Resource Niche and Trade-offs 173Crown exposure1.01.52.02.53.03.54.04.5Juvenile AdultCrown exposure10 m1234Crown exposure20 m, 40 m1234r = 0.68***r = −0.16 NS(a)(b)Height (m)0 1020304050Crown exposure1.01.52.02.53.03.54.04.5(c)Figure 10.5 Ontogenetic trajectories of
crown-exposure index for 53 Liberian rainforest trees.
(a) Height-dependent change in crown exposure.
(b) Correlation between species’ crown exposure at 10
m height and at 20 m (closed symbols) and 40 m (open
symbols). (c) Changes in crown exposure between the
juvenile and adult phases. Each species is presented by a
line (a,c) or a symbol (b). From Poorteret al.(2005).
However, a species’ distribution in relation to
light environment by itself is only a snapshot that
merely suggests the light environment in which
its growth and survival are optimal. For exam-
ple, saplings may be shrinking in size, even when
they persist in shade by relyin gon carbohydrate
reserves accumulated when light availability is
temporarily high. Thus, for more rigorous eval-
uation, growth and survival patterns need to be
examined in relation to light, as well as relative to
patterns exhibited by other species. Indeed,Alseis
blackianadoes not shift relative position alon gthe
interspecific growth–survival trade-off relation-
ship between seedlin gand saplin gsta ges (Gilbert
et al.2006). Such a shift was observed in only
one of the 30 tree species for which growth–
survival trade-offs were analyzed for seedlings and
saplings;Inga marginatais a shade-tolerant species
with slow growth as seedlings, but a fast-growing
species with higher mortality as saplings (Gilbert
et al.2006).
Species differ in crown exposure when com-
pared at a similar size (Figure 10.5a). But to
what extent is this species rankin gin crown expo-
sure maintained when they increase in height?
The crown exposure is positively correlated when
trees of 10 and 20 m height are compared
(Figure 10.5b); species that have a high crown
exposure when small also have a high crown expo-
sure when tall. However, this consistency in rank-
in gdisappears when species approach the canopy
and attain similar full light levels. Furthermore,
rank reversals in crown exposure are common
between juveniles and adults (Figure 10.5c). Tall
species have higher population-level crown expo-
sures, higher adult crown exposures, and make
larger switches in their crown exposure from
juvenile to adults compared with small species.
Adult stature is therefore an important life-history
trait for which many species differentiate (Thomas
and Bazzaz 1999, Turner 2001). Tall species
should be very plastic in their traits, given the
large switch they make from low light as seedling
to high light as a canopy tree (e.g., Rijkerset al.
2000), but if there is a limit to the acclima-
tion potential of species, then it follows that tall
canopy species should be more light demanding
than shade-tolerant species (Thomas and Bazzaz
1999). Indeed, a positive correlation has been