[More information will be added to this section in the near future. Here are only a few examples of rainforest plants.]
Tropical and subtropical areas are home to 170,000 of the approximately 250,000 known species of vascular land plants, and most of these species are in rainforests. Half of these species are in the New World, with at least 60,000 species in the Amazon, 35,000 in Africa (+ 8500 in Madagascar), and 40,000 in Southeast Asia. Forty thousand species inhabit Colombia, Peru, and Ecuador alone on only 2% of the earth’s land surface. In contrast, the entire British Isles have 1380 plant species and Europe only 11,500 (Soepadmo, 1995). In the Atlantic forest of Brazil 427 species of trees were found by Cardoso da Silva and Tabarelli, (2000), but even more – 476 tree species in a 2½ acre plot (the highest recorded number in the world) – were identified in that forest by a group from the New York Botanical Garden (Brooke, 1996). In Malaysian rainforests, there are more than 800 species of trees exist on relatively small plots (Durning, 1989; Condit, et al., 2000); in Borneo, 700 tree species were found on 10 one-hectare plots (Wilson, 1988). Madagascar has 8000 endemic species of plants (Green and Sussman, 1990), out of a total of 10,000. Non-tree species are even more diverse. In Ecuador, there are (or were) more than 10,000 plant species in the lowlands and foothills west of the Andes. These forests are now almost gone; at the Rio Palenque Science Center only one square kilometer of primary forest remains, but that tiny remnant contains 1200 plant species, of which 43 are endemic to this site (Wilson, 1992). In Malesia, there are 16 families of flowering plants; each family contains more than 500 species. The largest group is the orchids, with 6500 species (Soepadmo, 1995). Plant diversity appears greatest in lowland areas with abundant and regular rainfall, while soil fertility seems to be a less important factor.
Tropical plants are very diverse, ranging from very tiny (some orchids, aquatic plants, and saprophytes) to the enormous (Southeast Asian dipterocarps). This phenomenal diversity is due to the moist warm climate, the availability of many and diverse habitats remaining from geological history, and the ability of indigenous plants to adapt, evolve and invade new habitats (see above). Remember, however, that along with the great species diversity of tropical forest plants goes a paucity of individuals of any given species.
Trees are the predominant form of vegetation in rainforests. They comprise 68% of (known) plant species in central Amazonia (Gentry, 1992). Each tropical rainforest region has certain predominant genera of trees. For example, in the New World there are many trees of the Brazil nut family. In Malesia, the forests are dominated by giant dipterocarps, which provide as much as 70% of canopy tree biomass and 80% of the tallest canopy trees. Some families are strictly tropical (as is the case with the nutmegs of Southeast Asia), while others are concentrated in the tropics (such as bananas and ebonies). Although there are some genera in common among the three large rainforest regions, they share almost no species. Each larger region has local variations in composition as well, and many species are found only in a certain circumscribed area of a forest. Such species are known as endemics. The number of tree species increases proportionally with rainfall, so that dry tropical forests are much impoverished compared to wet forests, with approximately threefold fewer species. African forests contain fewer families, genera and species than Southeast Asia or the Neotropics because they are largely seasonal forests with relatively low annual rainfall.
Rainforest trees come in all sizes – very tall emergent canopy trees, medium-sized trees with their canopies in the middle layers, and small, spindly trees (if any) in the lower layers of the forest. Tropical trees have many types of crowns – with a single apical shoot or many, for example, or they may form tufts at the base (bamboos, bananas), or have a single trunk. Roots, which are for anchorage of the tree in the soil and for absorption of water and nutrients, are very important elements in rainforest ecology. Some trees have a single deep tap root; others have “sinker” roots which descend from other roots or from buttresses. Nevertheless, in rainforests, most of the trees have relatively few deep roots; most of the root mass lies in the upper 0.3 m of soil. This is because tropical soils tend to be very shallow, and because rainfall is high, so that nutrients tend not to sink into the soil but to be leached out quickly. Therefore the roots must “snatch” nutrients and water before they run off. Most tree species grow intermittently; this is especially true in the seasonal forests.
There is no real distinction between deciduous and evergreen species in tropical forests. Leaves may fall continuously in some species, others may shed all their crown leaves periodically, or may bud new leaves before the old leaves fall – and there are all intermediate stages. It is not clear what triggers leaf loss in tropical forests, but it is not always related to seasonal changes (if any).
a. Palms: Palms form a very significant proportion of tree species in most tropical forests. In Amazonia, they comprise perhaps 20% of the total number of plant genera. Inventories of palms in Peruvian terra firme forests yielded 23 species within 0.27 hectares (Kahn and de Granville, 1992). Palms seem to be most diverse in terra firme forests, but are also found in wetter forests which are waterlogged intermittently or are permanently flooded (swamp forests). These latter forests provide more difficult living conditions for trees, and there are correspondingly fewer palm species in them than in drier areas. In swampy areas, palms tend to cluster together. In the Huallaga River valley in Peru, up to 207 palm individuals have been found within a single hectare. Mangrove forests, perhaps because of their acidic and anoxic waters, contain few palm species. The same is true of high elevations, but some lowland species manage to survive there, along with species endemic to montane forests. Despite the great diversity of palm species, palm communities in a particular area tend to be dominated by one or only a few species. In Peruvian terra firme forest (see above), two species of palm, Lepidocaryum tessmani and Jessenia bataua, form almost 80% of the palm community.
Palms are not among the tallest trees, but some of them are of substantial height, up to 50 meters. However, most are less than 40 m tall and form a large part of the understory of many rainforests. Palms may be single- or multiple-stemmed, have large or small leaves, be tall, short, climbing, or even prostrate. In the latter forms, the stem “creeps” along the ground. In some species, stilt (aerial) roots are produced from the stems. Other species which live in anoxic environments such as waterlogged or flooded areas may have “pneumatophores,” complexes of rootlets which protrude from vertical roots and absorb oxygen.
Palms are productive and prolific plants, and so are extremely important in rainforest ecosystems because their fruits provide an essential food source for many animals, mammals in particular. Some palms may fruit only every year or two, while others (such as Mauritania flexuosa of the Amazon basin), may produce two to six inflorescences annually. One tree of this species produced 2190 fruits in a single inflorescence (Kahn and de Granville, 1992). A more typical palm might still produce hundreds of fruits at a time. In the Amazon, at least, palms tend to flower at the end of the dry season and fruit during the ensuing rainy season (in the Amazon, the flood season). Most palms are dioecious, that is, produce either male or female flowers, so that it is necessary to maintain large tracts of palm trees in close proximity to each other to ensure that both male- and female-flowered plants will be present.
Because palms often occur in dense tracts, unlike most other rainforest trees, their leaves are a significant source of leaf litter, which enhances the levels of organic matter (and thus, soil fertility) in palm swamps. A hectare of Mauritania flexuosa produces approximately 15.8 tons of dry litter annually, whereas a terra firme forest produces about 7.8 tons per hectare (Kahn and de Granville, 1992). Because of the large amount of organic matter decaying in the waters of these swamps, swamp soils tend to be highly acidic (histosols).
Palms also provide habitats for a number of animals, especially arthropods, and are in turn pollinated by many species of insects – beetles, bees and flies.
Palms are an important component of secondary forests because some have rapidly-germinating seeds and seedlings which are tolerant of sun. Other species appear only when a canopy is established. Many palms are found in deforested areas because, being useful plants, they are often retained when timber trees are removed, and because some of them are resistant to burning. Thus, palms may comprise an unusually high proportion of secondary forest trees.
b. Figs (Ficus sp.): Figs are among the most important plants in rainforests. There are many species (450 in Malesia alone), and as there are always some species in fruit, they provide a major food source for many fruit-eating mammals and birds. Figs can be huge or dwarf; there are figs which are lianas, shrubs, and stranglers. Figs are interesting in that the trees are dioecious, as are most palms. Each fig species is pollinated by a particular species of wasp.
Many fig species are “stranglers,” a life style they share with a varied group of other climbing plants. These species begin life as seeds which have been dropped on the branches of trees, from which the sprouts send roots down to the ground. When the roots reach the soil, the fig plant enlarges in diameter and may send a network of branches around the trunk of its host, which becomes increasingly constricted as its unwanted guest grows in diameter. In this way the fig may eventually kill the tree, which earns it the title “strangler fig.”
c. Dipterocarps: Dipterocarps are generally very large trees which are the dominant vegetation form in forests from Sri Lanka and India across through the Philippines. In Peninsular Malaysia, 30% of the trees with a diameter of 30 cm or more were found to be dipterocarps (Manokaran, 2002). These trees are the major emergent and canopy trees and represent a large proportion of the plant biomass in Southeast Asian primary forests; they are only a very small proportion of secondary forests.
2) Other plants
There are many types of plants other than trees in rainforests. Approximately one-sixth of tropical plant species are epiphytes (plants which are not rooted in the soil), and up to 50% are shrubs and herbs.
a. Lianas: Lianas are woody vine-like plants which can grow quite thick (15 cm in diameter and 70 m in length is common) and which “climb” trees to reach the light, holding on by a variety of devices such as winding twigs, roots, thorns, tendrils, and hooks. They often reach the canopy and may have large crowns. Little is known of the function of lianas in the forest (other than ensuring their own survival), but they do provide protection for animals and stabilize trees against wind and other natural forces, perhaps also regulating the microclimate at the same time. There are many species of lianas, and they constitute approximately 8% of rainforest species. Rattans are examples of lianas with economic importance; so much so that in some areas (Southeast Asia) they have been collected so heavily that they are endangered.
b. Epiphytes: Epiphytes are non-woody plants that have no contact with soil, but grow entirely on trees, which they use as conduits to sunlight. Among them are orchids, some ferns, and bromeliads. They may or may not be partially or entirely parasitic. They provide many habitats for plants and animals – homes for ants, for example. They often have interesting adaptations. Those which climb into the upper canopy have structures which enable them to survive in an environment with high temperatures and low humidity. The leaves of such epiphytes may have thick cuticles and leathery leaves, and form a variety of types of water storage organs (such as leaf bases formed into tiny “water tanks” or nutrient traps). These plants are much more common than one might think; in western Ecuador, epiphytes constitute up to 25% of the plant species in wet forests (Gentry, 1992).
c. Bamboos: Bamboos are hollow-stemmed woody grasses, which may be quite tall, up to 30 meters in height. Bamboos are unusual for tropical rainforest plants because they occur in clumps, with closely-packed trunks and a thick subcanopy. Bamboos flower synchronously after growing asexually for many decades, and this event is followed by a die-off of the stems.
d. Epiphylls: These are lesser-known forest denizens, and include many “primitive” types of plants such as liverworts, mosses, and lichens.
e. Hemi-parasites: This group consists of plants which are partially parasitic, but which also provide some of their own nutrients by photosynthesis. Among these is the mistletoe family, of which there are more than 1100 species in the tropics.
f. Parasites: Some plants are complete parasites, like the spectacular Rafflesia which is a huge plant found on Borneo. Rafflesia and some other parasitic plants have a foul smell, which attracts flies, their pollinators.
g. Herbs: These ground-dwelling and sometimes inconspicuous plants are highly diverse in tropical forests. Gentry (1992) reports that herbs and shrubs constitute as much as 50% of plant diversity in Ecuadorian forests, and almost as many in Southeast Asian forests. Among forest plants which are herbaceous are bananas, gingers, and taro.
3) Flowering and fruiting
Many rainforest trees flower, although the popular perception of these forests is one of unbroken masses of greenery. The fruits of these trees provide essential foods for many forest animals. Mass flowering and fruiting is characteristic of many groups of tropical trees, especially the dipterocarps of Southeast Asia. Flowering generally does not occur annually, as in many temperate species, but at two- to ten-year intervals, with several species flowering more or less simultaneously.
Mass flowering can occur over small or very large areas, and during the flowering period, a great number of seeds are produced and many fruits set. This may be advantageous to the trees, since if many seeds are produced, the likelihood that some will survive and germinate is greatly enhanced. Although most of the seedlings will die, victims of competition for light, some are certain to survive and grow. The massive production of seeds is very important in forest ecology, since seeds are very nutritious and desirable foods, particularly for wild pigs and other mammals, major seed predators. What triggers these flowering events? The cues are not well known, but it is necessary for the tree to be emergent and have its crown in the light prior to the initiation of reproductive activity. A slight drop in temperature may provide such a cue (Ashton, Givnish and Appanah, 1988).
Some other trees and smaller plants produce flowers and fruits continuously and so are extremely important food sources for forest animals. Figs (Ficus) are among the most extensively distributed of these, and are found in both neotropical and Southeast Asian forests.
Many, if not most, plant species in tropical rainforests are irregularly distributed. If a species is found uniquely in a restricted area, it is called endemic. Endemism is greatest in isolated or unusual habitats – forest ridges, isolated valleys, and islands. In fact, areas of high endemism can be considered as “islands” of specialized species and are particularly important for conservation. Since they contain unique species, many will become extinct if these areas are logged or damaged. Thus, they are “extinction hotspots.” Endemism is very common in tropical rainforests, perhaps for historical reasons, and because there are so many localized habitats available.
5) Habitat specificity
Many tropical plant species are specialists, which is a bit surprising in these areas which appear so abundant, even profligate, in those factors which support life: water, light, warmth. But many tropical plants have adapted to quite specific local conditions. The result is that certain areas have a greater variety of plants than others; among these are coastal Brazil and Ecuador and parts of the Amazon; in Asia, northern Borneo, peninsular Malaysia and New Caledonia. These regions also have a great many endemic species, as many as 20% of the plants. These regions can be considered “evolutionary hotspots”; they are especially significant in that, if rainforests are removed from them, many more species will be lost than if the deforestation occurred elsewhere. Because of their great biodiversity, they are vital as genetic resources for evolution. (See Gentry, 1992, for a review of the above two topics.)