Tropical Forest Community Ecology

162 Kaoru Kitajima and Lourens Poorter

Adult^100

10

1

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Cumulative LAI

Anacardium excelsum

Cecropia longipes

678

(a) (b)

Sapling

Seedling

Seed

Low High

Light

Intermediate

Ontogenetic stage % light transmission

Figure 10.1 (a) Possible ontogenetic trajectories of light niches for tree species inferred by a crown-exposure index
(modified from Poorteret al.2005). Within a given horizontal plane, light availability is roughly classified into low,
intermediate, and high based on crown exposure. The preferred light environment of a species may remain constant
from seed to adult stage (vertical arrows) or may shift from one stage to the next (diagonal arrows). However, crown
position alone cannot infer the average leaf light environment, which is also a function of leaf display patterns.
(b) Species differences in % light transmission (on log scale) as a function of cumulative leaf area index (LAI) through
individual crowns of five canopy tree species in a seasonal dry forest in Panama (modified from Kitajimaet al.2005).
A pioneer,Cecropia longipes(open triangle at upper left) exhibits steep light extinction within a shallow crown of total
LAI less than 1. In contrast, long-lived dominantAnacardium excelsum(closed circle) creates less steep light extinction
through its crown, but its high total LAI casts much deeper shade underneath. Three other species exhibit
intermediate characteristics in relation to architecture and successional status.

FUNCTIONAL MECHANISM FOR

LIGHT COMPETITION

At the heart of niche theory is competitive asym-
metry. An established plant cannot be competi-
tively displaced by another plant, unless the latter
is sufficiently superior in competition for the most
limitin gresource in that particular location. Does
the functional basis for resource competitiveness
lie in the rate of biomass accumulation per unit of
the limitin gresource consumed, or in the thor-
oughness with which the limiting resource is
consumed? The latter is supported by the theoret-
ical and empirical studies of nitrogen competition
(Tilman 1982, Wedin and Tilman 1993). Growth
rates of individuals and populations decline as
competin gindividuals use up the resource in the
shortest supply relative to demand. The resource
level at which the net growth rate is zero is
known asR∗, and theory predicts that the lower

theR∗, the greater the competitiveness for that

resource (Tilman 1982, Chase and Leibold 2003).

R∗varies amon gspecies and is quantified as the

minimum level to which a monospecific stand of

this species eventually drives down resource avail-

ability (Wedin and Tilman 1993). However, does

this idea apply to the competitive interaction of

trees for light in a humid tropical forest?

Observations of light utilization by adult tree

crowns indeed suggest differences inR∗for light

between early and late successional tree species.

The light intensity received by the horizontal plane

above the forest canopy is conventionally called

“full sun,” that is, the reference relative to which

light intensity on an inclined surface or light

transmitted through the canopy is expressed. Both

in temperate (Canhamet al.1994) and tropical

forests (Kabakoff and Chazdon 1996, Kitajima

et al.2005), the percentage of light transmitted

below adult crowns of late successional trees is