Methods in Forest Canopy Researchby Margaret D. Lowman, Jerry Franklin, Timothy Schowalter
Poised between soil and sky, forest canopies represent a critical point of exchange between the atmosphere and the earth, yet until recently, they remained a largely unexplored frontier. For a long time, problems with access and the lack of tools and methods suitable for monitoring these complex bioscapes made canopy analysis extremely difficult. Fortunately,
Poised between soil and sky, forest canopies represent a critical point of exchange between the atmosphere and the earth, yet until recently, they remained a largely unexplored frontier. For a long time, problems with access and the lack of tools and methods suitable for monitoring these complex bioscapes made canopy analysis extremely difficult. Fortunately, canopy research has advanced dramatically in recent decades. Methods in Forest Canopy Research is a comprehensive overview of these developments for explorers of this astonishing environment. The authors describe methods for reaching the canopy and the best ways to measure how the canopy, atmosphere, and forest floor interact. They address how to replicate experiments in challenging environments and lay the groundwork for creating standardized measurements in the canopyessential tools for for understanding our changing world.
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Methods in Forest Canopy Research
By Margaret (Meg) D. Lowman, Timothy D. Schowalter, Jerry F. Franklin
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2012 The Regents of the University of California
All rights reserved.
SETTING THE STAGE
Canopy Research Emerges as a Component of Forest Science
Indeed, over all the glory there will be a canopy. It will serve as a pavilion, a shade by day from the heat, and a refuge and a shelter from the storm and rain.
Botany needs help from the tropics. Its big plants will engender big thinking.
E. J. H. CORNER, CAMBRIDGE UNIVERSITY, 1939
THE IMPORTANCE OF FOREST CANOPIES · 1 WHY SUCH A TIME LAG IN STUDYING FOREST CANOPIES? · 3 CANOPY DIVERSITY · 6 FUTURE DIRECTIONS IN CANOPY SCIENCE · 7
THE IMPORTANCE OF FOREST CANOPIES
Our ancestors were tree dwellers. Throughout human history, people have taken to the trees as safe havens, sites of special spiritual connection, and cornucopias for food, medicine, materials, and productivity (reviewed in Lowman 1999; Nadkarni 2008). In many tropical forest regions, indigenous people rely on forests for their livelihoods. Increasingly on a global scale, people and governments are beginning to recognize the importance of ecosystem services provided by forests, which link directly to human health (Perrings et al. 2010). Such benefits include medicine, food, shade, building materials, gas exchange capabilities, energy production, carbon storage, genetic libraries, water cycles, and spiritual/cultural heritage. In many religions, the clergy are stewards of the forests (e.g., Bossart et al. 2006; Cardelús et al. 2012; Lowman 2010; Wassie-Eshete 2007). Children in many cultures climb trees for recreation and build tree houses as an important component of their creative and spiritual links to nature (Louv 2005). With their billions of green leaves that produce sugars from sunlight, the treetops are the engines that support life and the basis of food chains throughout the planet. In an evolutionary sense, humans descended from ancestors in the treetops. Recent findings about ancient hominoids in Ethiopia indicate that human ancestors inhabited forests (not savannahs as was previously thought; White et al. 2009). Anyone who pauses at the zoo to watch a monkey cavorting in tree branches is amused, inspired, and subconsciously reminded of some arboreal sensation that tugs on our evolutionary memory banks.
In Papua New Guinea, a tribe called the Korowai still lives in the treetops, erecting amazing aerial houses accessible by twig ladders. It is speculated that their unusual habit of community tree houses evolved as a mechanism to escape enemies on the forest floor and provide a healthy environment above the dank, dark understory. Tree houses remain a recreational vestige of children and adults alike that inspires links between humans, their ancestry, and the natural world. Many famous people have escaped to childhood tree houses: John Lennon (of the Beatles), Winston Churchill, the Roman Emperor Caligula, and Queen Victoria when she was a young princess (Nelson et al. 2000). Recent medical findings indicate that children who play outdoors and learn about nature have better health and well-being (Louv 2005; Lowman et al. 2009).
Why do the treetops hold such a spiritual and scientific importance for cultures throughout the world? And why have scientists only recently begun to explore these heights for scientific discovery, after decades of studying a mere fraction of the forest understory? Relatively few unknown frontiers of exploration still exist in the twenty-first century, but the treetops are still considered a "black box" in science (Lowman 1999). The other "black boxes" for exploration include the ocean floor and soil ecosystems.
Forest canopies reputably house more than 40 percent of the biodiversity of terrestrial ecosystems (reviewed in Lowman and Rinker 2004; Wilson 1992). The combination of sunlight, fruits, flowers, and year-round productivity of the foliage in tropical rain forests provides the ideal conditions for an enormous diversity of inhabitants. Thousands of species of trees and vines produce a veritable salad bar for millions of insects that are in turn eaten by myriad reptiles, amphibians, birds, and mammals, and those primary consumers are eaten by secondary consumers such as harpy eagles, jaguars, and other carnivores. Finally, the cycle of life is completed when soil decomposers break down and recycle all matter for uptake of component nutrients into the canopy (Frost and Hunter 2007).
In addition to classic food chains using energy from sunlight to cycle water and nutrients through leaves to herbivores to carnivores/omnivores to decomposers and back to plants, forests house extra niches for other unique forms of life. Bromeliad tanks, tree crotches, leaf surfaces, and epiphyte communities host extra layers of life in forest canopies. For example, bromeliad tanks house virtual swimming pools in the sky that are home to an entire microcosm of microorganisms. Mosquito larvae, nematodes, tarantulas, katydids, shovel-tailed lizards, and canopy mammals live in and/or drink from these aerial watering holes. Some poison dart frogs trek all the way from the forest floor into emergent trees to deposit their eggs in phytotelms. Other unique canopy niches include the crotches of trees that provide germination sites for strangler figs and soil repositories that house many microarthropods usually associated with the forest floor. Strangler figs are the only trees known to start life "at the top" and send their aerial roots extending downward, eventually penetrating the soil below to expand and strangle their unwitting host plants. Epiphytes add an extra layer of biodiversity and productivity in the moist, sun-flecked branches. Even more amazing, the surfaces of canopy leaves provide a substrate for epiphylly, another extra layer of plant forms including lichens, mosses, and fungi, many of which grow exclusively on leaf surfaces. Within the "canopies" of these tiny epiphylls lives an entire microcosm of microinvertebrates and other microscopic organisms. Nothing rivals the forest canopy in terms of biodiversity—layers upon layers of life, all nurtured by sunlight, moisture, and warmth in a unique combination that fosters an extraordinary diversity and abundance of species.
This world of canopy plants, insects, birds, mammals, and their interactions remained relatively unknown and out of reach to scientists until as recently as twenty-five years ago (reviewed in Lowman 2004). William Beebe first introduced the world to the wonders of tropical rain forest canopies with his popular book High Jungle (1949), in which he described using a bow and arrow or climbers to hoist rope ladders into the high canopy. Field ecologists used slingshots to propel ropes into the treetops in the late 1970s, but at that time no one recognized that this green umbrella overhead was a critical component of global health. Exploration of both the deep sea and outer space was relatively commonplace prior to the exploration of forest canopies. But with the recent escalation of climate change and population pressures, canopies have become a proverbial "canary in the coal mine," since their declining health is a harbinger of environmental changes on a global scale, as well as a driver of further change (Foley et al. 2003; Janssen et al. 2008). Currently, forest canopy scientists—along with reef ecologists, ice physicists, soil biologists, water chemists, and others—are taking on the role of planetary physicians, working against a near-impossible timeline in hopes of unraveling the critical mysteries of how our planet functions. With access to forest canopies, our knowledge of the machinery of forest ecosystems has greatly expanded. And perhaps less appreciated in a technical sense, forest canopies enhance our sense of wonder and appreciation of the natural world, serving as drivers for more ambitious conservation agendas.
WHY SUCH A TIME LAG IN STUDYING FOREST CANOPIES?
Biologists in the nineteenth and twentieth centuries traditionally based their ideas about forests on observations made at ground level. These ground-based perceptions are summarized in a comment by Alfred R. Wallace in 1878: "Overhead, at a height, perhaps, of a hundred feet, is an almost unbroken canopy of foliage formed by the meeting together of these great trees and their interlacing branches; and this canopy is usually so dense that but an indistinct glimmer of the sky is to be seen, and even the intense tropical sunlight only penetrates to the ground subdued and broken up into scattered fragments ... it is a world in which man seems an intruder, and where he feels overwhelmed" (Wallace 1878).
Binoculars and telescopes were the first documented tools for canopy exploration. Charles Darwin, in the nineteenth century, looked into the tropical rain forest foliage, exclaiming,
Delight itself ... is a weak term to express the feelings of a naturalist who, for the first time, has wandered by himself in a Brazilian forest. The elegance of the grasses, the novelty of the parasitical plants, the beauty of the flowers, the glossy green of the foliage, but above all the general luxuriance of the vegetation, filled me with admiration. A most paradoxical mixture of sound and silence pervades the shady parts of the wood. The noise from the insects is so loud, that it may be heard even in a vessel anchored several hundred yards from the shore; yet within the recesses of the forests a universal silence appears to reign. To a person fond of natural history, such a day as this brings with it a deeper pleasure than he can ever hope to experience again. (Darwin 1883)
Ideas about forest canopies changed very little for almost a hundred years from Darwin's day until the 1950s, when a steel tower was constructed in Mpanga Forest Reserve in Uganda to study gradients from the forest floor to the canopy. Towers provided access to monitor insect vectors of human diseases, representing one of the first applied biological studies conducted in the forest canopy (Haddow et al. 1961). Other early canopy studies were mainly exploratory by nature: Operation Drake by Andrew Mitchell through the Oxford University explorers club, the first canopy walkway structure in Malaysia anchored among tree crowns (Muul and Liat 1970); ladders affixed to observe tree phenology and animal visitors (McClure 1966); Kamal Bawa's early chromosomal cytology of canopy trees in the Himalayas (Mehra and Bawa 1968; Mehra and Bawa 1969); and Steve Sutton's early exploration of Sulawasi (Sutton 2001). All these early canopy pioneers utilized methods that were relatively inexpensive and designed as creative solutions to overcome the challenges of gravity but were not necessarily devised to be easily replicated for experimental design purposes or for multiple users.
The late 1970s represented a golden age of canopy access, with development in 1978 of single rope techniques (SRT), independently designed for ecological data collection by Meg Lowman (1983) in Australia and Don Perry (1986) in Costa Rica. Whereas scuba equipment in the 1950s heralded the age of exploration for coral reefs (reviewed in Sale 2002), ropes and harnesses inspired the "race to the top" (of trees). This versatile toolkit of ropes, harness, and climbing hardware enabled scientists to reach the midcanopy with ease, suspended from a rope to observe pollinators, epiphytes, herbivores, birds, monkeys, and even sloths. Portable and relatively inexpensive, SRT and double rope techniques (DRT) allowed even budget-limited graduate students to survey life in the treetops (reviewed in Mitchell et al. 2002). Ropes were ineffective, however, to reach the leafy perimeters of tree crowns, since the ropes had to be looped over sturdy branches usually close to the tree trunk. To access the uppermost foliage of canopy trees, new devices were invented to overcome earlier limitations. For example, botanists in Indonesia devised the canopy boom, a horizontal bar with a bosun's chair at one end, which could be swung around the leafy canopy away from the woody trunks. In Pasoh, Malaysia, a combination of ladders, ropes, and booms launched research that solved the mystery of the pollination of dipterocarp flowers (Appanah and Chan 1981). Bawa (1969) used scaffolds and ropes to study pollinators in tropical trees in India. In temperate forests with their lower canopies, Lowman adapted construction scaffolding to survey the leafing phenology of birch trees in Scotland (Lowman 1978).
Engineers and creative canopy biologists partnered to construct canopy bridges and platforms in the 1980s. The first two canopy walkways were constructed nearly simultaneously: one in Malaysia, anchored in tree crowns by Ilaar Muul, and another in Queensland, Australia, supported by telephone poles (see Lowman 2009). Built in Lamington National Park, the Australian walkway was a collaboration of the ideas of Lowman and Peter O'Reilly, owner of the ecotourist lodge called O'Reilly's Guesthouse. Lowman's Earthwatch programs in Queensland rain forests required a canopy access tool that held groups of researchers (since ropes are solo operations). More than five years after the walkway construction in Australia and Malaysia, North America's first canopy walkway was built in 1992, using a suspension-bridge construction design suspended between oak trees in Massachusetts (Lowman and Bouricius 1995), and America's first public canopy walkway was constructed among Florida oak-palm hammocks in Myakka River State Park in Florida in 2000 (Lowman et al. 2006). Canopy walkways have since been replicated throughout the world, using a modular construction design developed by Canopy Construction Associates (http://www.canopyaccess.com), Greenheart, and a few other companies. Throughout the last decade, canopy walkways and ladders used in conjunction with climbing ropes, zip lines, and other tools have become popular ecotourism destinations as well as research tools. (See also http://www.treefoundation .org.) Subsequently, canopy walkways can provide sustainable income to local people and are especially useful in tropical rain forests because they provide an economy aside from logging (Lowman 2009).
Perhaps one of the most creative canopy access tools is the French-designed hot-air balloon, called Radeau des Cimes (trans. "raft on the rooftop of the world"). The balloon flies independently but also operates in conjunction with an inflatable raft (27 meters in diameter) that can be set on top of the canopy surface to serve as a base camp or platform atop the uppermost branches of tall trees (Hallé and Blanc 1990). In 1994, the Radeau des Cimes expedition team pioneered a new technique in French Guinea called the sled, or skimmer. This small (5 meters across), equilateral, triangular miniraft was towed across the canopy by the dirigible, similar to a boat with a trawling apparatus in the water column. The sled allowed rapid exploration between trees to compare pollinators, photosynthesis, herbivores, and the relative diversity/abundance of canopy life.
Construction cranes represent the most recent tool for safe access into the forest canopy (reviewed in Mitchell et al. 2002). In 1990, the Smithsonian Tropical Research Institute first erected a 40-meter-long crane in a Panamanian seasonally dry forest; since then, ten other crane operations have commenced operation in Australia, Switzerland, Germany, Japan, Indonesia, the United States, and Venezuela. Cranes are expensive to install and operate (usually ranging from $1 million to $5 million) but offer unparalleled, repeated access to the uppermost canopy within reach of the crane arm.
The forest canopy is defined as "the top layer of a forest or wooded ecosystem consisting of overlapping leaves and branches of trees, shrubs, or both" (Parker 1995). Tree canopies represent the hotspots of the forest—new leaves, flowers, pollinators, birds, arboreal mammals, orchids, lizards, mosses, and millions of insects. The bulk of energy captured from sunlight is concentrated in this region high above the forest floor. Oxygen is just one of the byproducts of this canopy engine, which represents the interface of Earth and atmosphere.
Studies of plant canopies typically include four organizational levels of approach: individual organs (leaves, stems, or branches), the whole plant, the entire stand, and the forest landscape. Canopy biology represents a relatively new discipline of forest science only formally launched over the past three decades (albeit linked inextricably to whole-forest biology). Canopy science incorporates the study of mobile and sessile treetop organisms and the processes that link them to the larger ecological forest ecosystem. Studying mobile bird populations is dramatically different in methods, tools, and temporal dynamics from studying sessile lichens anchored to leaf or bark surfaces. With the recent urgency surrounding climate-change research, forest canopies have emerged as an important interface between Earth and atmosphere; episodes such as insect outbreaks or canopy species extinctions serve as early warning signals indicative of hotter, drier climates.
Excerpted from Methods in Forest Canopy Research by Margaret (Meg) D. Lowman, Timothy D. Schowalter, Jerry F. Franklin. Copyright © 2012 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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Meet the Author
Margaret D. Lowman is Director of the Nature Research Center at the North Carolina Museum of Natural Sciences and Research Professor at North Carolina State University.
Timothy Schowalter is Professor and Department Head in the Department of Entomology at Louisiana State University Agricultural Center.
Jerry F. Franklin is Professor of Ecosystem Analysis in the College of Forest Resources at the University of Washington.
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