Final: Plantlife in the Canopy of the Tropical Rainforests of Costa Rica

This discussion topic submitted by Teresa Wildfong (twildfong@hotmail.com) at 7:36 pm on 8/5/99. Additions were last made on Thursday, January 2, 2003.

The forest is most often viewed from the floor as we walk through it, gazing up at the canopy high above. We miss the 3/4 of the plant life that resides in the canopy-well placed, basking in the sunlight the plants on the forest floor spend their entire life reaching for.

In the tropical forests epiphytes and lianas are a common entity, few forests in the world can say this. Epiphytes are aerial plants that refuse the traditional methods of roots in soil, but grasp their nutrients and water from the air around them. Many interesting methods have been develped by these canopy dwellers to get up there or begin life in the canopy that have few parallels in the plant world. I would like to explore these adaptions to the tropical forest that aren't readily apparent from the forest floor and are prime examples of evolutionary genius.

The trees of the tropical rainforest are as extreme as the weather that surrounds them; representing well the abundant rain and sunshine most plants would love to bask in. They reach an average height of 120 feet, 276-300 feet being the record, with Sequoias and Redwoods being the only toppers in height elsewhere in the world. This is no easy feat, requiring about 150 years to reach such a height. The starting of life on the forest floor is no easy task as there are many plants making up the lushness of a forest, also competing for the abundant resources of the tropical rainforest-making evolutionary feats not hard to find in such a competitive world.

Once the trees reach the canopy they can spread up to an acre across-not leaving much room for the strugglers on the forest floor reaching for light. It is no wonder that plants in such an environment have reached for the canopy, where light is abundant and struggles fewer. In fact, 1/4 of the biomass of the canopy consists of trees-the remaining 3/4 consists of vines and epiphytes. This 3/4 represents 45% of the nutrient storage of the rainforest as well-an important storage system for organic material in the ever recycled nutrients of the rainforest where virtually all of the nutrints are in the biomass.

There are specifically 15,500 species of epiphytes in Central and South America alone. One tree alone was recorded having 300 orchids and 2000 other epiphytes in the Neotropics. Examples of epiphytes would be: bromiliads, orchids, ferns, cacti, mosses, and lichens; all non-parasitic, they use trees, rocks and other plants for support. A generalization of such plants is that they use aerial roots to absorb dust, litter, rainwater, and slowly disolving bark with tricome cells-a special adaption on the roots for aerial absorbtion.

More detail will be given on the specific adaptions of the epiphytes previously mentioned. Aerial plants have moved from the soil to bask in the light intensity of the canopy, but because of this light intensity they have increased metabolism, thus increased transpiration and evaporation, therefore, epiphytes are generally experts at storing water.

Orchids are a prime example with their nocturnal absorbtion systems. Most plants absorb CO2 during the day, but orchids have closed stomata ( openings in leaves of plants through which gases are exchanged) during the day and open at night when the temperature is cooler. CO2 is absorbed then and stored as malic acid and then used in photosynthesis the next day. Other epiphytes reduce stomatal H2O loss with humidity sensitive hairs surrounding each pore. When the air is dry, they fold over the openings and retard evaporation. When humidity is high, they stand erect and open pores for a free exchange of gases. Epiphytes furthur reduce H2O loss through their exposed roots by surounding them with a layer of dead cells. These absorb H2O and expand in a spongelike manner when wet, then contract to aborb H2O.

Other adaptions orchids have developed to retain water include: bulbous stems to store water, thicker leaves than their ground dwelling cousins, and a dependancy on mycorrhizae fungus that greatly aids in the uptake of scarce minerals which are found on the roots-some orchids need these for survival.
Some orchids also have basket roots to deal with adverse conditions which are up and away from the plant, capturing organic material that decomposes and then can be absorbed by the plant.

Another variety of the epiphyte is the hemi-epiphyte, where the roots start in the ground and then seperate once the canopy is reached. This is a way to get around the millions of dust like seeds that are often put out by epiphytes with hopes to land on any small pile of organic matterial not yet consumed in the canopy.

Bromiliads are another variety, with 2000 new world species. They are part of the pineapple family, which can be seen by their blade-like leaves. They create a tree top swamp, as they are experts at storing water. To do this, they construct tanks with tightly overlapping leaves-some are known to store up to 30 gallons! Birds and monkeys that rarely leave the canopy are grateful for this water source. These tanks are also a host to a myriad of insects, frogs, salamaders, plus tree frogs, mosquitos, flatworms, snails, and salamanders that complete their life cycle here. These creatures are also beneficial to the bromiliad-leaving debris to be absorbed by their aerial roots.

Vines are also abundant in the tropics and rare elsewhere. 90% of all vines can be found in the tropics; on one hectare in Panama, 1,600 vines were found. They often start as a bush and grow up either in light patches or dark patches, depending on the type of vine. Structure input is less than trees, so they can grow to incredible lengths. Vines will use either opportunistic methods or skototropism to grow into the canopy. Oppertunistic vines grow in light gaps, while skototropistic vines grow towards dark shadows in the forest. Short lived light patches don't offer a strong future, and the darkest places in the forest are usually the bases of large trees, which is a gaurantee to the canopy above. Scrambling vines use tendrils and hooks in a self trellising system in light gaps along river banks or in the forest. They sacrifice long life and size by requiring light gaps for growth. Woody lianas use skototropism, and throught this can eventually reach 3000 feet. These vines can be found sprawled all across the top of the canopy. They also attach using hooks and tendrils to climb up. They have hormones which bend the vine on the stimulated side of the tendril once it hits a solid something (hopefully a tree) in a dark patch, which triggers a curl reaction, starting the vine on it's way upward.

Climbers are an opportunistic vine with an intersecting adaptation using leaf changes to adjust to larger or smaller amounts of light. They grow in sun patches up the sides of trees, and in the shady interior use leaves, but once the canopy is reached, drop the leaves where light is abundant. Some will grow small rounded leaves that grow up and flat on the sides of trees in lowland forests, and once they reach sunlight the leaves grow into giant structures, sometimes 6 feet long, projecting outwards.

All of these adaptions are remarkable, but sometimes a problem for trees who also require the abundant nutrients and sunlight the canopy provides; the epiphyte's weight alone could be a problem, often making up 1/3 the weight of the tree. The tree itself will grow aerial roots into epiphytic mats to steal some nutrients. Rainforest leaves are pretty uniform in their smooth, oval shape with drip tips to discourage a hospitable environment for lichens and mosses. Trees often also have smooth bark to discourage crevices with organic matter in which epipytes could take root. A reassurance to this adaption is the tree peeling off huge sheets of bark on a regular basis to remove any epiphytes which may have adheared to them. Symbiotic relationships with ants have also evolved as a protection. The tree provides tunnels for the ants to live in in return for protection. There are aggressive varieties of ants that will swarm anything on the tree, clearing vegetation off and killing all growth around the base of the tree. Non-aggressive varieties leave unpalatable insect parts, wate and other debris for nutrients. Animals such as monkeys, sloths, anteaters , oppposums, help by keeping a network of paths clear in the canopy on main limbs and branches.

The unique combination of extreme climate with overabundance of basic resources for life are reflected in the amazing evolutionary advances at tapping these nutrients and defying the laws of life that exsist elsewhere in the world.


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