a net rate of energy capture that cannot be bettered by competitors with another ratio,
must depend on subcanopy coverage and the resulting probability of a tree being adjacent
to, and shaded by, a competitor. At very low subcanopy coverage, very broad
canopies that maximise energy gain in the absence of competitors also maximise energy
gain in the presence of competitors. At very high subcanopy coverage, narrow canopies
with a diameter/height ratio in accord with King's (1981) model are the only ones not
surely overtopped, and again maximise energy gain in the presence of competitors.
Canopies that expand at an intermediate diameter / height ratio should maximise energy
gain realised in subcanopies of intermediate coverage, reflecting a balance between the
energetic costs of outgrowing all possible neighbours, the chance of encountering a
neighbour, and the energetic benefits of a broad canopy. To the extent that stratal
coverage is lower in shade than in sun, and lower in the subcanopy than the forest
canopy, the diameter/height ratio of individual trees should be greater in subcanopy
trees and shade-adapted canopy species.
Data to test these ideas were culled from the register of 'champion trees' for North
America (Hunt 1986). Unusually large individuals of each species qualify as 'champions'
based on a combination of their girth, height and canopy spread. Undoubtedly,
most grow on sites that are, or have been, more open than is typical for theit: species;
in that sense, the use of such individuals may reduce somewhat the effects of systematic
differences between shade-tolerant and shade-intolerant species in the environmental
conditions experienced during ontogeny. Canopy diameter:height ratios were calculated
for all tree species occurring in eastern North America that could unequivocally
be assigned to one of three classes: obligate subcanopy species, shade-tolerant canopy
species, and shade-intolerant canopy species (Table 3). The expectation was that obligate
subcanopy species, experiencing shady conditions throughout their lives, would
have the greatest diameter/height ratios; that shade-tolerant species that achieve the
canopy would have somewhat lower ratios; and that shade-intolerant canopy species
would have the lowest ratios. This is exactly what is seen (Table 3): the mean diameter /
height ratio for subcanopy species is 1 -09 f 0.47 (n = 8); for shade-tolerant canopy
Table 3. Ratio of canopy diameter to height in selected 'champion' trees of eastern North America (see