General structure of the rainforest

Plants with similar stature and life-form can be grouped into categories called synusiae, which make up distinct layers of vegetation. In tropical rainforests the synusiae are more numerous than in other ecosystem types. They include not only mechanically independent forms, whose stems are self-supporting, and saprophytic plants but also mechanically dependent synusiae such as climbers, stranglers, epiphytes, and parasitic plants. An unusual mix of trees of different sizes is found in the tropical rainforest, and those trees form several canopies below the uppermost layer, although they are not always recognizably separate layers. The upper canopy of the tropical rainforest is typically greater than 40 meters above ground.

The tropical rainforest is structurally very complex. Its varied vegetation illustrates the intense competition for light that goes on in this environment in which other climatic factors are not limiting at any time of year and the vegetation is thus allowed to achieve an unequaled luxuriance and biomass. The amount of sunlight filtering through the many layers of foliage in a tropical rainforest is small; only about 1 percent of the light received at the top of the canopy reaches the ground. Most plants depend on light for their energy requirements, converting it into chemical energy in the form of carbohydrates by the process of photosynthesis in their chlorophyll-containing green tissues. Few plants can persist in the gloomy environment at ground level, and the surface is marked by a layer of rapidly decomposing dead leaves rather than of small herbaceous plants. Mosses grow on tree butts, and there are a few forbs such as ferns and gingers, but generally the ground is bare of living plants, and even shrubs are rare. However, tree seedlings and saplings are abundant; their straight stems reach toward the light but receive too little energy to grow tall enough before food reserves from their seeds are exhausted. Their chance to grow into maturity comes only if overhanging vegetation is at least partially removed through tree death or damage by wind. Such an occurrence permits more solar radiation to reach their level and initiates rapid growth and competition between saplings as to which will become a part of the well-lit canopy.

Gaps in the canopy of a tropical rainforest provide temporarily well-illuminated places at ground level and are vital to the regeneration of most of the forest’s constituent plants. Few plants in the forest can successfully regenerate in the deep shade of an unbroken canopy; many tree species are represented there only as a population of slender, slow-growing seedlings or saplings that have no chance of growing to the well-lit canopy unless a gap forms. Other species are present, invisibly, as dormant seeds in the soil. When a gap is created, seedlings and saplings accelerate their growth in the increased light and are joined by new seedlings sprouting from seeds stored in the soil that have been stimulated to germinate by light or by temperature fluctuations resulting from the sun’s shining directly on the soil surface. Other seeds arrive by various seed dispersal processes (see below). A thicket of regrowth rapidly develops, with the fastest-growing shrubs and trees quickly shading out opportunistic, light-demanding, low-growing herbaceous plants and becoming festooned with lianas. Through it all slower-growing, more shade-tolerant but longer-lived trees eventually emerge and restore the full forest canopy. The trees that initially fill in the gap in the canopy live approximately one century, whereas the slower-growing trees that ultimately replace them may live for 200 to 500 years or, in extreme cases, even longer. Detailed mapping of the trees in a tropical rainforest can reveal the locations of previous gaps through identification of clumps of the quicker-growing, more light-demanding species, which have yet to be replaced by trees in the final stage of successional recovery. Local, natural disturbances of this sort are vital to the maintenance of the full biotic diversity of the tropical rainforest (see Sidebar: Rainforest Regeneration in Panama).

Just as tropical rainforest plants compete intensely for light above ground, below ground they vie for mineral nutrients. The process of decomposition of dead materials is of crucial importance to the continued health of the forest because plants depend on rapid recycling of mineral nutrients. Bacteria and fungi are primarily responsible for this process. Some saprophytic flowering plants that occur in tropical rainforests rely on decomposing material for their energy requirements and in the process use and later release minerals. Some animals are important in the decomposition process; for example, in Malaysia termites have been shown to be responsible for the decomposition of as much as 16 percent of all litter, particularly wood. Most trees in the tropical rainforest form symbiotic mycorrhizal associations with fungi that grow in intimate contact with their roots; the fungi obtain energy from the tree and in turn provide the tree with phosphorus and other nutrients, which they absorb from the soil very efficiently. A mat of plant roots explores the humus beneath the rapidly decomposing surface layer of dead leaves and twigs, and even rotting logs are invaded by roots from below. Because nutrients are typically scarce at depth but, along with moisture, are readily available in surface layers, few roots penetrate very deeply into the soil. This shallow rooting pattern increases the likelihood of tree falls during storms, despite the support that many trees receive from flangelike plank buttresses growing radially outward from their trunk bases. When large trees fall, they may take with them other trees against which they collapse or to which they are tied by a web of lianas and thereby create gaps in the canopy.

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Tree growth requires substantial energy investment in trunk development, which some plants avoid by depending on the stems of other plants for support. Perhaps the most obvious adaptation of this sort is seen in plants that climb from the ground to the uppermost canopy along other plants by using devices that resemble grapnel-like hooks. Lianas are climbers that are abundant and diverse in tropical rainforests; they are massive woody plants whose mature stems often loop through hundreds of meters of forest, sending shoots into new tree crowns as successive supporting trees die and decay. Climbing palms or rattans (Calamus) are prominent lianas in Asian rainforests, where the stems, which are used to make cane furniture, provide a valuable economic resource.

Epiphytes are particularly diverse and include large plants such as orchids, aroids, bromeliads, and ferns in addition to smaller plants such as algae, mosses, and lichens. In tropical rainforests epiphytes are often so abundant that their weight fells trees. Epiphytes that grow near the upper canopy of the forest have access to bright sunlight but must survive without root contact with the soil. They depend on rain washing over them to provide water and mineral nutrients. During periods of drought, epiphytes undergo stress as water stored within their tissues becomes depleted. The diversity of epiphytes in tropical deciduous forests is much less than that of tropical rainforests because of the annual dry season (see Sidebar: Life in a Bromeliad Pool).

Parasitic flowering plants also occur. Hemiparasitic mistletoes attached to tree branches extract water and minerals from their hosts but carry out their own photosynthesis. Plants that are completely parasitic also are found in tropical rainforests. Rafflesia, in Southeast Asia, parasitizes the roots of certain lianas and produces no aboveground parts until it flowers; its large orange and yellow blooms, nearly one meter in diameter, are the largest flowers of any plant.

Stranglers make up a type of synusia virtually restricted to tropical rainforests. In this group are strangler figs (Ficus), which begin life as epiphytes, growing from seeds left on high tree branches by birds or fruit bats. As they grow, they develop long roots that descend along the trunk of the host tree, eventually reaching the ground and entering the soil. Several roots usually do this, and they become grafted together as they crisscross each other to form a lattice, ultimately creating a nearly complete sheath around the trunk. The host tree’s canopy becomes shaded by the thick fig foliage, its trunk constricted by the surrounding root sheath and its own root system forced to compete with that of the strangling fig. The host tree is also much older than the strangler and eventually dies and rots away, leaving a giant fig “tree” whose apparent “trunk” is actually a cylinder of roots, full of large hollows that provide shelter and breeding sites for bats, birds, and other animals (see Sidebar: Apartments of the Rainforest). Stranglers may also develop roots from their branches, which, when they touch the ground, grow into the soil, thicken, and become additional “trunks.” In this way stranglers grow outward to become large patches of fig forest that consist of a single plant with many interconnected trunks.