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.

ISAIAH 4:5–6

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...