Treefall Gaps and Plant Species Diversity in Forests 201(and thus microclimate), Brown (1993) argued
that gap-size partitioning is likely to be uncom-
mon because the relationshi pbetween ga psize
and microclimate is unpredictable due to large
spatial and temporal variation in microclimate.
For example, in a Malaysian tropical forest, Brown
and Whitmore (1992) and Whitmore and Brown
(1996) found no evidence to support the hypoth-
esis that pioneer tree species specialized on gaps
of different sizes. Nevertheless, there is no dis-
agreement about the general necessity of gaps
to maintain light-demanding pioneer trees in
tropical forests.
Is shade-tolerant tree diversity maintained by
treefall gaps via within- or among-gap resource
partitioning? Plants require light at a level that
compensates for a minimum level of metabolism
(i.e., light compensation point), and empirical
studies demonstrate that many tree species vary
in their growth rate or have a trade-off between
growth and survival that is dependent on their
light compensation point (Swaine and Hall 1988,
Bariket al. 1992, Kitajima 1994, Raoet al.
1997, Kobe 1999, Poorter 1999, Kitajima and
Poorter Chapter 10, this volume). While pioneer
and shade-tolerant species differ substantially in
their light compensation requirements, lesser dif-
ferences among shade-tolerant species may also
be sufficient to allow their coexistence along
light gradients, which are known to exist within
and among gaps, as well as in the intact forest
(Chazdon and Fetcher 1984, Beckeret al. 1988,
Liebermanet al. 1989, 1995, Montgomery and
Chazdon 2002). Thus, diversity of shade-tolerant
species could possibly be maintained by resource
partitioning; however,in situdemonstrations of
niche partitioning leading to species coexistence
are necessary to substantiate this claim.
Currently, there is only limited evidence that
shade-tolerant tree species partition resources
within or among gaps (Brandaniet al. 1988, Clark
and Clark 1992, Clarket al.1993, Zanne and
Chapman 2005). For example, Brandaniet al.
(1988) reported that several shade-tolerant tree
species were non-randomly associated with dif-
ferent areas within gaps (e.g., the root, bole,
and crown zones; but see Brown and Whitmore
1992, Brown 1996, Whitmore and Brown 1996).
Collectively, however, there is little conclusive
evidence that gaps maintain more than a few of
the thousands of shade-tolerant tree species in
tropical forests via within- or among-gap resource
partitioning (see reviews by Brown and Jennings
1998, Brokaw and Busing 2000). Consequently,
the ga phy pothesis as a mechanism to maintain
tree species diversity has received considerable
criticism because it apparently fails to explain the
maintenance of diversity for a predominant group
of species: the shade-tolerant tree species (Welden
et al.1991, Whitmore and Brown 1996, Brown
and Jennings 1998, Hubbellet al.1999, Brokaw
and Busing 2000; but see Chazdonet al. 1999,
Kobe 1999).
For example, Hubbellet al.(1999) conducted
a census of saplings in over 1200 canopy gap
and non-ga psites in a permanent 50 ha old-
growth forest plot on Barro Colorado Island (BCI)
in central Panama. After correcting for density
between ga pand non-ga psites, they concluded
that gaps played a “relatively neutral role in main-
taining [tree] species richness.” Similarly, Brown
and Jennings (1998) questioned whether a gradi-
ent in light availability was a viable axis for niche
differentiation for the majority of tropical trees.
These authors further argued that the “excessive
emphasis” on treefall gaps has deflected atten-
tionawayfrom other important processes that
more likely determine community composition.
Consequently, there remain sharply contrasting
views regarding the viability of the ga phy pothesis
as an explanation for the maintenance of species
diversity in tropical forests.Have we missed the forest for
the trees?Most studies of gaps have failed to consider impor-
tant plant groups other than trees (Brokaw and
Busing 2000, Schnitzer and Carson 2000), and
have failed to consider the impact of gaps on
plant reproduction. For example, although lianas
are rarely considered in ga pstudies, they are an
important component of many tropical forests in
terms of high stem density and leaf area, are
highly diverse, and directly and uniquely impact
gap-phase regeneration (Gentry 1991, Schnitzer
and Bongers 2002). In many tropical forests,