Chapter 12

TREEFALLGAPSANDTHE

MAINTENANCEOFPLANTSPECIES

DIVERSITYINTROPICALFORESTS

Stefan A. Schnitzer, Joseph Mascaro, and Walter P. Carson

OVERVIEW

Treefall gaps, one of the key forms of disturbance in tropical forests, are hypothesized to maintain species diversity via
three main and non-mutually exclusive ways. First, they create high light habitats, providing a regeneration niche for
early successional shade-intolerant and intermediate-tolerant species to reach reproductive maturity, and thus prevent
their competitive exclusion by more shade-tolerant species. Second, species may specialize on and partition resources
along resource gradients that vary strongly from the gap center to forest interior, thus permitting species coexistence.
Third, species may specialize along a gradient of gap sizes, with some species regenerating in small gaps and others in
large gaps, which would also permit stable species coexistence. Support for the gap hypothesis is mixed, but evidence
suggests that some plant groups may benefit from gaps more than others.
Pioneer tree species and at least some species of lianas appear to require or capitalize on gaps for successful
regeneration or to reach reproductive maturity. This may also be true for shrubs and herbaceous species, but these
growth forms are rarely considered in studies of gap dynamics. Gaps provide not only an essential regeneration niche
for some growth forms, but they also may provide the resources necessary for reproduction; this latter aspect of gap
dynamics has been largely ignored. In contrast, shade-tolerant tree diversity does not appear to be maintained by gaps,
possibly due to a combination of seed, dispersal, and recruitment limitation, the last possibly due to competition with
other plants, particularly lianas. Nevertheless, treefall gaps maintain the diversity of some plant groups, which, in
many tropical forests, may comprise a large proportion of the vascular plant community.

INTRODUCTION

The formation of treefall gaps and their influence
on forest regeneration and dynamics has a long
history in ecology. Whitmore (1989) suggested
that “gaps, openings in the forest canopy, drive the
forest cycle,” and that “the ga p phase is thus
the most important part of the growth cycle
for the determination of floristic composition.”

The genesis of the ga phy pothesis began with the

studies of Watt (1925), Aubréville (1938), and

Jones (1945), who described the patchy nature

of mature forest communities (cited in Swaine

and Hall 1988, Peet 1991). Watt (1947) extended

these ideas by describing “the ga p phase” as a gen-

eral phenomenon whereby succession occurred

withinsmallpatchesof relativelystableplantcom-

munities, regardless of the specific ecosystem. In

the 1950s, treefall gaps became recognized as