Foundation of the Food Web
By Fran Zak
Getting one's leaves to the sun is a central theme for plants so they can absorb enough sunlight to carry on photosynthesis to make sugar. In most mature rainforests, only 1% of incoming light reaches the forest floor (Leigh, 2002). This was clearly obvious in Panama tropical rainforests where a walk through the rainforest on a bright, sunny day around noon gave me the feeling that it was approaching dusk because of such low light levels penetrating through the tree canopy.
The tropical forests in Northeastern Australia do not seem to have nearly as dense a leaf canopy and more light reaches through the canopy to the forest floor. Could this be because there has been so much destruction of the Australian rainforest ecosystem? Only 20% of the original rainforest remains and only 2% of what remains is pristine (Jonathan Munro, pers. comm.), so most of the rainforests I visited were not completely original, but were in various stages of restoration.
I saw many striking similarities between Panama and Australia rainforest flora species orienting their leaves to absorb the maximum amount of sunlight possible. There are several mechanisms a plant can use to maximize the absorption of sunlight for photosynthesis:
A plant that grows tall quickly and gets its leaves above the canopy will get a good share of the sunlight. Lianas (vines) in both Panama rainforests and Australia rainforests use this technique to reach the sunlight. Lianas climb up other trees, using the trees as support so they do not need to invest a lot of energy to produce thick wood to make strong trunks like slower growing trees do. It might be interesting to conduct a study to compare the number of Lianas and their diameter in two comparable tropical forest ecosystems on two different continents.
Some rainforest plant species have leaves with much larger surface areas than temperate plants. This is a good way to maximize the absorption of sunlight, but it also increases the problem of pest damage to the larger exposed surface area.
Epiphytes are plants that grow on top of other plants to be nearer to sunlight. I saw epiphytes in both Panama and Australia rainforests that had landed in a nice V-notch in a tree trunk or branch. They are usually high up on the trunk where sunlight is plentiful so they can absorb the sun’s rays. Epiphytes are plants like some ferns, orchids, bromeliads, or even cactus. Epiphytes do not "waste energy" building thick, strong stems like trees do. However, living far above the soil means that an epiphyte has the difficulty of finding ways to get nutrients. The epiphyte often has a basket formed by a network of roots to collect leaf litter that it can recycle into useable nutrients. This basket also serves as a good basin for collecting water. Epiphytes also have to deal with lack of anchorage in strong winds. If the wind blows them out of the tree, they are likely to die after they tumble to the ground. I was amazed to see that epiphytes can grow very large. Some were almost as tall as me!
Several rainforest plant species that I saw in Australia grow very slowly (1-2 mm per year) and remain as small seedlings for many, many years. Then, as soon as something occurs that increases the amount of light filtering through the canopy (like a tree falling down), they suddenly sprout and grow rapidly toward the sun.
The above examples show the importance of plants and trees finding a way to maximize absorption of sunlight that is necessary to make sugar during photosynthesis. These plants are the basis of the terrestrial food web because the energy they capture gets converted in useable food sources that are eaten by animals.
Leigh, Egbert. A Magic Web. New York:Oxford University Press, 2002.
Munro, Jonathan. Personal Interview. July, 2003. www.wildwatch.com.au
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.