Canopy Crane Access System

Forest Canopies

Natural forests cover about one quarter of the global landmass, or 37 million km2 (FAO, 2001). On a large scale forest ecosystems provide services that include pollution mitigation, carbon sequestration, disaster prevention (by stabilising soils and reducing wind and soil erosion) and regular and clean water supply, just to mention a few. Many regional and global ecological processes depend crucially on the integrity of the forest canopy. For example, forest influences the quantity and quality of water yielded from watersheds, moderates variation in stream flow between the high and low flows during a year, and stabilizes the soil, lowering levels of soil mass movements and surface erosion (FAO, 2003).

Forest canopies also play a key role in other ecosystem processes, including energy flows, biogeochemical cycles and the dynamics of regional and global climates. Forest canopies both control regional climate and play an important role in regulating global climate (Shukla et al., 1990). The forest canopy is the principal site of energy assimilation in primary production, which involves the interchange of oxygen, water vapour and carbon dioxide between plants and the atmosphere. Most photosynthetic activity occurs in the canopy, and forests account for almost half of the carbon stored in terrestrial vegetation. For example, the annual net carbon production of tropical forests is 18 gigatons (a million million kilogrammes), accounting for 31% of global terrestrial photosynthesis (Malhi & Grace, 2000). Ecophysiological studies in forest canopies will be crucial to predict the impact of increasing atmospheric concentrations of carbon dioxide for carbon uptake by forests.

Forest canopies have low albedo, which makes most solar energy available to plants to fuel photosynthesize and transpiration. The high evapotranspiration increases the water vapour in the air, which is released to the upper layer of the atmosphere where it cools and condenses in clouds, forming water drops. The water drops then return to the ground as precipitation. High evapotranspiration also leads to a high latent heat loss that cools the surface. In deforested areas, the higher albedo of the bare soil reduces the amount of energy absorbed at the surface. Latent heat loss is reduced severely and the surface warms, since there is no means of removing the excess energy through the transpiration of plants (Ahhuf, 2002; Foley et al., 2003).

In addition, forest canopies, particularly in the tropics, sustain countless species of animals and plants, the majority representing an unknown and unexploited resource. 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 disturbance. For example, 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.

Aerial view of the canopy in Gabon


Background and definitions

Today, the study of forest canopies is a burgeoning and exciting field, as evidenced by the ever increasing number of publications concentrating on these habitats (Nadkarni & Parker, 1994; Nadkarni et al., 1996). The vitality of canopy science can be traced back to a series of studies about the canopy flora and fauna in tropical forests performed twenty-five years ago (Hallé et al., 1978; Perry, 1978; Erwin & Scott, 1980; Nadkarni, 1981; Lowman, 1984). These studies sparked continuing scientific interest in forest canopies and their inhabitants (Stork & Best, 1994; Lowman & Nadkarni, 1995; Stork et al., 1997a, 1997b; Linsenmair et al., 2001; Basset et al., 2003; Ozanne et al., 2003; Roslin, 2003).

Collecting insects from traps

The term ‘canopy’ has been used by different authors to mean rather different things. Although it indicates the treetop region in general, its precise definition includes the aggregate of every tree crown in the forest, including foliage, twigs, fine branches and epiphytes, in short all elements of the vegetation above the ground (Nadkarni, 1995). If this highly inclusive view of ‘canopy’ is adopted then we need a careful set of definitions for the height sub-divisions to which we must refer for descriptive clarity.

The ‘understorey’ may be defined as the vegetation immediately above the forest floor and capable of being reached by the observer or, if such measurements are available, the zone with less than 10% light transmittance (Parker & Brown, 2000). The French word canopée denotes the interface between the uppermost layer of leaves and the atmosphere (Hallé & Blanc, 1990). It has been translated as ‘canopy surface’ (Bell et al., 1999) or ‘outer canopy’ (Moffett, 2000). Further, the ‘upper canopy’ refers to the canopy surface and the volume immediately below (a few metres). This zone may be distinct only in tall, wet and closed tropical forests, where this layer experiences water condensation during the night (Blanc, 1990). Most of the biological activity and species diversity within tropical rain forests appears to be concentrated in the upper canopy, in comparison with other foliage layers (Hallé & Blanc, 1990; Parker, 1995; Basset et al., 2003). Emergent trees and the air above the canopy may be termed the ‘overstorey’. Use of these different terms may be important when studying tropical forests, since their stratification is often more pronounced than that in temperate forests (Smith, 1973; Richards, 1996).

Different sub-zones of the forest canopy

Crane sites in Panama


References cited

Anhuf, D. (2002).
Forest canopies and climatic change. The Global Canopy Handbook. Techniques of Access and Study in the Forest Roof. A. W. Mitchell, K. Secoy and T. Jackson. Oxford, The Global Canopy Programme: 209-214.

Basset, Y., Novotny, V., Miller, S. E. & Kitching, R. L., Eds. (2003).
Arthropods of Tropical Forests. Spatio-temporal Dynamics and Resource Use in the Canopy. Cambridge, Cambridge University Press.

Bell, A. D., Bell, A. & Dines, T. D. (1999)
Branch construction and bud defence status at the canopy surface of a West African rainforest. Biological Journal of the Linnean Society, 66, 481-499.

Blanc, P. (1990).
Bioclimatologie comparée de la canopée et du sous-bois. Biologie d'une canopée de forêt équatoriale. Rapport de Mission Radeau des Cimes Octobre-Novembre 1989, Petit Saut - Guyane Française. F. Hallé and P. Blanc. Montpellier/Paris, Montpellier II et CNRS-Paris VI: 42-43.

Erwin, T. L. & Scott, J. C. (1980)
Seasonal and size patterns, trophic structure and richness of Coleoptera in the tropical arboreal ecosystem: the fauna of the tree Luehea seemannii Triana and Planch in the Canal Zone of Panama. The Coleopterists' Bulletin, 34, 305-322.

FAO (2001).
State of the World's Forests 2001. Roma, Food and Agricultural Organization of the United Nations.

FAO (2003).
State of the World's Forests 2003. Roma, Food and Agricultural Organization of the United Nations.

Foley, J. A., Costa, M. H., Delire, C., Ramankutty, N. & Snyder, P. (2003)
Green surprise? How terrestrial ecosystems could affect earth's climate. Frontiers in Ecology and the Environment, 1, 38-44.

Hallé, F. & Blanc, P., Eds. (1990).
Biologie d'une Canopée de Forêt Equatoriale. Rapport de Mission. Radeau des Cimes Octobre-Novembre 1989, Guyane Française. Montpellier/Paris, Montpellier II et CNRS-Paris VI.

Hallé, F., Oldeman, R. A. A. & Tomlinson, P. B. (1978).
Tropical Trees and Forests, an Architectural Analysis. Berlin, Springer-Verlag.

Linsenmair, K. E., Davis, A. J., Fiala, B. & Speight, M. R., Eds. (2001).
Tropical Forest Canopies: Ecology and Management. Dordrecht, The Netherlands, Kluwer Academic Publishers.

Lowman, M. D. (1984)
An assesment of techniques for measuring herbivory : is rainforest defoliation more intense than we thought ? Biotropica, 16, 264-268.

Lowman, M. D. & Nadkarni, N. M., Eds. (1995).
Forest Canopies. San Diego, Academic Press.

Malhi, Y. & Grace, J. (2000)
Tropical forests and atmospheric carbon dioxide. Trends in Ecology and Evolution, 15, 332-337.

Moffett, M. W. (2000) What's "up"? A critical look at the basic terms in canopy biology. Biotropica, 32, 569-596.

Nadkarni, N. M. (1981)
Canopy roots: convergent evolution in rainforest nutrient cycles. Science, 213, 1024-1025.

Nadkarni, N. M. (1995)
Good-bye, Tarzan. The science of life in the treetops gets down to business. The Sciences, January / February 1995, 28-33.

Nadkarni, N. M. & Parker, G. G. (1994)
A profile of forest canopy science and scientists - Who we are , what we want to know, and obstacles we face: results of an international survey. Selbyana, 15, 38-50.

Nadkarni, N. M., Parker, G. G., Ford, E. D., Cushing, J. B. & Stallman, C. (1996)
The international canopy network: a pathway for interdisciplinary exchange of scientific information on forest canopies. Northwest Science, 70, 104-108.

Ozanne, C.M.P., Anhuf, D., Boulter, S.L., Keller, M., Kitching, Roger L., Korner, C., Meinzer, Frederick C., Mitchell, A.W., Nakashizuka, Tohru, Dias, Silva, Stork, Nigel E., Wright, S. Joseph & Yoshimura, M. (2003)
Biodiversity meets the atmosphere: global view of forest canopies. Science, 301, 183-186.

Parker, G. G. (1995).
Structure and microclimate of forest canopies. Forest Canopies. M. D. Lowman and N. M. Nadkarni. San Diego, Academic Press: 431-455.

Parker, G. G. & Brown, M. J. (2000) Forest canopy stratification - Is it useful? The American Naturalist, 155, 473-484.

Perry, D. R. (1978)
A method of access into crowns of emergent and canopy trees. Biotropica, 10, 155-157.

Richards, P. W. (1996).
The Tropical Rain Forest. An Ecological Study. Second Edition. Cambridge, Cambridge University Press.

Roslin, T. (2003)
Not so quiet in the high frontier. Trends in Ecology and Evolution, 18, 376-379.

Shukla, J., Nobre, C. & Sellers, P. (1990)
Amazon deforestation and climatic change. Science, 247, 1322-1325.

Smith, A. P. (1973)
Stratification of temperate and tropical forests. The American Naturalist, 107, 671-683.

Stork, N. E., Adis, J. & Didham, R. K., Eds. (1997b).
Canopy Arthropods. London, Chapman & Hall.

Stork, N. E. & Best, V. (1994)
European Science Foundation - Results of a survey of European canopy research in the tropics. Selbyana, 15, 51-62.

Stork, N. E., Wright, S. J. & Mulkey, S. S. (1997a)
Craning for a better view: the Canopy Crane Network. Trends in Ecology and Evolution, 12, 415-420.

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