Published in Selbyana 2001.

Mark W. Moffett, Museum of Vertebrate Zoology, University of California, 3101 Valley Life Sciences, Berkeley, CA 94720, U.S.A., moffett@uclink4.berkeley.edu

Seeing the Forest for the Herbs

Whereas often the word "canopy" is applied to the upper parts of forest ecosystems, I previously argued for approaching canopy biology broadly, so as to incorporate the literature on all aerial parts of any terrestrial plant community. We should adhere to this approach except when dealing with "concepts or situations necessarily restricted to trees" (Moffett 2000). In fact, no one to date has specified any rationale for the common practice of restricting the scope of canopy discussions to trees or to forests (as in the useful distinction made between "tree canopies" and "forest canopies" by Shaw 1996). Consider five criteria by which forest canopies could merit this kind of separate attention:

1) Humans have unique interactions with or concerns about forest canopies. There are grave concerns about extinction of canopy species, and there is great interest in the value of canopy products to societies past, present, and future. Because conserving the top of a forest is not possible without conserving its bottom, however, conservation issues are sensibly considered not specifically as canopy biology, but under the general rubric of ecology, which encompasses aspects of economics, ethnography, and conservation.

2) There is a substantive basis for treating trees as a distinct category. The diverse suite of characteristics associated with trees serves "as an example of the molding of the entire phenotype by selection pressures" that has come about convergently in numerous lineages (Niklas 1997). The ecological impetus discussed most often in reference to plant height is competition, especially for light (Tilman 1988, Givnish 1995, Leigh 1999), but the basic question of "treeness" per se may be primarily biomechanical (Niklas & Kerchner 1984). Any self-supporting terrestrial plant growing beyond a certain height appears to be channeled into evolving a main vertical trunk built of the stiffest available structural elements (especially around its perimeter), surmounted by a branched crown. In short, it becomes a tree. The transformation seems to occur in a similar way under diverse environmental conditions, and is a result of shifts from small herbaceous structural designs to one that allows large upright plants to cope with bending or torsion (Niklas 2000). If there is a critical point in this transformation at which many of these changes occur synchronously during the evolution of increasing mass or stature, e.g., 3-5 m in height (Givnish 1983), then the tree bauplan could be sufficiently distinct to treat the study of the canopies in tree-dominated ecosystems (forests) as an independent research discipline. Although this issue appears central to our very conception of "tree," it remains unresolved (Givnish 1984). Consider that trees allocate a large portion of their photosynthate to supportive tissues, and they pay a high price in aerodynamic drag, friction during fluid transport, and increased potential for structural failure if they have high crowns (see Vogel 1996). Given the character of woodiness and apparently also treeness has been labile in plant evolution (Judd et al. 1994, Dodd et al. 1999), because of the costs we would expect that plants would readily lose the tree bauplan where doing so would increase fitness. This would be true even if this growth form was once adaptive, for example by being the heritage of a forest-dwelling past. Nevertheless, trees in deserts, savannas, and other open ecosystems grow extremely tall even though they occur widely separated from neighbors and so by ecological criteria seem conspicuously overbuilt. Trees in these situations may be large because they store water in their trunks (Holbrook 1995); depend on height to avoid herbivory, as arborescent cacti do from tortoises (Dawson 1966) and acacias do from giraffes (Brooks & Owen-Smith 1994); are maximizing reproductive dispersal (Richards 1986); are avoiding damage from fast-moving ground fires (Givnish 1995); or are shading out grasses that compete with them for water (Walter 1973). Yet such factors appear neither pervasive nor severe enough to explain the almost ubiquitous occurrence of dispersed trees that are radically taller than other plants in their communities. For example, giraffes always forage below 5 m, whereas savanna acacia trees often exceed 20 m in height (T. Young, pers. comm.). Plant evolutionary mechanics could hold the key to this apparent mystery, and further investigations in this area might thereby shed light on the nature of "treeness" itself.

3) There is a substantive basis for treating trees as a distinct category within forests. Although adult trees are usually assigned to separate strata from other plants in a forest, the question of whether they are distinct as a group or are part of a continuum with other, smaller forest plants has not been clearly resolved, in part because of inconsistencies among the research approaches to stratification (Parker & Brown 2000). In a frequency distribution of the size of mature vascular plant individuals in a forest, is there a distinct peak corresponding with trees? Size-frequency distributions are common in studies of animal diversity but apparently are absent for plant communities, presumably because modular construction and indeterminate growth can make plant size difficult to assess.

4) There is a substantive basis for treating forests as a distinct category. Forests could be considered a distinct category if by some parameter of community physiognomy they can be separated out nonarbitrarily from other terrestrial ecosystems. For example, I have the impression (perhaps it is merely the observational bias of a human-size species) that when scarce height extremes are excluded (recent treefall gaps in mature forests or trees in savannas), most communities are either much shorter or much taller than human height. Suppose we graph some measure of overall community height, say, the modal height reached by the vegetation averaged over randomly chosen points on the ground for each major community type in a classification of ecosystems. Suppose the distribution indeed turns out to be bimodal, such that forest systems represent a distinct peak. This would suggest that forests are more than an arbitrary construct that humans have split off from a continuum of natural communities. Perhaps forest canopies can be distinguished as a separate research discipline on that basis. But in fact no information seems to exist on patterns of overall height across communities. A practical difficulty to such an effort would be if available classifications are biased with respect to height (for example, if ecosystem taxonomists have been "splitters" with respect to forest communities), or if community categories are largely artificial, at best representing opportunistic associations of species (Brown 1995) .

5) Attributes of tree crown residents or of ground-rooted plants in a forest prove distinct. If future studies of scaling effects on canopy residents demonstrate that trees harbor communities distinct in some fundamental and reasonably abrupt way from those dwelling in progressively shorter kinds of vegetation, that might be taken as evidence for distinguishing forest canopies as a distinct kind of biological entity. This seems unlikely, however, given that most resident canopy organisms respond not to height but to environmental factors that happen to correlate with height (Moffett 2000). Thus epiphytes seemingly associated with high forest canopy situations occur closer to the ground where conditions allow (McCune 1993, Benzing 2000). Biodiversity in forest canopies can be extreme, but given that most inventories of biodiversity to date have been made in tree crowns (e.g., Stork et al. 1997), the relation between species diversity and community scaling is likewise open to question. For any given latitude, how much of the high diversity of forest-canopy-dwelling species can be attributed to these canopies offering a relatively large overall mass, surface area, productivity, or microhabitat richness? There is also little basis to date for asserting that the organizational principles manifested by ground-rooted plants in forests (such as in the way the trees distribute horizontally or vertically) could be distinct from those operating in other communities, beyond matters of scaling that might be expected to vary in a reasonably continuous manner with successively shorter vegetative types (Moffett 2000).

In summary, there seems to be no unequivocal basis for the common practice of treating forest canopies independently from the study of the canopies of other terrestrial plant communities, although further investigations of the nature of trees and forests may prove me wrong.



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.