Tropical Forest Community Ecology

376 Chris J. Peterson and Walter P. Carson

Holl (2002) demonstrated that while woody
species (shrubs) overall facilitated tree establish-
ment into pastures in Costa Rica, this was the
product of positive, negative, and neutral effects
atdifferentstagesof treeregeneration.Specifically,
shrubs enhanced seed input and seedlin gsurvival
of animal-dispersed species, but seed predation
was greater beneath shrubs. Germination did not
vary between habitats. This work confirms the
conceptual problems pointed out by Pickettet al.
(1987) with tryin gto apply the models of Connell
and Slatyer (1977) in a mutually exclusive way;
nonetheless net effects (i.e., facilitation) were still
apparent. Regardless, there appears to be severe
dispersal and colonization limitation into these
early successional habitats and resident woody
species generally ameliorate this constraint and
likely alter rates of woody species succession as
well as composition.

Inhibition caused by resident
vegetation

Followin gabandonment, all types of a gricul-
tural land uses leave some residual herbaceous

vegetation. Typically, this residual vegetation

reduces woody species germination and establish-

ment (Nepstadetal. 1996, Slocum 2000), though

less so for large-seeded species (Figure 22.2).

For example, Hollet al. (2000) found that in

montane pastures in Costa Rica, species richness

and cover of broad-leaved species was up to five-

fold greater after just 6 months in sites where

graminoids had been removed versus where they

were left intact (see also Peterson and Haines

2000). In African post-agricultural grasslands

and plantations, Zanne and Chapman (2001)

reported significant negative correlations between

the number and richness of new woody stems

and the abundance of grasses and forbs (see also

Fergusonet al. 2003). The impact of pasture

graminoids, however, on woody species establish-

ment is not uniformly negative and can be species

specific. For instance, Zimmermanet al. (2000)

introducedseedsof 11woodyspeciesintoaPuerto

Rican pasture; germination across all species

went from 32% in natural vegetation to only 17%

in removal plots. Four species had significantly

lower germination where the graminoids had

been removed. From the same experiment, sur-

vivorship of two species was significantly higher

e−^8

1.0

0.8

y = 0.294 + 0.036x

R^2 = 0.23 n = 27 P=0.02

Seed mass (g), in scale

0.6

Proportion of seeds germinated

0.4

0.2

0

e−^7 e−^6 e−^5 e−^4 e−^3 e−^2 e−^1 e^0 e^1 e^2 e^3 e^4 e^5

Figure 22.2 Relationship between seed mass (natural lo gscale) and germination for 27 species sown into intact
grassland or pasture vegetation. Pooled from Zimmermanet al.(2000) and Hooperet al.(2002).