Canopy Crane Access System

Futures Studies

The important data that individual crane sites can provide to the scientific community concern issues such as how biodiversity is maintained locally, which biotic interactions appear primordial to maintain the integrity of the forest ecosystem and its functioning, how plant metabolism realistically operates at the top of the canopy, and how plants and primary consumers will respond to elevated air temperature and increased levels of CO2 and pollutants in the atmosphere. All of these represent key parameters needed to improve knowledge of environmental threats identified in international conventions. A comparative and collaborative approach within the network of canopy cranes could greatly enhance the flow of information relevant to these parameters.

 

Relationships between the forest canopy and the forest ecosystem

Some researchers consider the forest canopy, particularly the upper canopy, as a distinct habitat from the rest of the forest and, consequently, study it in isolation. Canopy science emphasizes the importance of the canopy, not that the canopy is distinct from the rest of the forest ecosystem. Canopy processes must be linked to other forest component for a sound understanding of the functioning of the forest ecosystem. For example, studies to quantify herbivory and frass (faeces) fall in conjunction with decomposition on the forest floor are likely to enable a better mapping of nutrient cycling than if these components are studied in isolation (Lowman, 2001). In particular, defoliation may influence positively decomposition on the forest floor by increasing carbon, nitrogen, and phosphorus available to the decomposer community. This may occur via the fall of insect frass, greenfall (leaf fragments dropped by defoliators), prematurely-abscised leaves, and rainfall which collects dissolved insect frass and modified leachate from damaged foliage (Rinker et al., 2001). Indeed, Schowalter and Sabin (1991) reported increases in litter arthropod diversity and abundance following defoliation of saplings.

Moffett (2002) rightly remarks that it is unlikely that a forest will harbour a significant canopy flora and fauna in isolation of the ground and soil biota. In fact, levels of biodiversity above and below ground are often correlated (Hooper et al., 2000). Perhaps the necessity to study the canopy in conjunction with other forest habitats is best illustrated in entomological studies. Many insect herbivores, such as some leaf beetles and weevils, feed on roots as larvae and later migrate in the canopy to feed as adults on leaves. In inundation forests in Brazil, some soil and leaf litter insects migrate to the canopy when the forests are flooded (Adis, 1997; Gauer, 1997). Although it is relatively easy to report differences in the occurrence of particular species of beetles in the adult stage either in the soil or in the canopy, our understanding of the relationships between the canopy and soil should also proceed by assessing how many insect species depend on the soil/litter habitat during their juvenile stages and on the canopy during their adult phase. Understanding the distribution of adult insects in the canopy may require solid data on their distribution as larvae in the soil (Basset & Samuelson, 1996). Surprisingly little analysis of the use of different strata (soil, leaf litter, tree trunks, canopy) by different insects and their life stages in European temperate forests has been carried out. Further, comparison between the litter and canopy faunas may emphasize specific adaptations of arboreal invertebrates which may be important from a conservation viewpoint. Nevertheless, multi-method, multi-habitat studies are essential if statements are to be made about the overall arthropod diversity of forests: the assumption, tacit since Erwin & Scott's (1980) article, that the species richness of the forest is totally canopy-dominated is certainly not true. Thus, sound understanding of biotic relationships in the canopy may require baseline knowledge of the entire rainforest ecosystem, an additional challenge in itself (Basset et al., 2003).

 

Canopy structure

At present, measurement of canopy structure, such as those obtained with remote sensing (Lefsky et al., 2002), are mostly used for descriptive purposes. In fact, these measurements should be at the core of multidisciplinary studies, since mapping canopy structure is equivalent to mapping forest productivity. A description of canopy structure can also be used to study the finer distribution of organisms, or variation in plant ecophysiological processes. One may imagine studying the relationships between leaf area density or woody structures and the vertical flight distribution of insects, for example. In this case the priority of the entomological protocol would be on spatial replication, with the use of numerous, small collection devices placed throughout the vertical profile of the forest. Such a replicated study at different crane sites would be able to compare patterns of insect stratification among different forest types and canopy structures. Forest structure measurements could also be crucial in ecophysiological studies concerned with the scaling up of processes from the leaf to the forest stand levels. Possibly, studies of canopy structure could evaluate the intriguing 'inversion surface' (surface d'inversion) of Oldeman (1974) in tropical rainforests. It is surprising that the biological significance of this concept has never been assessed. Briefly, this is the zone where water becomes the limiting factor for tree growth and where photosynthesis is performed with a minimum of transpiration. It roughly corresponds to the first branching of dominant trees (Oldeman, 1974). In French Guiana, Sterck and Bongers (2001) recently showed that regressions of plant traits against tree height were linear with study trees below 25 m in height, and become non-linear with taller trees. Light availability was not considered to be an important selection force acting on architectural changes with tree height. Forest structure measurements might help to describe such branching patterns, and, further, these data might conveniently be related to ecophysiological measurements with the help of canopy cranes.

See also: Parker, G. G., Lefsky, M. A. & Harding, D. J. (2001)
Light transmittance in forest canopies determined using airborne laser altimetry and in-canopy quantum measurements. Remote Sensing of Environment, 76, 298-309.

G. Parker & A. Smith measuring canopy structure at Parque Natural Metropolitano


Biodiversity and biotic interactions

With regard to international conventions on global environment and biodiversity, one may remark that estimates of rates of carbon sequestration, or of gaseous pollutants in the atmosphere, have notably improved since the 1990’s, whereas estimates of rates of species loss remain vague and controversial. In part, this may have resulted from funding being channeled towards studies of global change rather than towards biodiversity studies. The magnitude of the effort necessary for adequate surveys of biodiversity and implementation of conservation policies may also be overwhelming to many organizations, institutions and individuals.

In a sense, it is puzzling to observe that scientists have managed to focus public attention on complex problems of gas exchange and nutrient cycling (involving many primary reactions not visible with the bare eye), such as the ‘ozone hole’, whereas they have largely failed to focus public attention on the fundamental and unanswered question of how many species inhabit Earth, what proportion of these have been named and described, and how widely they are distributed. One of the reasons accounting for this state of affairs may be that global cycles of gases and nutrients can now be well explained. In contrast, the local maintenance of biodiversity is far from being reasonably captured. For example, knowledge of local food-webs, especially in the tropics, is still rudimentary (e.g., Godfray et al., 1999). In short, the inability of the scientific community to document species diversity, and hence its changes including decline, is hugely detrimental to the credibility of the conservation movement (e.g., Mann 1991).

Promoting local studies of food-webs may represent one way to improve knowledge about the maintenance of biodiversity (the study of regional and historical factors represents another). There have been repeated pleas to initiate large-scale inventories of invertebrates, particularly in the tropics (e.g., Janzen, 1993; Stork, 1994; Godfray et al., 1999; Basset, 2001). Smaller inventories, but with convenient access to all levels in the forest, such as within the perimeters of canopy cranes, may be easier to implement. Such inventories could be coupled with detailed studies of ecophysiology and energy budgets, and efficiently compared among different biogeographical regions (crane sites for example). There is nothing really new with this methodology (e.g., Morawetz, 1998), but in practice, it has yet to be implemented.

Many studies within the world network of canopy cranes are concerned with the vertical distribution of organisms. Many reported that the occurrence of organisms along vertical transects is linked to specific abiotic and biotic conditions. One interesting question is to know whether rising air temperatures (with concomitant changes in plant metabolism) may alter this equilibrium and result in further species extinction. For example, rising temperature may cause spatial shifts of arthropod species, particularly sedentary ones, that may threaten their survival (e.g., specialized species of the upper canopy may move down, to cooler levels where their resources may be less abundant). These effects are likely to be of greatest magnitude in tall, closed tropical rainforests, where biodiversity is also greatest.

Lawler et al. (2001) considered several ways in which studies of biodiversity could best address conservation-related problems. The two following items appear to be of particular relevance to the study of forest canopies: (a) research that builds tools for predicting which sort of ecosystem failures are likely with the loss of particular species or functional group of species; and (b) the role of exotic species. Research into item (a) could be promoted by studying the effects of canopy opening by logging. These effects have not been well investigated to date. For example, specialized insect herbivores of the upper canopy are unlikely to fare well in the understorey since forest gaps typically include different set of plant species (pioneers) than are present in the mature canopy (shade-tolerant species). Taxa less tied to resources occurring specifically in the upper canopy, such as dung beetles, do not appear to suffer much from canopy loss and survive well in the understorey of disturbed forests (Davis & Sutton, 1998). Some crane sites could be used as controls of undisturbed canopy, in comparison to natural and anthropogenic forest gaps. With regard to (b) an important question would be to evaluate the ecological consequences of a canopy constituted mostly of exotic species, such as in certain plantations. Ideally this would include establishing a crane in a plantation not far away from a control crane-site, but perhaps manipulative experiments with smaller plants, such as epiphytes, may also be possible.

Studies of biotic interactions in the forest canopy should also move towards assessing the influence of biodiversity (species richness) on forest productivity and functioning. This might be best investigated by coupling studies of herbivory, predation, parasitism and plant productivity, and comparing patterns among crane sites.

L. Cizek & D. Hauck collecting insects with large nets within the canopy at San Lorenzo


 

Plant ecophysiology and the changing climate

Beside the effects of rising pollutants in the atmosphere, one worrying aspect of global climate change is the increase in air temperature per se. Rising air temperatures are likely to involve cascading effects starting from the level of the leaf (e.g., Clark et al., 2003), continuing at the level of primary consumers (e.g., Percy et al., 2002), and culminating at the level of ecosystems. The hypothesis that the control of stomatal closure will be greatly affected in tropical trees by rising air temperatures warrants further investigations, since this affects the likelihood that tropical forests act as either sources or sinks of carbon (Clark et al., 2003). Therefore, studies related to this topic and performed in different forest types (temperate, tropical) and at different ambient temperatures would be instructive.

More generally, the above remark emphasizes the crucial effect of changes in limiting resources for plants. Another example is the negative effects of increased nitrogen deposition. Anthropogenic burning of fossil fuels and addition of fertilizers have more than doubled the nitrogen flux through natural ecosystems. The problem is particularly acute in densely populated and heavily industrialized areas, and is largely limited to higher latitudes where nitrogen limits most natural systems (Galloway & Cowling, 2002). With nitrogen limitation removed, species must operate under novel constraints such as inadequate phosphorus and water supplies. How are the performance of organisms and the operation of larger ecological processes affected by rapid changes in their chemical environment for which they have no evolutionary background and to which they are not adapted (Vitousek et al., 1997)?

Körner and Zotz (2003) have stressed the importance to adopt a multi-species approach to study biotic reactions to global change. Natural forest canopies accessible from canopy cranes represent an ideal arena for such studies, in contrast to artificial systems such as, for example, the Ecotron, an integrated series of environmental chambers in which physical conditions are controlled (Lawton et al., 1993). These artificial systems can include only few species and appear very simplified in comparison with natural systems.

See also: 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.

Stork, N. E. (2001)
The management implications of canopy research. Plant Ecology, 153, 313-317.

 

References cited

Adis, J. (1997)
Terrestrial invertebrates: survival strategies, group spectrum, dominance and activity patterns. The Central Amazon Floodplain. Ecological Studies, Vol. 126. W. J. Junk. Berlin, Springer - Verlag: 318-330.

Basset Y. (2001)
Invertebrates in the canopy of tropical rain forests: how much do we really know? Plant Ecology, 153, 87-107.

Basset, Y., Novotny, V., Miller, S. E. & Kitching, R. L. (2003)
Conclusion: arthropods, forest types and interpretable patterns. Arthropods of Tropical Forests. Spatio-temporal Dynamics and Resource Use in the Canopy. Y. Basset, V. Novotny, S. E. Miller and R. L. Kitching. Cambridge, Cambridge University Press: 394-405.

Basset, Y. & Samuelson, G. A. (1996)
Ecological characteristics of an arboreal community of Chrysomelidae in Papua New Guinea. Chrysomelidae Biology. Volume 2. Ecological Studies. P. H. A. Jolivet and M. L. Cox. Amsterdam, SPB Academic Publishing: 243-262.

Clark, D. A., Piper, S. C., Keeling, C. D. & Clark, D. B. (2003)
Tropical rain forest tree growth and atmospheric carbon dynamics linked to interannual temperature variation during 1984-2000. Proceedings of the National Academy of Sciences, 100, 5852-5857.

Davis, A. J. & Sutton, S. L. (1998)
The effects of rainforest canopy loss on arboreal dung beetles in Borneo: implications for the measurement of biodiversity in derived tropical ecosystems. Diversity and Distributions, 4, 167-173.

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.

Galloway, J. N. & Cowling, E. B. (2002)
Reactive nitrogen and the world: 200 years of change. Ambio, 31, 64-71.

Gauer, U. (1997)
Collembola in Central Amazon inundation forests - strategies for surviving floods. Pedobiologia, 41, 69-73.

Godfray, H. C., Lewis, O. T. & Memmott, J. (1999)
Studying insect diversity in the tropics. Philosophical Transactions of the Royal Society, Series B, Biological Sciences, 354, 1811-1824.

Hooper, D. U., Bignell, D. E., Brown, V. K., Brussaard, L., Dangerfield, J. M., Wall, D. H., Wardle, D. A., Coleman, D. C., Giller, K. E., Lavelle, P., Van der Putten, W. H., De Ruiter, P. C., Rusek, J., Silver, W. L., Tiedje, J. M. & Wolters, V. (2000)
Interactions between aboveground and belowground biodiversity in terrestrial ecosystems: patterns, mechanisms, and feedbacks. BioScience, 50, 1049-1061.

Janzen, D. H. (1993)
Taxonomy: Universal and essential infrastructure for development and management of tropical wildland biodiversity. Proceedings of the Norway/UNEP Expert Conference on Biodiversity. O. T. Sandlund and P. J. Schei. Trondheim, Norway, Directorate for Nature Management and Norwegian Institute for Nature Research: 100-113.

Körner, C. & Zotz, G. (2003)
Basel, Switzerland. In: Y Basset, V Horlyck, SJ Wright (eds) Studying forest canopies from above: The International Canopy Crane Network. Smithsonian Tropical Research Institute and UNEP, Panama. pp 67-70.

Lawler, S. P., Armesto, J. J. & Kareiva, P. (2001)
How relevant to conservation are studies linking biodiversity and ecosystem functioning? The Functional Consequences of Biodiversity. Empirical Progress and Theoretical Extensions. A. P. Kinzig, S. W. Pacala and D. Tilman. Princeton and Oxford, Princeton University Press: 294-313.

Lawton, J. H., Naeem, S., Woodfin, R. M., Brown, V. K., Gange, A., Godfray, H. J. C., Heads, P. A., Lawler, S., Magda, D., Thomas, C. D., Thompson, L. J. & Young, S. (1993)
The Ecotron: a controlled environmental facility for the investigation of population and ecosystem processes. Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences, 341, 181-194.

Lefsky, M. A., Cohen, W. B., Parker, G. G. & Harding, D. J. (2002)
Lidar remote sensing for ecosystem studies. BioScience, 52, 19-30.

Lowman, M. D. (2001)
Plants in the forest canopy: some reflections on current research and future directions. Plant Ecology, 153, 39-50.

Mann, C. C. (1991)
Extinction: are ecologists crying wolf? Science, 253, 736-738.

Moffett, M. W. (2002)
The highs and lows of tropical forest canopies. Journal of Biogeography, 29, 1264-1265.

Morawetz, W. (1998)
The Surumoni project: the botanical approach toward gaining an interdisciplinary understanding of the functions of the rain forest canopy. Biodiversity: A Challenge for Development Research and Policy. W. Barthlott and M. N. Winiger. Berlin, Springer-Verlag: 71-80.

Oldeman, R. A. A. (1974)
Ecotypes des arbres et gradients écologiques verticaux en forêt guyanaise. La Terre et la Vie, 28, 487-520.

Percy, K. E., Awmack, C. S., Lindroth, R. L., Kubiske, M. E., Kopper, B. J., Isebrands, J. G., Pregitzer, K. S., Hendrey, G. R., Dickson, R. E., Zak, D. R., Oksanen, E., Sober, J., Harrington, R. & Karnosky, D. F. (2002)
Effects of elevated CO2 and O3 on plant-insect interactions in a trembling aspen forest. Nature, 420, 403-407.

Rinker, H. B., Lowman, M. D., Hunter, M. D., Schowalter, T. D. & Fonte, S. J. (2001) Canopy herbivory and soil ecology - the top-down impact of forest processes. Selbyana, 22, 225-231.

Schowalter, T. D. & Sabin, T. E. (1991)
Litter microathropod responses to canopy herbivory, season and decomposition in litterbags in a regenerating conifer ecosystem in western Oregon. Biology and Fertility of Soils, 11, 93-96.

Sterck, F. & Bongers, F. (2001)
Crown development in tropical rain forest trees: patterns with tree height and light availability. Journal of Ecology, 89, 1-13.

Stork, N. E. (1994)
Inventories of biodiversity: more than a question of numbers. Systematics and Conservation Evaluation. P. I. Forey, C. J. Humphries and R. I. Vane-Wright. Oxford, Clarendon Press: 81-100.

Vitousek, P. M., Aber, J. D., Howarth, R. W., Likens, G. E., Matson, P. A., Schindler, D. W., Schlesinger, W. H. & Tilman, D. G. (1997)
Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications, 7, 737-750.


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