Rain Forest Canopy

The rain forest canopy consists of the treetop region or, more precisely, of the aggregate of every tree crown in the rain forest, including foliage, twigs, fine branches, and epiphytes . The upper canopy represents the interface between the uppermost layer of leaves and the atmosphere, and, for practical purposes, many researchers consider this layer to be only a few meters deep. Most of the biological activity in and biodiversity within tropical rain forests appears to be concentrated in the upper canopy.

Many abiotic and biotic characteristics of the canopy are different from the understory beneath. Its higher illumination levels promote more rapid rates of photosynthesis, which, in turn, promote higher vegetal production, and consequently sustain a more abundant and diverse community of animals than in the understory. In a much publicized article in 1983, Terry Erwin termed the canopy of tropical forests "the last biotic frontier," referring to the vast, but poorly studied, richness of organisms, particularly arthropods, resident in the canopy.

Many regional and global ecological processes depend crucially on the integrity of the rain forest canopy, which possesses features unique to this environment. Hence, canopy science represents a young, but blossoming, discipline in the field of natural sciences.

Significance of the Rain Forest Canopy in Ecological Processes

Canopies of all types, including boreal and tropical forests, play a crucial role in the maintenance of ecological processes, although thus far, the attention of researchers has tended to concentrate on those in the tropics, rather than those in temperate climates.

The forest canopy is the principal site for the interchange of heat, oxygen, water vapor, and carbon dioxide. It has been estimated that most photosynthetic activities in the biosphere occur in the canopy. Forest canopies account for almost half of the carbon stored in terrestrial vegetation and fix more carbon per year than any other habitat. Ecophysio-logical studies are therefore crucial to predict the impact of increasing atmospheric concentrations of carbon dioxide in global warming. Thus, forest canopies both control regional climate and play an important role in regulating global climate.

Rain forest canopies sustain countless species of animals and plants, and the majority of them are undiscovered and potentially unexploited resources. This important reservoir of genetic diversity ensures that vital ecological processes are performed by a variety of species, rather than a few, thus maintaining the integrity of the forest ecosystem in case of light disturbance. Adequate pollination and seed dispersal by a variety of organisms ensure the regeneration of the forest, whereas herbivory hastens the return of nutrients to ground level and their recycling; all three processes are prevalent in the canopy.

Unique Features of the Rain Forest Canopy

The uppermost canopy leaves are typically thicker, more upright, and have higher specific leaf mass and higher photosynthetic rates than under-story leaves. In closed tropical forests, the upper canopy is more akin to chaparral shrub vegetation than to rain forest understory vegetation. Leaf area density and the abundance of young leaves, flowers, and seeds are also higher in the canopy than in the understory. Microclimatic conditions differ markedly between the canopy and the understory; illumination, air temperature, wind, fluctuation of relative humidity, and water condensation at night are appreciably higher in the former.

Further, the array of tree crowns in the canopy are rather heterogeneous, including different species, size, phenologies (e.g., flowering and leaf flushing), and age state. Thus, forest canopies are best considered as spatially complex, three-dimensional structures that are temporally dynamic. Such systems are particularly conductive to the stratification , niche differentiation, and habitat selection of canopy organisms.

Indeed, the rain forest canopy may represent one of the most biodiverse biotas , perhaps containing between 50 and 80 percent of terrestrial species, depending on estimates. Besides the support trees, not only are many epiphytic plants (such as lianas, ferns, and orchids), arboreal mammals and reptiles, birds, and bats encountered, but unrivaled numbers of species of insects, spiders, mites, and other arthropods are also present. Ants represent the most regularly abundant animal group in the canopy, both in terms of numbers and biomass , whereas the most species-rich groups appear to be rove beetles (Staphylinidae) and weevils (Curculionidae). Typically, arthropod abundance and diversity are between two and four times higher in the canopy than in the understory.

Many of these organisms show distinct physical or behavioral adaptations to arboreal life. These include the canopy root system of several tree species that tap into the humus accumulated within epiphytes; the coalescing roots of strangling figs; the prehensile tails and gliding membranes of various arboreal mammals; the foraging behavior of particular bird species visiting epiphytes to search for various food resources; or the many peculiar life cycles and specializations (e.g., symbiotic associations) of a multitude of arthropod species. In particular, it is probable that herbivorous insects in the canopy are more host-specific to their host plants than their counterparts in the understory. Interactions between canopy organisms are often complex, due to heterogeneous substrates and patchy food resources, often resulting in intriguing mutualisms , such as ant gardens, in which ants harvest leaves to feed to cultures of fungi maintained by the colony. However, very little is known of most canopy organisms and their interactions with the canopy environment.

Canopy Access

The means for gaining access to the canopy, a major impediment to canopy science, was developed in the tropics. A pioneering attempt to study the canopy in situ by means of ladders and pulley systems was utilized during Oxford University's expedition of 1929 in Guyana, led by Major R. W. G. Hingston. The few studies performed before the late 1970s used fixed systems such as various towers, platforms, walkways, and ladders. In 1978, Donald Perry reported the inexpensive adaptation of a single-rope technique (used by cave explorers to ascend vertical shafts) to the safe climbing of tall forest trees. This led to an expansion of canopy studies, augmented in the following decade with newer methods permitting access to the upper canopy, including the canopy raft (and accompanying sledge) and canopy cranes. In addition, entomologists collect large quantities of canopy arthropods by insecticide knockdown or by hoisting various designs of traps into the canopy. Landscape ecologists also study the canopy with satellite remote sensing. By December 1999, the canopy raft had completed four successful missions, and four canopy cranes were in continuous use in the tropics. The scientific exploration and study of one of the most significant, exciting, and endangered habitats on Earth has only just begun.

see also Plant Prospecting; Rain Forests.

Yves Basset

Bibliography

Erwin, Terry L. "Tropical Forest Canopies: The Last Biotic Frontier." Bulletin of the Entomological Society of America 29 (1983): 14-19.

Hallé, Francis. The Canopy Raft. 1999. [Online] Available at http://www.radeaudes-cimes.com/.

Lowman, Margaret D. Life in the Treetops. Adventures of a Woman in Field Biology. New Haven, CT: Yale University Press, 1999.

——, and Nalini M. Nadkarni, eds. Forest Canopies. San Diego, CA: Academic Press, 1995.

Mitchell, Andrew W. The Enchanted Canopy: Secrets from the Rainforest Roof. London: Collins, 1986.

Moffett, Mark W. The High Frontier: Exploring the Tropical Rainforest Canopy. Cambridge, MA: Harvard University Press, 1993.

Mulkey, Stephen S. The Panama Canopy Crane at the Smithsonian Tropical Research Institute. 1997. [Online] Available at http://atb.botany.ufl.edu/crane/crane.html.

Nadkarni, Nalini M. The International Canopy Network. 1999. [Online] Available at http://192.211.16.13/individuals/nadkarnn/info.htm.

Perry, Donald R. "A Method of Access into Crowns of Emergent and Canopy Trees." Biotropica 10 (1978): 155-57.