Epiphytes. In a single word you just can’t grab all the complexity of life above the forest floor in tropical forests. Epiphytes are those plants that grow on the surface of other plants. They include orchids, ferns, cacti, vines, shrubs, and even other trees. Close your eyes and imagine the raucous greenery growing in a tropical forest. An awful lot of that stuff is made up of epiphytes. They are so abundant and so complex that up in the treetops you basically have a whole other forest growing, double decker style. For more on these wonderful plants and how they add to biodiversity in a rain forest, check out an earlier blog on the topic.
Simply stated, epiphytes add life to the forest. One thing that is less clear is how the forest adds life to epiphytes. These plants can grow hundreds of feet up in the forest. Really! How do they do that? I’d like you to meet Sybil Gotsch, because she studies the snot out of these things and she has the answers. Sybil is a professor at Franklin and Marshall College but her “office” is in the treetops in the fabled Monteverde Cloud Forest Reserve in Costa Rica.
In case you don’t know, Monteverde is a type of “tropical montane cloud forest.” That means the forest grows on the slopes of mountains that are constantly covered in clouds and mist. Epiphyte density is off the charts in cloud forests. There are about 800 species of epiphytes just in Monteverde (!!!), and it is estimated that 100 square meters of Monteverde cloud forest is home to almost TWO TONS of epiphytes. As much as 35% of all the leaves, and 45% of all the nutrients in the forest canopy, are made up of epiphytes. If you want to study the life of epiphytes, Monteverde is a good place to do it.
One of the riddles that Sybil has answered is just how epiphytes deal with water in the treetops. Think about it for a second. Epiphytes aren’t rooted in the ground and have to get all their water from up in the canopy, where it is windy, sunny, and surviving these near-drought conditions can be a major challenge. One old assumption is that epiphytes absorb water from all the clouds and mist through their leaves. It’s a logical assumption to make for plants that spend half the year immersed in clouds. Well, dang-it, scientists don’t just assume, they ask questions, take names, and kick butt. Sybil has solar-powered weather stations installed in the canopy. She dissects leaves under microscopes. She injects pressurized nitrogen and water into sections of stems to understand the flow of sap. She has an entire crew swinging from ropes, Tarzan style, conducting this research. And Sybil has answers.
Answer #1) All the epiphyte species she studied can and do absorb water directly through their leaves. But some are better at it than others. The old assumption holds true, but the answer is qualified: how well an epiphyte absorbs water depends on the type of plant.
Answer #2) Some epiphytes have special tissues in their leaves that store water. (Can you say hydrenchyma?) Absorbing water through your leaves is one answer to surviving drought, but being able to store water is another strategy.
Answer #3) Epiphytes refill the water in their leaves at night. It’s cooler at night, and often cloudy, and the water that is lost during the sunny, windy days gets replenished during the cool, wet hours. Plants aren’t dumb!
These are just a few of Sybil’s discoveries. You’d fall asleep if I spelled them all out, so here’s what you need to know: First, epiphytes have a bunch of strategies to survive water stress in the canopy. They come in different sizes, some are woodier while others are gentle herbs, leaves can be thick or thin, and so on. It’s the diversity of strategies that allows 2 tons of epiphytes to coexist in a 100-m2 plot. Diversity is the hallmark of tropical forests. Epiphytes have figured out the water problem, and in doing so they contribute to the amazing biodiversity of tropical forests.
Second, because epiphytes capture water out of the air, they in fact add water to the whole forest ecosystem, including the water that reaches the soil and then the streams and rivers. Every human being who lives downstream from a cloud forest depends on epiphytes for the water they drink, cook with, and for watering their crops.
Last, global climate change stands to ruin all that. In Monteverde there are more days in the year without clouds than there used to be, and the base of the cloud layer keeps moving up in altitude. Simply stated, global climate change is depriving epiphytes of water. And when that happens, the forest, wildlife, and humans have less water too.
Gotsch, S. G., N. Nadkarni, A. Darby, A. Glunk, M. Dix, K. Davidson, and T. E. Dawson. 2015. Life in the treetops: ecophysiological strategies of canopy epiphytes in a tropical montane cloud forest. Ecological Monographs 85:393-412.