174 Kaoru Kitajima and Lourens Poorter
found between adult stature and juvenile light
requirements (e.g., Poorteret al.2003, 2006,
Sheilet al.2006), but the relationship is not tight,
Furthermore, functional traits of adult leaves can
be explained better by regeneration niches rather
than adult niches, suggesting long-lasting selec-
tive importance of seedling regeneration stage
(Poorter 2007).
In summary, high heterogeneity of light in
time and space creates many niche opportuni-
ties through various types of trade-off, including
trade-offs between fast growth versus high sur-
vival, current versus future light interception,
and early maturity versus large fecundity. Along
each type of trade-off, multiple suites of traits
are associated in a convergent manner. The rel-
ative position alon gthe growth–survival trade-off
is indicative of the species’ preferred light envi-
ronment at a given size class. This position is
generally concordant through ontogenetic stages
even though notable exceptions exist (Gilbertet al.
2006). Still, light is but one ecological factor that
impacts growth and survival. Do we see simi-
lar growth–survival trade-offs in relation to the
species’ position alon gniche axes defined by other
resources? Will there be more niche opportunities
when other ecological factors interact with light?
NICHE HYPERSPACE
The total volume of niche hyperspace that allows
coexistence of similar life-forms may be expanded
in a multiplicative manner if niche axes are
orthogonal to each other (i.e., varying indepen-
dently). If two or more resources limit plant
performance, then the total number of perfor-
mance rank reversals may be greater than the
number of reversals that occur alon ga sin gle
resource gradient (Latham 1992, Burslemet al.
1996, Walters and Reich 1996). This requires
orthogonality of not only resource gradients, but
also functional traits; adaptations in relation to
one resource gradient may be independent of
adaptations in relation to another resource gradi-
ent. Orthogonality of niche axes may be suggested
in the multivariate space defined by functional
traits of potentially competin gspecies. Princi-
pal components analysis and other multivariate
statistics reduce the dimensionality of multiple
trait spaces, often to just two dimensions defined
by the first and second principal components that
are orthogonal to each other. The traits asso-
ciated with growth–survival trade-offs, such as
SLA, LAR, leaf lifespan, photosynthetic rates per
unit leaf mass, and nitrogen per unit mass, form
the first principal component axis, while photo-
synthetic water use efficiency and nitrogen per
unit area form the second principal component
axis (Poorter and Bongers 2006).Thus, functional
traits correlated with light gradients form the
first principal component axis, while traits asso-
ciated with use of soil resources form the second
principal component axis. Orthogonality of adap-
tations to two different resource axes can also be
shown experimentally; shade and drought toler-
ance were uncorrelated, and therefore orthogonal
to each other, amon gtemperate shrubs (Sacket al.
2003).
Yet, availabilities of light, nutrients, and water
may not vary independently of each other in
the field. Nutrient and water availabilities are
strongly associated with each other in relation to
topography, such that moist sites tend be more fer-
tile (Svenninget al.2004). Likewise, the rainfall
gradient strongly influences nutrient regimes in
tropical forests (Schuur and Matson 2001, Santi-
agoet al.2004), such that very wet forests tend
to be infertile due to greater degrees of leach-
ing. Thus, niche differentiation due to moisture
and nutrient availabilities may be difficult to dis-
tinguish, as they change together in relation to
topography, soil texture, and rainfall. Light gradi-
ent is also not totally independent of soil resource
availability,aswetterforeststendtosupportdenser
vegetation leading to darker understories. Certain
combinations, such as “fertile and very open for-
est” or “infertile and very dark forest,” are very
rare.
Distribution of species in relation to topogra-
phy may not reflect adaptations to contrasting
nutrient or water availability, but rather adapta-
tions to other ecological factors that change with
topography. The difference in distribution of two
Moraspecies in relation to topography in Guyana
could be explained not by seedlin gdrou ght tol-
erance, but by differences in flood tolerance of
seeds (ter Steege 1994). In dry forest and savanna