Tropical Forest Community Ecology

202 Stefan A. Schnitzeretal.

0

5

10

15

Species or individuals per 20 m

2

Shade-tolerant densityShade-tolerant richness

Pioneer densityPioneer richness

Liana densityLiana richness

Non-gap

Gap

Figure 12.3 Mean density and species richness in
ga pversus non-ga psites (n=17) for shade-tolerant
trees, pioneer trees, and lianas. Asterisks represent
significant differences (∗=P<0.05,∗∗=P<0.01,
∗∗∗=P<0.001). Modified from Schnitzer and Carson

(2001).

lianas commonly compose approximately 25% of

the woody stems and nearly 30% of the woody

species in intact forest (Gentry 1991). In a test of

the ga phy pothesis in the same 50 ha area of for-

est on BCI where Hubbellet al.(1999) found no

difference in tree diversity between ga pand non-

ga psites, Schnitzer and Carson (2001) re ported

that both liana and pioneer tree abundance (den-

sity) and richness were significantly higher in both

5- and 10-year-old gaps than in non-gap sites on

both a per area and per stem basis (Figure 12.3;

Schnitzer and Carson 2001). Lianas and pioneer

trees combined can represent more than 40%

of the woody species diversity in many tropical

forests, demonstrating that gaps are important

for maintaining woody species diversity, even if

they fail to maintain shade-tolerant tree diversity

(Schnitzer and Carson 2000, 2001).

Lianas may be particularly abundant and

diverse in gaps because they can colonize gaps

in four ways, whereas trees typically use only

two. Lianas colonize gaps both from seed and

via advance regeneration, as do trees. Lianas can

compose from 18 to 32% of the advance regen-

eration (<2 m tall) under the intact canopy in

tropical forests (Putz 1984a, Putz and Chai 1987,

Schnitzer and Carson unpublished). Lianas can

also colonize gaps as adults, which is less common

in other growth forms (but see Bond and Midgley

2001). Putz (1984a) reported that approximately

90% of the lianas that were pulled into a gap dur-

ing the treefall survived and regenerated in the

gap. Lianas can also colonize gaps from the intact

forest by growing along the forest floor (Peñulosa

1984). Upon arrival in a gap, lianas vigorously

grow and produce new stems at a rapid rate (Putz

1984a, Schnitzeret al. 2000, 2004), which may

promote greater survivorship. Even though many

lianas may be shade tolerant (Gilbertet al. 2006),

because they can arrive in high numbers and sur-

vive in gaps for a long period of time, gaps may be

integral to maintaining their diversity.

Liana species richness may be maintained by

gaps via two main mechanisms. First, lianas

may be able to partition the abundant and het-

erogeneous light resource in gaps (sensuRicklefs

1977, Denslow 1980). While this mechanism

lacks strong empirical support, liana diversity was

higher in gaps than in comparable-sized areas

of intact forest, even after correcting for den-

sity (Schnitzer and Carson 2001), suggesting that

resource partitioning in gaps is possible. Second,

although lianas are sometimes considered to be

gap-dependent pioneers (Peñulosa 1984, DeWalt

etal.2000),theyactuallyappeartohaveattributes

of both pioneer and shade-tolerant species due to

their tolerance of low light (Gilbertet al. 2006)

and exceptionally rapid growth rates in high light

(Schnitzeret al. 2004). This rapid growth rate is

probably related to their reputedly high ratio of

photosynthetictostructuraltissue(Gartner1991,

Schnitzeret al. 2008), which allows them to fix

more carbon per unit biomass compared with

other growth forms. Consequently, treefalls may

increase liana diversity forest-wide by creating

ideal habitats for these species. Only detailed stud-

ies of the life history of numerous liana species