Text by David Anderson. This week’s guest photographer – Will Koomjian.
I plant a vegetable garden every year and I know a fair bit about how plants grow. Seeds germinate in the soil, the stalk grows up into the air as the roots grow into the ground. Nutrients and water are absorbed from the soil. Ask any grade school student and they can confirm these simple facts.
Turns out we’re all wrong. Go to a rain forest anywhere in the world – Washington, Brazil, Borneo, Ivory Coast – and you get a whole different story. Plants don’t have to be rooted in the ground, or even in the soil, but can grow right up in the tree tops in an amazing variety of life and leaf forms. They contribute to biodiversity in important ways, and are the subject of this week’s blog.
You ask: If a plant isn’t rooted in the ground, and it’s growing as high as 150 feet (50 m) in the air, then where the heck is it growing? A lot of plants that grow in old forests, especially rain forests, simply grow on the surfaces of other plants. Any plant that relies on another for mechanical support (read, “a place to grow”) in this way is called an epiphyte. In some forests you will find the trees so covered in epiphytes that you can’t actually see the trunk or branches of the tree, and it is hard to discern where the tree ends and the epiphytes begin.
This may start to sound weird and foreign, but you have probably seen them before. Spanish moss that hangs on old trees in the southern U.S.A. (and a lot of other countries) in weeping curtains of drab green and gray? It’s not moss; it’s a flowering plant. Look closely and you can see the flowers for yourself. Do you have a showy orchid in your house that you bought at the grocery store? More than likely, if found in it’s natural environment it was growing on a tree somewhere.
One of my favorite epiphytes is the strangler fig. As the name implies, it really is a fig tree, but this one is pretty special. Strangler figs occur in tropical rain forests around the world, and birds love to eat their juicy fruits. When a bird flies off with figs in its belly, sooner or later it has to poop (true story!), and a seed that falls on the branch or trunk of another tree can sprout there. Gradually the roots grow down to the ground in a delicate and spreading lattice, until finally they root in the soil. As the roots continue to grow they encompass the parent tree and strangle the life out it. The parent tree dies, rots, and is absorbed by the assassin. Birds eat the fig fruits of the strangler, and the story repeats itself all over the forest. Tropical intrigue at its best!
This strangler fig story brings up several important points on the biology of epiphytes.
1) Epiphytes have all kinds of growth habitats that allow different species to live at all levels of the forest. Some epiphytes, like the strangler fig, germinate on the surface of a tree and grow down to the ground. Others sprout on the ground and grow up into the tree. Some sprout on the tree and stay there, never touching the ground in their entire lifetime. Also, they have different needs based on their growth habits. Some need shade, others sun. Some can dry out and be O.K., but some need to stay moist. Some catch rain and store large amounts of water, others catch debris falling from trees and make soil, or trap nutrients like nitrogen and phosphorous from the atmosphere.
2) Because they have so many life strategies, epiphyte diversity in a tropical forest can be sky high (pun intended). At some sites 1/3 of all plant species are epiphytes. There can be hundreds of species in a square hectare (100 meters x 100 meters) and as many as 107 species have been found on a single tree. A single tree! I don’t think that there are 107 species of trees in the state I live in. Epiphytes can be almost any type of plant: orchids, ferns, mosses, shrubs, trees, vines. Yes I said trees. I once climbed a massive Ceiba tree – biggest tree I ever climbed, in fact – that had trees growing a hundred feet high in its branches that were larger than other trees growing down on the ground. Epiphytes have bright flowers, nice fruit, spores. They do it all.
3) Epiphytes are critical for forest biodiversity. Epiphytes create entire microcosms in their leaves, teaming with tadpoles, worms, mosquito larvae, even crabs living in there and living off each other. Hundreds of bird species feed on the fruits and nectar. Bees collect the pollen. If you went into a forest and stripped off all the epiphytes from the trees in a 100 x 100 meter plot, you would literally have tons of debris in your lap and you’d convert the forest canopy into a desert.
And here is where we see a forest for the trees. By some estimates it takes hundreds of years to develop a full epiphyte community in the canopy of a forest (depending on the forest). You can grow a tree farm in 100 years. It will have some epiphytes in it. You need 100’s more years to get the full, naturally occurring compliment of epiphytes in all their abundance, and all the biological diversity that depends on those epiphytes. Nature takes care of herself – if we let her.