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.
Canopy
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.
Understory
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%.
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