second growth (Williams-Linera 1990; Kapos et al. 1997), making them less
permeable to lateral light penetration and the penetration of hot, dry winds
from adjoining agricultural lands.
Forest structure is also markedly altered, especially by an increase in tree
mortality rates. When a new edge is created, some trees simply drop their
leaves and die standing, possibly because of sudden shifts in relative humidity,
temperature, or soil moisture that exceed their physiological tolerances (Love-
joy et al. 1986). Other trees are snapped or felled by winds, which accelerate
over cleared land and then strike forest edges (Ferreira and Laurance 1997;
Laurance 1997). Finally, lianas (woody vines)—important structural parasites
that reduce tree growth, survival, and reproduction—increase markedly near
edges and may further elevate tree mortality (Laurance et al. 2001).
This abrupt rise in tree mortality fundamentally alters canopy gap dynam-
ics (Williams-Linera 1990; Ferreira and Laurance 1997; Laurance et al.
1998a), in turn affecting forest composition and diversity (Brokaw 1985;
Hubbell and Foster 1986; Denslow 1987; Viana and Tabanez 1996). Smaller
fragments often become hyperdisturbed, leading to progressive changes in
floristic composition. In the Amazon, new trees regenerating within 100 m of
forest edges are significantly biased toward disturbance-loving pioneer and sec-
ondary species and against old-growth, forest interior species (Laurance et al.
1998b). Pioneer species such as Cecropia sciadophyllacan increase in density
by several thousand percent in fragmented landscapes (Laurance et al. 2001).
Because pioneer trees have a higher leaf turnover (Coley 1983), rates of litter
fall tend to increase near forest edges (H. L. Vasconcelos, unpublished data,
2002). Litter depth typically is greater within 100 m of the forest edge than in
the forest interior (Carvalho and Vasconcelos 1999; Didham and Lawton
1999). Changes in litter cover along forest edges not only have important
effects on plant (Bruna 1999) and animal communities (Carvalho and Vas-
concelos 1999) but also make forests vulnerable to devastating surface fires
during droughts (Cochrane et al. 1999).
Despite an initial increase in tree seedling recruitment especially of pioneer
species (Sizer 1992), just after forest edges are created, seedling density tends to
decrease near edges (Benitez-Malvido 1998). The reasons for this are not com-
pletely clear but probably involve reductions in seed rain and dispersal and
greater seed and seedling mortality near edges. In particular, the development
of a dense layer of secondary vegetation along forest edges may increase seedling
mortality by diminishing light availability and increasing damage from heavy
litterfall (Benitez-Malvido 1998). Microclimatic changes, especially reduced
soil moisture, may also be involved. Studies of the understory shrub Heliconia
acuminatashow that even when seedlings are protected from litter damage, sur-
vival is still lower near fragment edges than in interiors (Bruna 1999).
Changes in forest structure along edges have diverse consequences for for-
est fauna. Some species are insensitive to these changes and readily use edge
38 I. Conservation Biology and Landscape Ecology in the Tropics