Imagine being in a skyscraper. Close the windows, and would you still be able to tell if you were on the first floor or the hundredth? Forest canopies are different – the top and bottom are entirely different world. Let’s go on a tour of Puerto Rican moist tropical forest, and climb twenty or thirty meters up into another world.
Branches strive ever-upward, placing leaves in a position high enough to capture sunlight without being shaded by another plant. You can see a wide range of strategies. Some, like this Cecropia, have very open branching and large leaves (bigger than your face) on long petioles. This strategy works well for deploying a large amount of photosynthetic surface area at a low cost, and for inexpensively reaching to the highest points of the canopy. Moreover, each leaf has a low probability of shading another leaf from the same plant. On the other hand, species like the Prestoea palm below, don’t reach such heights but cover large areas with their multi-meter leaves, and effectively shade out competition from below. The canopy of a forest is a constant war between plants, played by rules set by the environment. Even the latitude of the site matters – more polar forests get more sunlight from the side, rather than from overhead, so different branching strategies begin working better.
To see the top of a canopy, one has to first get there. One way up is to climb – here are some aerial roots, probably of a Philodendron growing high in the canopy of another tree (photo credit Colby Sides). Lianas and hemiepiphytes are structural parasites that spend most of their carbon on making leaves, and very little on building strong trunks that can reach the high parts of the canopy. Of course, they still do need to access water – hence these roots. These roots were easily strong enough to support my weight, and probably much more. Of course, this kind of climbing gets increasingly interesting the higher one goes, so we usually use safer techniques.
Here is the top of a canopy tower – a metal structure designed to permit access to the high canopy. These are usually rickety affairs, bolted together and held in place with long guywires attached to concrete blocks buried in the ground. It can be a long way down, if the forest canopy is high.
From above the canopy, you can easily see the multiple layers of leaves that comprise a forest. Each layer captures some fraction of the light, leaving less and less available to all the plants below. In the upper right corner, you can see a Marcgravia liana, cheating its way to the top.
From below, the effects of this shading are immediately apparent. The topmost plants can use most of the light, and the bottommost ones get very little.
Shade in the forest floor can make growth difficult. When a large tree dies, it opens a gap in the forest, and suddenly light and rainwater will reach the forest floor. An immediate response is the growth of new seedlings that are able to take advantage of this resource pulse. As these seedlings grow, they will close the canopy, and so the cycle repeats in a dynamic sort of equilibrium. But the game is hard one to play – less than one out of every thousand of these seedlings will die before reaching adulthood. This should make sense, because there is only a finite amount of resources that can be divided up between a few large individuals or many small ones. These deaths are inevitable and natural.
So from darkness to light, from cheaters to tall strivers, you can see that no part of the forest is the same at all.
I’ll end the post with one digression – layers of forest canopy don’t uniformly block out light. Sometimes little pulses of light called sunflecks will penetrate all the way to the forest floor. And sometimes, this light illuminates a spider sitting in its web, and makes the world seem a little more magical.