Chapter 10

FUNCTIONALBASISFOR

RESOURCENICHEPARTITIONING

BYTROPICALTREES

Kaoru Kitajima and Lourens Poorter

OVERVIEW

The resource niche, that is, specialization alon gresource availability gradients, is one of the frequently hypothesized
mechanisms for coexistence of tropical tree species. Here, we evaluate physiological mechanisms that may lead to
partitionin gof resource gradients by tropical trees, with particular attention to li ght as the key limitin gresource.
The functional basis for light competitiveness is the extent to which individual tree crowns reduce light. Pioneer trees
cannot invade the shaded space occupied by shade-tolerant tree crowns that maintain multiple layers of terminal
shoots and leaves. Within the lowest stratum of a forest, light niche specialization by seedlings is better explained by
the growth–survival trade-off, rather than by performance rank reversal between low and high light environments.
In other words, seedlings of early successional species that specialize for treefall gaps tend to grow faster in both high
and low light but suffer higher mortality than late successional species that regenerate in the shaded understory. As
individuals grow beyond the seedling stage, ontogenetic shifts in light niche may contribute significantly to tree species
coexistence in forests. Analysis of crown exposure index in relation to individual height demonstrates that nearly all
species experience higher light levels when they grow towards the canopy, with frequent ontogenetic shifts of light
niches. Adult size is also an important species-specific trait that influences fundamental light niches of trees. Compared
with tall species, small species that have to invest less into support have sufficient carbon surplus to reproduce in shade
of canopy dominants. Future studies need to address whether similar mechanisms underlie niche specialization
alon gother resource gradients, how interaction of multiple resource axes and onto genetic shifts may expand niche
hyperspace, and how biotic factors modify the realized niche space alon gresource gradients.

NICHE – DEFINITIONS AND

IMPLICATIONS

The ecological niche can be defined as a set of
environmental factors that a species requires in
order to persist in a community, as an integrative
result of the collective impacts of environmental
factors on the focal species as well as the focal
species’ impacts on its environment (see Chase
and Leibold 2003 for variations from this defini-
tion). The niche of a species may be viewed as its
position in the ecological multivariate space, often
referred to as niche hyperspace (or hypervolume)

defined by the relevant multiple environmental

factors. In particular, specialization to a limited

range of resource availability has been consid-

ered central to the niche theory that attempts

to explain species coexistence (e.g., MacArthur

and Levins 1967, Ricklefs 1977, Denslow 1980,

Chesson 2000, Chase 2005). For plants, resources

that potentially constrain population growth rates

include light, water, and soil mineral nutrients.

The competitive exclusion principle, historically

attributed to Gause (1936), states that only one

species can persist as the competitive winner when

two species consume a single resource. Two or