198 Stefan A. Schnitzeretal.(Putz 1984a, Vandermeeret al. 1996, 2000, Bond
and Midgly 2001, Schnitzeret al. 2004).
4 From spreading laterall yinto the gap from the sur-
rounding intact forest.Some growth forms, typically
lianasandsomeherbaceousvines,canrecruitinto
and fill treefall gaps by growing laterally from the
adjacent intact forest (Peñulosa 1984, Schnitzer
et al. 2000).
The first two regeneration pathways and occa-
sionally the third are common to trees, whereas
lianas may exploit all four pathways due to their
ability to disperse their seeds throughout the for-
est (Wrightet al. 2007), persist in the shaded
understory (Gilbertet al. 2006), rapidly produce
clonal shoots from their fallen stems (Putz 1984a,
Schnitzeret al. 2000, 2004), and maintain pos-
itive growth rates in the understory (Schnitzer
2005). Lateral growth of tree crowns may also fill
small gaps from above and, while crown growth
is not a true recruitment pathway in the sense of
those mentioned above, it is likely responsible for
the partial or complete closure of small gaps orig-
inating from the death of a single small tree or a
large limb (Hubbell and Foster 1986).
Following ga pformation, plant recruitment
and growth are exceptionally high for the first
few years. Thinning, or the reduction in total
stem number, typically begins within 3–6 years
of ga pformation, as individuals increase in size
and close the canopy, thereby decreasing light
reaching the understory and increasing mortality
(Brokaw 1985a, Hubbell and Foster 1986, Fraver
et al. 1998). Gap-phase regeneration is complete
when one or a few large trees attain the height
of the intact canopy, thus effectively closing the
gap. Gap-phase regeneration can be a rapid pro-
cess in relatively productive tropical forests, with
the height of the canopy increasing by as much as
5–7 m per year when certain pioneer trees are
present (e.g.,Trema micrantha,Cecropia insignis,
andZanthoxylumspp.; Putz 1983, Brokaw 1985a,
1987), and by several meters per year for non-
pioneer trees (e.g.,Simarouba amara,Virola sebifera,
andProtium panamensis; Brokaw 1985a).
Gap-phase regeneration, however, can also
become stalled or arrested at a low canopy height
for many years (Figure 12.1). This alternative
regeneration pathway can occur when lianas
are in high abundance initially and the falling
trees drag even more lianas into the gap. Most
lianas survive the initial treefall (Putz 1984a),
after which they copiously produce new stems in
the high resource environment, forming a dense
tangle of vegetation (Babweteeraet al. 2000,
Schnitzeret al. 2000, 2004). Lianas colonizing
the ga pfrom seed or from advance regeneration,
or growing into gaps from the intact forest all
contribute to these liana tangles (Appanah and
Putz 1984, Peñulosa 1984, Putz 1984a, Putz
and Chai 1987), which can continue to expand
in size and density if lianas fail to find trellises
(Peñulosa 1984). Once a liana tangle forms, it will
block and delay gap-phase regeneration of trees by
some combination of below-ground competition,
light pre-emption, and mechanical interference
(Schnitzeret al. 2000, 2004, 2005, Tabanez and
Viana 2000). Gap-phase regeneration may be
stalled at a low canopy height by lianas for at least
13 years, and probably much longer (Schnitzer
et al. 2000). Eventually, some trees (usually pio-
neers; Putz 1984a, Schnitzeret al. 2000) may
ultimately escape vertically through the liana tan-
gle and begin to close the canopy. The legacy of
these formerly arrested gaps is often an impene-
trable thicket of liana stems that remain in the
understory (Figure 12.2).
Palms may cause a completely different suc-
cessional trajectory of gap-phase regeneration,
when they are abundant in a newly formed gap
(Figure 12.1). While the ga pdynamics of palms
has not been explicitly studied, understory palms
suppress seedling regeneration by casting deep
shade and dropping large fronds that decay slowly
and smother seedlings (Denslowet al. 1991). Con-
sequently, there is often a depauperate seedling
layer beneath palms in intact forests prior to
ga pformation (Farris-Lo pezet al. 2004, Peters
et al. 2004, Wang and Augspurger 2006). Palms
may also act as a filter to regeneration, whereby
only very shade-tolerant species that are also
resistant to mechanical damage (sensuClark and
Clark 1991) are available to fill the newly opened
gap. Thus, the suite of species found in palm-
dominated gaps may be very different than that
in either liana-dominated or tree-dominated gaps,
and the pathway of gap-phase regeneration in
a given forest will likely depend on the relative
abundance of the different growth forms.