Resource Niche and Trade-offs 171small seeded, and they deploy their leaf area
rapidly to become autotrophic (Kitajima 2002).
Interspecific differences in LAR are especially
marked at the seedlin gsta ge, but tend to decrease
over time when plants increase in size (Poorter
and Rose 2005). Larger plants with larger leaves
require greater support, causing ontogenetic
declines in SLA, leaf mass ratio (=ratio of leaf
mass to the total biomass), and LAR (Boot 1996,
Veneklaas and Poorter 1998, Delagrangeet al.
2004). These changes, in turn, cause a reduc-
tion in RGR. The size-dependent decline in LAR
has important consequences for the whole-plant
light compensation point. As the ratio of photo-
synthesizin gto respiratory tissue decreases, the
plant needs to encounter brighter light condi-
tions to support the greater respiratory mass
(Givnish 1988). All species exhibit size-dependent
shifts in allocation patterns that should result in
slower growth rates under exactly the same light
availability when they are larger. Yet, juveniles
of all six species examined by Clark and Clark
(1992) exhibit increasin gsurvival and growth
rates as taller individuals, partly because they
receive more light at higher strata of the forest.
The critical question is whether such ontogenetic
declines in LAR are predictably faster for pioneers
than for shade-tolerant species, so as to result in
ontogenetic shifts of species ranking.
Lusk (2004) analyzed ontogenetic changes in
LAR for four temperate rainforest species grow-
in gin the understory. Pioneers started out with
a higher LAR, because of a high SLA. Their
LAR declined rapidly with height due to their
fast leaf turnover, whereas shade-tolerant species
maintained their LAR because of their long
leaf retention times. Consequently, shade-tolerant
species have a consistently higher LAR than light-
demandin gspecies at the saplin gsta ge (Lusk
2002). Interestingly, the higher carbon gain rate
per mass expected from the higher LAR did not
lead to differences in growth rates, suggesting
possible interspecific differences in allocation to
storage and defense. Pioneer species continue to
produce new leaves and to extend shoots, in an
attempt to receive more light available at greater
height. Such a strategy may pay off in dense
gap vegetation that creates a steep increase in
light with height (Denslow 1995). It may fail,
however, in the understory, where the vertical
light gradient is less steep (Montgomery 2004).
Pioneer species may be unable to sustain such
a rapid leaf turnover in the light-limited under-
story and literally may grow themselves to death
when their LAR falls below a critical threshold
level.
Differences in size at maturity may also con-
tribute to partitionin gof the vertical hei ght gra-
dient and species coexistence (Richards 1952,
Terborgh 1985). Compared with the gap–shade
paradigm, the small–large paradigm has received
considerably less attention, despite its importance
for many aspects of the life cycle of a tree
(Kohyama 1993, Westoby 1998, Thomas and
Bazzaz 1999, Turner 2001, Falster and Westoby
2005, Kinget al.2006). Yet, niche specializa-
tion of adults alon gthe vertical hei ght gradient
may be evolutionarily more important than light
niche preference of juveniles. Phylogenetic con-
straints may make related species to occupy simi-
lar height niches. Different families prefer different
canopy positions (e.g., species in Annonaceae and
Rubiaceae tend to be small understory specialists,
and those in Dipterocarpaceae and Fabaceae tend
to mature as canopy dominants). At the same
time, niche diversification may be important for
related taxa with similar ecological requirements
to avoid competition. Indeed, species-rich genera
in Malaysia can show remarkable size variation
amon gsympatric species (Thomas 1996a).
A trade-off between maximization of current
versus future light interception is one of the func-
tional mechanisms leadin gto vertical li ght niche
partitionin gby adult trees. Species that mature
in the forest understory often differ from canopy
species in their architecture, light utilization strat-
egy, and shade tolerance. Understory species max-
imize current light interception by making wide
crowns, whereas canopy species maximize future
light interception by making narrow crowns,
which allows them to grow quickly to the canopy
(Kin g1990, Kohyama and Hotta 1990). Under-
story species have relatively thick stems to support
the wide crowns and resist dynamic loadin gdue to
fallin gdebris (Kin g1986, van Gelderet al.2006),
whereas canopy species have slender stems, to
rapidly attain the canopy at low costs for construc-
tion and support (Kohyamaet al.2003, Poorter