Tropical Rainforest Plants

More than two thirds of the world plant species are found in the tropical rainforests: plants that provide shelter and food for rainforest animals as well as taking part in the gas exchanges which provide much of the world oxygen supply. Rainforest plants live in a warm humid environment that allows an enormous variation rare in more temperate climates: some like the orchids have beautiful flowers adapted to attract the profusion of forest insects. Competition at ground level for light and food has lead to evolution of plants  which live on the branches of other plants, or even strangle large trees to fight for survival. The aerial plants often gather nourishment from the air itself using so-called air roots? The humidity of the rainforest encourages such adaptations which would be impossible in most temperate forests with their much drier conditions.

Most tropical rain forest plants are exotic and very beautiful. Orchids and bromeliads for example are found throughout the canopy and under-story. The flowering Rafflesia arnoldi which grows on the forest floor has the largest flower in the world measuring up to 1 metre across. Unfortunately it smells like rotting meat! However the odour attracts flies which carry out the necessary pollination.

The huge top layer trees are also quite strange. Many of them have huge base fins known as buttresses, which help support them in the poor soil, and prevent them being blown over by the high winds that can accompany the monsoon. Other trees send their roots down from their branches to provide extra support. Many trees have also evolved protection from leaf eating insects and animals, as they produce disagreeable chemicals in their leaves making them unpalatable. Others grow spines on their trunks and branches making it hard for animals to reach their leaves. Some have hollows in their branches for ants to nest in, and they return the favour by attacking those insects and vines that can harm the tree.

Most tropical forest soils are very poor and infertile. Millions of years of weathering and torrential rains have washed most of the nutrients out of the soil. More recent volcanic soils, however, can be very fertile. Tropical rain forest soils contain less organic matter than temperate forests and most of the available nutrients are found in the living plant and animal material.

The soil of a tropical rainforest is only about 3-4 inches (7.8-10 cm) thick and is ancient. Thick clay lies underneath the soil. Once damaged, the soil of a tropical rainforest takes many years to recover. Temperate rainforests have soil that is richer in nutrients, relatively young and less prone to damage.

Constant warmth and moisture promote rapid decay of organic matter. When a tree dies in the rainforest, living organisms quickly absorb the nutrients before they have a chance to be washed away. When tropical forests are cut and burned, heavy rains can quickly wash the released nutrients away, leaving the soil even more impoverished.

Comparison of Where Nutrients Are Found in an Ecosystem Based on the Averaging of Major Nutrients.

Tropical Rainforest	Temperate Deciduous Forest
52% in Vegetation	31% in Vegetation
48% in Soil	69% in Soil

Common characteristics of tropical trees

Buttress roots: Soils of the typical lowland rainforest are often shallow with all the nutrients available largely remaining at surface level. Many roots stretch out over the surface, rather than burrowing underground. Many species have broad, woody flanges at the base of the trunk. Originally believed to help support the tree, now it is believed that the buttresses channel stem flow and its dissolved nutrients to the roots.

Prop and Stilt Roots: In other places, so-called “prop” or “stilt” roots emerge like slanting rods from the main trunk 1 to 2 meters above the ground to help support the trunk. This particular kind of root is often found in flooded or mangrove forests, where it also protects the tree against waves and currents. They develop several aerial pitchfork-like extensions from the trunk which grow downwards and anchor themselves in the soil trapping sediment which helps to stabilize the tree. Although the tree grows fairly slowly, these above-ground roots can grow 28 inches a month.

Drip tips: Rainforest leaves have “drip tips”—a pointed shape which helps drain excess water from the leaf and reduces vulnerability to mold and predation (to promote transpiration). They facilitate drainage of precipitation off the leaf. They occur in the lower layers and among the saplings of species of the emergent layer. Researchers often speak of how different plants try to prevent ”herbivory”—the eating away of vegetation by insects or other parasites—and minimizing moisture with drip tips is one such strategy. When it fails, leaves become a lacy net of holes and fibers.

Bark: In drier, temperate deciduous forests a thick bark ( often only 1-2 mm thick, sometimes armed with spines or thorns) helps to limit moisture evaporation from the tree's trunk. Since this is not a concern in the high humidity of tropical rainforests, most trees have a thin, smooth bark. The smoothness of the bark may also make it difficult for other plants to grow on their surface. The bark of most trees looks very similar. This similarity is very frustrating for botanist-it makes trees more difficult to identify in the rainforest.

Mutualistic relationships: With the constant fight for light and water, nutrients and energy, many rainforest species come to rely on each other, and develop intimate and exclusive relationships. Some plants, for example, provide “ant houses,” home to a particular species of ant whose soldiers defend the leaves against other would-be insect predators. Other plants provide leaves with tiny feeding troughs, pools of sugar-solution, enticing ant patrols with sweet rewards. 

Other characteristics that distinguish tropical species of trees from those of temperate forests include  Cauliflory: the development of flowers (and hence fruits) directly from the trunk, rather than at the tips of branches; Large fleshy fruits attract birds, mammals, and even fish as dispersal agents.

In every rain forest there are many kinds of plants. In fact, inside a single hectare (2.47 acres) you can find up to 750 types of trees and 1,500 types of plants! But this entire range of species can easily be broken down into four categories, grouped by how they take up nutrients: Carnivorous plants eat small animal; Saprophytic plants eat decaying matter; Parasitic plants take nutrients directly from other living plants; Autotrophs take nutrients from the soil.

Carniverous Plants: Some plants are adapted to obtain nutrients from animal matter. The best known of these is probably the Venus fly trap. The venus fly trap grows in poor soil swamps. To ensure they get enough nitrogen they hunt insects by giving off an odor that attracts them. An insect lands on the leaf which triggers it to snap shut. This one has caught an insect. Venus fly traps taken and grown in other parts of the world have even been known to eat small bits of raw ground beef. They're obviously not picky eaters.However, more impressive is the pitcher plant Nepenthes rafflesiana, found in southeast Asia. This plant grows to 30 feet tall and may have pitchers 12 inches in length, usually crammed full of digested insects. Pitcher plants also eat small mammals and reptiles that attempt to steal the insects from the pitcher.

Saprophytes: Many saprotrophs are so small, called microbes, that they cannot be seen with the naked eye. Other decomposers, which include insects, grubs, snails, slugs, beetles and ants, aid in recycling valuable nutrients from dead organic matter which is then released back into the soil to be reabsorbed rapidly by plants and trees. Decayed matter contains essential nutrients like iron, calcium, potassium and phosphorous all of which are necessary to promote healthy rainforest growth. Thus decomposers must work continuously to release these and other elements into the soil. Saprophytes are the organisms that act as the rainforests decomposers, competing with the heavy rainfall which constantly washes away nutrients on the forest floors. Some fungi, called mycorrhizals, are examples of plant life that carry out this function. Decomposers work extremely efficiently and, together with the warmth and wetness which helps accelerate decomposition, can often break down dead animals and vegetation within 24 hours.  Decomposition in montane forests, which are colder and less humid, however, can sometimes take up to six weeks.

Growthforms

Various growthforms represent strategies to reach sunlight:

Vines: Vines are not a plant species, but a category of various forms of rainforest vegetation that, in the words of biology professor and write John Kricher, “literally tie the forest together.” These include lianas, bole climbers and stranglers. Vines are everywhere in the rainforest and compete with other plants for light, water and nutrients. Some, in turn, are a food source for other creatures. They are climbing woody vines that festoon rainforest trees. Ninety per cent of the world's vine species grow in tropical rainforests. They do this by attaching themselves to trees with sucker roots or tendrils and growing with the young sapling, or they climb by winding themselves round the tree's trunk. When they reach the top of the canopy they often spread to other trees or wrap themselves around other lianas. This network of vines gives support against strong winds to the shallow-rooted, top-heavy trees. However, when one tree falls several others may be pulled down also. They have adapted to life in the rainforest by having their roots in the ground and climbing high into the tree canopy to reach available sunlight. 

Lianas are woody vines grow rapidly up the tree trunks when there is a temporary gap in the canopy and flower and fruit in the tree tops. Although they begin life on the ground and their woody stems remain rooted to the forest floor, tendrils attach to neighboring trees, and, entwined around their host, these hitchhikers climb to the canopy on the backs of other trees. Up in the treetops, lianas spread and a single vine can loop its way through a number of trees. Lianas are springy and strong, able to support an adult’s weight. Some have hollow stems that contain water that can be gotten with the aid of a machete. Lianas can weigh down a tree so much that wind and weather collapses it more easily. A fallen liana just moves and grows on another tree. Lianas include rattan palms, philodendron and Strychnos toxifera (from which the deadly poison strychnine is obtained). Rattans, the Asian lianas, have thorny stems and can reach heights of 650 feet (200 m). They are used to make a variety of things including baskets, ropes and wicker furniture.

Climbers: Green-stemmed plants such as philodendron that remain in the understory. Many climbers, including the ancestors of the domesticated yams (Africa) and sweet potatoes (South America), store nutrients in roots and tubers.

Heterotrophs: non-photosynthetic plants can live on the forest floor. Parasites derive their nutrients by tapping into the roots or stems of photosynthetic species. Rafflesia arnoldi, a root parasite of a liana, has the world's largest flower, more than three feet in diameter. It produces an odor similar to rotting flesh to attract pollinating insects.

List of plants