What is the forest canopy?  Defining the canopy, Part I.

Here’s a question. If we are planning a series of Canopy Watch events in 2015, with research expeditions and the posting of videos and other media from the forest canopy, then we had better know exactly where the “canopy” is, right?

Let’s put it another way. If someone climbs a tall ladder or some vines and they get 20 feet (6 meters) up into the trees and they say they are working in the canopy, but the forest is 180 feet (55 meters tall), are they really in the canopy? Is the forest the same at the lowest 20 feet and the top 20 feet? Heck no!!

This argument may sound silly, but forest ecologists can’t agree on a definition of what constitutes the forest canopy. We know what a tree trunk is, or a root, but defining the limits of the canopy is a lot trickier. Let’s start with what we do know.

Have you ever noticed while walking under a tall, dense forest on a hot, sunny day that it’s nice and shady and cool in the forest, but when you walk out in the open the sun beats down and it’s hot?  Sure, everybody knows it’s cooler in the shade.

Now, have you ever tried this at night?  Under the forest the air remains a little warmer and still, but in the clearing that is hot in the day time, it gets cooler and breezier at night. This is because the trees with their branches, trunks, and leaves act as a buffer against the sun, wind, rain, and the weather in general, and protect the environment – or microclimate – close to the ground.

When was the last time you walked – or climbed – in a tropical forest? You haven’t? Well, that’s why we’re here. Look at the photo below of the ground level of a tropical forest and think of three words that describe what you see.

 

What are your three words? I think of shady, moist, looks cool, there are lots of plants with big leaves, it’s dense.

But have you ever climbed to the top of a forest? Well, if not, here is what it’s like. Pause and think of three words to describe this photo.

 

All that buffering that protects the understory is lost the higher you climb in a forest. During the day the top of the forest gets the full brunt of the sun and wind. It can be quite hot and dry. Find yourself on a branch during a rainstorm and you get soaked. At night it’s the reverse: there is no vegetation above to trap the heat from the daytime sun, and the canopy cools off. Said another way, the microclimate of the forest canopy is more variable and more prone to extremes than the microclimate of the forest understory.

“SO WHAT?” you ask. Well, it makes a big difference in the biology of a forest. Think of the forest in terms of gradients. Plants that need a more stable environment, or that require less sunlight, live closer to the ground, whereas plants that like to be dried out periodically, or that require more sunlight, or that can take extremes in weather, will be found growing higher up. Insects find their preferred foods at the right heights, and so do birds. In a tropical rainforest you will see birds like the Lovely Cotinga only in the canopy.

 

Meanwhile, there are birds like this Blue-crowned Manakin that live in the understory and are rarely or never seen higher up in the canopy.

 

The Lovely Cotinga eats fruits that are only found in the canopy, and likewise, all the plants, insects, and birds interact in their own little ecosystem that is totally distinct from what happens nearer the forest floor.

Now that we all accept that the forest canopy is its own unique space, defining where that space starts and stops should be easy, right? Not so fast. Forest ecologists have a hard time agreeing on what part of the forest is the canopy. There are biological gradients along a vertical axis in the forest, but there aren’t distinct layers, as in rocks, which are easy to identify and which we can call A, B, and C. Where do we look for a definition of the forest canopy?

Well, let’s try the 2004 book Forest Canopies, edited by Lowman and Rinker: “the combination of all foliage, twigs, and fine branches in a stand of vegetation.” In other words, if it’s a plant and growing anywhere above the level of dirt, it counts as “canopy.”

FOUL!  Although some ecologists prefer this simplistic definition, I don’t buy it. The top of a forest is quite a different place than the bottom. The birds know it. The plants know it. Let’s listen to them as our teachers. Instead, I’m going with a definition I found written by Frans Bongers in 2001. (Evidently I’m not the only one who is “bongers” for the treetops.) In a paper published in the scientific journal Plant Ecology he set out to define methods that ecologists can use to define the structure of a rainforest. Bongers realized that ecologists needed a language to use in describing the forest canopy, and he set out to define that language and reduce the confusion. I created the figure below to help explain.

In the simplistic definition of a forest canopy, any plant growing above the ground is “canopy.” That is everything in the diagram. Bongers gives us an alternate definition that I like better: if a tree crown touches the sky (and the sun, the wind, and the rain), then the whole crown of that tree is part of the canopy – what I have labeled “canopy” in green, above.  As someone who has climbed to the tops of hundreds of tropical trees for research and to simply watch life in the canopy, it makes sense, because I already know that I’ll find some birds, plants, and other living things only in these tree tops. Together they form the unique realm, The Forest Canopy.

 

REFERENCES

Bongers, F. 2001. Methods to assess tropical rain forest canopy structure: an overview. Plant Ecology 153: 263-277.

Lowman, M. D., and H. B. Rinker. 2004. Forest Canopies. 2^nd edition. Elsevier Academic Press. Burlington, Massachusetts.