Published in Selbyana 2001.

Mark W. Moffett, Museum of Vertebrate Zoology, University of California, 3101 Valley Life Sciences, Berkeley, CA 94720, U.S.A*

Current address: 155 Sterling Street, Greenport, NY 11944, U.S.A.

Abstract

The way ecologists studying forests choose to define canopy biology could lead them to overlook other communities of spatially-fixed organisms that may have properties usefully compared to or contrasted with forest canopies. This paper represents a series of reviews on the possible nature and limits of canopy biology and introduces the prospect for a general comparative science of biological canopies.

* Five rationales for the common practice of limiting discussions of canopy biology to terrestrial forests are considered: if people have unique interactions with or concerns about forest canopies; if a substantive basis exists for treating trees as a distinct category of plants; if a substantive basis exists for treating trees as a distinct category within forests; if a substantive basis for treating forests as a distinct category of terrestrial community; or if attributes of tree crown residents or ground-rooted plants in a forest prove distinct. In no case is there unequivocal evidence for the usefulness of separating the study of forest canopies from the study of the aerial parts of other terrestrial plant communities.

* Instead of restricting the sphere of canopy biology to plants in terrestrial systems, "canopy" can be defined in terms of any sessile organisms and the structural products derived from them. This opens the field to a range of communities that could share many properties with forest (or plant) canopies. A brief review is made of the canopy literature on kelp forests, algal turfs, periphyton, biofilms, and coral reefs. The word "canopy" has already been applied to each of these ecosystems, and biofilms in particular have great potential as model systems for studying assembly rules for canopy physical structure.

* In mainstream ecology the organisms of a community are typically studied in two dimensions, or as points on the earth. Canopy biology can in large part be distinguished as the science of treating plants (or other sessile hosts) as three dimensional, although the discipline encompasses all aspects of scientific study of the portion of a community that project into a medium.

* In many studies involving canopy species, the organisms' relationship to the canopy is treated as incidental. "Putting the canopy into canopy biology," that is, contributing fundamentally to canopy biology as an independent discipline, requires that aspects of biology specific to the canopy form an integral part of the research. Six approaches are proposed to putting the canopy into canopy biology: issues of community ecospace; properties emerging from a community in aggregate, such as stratification of microclimate; host distributions; host architectures; properties of a canopy’s structural elements; and characteristics of the open spaces within a community.

* Much of the language and thinking of terrestrial canopy biology has been predicated on notions of plants as supports for other organisms. Structural support between individuals occurs as well in the rhizosphere, reminding us that distinctions between aboveground and belowground plant parts can be arbitrary. For example, in many ways it would be logical to define words such as "epiphyte" so that they apply to a plant in its entirety. For many research concerns, this idea suggests the validity of transcending canopy biology to fashion a more comprehensive science of plant associates, referred to here as "structural ecology," congruent with the approach used in animal parasitology or marine epibiosis.

Keywords: algal turf, biofilm, biodiversity, biomechanics, canopy, community, coral reef, ecology, ecosystem, epiphyte, forest, kelp, periphyton, rhizosphere, stratification, structural support, tree.

Words compartmentalize information, and that has many repercussions (Lakoff 1987). This is true for how we identify subjects for academic study as well (Bates 1960, Hull 1988). The word "canopy" has been applied to vegetation in varied ways (Moffett 2000), leading to varied interpretations of the domain of canopy biology. To pick one example, if by "canopy" we mean the highest plant surfaces in a forest, as many authors do (e.g., Kritcher 1997; the "outer canopy"), it is likely that, out of habit, we as canopy biologists will develop a search image effective at picking out information on that subject. Our knowledge of parallel and potentially useful studies on shrubs and herbs, whether in forests or in other systems, or studies on the lower parts of forest trees, is likely to be marginalized. Philosopher W.T. Jones (in Bateson 1972) describes this as the "topography of ignorance." This intellectual fragmentation is borne out by the literature. For example, citations on stratification in herbaceous communities such as Monteith 1975, 1976 tend to be scarce in forest canopy publications. A consequence of this can be independent discoveries along parallel research tracks by different academics, such those working in agriculture, economic entomology, or landscape ecology, needlessly wasting time and effort.

How then to define the boundaries for a discipline? While any "conceptual framing" can be arbitrary and subjective (Bohr 1955), criteria most likely to be compelling are those of general import to practitioners across a field for which relatively abrupt or unambiguous shifts occur in content or perspective. This is the case for the shift from mind to community that distinguishes psychology from sociology, or the shift from molecules to structural integration that distinguishes biochemistry from cell biology. In this respect, it makes little sense to grant canopy biology an independent status as a discipline if by "canopy" we specifically mean "outer canopy," given that most aspects of the biology of that part of the forest grade in a continuous way with those found among the plant organs lying beneath the topmost foliage. This is not to deny, however, that some biologists will need to concentrate on the outer canopy because of its relatively pronounced features, such as the disproportionate significance of the upper leaves to forest productivity.

Is there any basis then for a preference on how we delineate canopy biology as a discipline? I will pursue the possibility that trees or forests are distinct from other terrestrial vegetation. Then I consider further modifications of the definition of "canopy" that would encourage canopy biologists to compare environments previously ignored by forest-working ecologists, such as coral reefs and bacterial films. Following this I present unifying themes for an expanded canopy biology. To conclude I address the ramifications of metaphors about structural support and physical intimacy that can underlie many of our views about canopy residents.


Continue reading this paper:

Abstract

Seeing the Forest for the Herbs

More to Pond Scum Than Meets the Eye

The Geometry of Canopy Biology

Getting to the Root of the Matter

Conclusions

© Mark W. Moffett, please e-mail naturalist@erols.com to obtain a complete reprint.