By Fran Zak
Rainforest soil is typically not very rich in nutrients. The nutrient-containing part of the soil in Panama is often only a few inches deep, and in Australian tropical rainforests it may be a little deeper. Because of the shallow soil, rainforest trees have root adaptations that improve the plant ability to anchor, which is especially important during heavy winds. They may also need special aids to absorb water and nutrients, especially during the dry season. Tropical tree roots have special coatings of symbiotic fungi that aid in moving nutrients from fallen leaves on the soil surface to the living leaves on trees.
I saw several root adaptations in both Panamanian and Australian tropical rainforests that were similar. Fig trees often showed extensive buttress roots , which are vertical wall-like panels projecting sideways from the trunk. These "above-ground" roots help support and anchor the tree, much as a buttress on a fort would support the walls of the fort.
Leaf litter collects between these buttress roots so the tree has access to more nutrients when the collected leaves decompose. Buttress roots also absorb oxygen directly from the air. This is a big help to the tree because it is difficult for the underground roots to absorb oxygen in the often waterlogged shallow soil of the rainforest. This photo shows Karen and myself leaning against buttress roots with an aerial root hanging down between us. Look at how large those buttress roots are-they come up to our shoulders! Sometimes these buttress roots are far enough off the ground that people can crawl under them.
I saw some fig trees in Australia that had very long, serpentine roots that extended more than 5 meters from the tree trunk. These serpentine aboveground roots were longer than any I had seen in comparable rainforests in Panama. I suspect that their length and the fact that they "snaked" along the forest floor helped the roots with absorption of water and minerals (and perhaps oxygen) for the tree.
I also saw interesting root adaptations on the mangrove trees in Australia. Mangrove trees (left) typically grow very near salt water (like in a bay or marsh) and their intertwined root systems are necessary to both decrease erosion of the fragile mangrove swamp "land" and they also afford great protection for young developing fish, arthropods, and mollusks. The mangrove trees I saw in Australia were far taller than any I have ever seen in Florida. The trees were more than 10 meters high and grew in really waterlogged marshy soil. Parts of the roots were exposed above the soil, looking like "knees". These knees allowed for easy absorption of oxygen gas from the air. I've outlined the root knees in this photo in a white rectangle to make it easier for you to see them.
Another type of aerial root I saw both in Panama and Australia tropical rainforests is pictured in this photo. Perhaps these roots can pick up water during a good rain. Finally, the most dramatic roots that I saw were on fig trees. Strangler figs have seeds that are dispersed by animals that live in treetops. When an opossum, for example, eats the fig fruits, the seeds inside the fruit are not digested. The seeds are unharmed and pass out of the opossum in its feces. If the opossum defecates on a branch, the seeds remain there and sprout. The seedling figs wrap their roots around the tree and eventually grow taller and shade the host tree. The figs are "support parasites" that use the trunk of the host tree to extend high into the sunlight. The strangler fig eventually causes the death of the tree that it has encircled and over-topped. I saw strangler figs in all stages of "strangling" their host trees; some that had just started, some a little farther along, and others where the host tree was completely gone, as you can see by the light coming through this fig tree. Notice in this photo that I am sitting inside the fig tree (right) since the host tree is gone. These fig trees form 2 basic designs of roots:
- Lattice roots that criss-cross each other and form a network.
- Curtain roots that hang down parallel to each other, seeking the soil for nutrients.
There is a famous landmark "Curtain Fig Tree" near Cairns, Australia that we visited where the roots hang down more than 8 meters. I did not remember seeing so many dramatic aerial roots so high up on trees in Panama as there were on trees in Australia. These aerial roots did not seem to be of any value in anchoring the plant, so I suppose they are for gathering nutrients from fallen leaves and moisture.
"Rainforest trees invest far less of their stock of vegetative matter in roots to gather water than dry forest species," says Dr. Egbert Leigh (2002). Yet, some rainforest tree roots are very impressive because they are visible aboveground. Trees show similar root adaptations based on their environment, as I noticed when I compared trees in rainforests in two different countries (Panama and Australia).
Leigh, Egbert. A Magic Web . New York: Oxford University Press, 2002.
Pascack Valley HS
Jackie Willis, Ph.D.
Montclair State University
Upper Montclair, NJ
Special thanks to Professional Resources in Science and Mathematics (PRISM) at Montclair State University and Dr. Jackie Willis for making these ecology trips possible and for sharing her wealth of knowledge, her expertise, and her photographs with us.