172 Kaoru Kitajima and Lourens Poorter
et al. 2003). Indeed, species with slender stems
and narrow crowns show a faster height-related
increase in crown exposure (Poorteret al.2005).
For these species the extension function of archi-
tecture is more important than the light inter-
ception function; they gamble upon reaping the
benefits of a better and brighter future in the
canopy. Accordingly, Kohyama (1987) referred to
these species as “optimists,” whereas the under-
story species were referred to as “pessimists.”
A theoretical model by Kohyama (1993) pre-
dicts that small species are able to coexist with
tall species only if the former have a higher
recruitment rate. Such a relationship was indeed
observed for 27 tree species that co-occurred in a
Bornean dipterocarp forest (Kohyamaet al.2003).
Saplin grecruitment rate per adult basal area was
negatively correlated with adult height (cf. King
et al.2006). Similarly, in 45 Costa Rican wet forest
species, the per capita recruitment rate was neg-
atively correlated with tree lifespan (Lieberman
et al.1985), which is closely associated with the
maximal size of the species.
The second mechanism underlyin gthe trade-
off between small versus tall adult size is the cost
of reproduction.Treeheightincreasessteeplywith
diameter at breast height (dbh), and levels off
when species start to reproduce (Thomas 1996a).
Small species start to reproduce at a smaller dbh
than large species (Liebermanet al.1985,Thomas
1996b, van Ulft 2004, Wrightet al.2005). Carbon
allocation to reproduction cannot be invested in
height growth, and small species are thus left
behind in the race for the canopy (Turner 2001).
Large species often delay their reproduction until
they are in the canopy, and can expand their
tree crown. Greater photosynthetic productivity
of large and well-exposed crowns enables these
species to produce large seeds (Hammond and
Brown 1995, Metcalfe and Grubb 1995) and/or a
large seed crop (van Rheenen 2005). Annual seed
production is therefore positively correlated with
the adult stature of the species (Davies and Ashton
1999). It might well be that the high seed produc-
tion balances the delayed reproduction, leadin gto
a similar lifetime seed production for small and
large species (Moleset al.2004). Yet, good com-
parative data to support this hypothesis are still
lackin gfor tropical rainforest trees.
The two mechanisms described above, that
is, the trade-off between current versus future
light interception, and the trade-off between early
versus late reproduction, can explain vertical
niche segregation of short versus tall species.
While light availability is positively correlated
with height in general (Yoda 1974), the exact
future light environment is unpredictable for a
given seedling because of unpredictability asso-
ciated with overstory canopy characteristics and
dynamics.Thus, temporal unpredictability of light
availability may equalize fitness associated with a
variety of ontogenetic trajectories of light prefer-
ence and contributes to species coexistence. How
do tree species vary in ontogenetic trajectories
for light environment within each adult stature
class (e.g., among canopy tree species), as well
as between adult stature classes (e.g., between
subcanopy versus canopy species)?
There are many different ways for trees to
grow and mature (Figure 10.1a). Poorteret al.
(2005) evaluated the height–light trajectories of
53 co-occurrin gLiberian wet forest tree species,
usin ga crown-exposure index (Dawkins and Field
1978). Nine different height–light trajectories
were distinguished based on the light environ-
ments of juveniles and adults, compared with
the average vertical light profile in the forest
canopy. The majority of the species simply fol-
lowed the vertical light profile in the forest canopy
(Figure 10.5a). Only one species occurred con-
sistently at higher light levels than the aver-
age light profile (whole-life light demander), and
one species occurred at consistently lower light
levels (whole-life shade tolerant). One species
(Syzygium gardneri) experienced decreasin gli ght
when growing in height. This species germinates
inthehighlightenvironmentof gaps,butbecomes
quickly overshaded by faster-growing neighbors.
It therefore switches from a light demander as a
seedlin gto a shade tolerant as a saplin g. A similar
strategy has been observed forAlseis blackianain
Panama (Dallinget al.2001). Species with such
behavior are also known as “gamblers” (Oldeman
and van Dijk 1991) or “cryptic pioneers”
(Hawthorne 1995). Some species exhibit more
complicated trajectories, appearin gto be relatively
shade tolerant in the middle stage (Clark and
Clark 1992).