystem.) Temperatures here reach 32 degrees C (96 F) but the humidity is only 60% (compared to 90% down below.) Since this is where photosynthesis occurs, this is also where productivity is greatest: each year a tropical rainforest produces about 25-30 tonnes of new growth per acre (10-12 tons), twice as much as a temperate oak forest.

In virgin rainforest, the understory is not the “jungle” of tangled vines seen in old movies, or observed from boats on one of the thousands of rivers and tributaries also nourished by the heavy rains. Undisturbed rainforest is surprisingly clear of vegetation close to the ground in part because so little light filters down through the canopy, sometimes only 1 to 2 per cent of the sun’s original intensity. But even here life’s struggles are intense, and insects, fungi and roots all fight for access to energy the raw materials of existence. Here temperatures are cooler, averaging about 28 degrees C (82 F) but humidity is higher, about 90%.