Tropical Forest Community Ecology

Treefall Gaps and Plant Species Diversity in Forests 199

Palms win

Trees

win

Lianas win

Palm-dominated

low-canopy

gap

Palms and

trees escape

low-canopy

status

Gap remains

palm-

dominated

low-canopy

Gap remains

liana-

dominated

low-canopy

Liana-dominated

low-canopy

gap

Pioneers

escape

liana

domination Non-pioneers

escape liana

Tree-, palm-, orliana-dominated domination

high-canopy

Open gap

Figure 12.1 Model of possible pathways of gap-phase regeneration. Following a treefall, gaps are assumed to go

from a low-canopy, open gap through a series of progressive stages, culminating at a high-canopy, intact forest

condition. However, alternative pathways of gap-phase regeneration also may occur. Modified from Schnitzeret al.

(2000).

HYPOTHESES AND MECHANISMS

BY WHICH TREEFALL GAPS ARE

PROPOSED TO MAINTAIN

DIVERSITY

Theories that invoke disturbance as a mechanism
to maintain diversity often assume that commu-
nities never reach an equilibrium state because
disturbance prevents competitively dominant
species from excluding competitively inferior, early
successional species (e.g., Connell 1978). The gap
hypothesis may be considered a non-equilibrium
mechanism for the maintenance of diversity
because the death of a canopy tree and sub-
sequent formation of a treefall ga pinitiate a
successional sequence that begins with pioneer
species and eventually transitions to dominance
by shade-tolerant species. Thus, treefall gaps pro-
vide a regeneration niche for shade-intolerant

pioneer species or intermediate shade-tolerant

species to establish and regenerate, preventing

their competitive exclusion from the community

(e.g., Swaine and Whitmore 1988, Whitmore

1989,Dallingetal.2001).Thisisthesimplestform

of the gap hypothesis, which, at the landscape

scale, permits the coexistence of both early suc-

cessional species occupying gaps and late succes-

sional species occupying the surrounding matrix

of intact forest (Whitmore 1978, Connell 1979).

The gap hypothesis also provides an equi-

librium and stabilizing (sensuChesson 2000)

niche-based explanation for the maintenance of

diversity. Resources (e.g., light, soil moisture,

and soil nutrients), which vary strongly from

the edge to the interior of gaps (Ricklefs 1977,

Denslow1980,1987,ChazdonandFetcher1984,

Beckeret al. 1988), may be partitioned by species

with different regeneration requirements. If each