FOREST CANOPIES, PLANT DIVERSITY Nalini M. Nadkami,* Mark C. Mewin,* and Jurgen Niedert "The Evergreen State College and tBotanisches Institiit der Universitat Bonn I. Introduction water balance independent of fluctuating environ- 11. Categories of Canopy Plants mental conditions. 111. Canopy Plant Taxa Diversity lichen Composite organism consisting of a fungus (the IV. Diversity of Habitats of Canopy Plants mycobiont) and an alga and/or a cyanobacteria (the V. Diversity of Growth Habits in Canopy Plants phycobiont) that live in a symbiotic relationship. VI. Canopy Plant Biodiversity and mistletoe Woody parasite that taps the xylem of a tree, Conservation Biology but is capable of photosynthesis. VII. Areas for Further Study obligate epiphyte Plant that always grows on another plant for structural support, but derives no nutrients from the host. parasite Woody or nonwoody plant that taps into the GLOSSARY vascular system of a host plant and derives energy and/ or nutrients from it, often to the detriment of the host. accidental epiphyte Plant that normally grows terres- poikilohydry Condition of internal water balance vary- trially but that occasionally grows to maturity in a ing with changes in ambient humidity. tree crown, usually in terrestriallike microsites such primary hemiepiphyte Plant that begins its life cycle as the crotches of branches. anchored in a tree crown and ultimately becomes bryophyte Nonvascular plant of the division Bryophyta rooted in the ground (e.g., strangler fig). (a moss, liverwort, or hornwort). secondary hemiepiphyte Plant that begins its life cycle cryptogam Plant that reproduces by spores or gametes as a terrestrial seedling, ascends a tree, and can later rather than seeds; includes bryophytes and lichens. lose root connections with the ground, including (a) epiphyll (folicolous) Plant that grows on the leaf sur- lianas, woody climbing plants with relatively thick face of another plant. stems that generally grow in mature habitats, and (b) epiphyte Nonparasitic plant that uses another plant as vines, herbaceous climbing plants that regularly grow mechanical support but does not derive nutrients or in disturbed habitats or forest edges. water from its host. facultative epiphyte Plant or lichen that commonly grows epiphytically and terrestrially, usually exhib- iting preference for one or the other habit in a partic- THE IMPORTANCE OF THE PLANTS THAT DWELL ular habitat. IN FOREST CANOPIES is becoming increasingly recog- homoiohydry Ability to maintain a constant internal nized in relation to understanding biodiversity. The Encyclopedia of Biodiversity, Volume 3 Copyright O 2001 by Academic Press. All rights of reproduction in any form reserved. 28 FOREST CANOPIES, PLANT DIVERSITY upper tree canopy of many forest ecosystems fosters Rhoades, 1995). Here, we describe the diversity of many extremely diverse plant communities, which include types of canopy-dwelling plants. Included in this review vascular and nonvascular epiphytes, hemiepiphytes, are epiphytic vascular plants, epiphytic cryptogams and parasites. Canopy-dwelling plants contribute sub- (nonvascular plants that include lichens and bryo- stantially to overall forest biodiversity and biocomplex- phytes), primary and secondary hemiepiphytic vascular ity by providing resources for arboreal vertebrates, in- plants (lianas and vines), and arboreal parasitic vascular vertebrates, and microbes, and by participating in plants. Arboreal fungi and free-living algae are so poorly nutrient and water cycling and gas and energy ex- known that there is little to review. We place greatest changes. emphasis on obligate and facultative epiphytes, and exclude "accidental epiphytes" from this review. The term "diversity" in the following will be based I. INTRODUCTION on species as the unit of biological diversity, since as- sessment of other aspects of biodiversity is virtually A. Definition of the Forest Canopy nonexistent in the case of canopy plants. We first review the systematic distribution of canopy taxa and provide The forest canopy has been called "the last biotic fron- species counts based on the state of current knowledge. tier" (Erwin, 1988). It presents a habitat conducive to We discuss gradients of canopy plant diversity of mi- the evolution of literally thousands-perhaps mil- crosites within the canopy at various spatial scales, lions-of species of plants, microorganisms, insects, spanning a single microsite within a tree (e.g., twig, birds, and mammals that are rarely or never encoun- branch bifurcation) to regional and global levels. Bio- tered on the forest floor. Although forest canopies have geographical analyses of canopy-dwelling taxa are then been among the most poorly understood regions of our considered, as well as some of the major evolutionary planet, their mysteries are being explored by increasing elements that have influenced their distribution and numbers of biologists. Canopy communities are now abundance. "Habit diversity" (the diversity of morpho- believed to be important in maintaining the diversity, logical and physiological features) of arboreal taxa will resiliency, and functioning of the forests they inhabit. then be described. Finally, we discuss conservation ef- The forest canopy is a structurally complex and eco- forts that involve canopy plants and suggest future re- logically important subsystem of the forest. It is defined search possibilities. as "the aggregate of all crowns in a stand of vegetation, which is the combination of all foliage, twigs, fine C. Historical Roots and Sources branches, epiphytes as well as the interstices (air) in a forest" (Parker, 1995). The forest canopy is the primary of Information site of gas exchange between the atmosphere and vege- In 1832, Charles Darwin first described what he termed tation and fosters many ecosystem processes that are the great diversity and profusion of "parasitical plants" crucial to the maintenance and diversity of the forest (that we now understand to have been epiphytes), as a whole (Lowman and Nadkarni, 1995). which he encountered in abundance in the coastal for- ests of Brazil. In the late nineteenth century, the German botanist A. F. W. Schimper first described epiphytes B. Scope of This Article and outlined their importance to tropical botany. Trees are the most obvious structural component of Historically, canopy studies have been dominated by forest canopies. Their trunks, branches, and leaves con- people who sought the thrill of climbing and followed stitute the infrastructure of the canopy and provide the lure of discovering new species. Early European mechanical support for thousands of species of arboreal explorers hired climbers and trained monkeys to collect plants and animals. Tree species diversity is dis- specimens of "exotic" air-plants that grew out of reach. cussed elsewhere. Pioneering work in old-growth forests of the Pacific Although much has been published on canopy Northwest contributed to the application of mountain- plants, the question of the global importance of epi- climbing techniques for safe and reliable access to the phytes for the biodiversity of tropical forests in general canopies of tall trees. Since 1980, the innovation of and the canopy in particular has not been explicitly high-strength and low-cost canopy access equipment addressed. Previous reviews of canopy biodiversity have has made canopy study more viable as an option for primarily dealt with arthropods (Erwin, 1988), or with scientific research. There are now a wide variety of specific subgroups of canopy plants (Kress, 1986; access tools from which to choose, depending on the FOREST CANOPIES, PLANT DIVERSITY 29 questions being addressed and the available budget cling and in providing arboreal and terrestrial animals (Lowman and Nadkarni, 1995). With the development with food, water, and nesting materials (Nadkarni, of effective technological climbing methods such as the 1994). Ecophysiologists recognize the varied structures "canopy raft" and the canopy crane, and of ground- and mechanisms that protect vascular epiphytes from based methods such as insecticidal fogging, researchers drought (Benzing, 1990). now spend less time working on how to prudently work Some of the characteristics for regular occurrence in the treetops and more time pondering the difficulties on bark and associated aerial substrates are obvious in recording meaningful canopy data, analyzing it, and (e.g., holdfast roots and wind-dispersed propagules), interpreting the results. but others are more subtle. In an extensive review of A remarkable burgeoning of scientific interest in the vascular epiphytism, Benzing (1990) outlined a variety C canopy has occurred within the last decade. This is of characteristics that are exhibited by vascular epi- related to increasing concerns with such conservation phytes (Table I). issues as biodiversity, global atmospheric change, and Vascular epiphytes are mainly restricted to the low management of tropical rain forests. The number of latitudes and within the tropics. They reach their great- scientific publications on canopy structure has grown est abundance and diversity at low to mid-montane at a disproportionately rapid pace relative to the general elevations (Madison, 1977; Benzing, 1990). Ferns occur field of biology (Nadkami, 1994). Aspects of the canopy in higher latitudes along the margins of the Pacific, and have been the focus of many recent symposia, scientific a few hardy bromeliads and orchids occur in the mild books, and popular articles