Climbing to Success—Rainforest Strata
Rainforests are often classified by strata—layers—just like sedimentary rocks. The various layers of the rainforest are differentiated by the amount of light the organisms receive there, by their “energy budget.” Many organisms live their entire lives in one stratum of the rainforest.
At the bottom of the rainforest is the litter layer, often called the “brown food web.” This area may exist in semi-darkness all of the time. There, plant roots merge with mycorrhizal organisms; this partnership allows plants to get nutrients quickly and efficiently. (You already know how shallow the soil is, and how difficult it is for the decomposers on the forest floor to free those nutrients for re-use.
From 5 to 15 m, the lower (trunk) zone of the forest is characterized by stilt roots and the organisms that grow on them. (Remember the mangrove.) Here you’ll find lianas (vines) that exploit their tree partners. They may have stems as thick as 20 cm (8 in) in diameter, but are often very pliable, following the growth of the tree. You’ll also find epiphytes, which have invented the most energy-efficient method of getting the light energy they need—living their whole life cycles perched on the stems or trunks of other trees. Examples include orchids, bromeliads, mosses, liverworts, lichens (really algae and fungus living together in a pseudo-organism), algae and mosses. The challenge for epiphytes is to get water, but as long as the canopy is continuous, the humidity is usually high enough for that to occur.
From 15 to 30 m, the continuous canopy of the rainforest collects most of the available light. That’s why some 70% of the plant species in the rainforest are trees. Canopy leaves are often compound, with thick cuticles and drip tips. You’ll remember that the veins are often quite prominent (C4 photosynthesis) too. The leaves in the lower canopy are often redder, to take advantage of the light that the upper canopy misses.
The emergent layer features the crowns of a few trees (perhaps 40 m/130 ft above the forest floor.) It is often discontinuous. These trees have to withstand the high heat, the wind and storms of the tropics without the buttressing effects of the canopy.
Vive’ le Difference—The Value of Biodiversity
In the rainforest, it’s impossible not to be amazed and confused by the diversity you experience. Remember that forest in Illinois where we imagined ourselves at the start of this course? There might have been 3 or 4 species of trees in an acre. In a tropical rainforest, there are seldom fewer than 40 species per hectare. (The unit has changed since most tropical areas measure land in metrics. A hectare is about 21/2 acres.) A single hectare of rainforest might have 1200 or more species of butterflies. (That’s twice the number in the United States and Canada combined.) Some of that biodiversity may have been deliberately created by humans thousands of years ago. We'll read about that in Week 3.
Of the world's twenty most biodiverse countries, 15 (Brazil, Colombia, Indonesia, Mexico, Venezuela, Ecuador, India, Peru, Bolivia, Malaysia, Costa Rica, Thailand, Papua New Guinea, the Democratic Republic of Congo, and Madagascar) are mostly tropical rainforest. In theory, that should make them more stable and more resistant to destruction. Here’s the basis for that model: Imagine you were building a structure—would ten times the number of joists make it stronger? Or a space ship? How many redundant systems would be necessary before you felt confident enough to travel to Mars?
But at the root of all that biodiversity in South America is the substrate. (Remember the last lesson in Week 2, “The Roots of the Rainforest”?) The rainforest soil is very thin. Even Darwin noted that. Beneath that fragile layer of available nutrients is impermeable clay. So it’s a bug-eat-bug world in there.
In any production line, there is a limiting factor. (Imagine you want to duplicate 50 final examinations, but only have 35 copies of page 6. You aren’t going to get there! Students who want A’s are going to compete for full copies.)
In the rainforest, you may have all the sunlight and moisture you can handle, but if there is not enough nitrogen (or sulfur, or magnesium, or even enough space to sink a root) your ability to reproduce is limited. If the soil is highly alkali, the odds favor certain plants. (There are places in Columbia where the soil is so caustic that the only crop that will grow is coca. So when the “war on drugs” defoliates rainforests, we create situations where the only way for indigenous people to make a living is to grow the raw materials for cocaine.) If the noise is too high, the pollution too great, the incursion of exotic species too frequent, you lose.
So in the rainforest it’s a race. Biodiversity=creativity ≠ stability.