The Ecology of Tropical Rain Forest Canopies
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The Ecology of Tropical Rain Forest Having solved many of the prob- lems of access, canopy biologists Canopies are now designing new sampling techniques and formulating hy- Margaret D. Lowman and Mark Moffett potheses. 'They face the difficulty of working in a large three-dimensional space. How are organisms detected With the advent 01 increasingly sophisti- Biological information about can- and sampled in such a hetero- cated techniques /or access, tropical lorest opies changed very little from geneous environment, where humans canopy research has burgeoned in the last Wallace's day until - exactly one are rendered less agile? In a lew years. Although an enormous amount hundred years later in 1978 - Don scenario similar to the expansion of of basic descriptive work remains to be Perry published a method of climb- coral reef fish ecology in the 1970s done, canopy research is now entering a ing into tropical tree canopies with the advent of SCUBA, canopy more advanced and ecological phase. using ropes and technical climbing biologists are developing sampling apparatus2. Although Perry was by protocols to account for the spatial, no- means the first researcher to temporal and substrate heterogen- Until recently, most of our knowl- climb into canopies, the use of single- eity of their en~ironment~,~. edge about forest ecosystems has rope technologies heralded a rapid The development of canopy been based on observations from expansion of canopy research in- research has been affected by sev- ground level. These ground-based volving a range of apparatus includ- eral spatial and temporal con- perceptions are summarized in a ing towers, walkways, platforms, straints of this habitat, including: comment by Alfred R. Wallace' cranes and dirigi bleP5. Having (I j differential use of this geometric Overhead, at a height, perhaps, overcome many of the logistic limi- space by canopy organisms; (2) het- of a hundred feet, is an almost tations of access into tall trees, we erogeneity of substrate; (3) varia- unbroken canopy of foliage can now do field work and bility in ages within the canopy formed by the meeting together of formulate hypotheses in an above- (e.g. soillplant communities accru- these great trees and their inter- ground heterogeneous three- ing in uneven layers on branches, lacing branches; and this canopy dimensiona! system. leaf cohorts between sun and is usually so dense that but an Historically, most ideas about shade regions), (4) variability in indistinct glimmer of the sky is to forest ecology were developed in microclimate of the atmosphere- be seen, and even the intense temperate regions. By contrast, canopy interface; (5) the high diver- tropical sunlight only penetrates most work on forest canopies has sity of organisms (many unnamed); to the ground subdued and bro- been pioneered in the tropics. The (6) development of protocols to ken up into scattered fragments . reasons for such sudden interest in quantify processes in the canopy it is a world in which man seems tropical canopy research are two- environment. Many aspects of an intruder, and where he feels fold. First, tropical tree canopies canopy research are so new that overwhelmed.. are the most complex of any forest results are not yet published. type. Second, the threatened extinc- In this review, we highlight sev- Margaret Lowman is at the Selby Botanical Gardens, tion of tropical organisms (many of eral areas of research that have been Sarasota, FL 14216, USA; Mark Moffett is at the which live in the canopy) has pro- enhanced by canopy access. We Museum of Comparative Zoology, Harvard Uni- vided incentive to study them be- define three major types of canopy versity, Cambridge, MA 02118, USA. fore they disappear6 (but see Ref. 7). research, each of which requires 0 1993, Elsevier Science Publishers Ltd (UK) 0169-5347/931$06.00 TREE vol. 8, no. 3, March 7993 different logistics and experimental Branching patterns, in turn, are diversity of gliding animals in Asia design: studies of plants, studies of indirectly affected by the location has been attributed to the relative animals and studies of canopy of a tree. Berner, who is studying scarcity of lianas in this region, processes (e.g. photosynthesis, the interactions between branch which many animals use as 'high- herbivory and nutrient cycling). growth patterns, disturbance and ways' to cross from one crown to the plant community dynamicsi7,found nextz2. As crowns become more Sessile organisms: trees, vines, that trees on slopes produce more widely dispersed (particularly in epiphytes and epiphylls asymmetrical branch growth pat- windy regions), lianas themselves Studies of sessile organisms in terns as com~aredto trees on level have more difficulty extending lat- forest canopies pose fewer logistic ground, due to increased light influx erally from tree to tree2'. Vines may difficulties than other aspects of into tree crowns on hillsides. But comprise one quarter of all leaves canopy biology. The biggest ob- the steeD slo~esalso result in in the forest of Barro Colorado stacle is access to growing shoots more disiurbance and higher mor- Island, and one individual of and reproductive parts, many of tality for trees growing there. Entada monostachya has been which occur in the uppermost can- Similar differential tree growth and recorded to connect the crowns of opy. Some methods (e.g. raft5 and mortality occur around the margin 64 canopy trees2'. Indeed, to crane8) facilitate access to these of a tree gap, apparently prolong- describe vines as sessile is some- upper regions. Shade-tolerant ing the successional processI8. times inappropriate, because of plants such as bromeliads and The first comparisons between their fast growth, mobility and for- other epiphytes are often access- ground-level observations and di- aging behavior as they search for ible in the mid-canopy region. rect measurements of canopy lightZ4. Techniques other than Epiphytes and epiphylls colonize architecture are under way in climbing have been employed to branches and leaves, respectively, Panama8.The 'surface' of tropical for- study vines, such as the use of in moist canopy regions. Never- ests appears much more irregular winches in Australia to haul Calamus theless, their diversity, distribution and dynamic than most measure- down from the canopy and measure and abundance is not well docu- ments from the ground would indi- its growth25.We are only beginning mented', and data on growth, cate, and is more heterogeneous to understand the complex dy- recruitment and survival are few than in temperate forests, partly namics of tree and vine growth in (but see Refs 10, l l ). because of the larger number of relation to canopy processes. Trees are the major substrate of tree species. This has implications the canopy ecosystem, and tree for canopy-atmosphere interactions, Mobile organisms in canopies species - their architecture, limb and for population dynamics of Most studies of vertebrates have strength, surface chemistry and tex- organisms in the upper canopy. For been made from ground level - an ture - play fundamental roles in instance, precipitation reaching the adequate vantage point for diurnal shaping the canopy community. understory layers can vary several- mammals and some birds. But Tree architecture is far more varied fold depending on the angle of access into the canopy has led to the in the tropics than in the temper- incidence of rainfall1'. Canopy discovery of unexpectedly arboreal ate zones, and the patterns of topography affected the flux of proclivities in some rodents, whose reiteration of canopy branches and wind-blown insects in Puerto Rican behavior was not obvious from the their implications for canopy rain forest canopies, contributing ground. Malcolm used the peconha processes are not well understood to the regulation of Anolis lizard Indian method (strap between the in either regi~n'~,'~.Over time, as populations (R. Dial, PhD Thesis, feet) to look at edge effects on small canopy branches grow, the com- Stanford University, 1992). Other mammals in the canopy of lowland munities within them increase in environmental (e.g. light, sunflecks, forest near Manaus, Brazil2" He complexity. For example, patches of wind-below-crown level) and bio- found that species exhibit distinct leaves, heterogeneous in their age logical factors (e.g. density of vines, height preferences, and more mam- structure, foliage quality and distri- distribution of flowers, populations mals were arboreal than terrestrial. butionI4, attract different popu- of canopy leaves and subsequent In a Costa Rican cloud forest, lations of insects both within and organisms that inhabit them) are Langtimm also found stratified between tree crowns: herbivores affected by tree growth and canopy height preferences for different prefer shade leaves over sun architecture. species of small mammals2'. leave^'^,'^; and patches of canopy Crown shyness gaps between Ornithologists face the challenge vegetation (e.g. palms versus trees arise from dieback of the of trying to capture (as well as to vines) may host entirely different outermost branches due to wind- observe) birds in tree crowns. In populations of insectsI6. Similarly, shearingZ0 or shading of adjacent New Guinea, Bechler hoisted nets branching patterns affect the com- crowns2'. The amount of spacing up and down tall poles to quantify munities that form around them. between tree crowns may have birds of paradise in the canopy Branches that are steeply inclined profound effects on the dispersal (B. McP. Bechler, PhD thesis, Prince- have less accumulation of canopy of canopy organisms, providing ton University, 1983). More recently plants (and consequently canopy pathways for flying organisms both in Peru, Munn used a large sling- soils and insects) than branches between tree crowns and between shot to position aerial mist nets in that grow horizontally (S.W. Ingrarn, canopy layers, but inhibiting the emergent trees as high as MA Thesis, University of California horizontal passage of climbing ani- 40-60 m28.Bierregaard and Lovejoy at Santa Barbara, 1989).