Environmental Engineering Reference
In-Depth Information
soils, such as andisols and inceptisols. After human
use, soils may be highly compacted, which impedes
root growth and water holding capacity. Many tropical
soils are phosphorus defi cient (Vitousek & Sanford
1986), but nitrogen and other nutrients may also be
limiting in some sites. Moreover, in certain types of soil,
aluminium or iron may be present in suffi ciently high
levels as to be toxic to plants and therefore to slow or
inhibit tree growth (Davies 1997). Finally, many tropi-
cal trees form mycorrhizal associations, which facili-
tate phosphorus uptake, but agricultural land uses
may substantially alter microbial communities (Car-
penter et al . 2001; M.F. Allen et al . 2005 ), which in
turn affects nutrient cycling .
Seed predation and seedling herbivory can also
be major obstacles to recovering agricultural lands.
Common seed predators in the tropics include small
mammals, bruchid beetles and leaf-cutter ants, which
can consume more than 80% of seeds of some species
within a few weeks (Nepstad et al . 1990 ; Jones et al .
2003). Likewise, herbivores - primarily insects but
also mammals - can cause extensive damage to certain
plants (Nepstad et al . 1990 ; Holl & Quiros - Nietzen
1999 ).
Finally, in some cases, plant pathogens cause exten-
sive mortality of certain trees planted in restoration
projects. For example, M.F. Allen et al . (2005) recorded
>50% mortality of one tree species ( Cochlo spermum
vitifolium ) planted to restore tropical dry forest in
Mexico, apparently caused by Fusarium sp., a wide-
spread pathogenic fungus. Seed predation, seedling
herbivory and plant pathogens are notoriously variable
among species and over time, and thus can have a
strong impact in some cases, but are less commonly the
crucial factors limiting forest recovery.
9.2.2 Overarching factors affecting the
rate of colonization and establishment of
forest plants
The relative importance of the above-mentioned proc-
esses in infl uencing dispersal and establishment limita-
tion, as well as the rate of natural recovery within
specifi c sites, is infl uenced by the adaption of specifi c
systems to local abiotic conditions, the past land use
type and intensity and the surrounding landscape
matrix (Holl 2007; Figure 9.1). In the tropics, the rate
of natural forest recovery is largely related to the abiotic
conditions with which these systems have evolved,
including rainfall, temperature and soil type (Holl
2007 ). Natural resilience is often higher in drier than
wetter tropical forest, in part because dry tropical
forests have a much greater percentage of trees, shrubs
and vines with wind-dispersed seeds (Vieira & Scariot
2006), which more readily disperse into open areas.
Moreover, resprouting is more common in tropical dry
forests (Vieira & Scariot 2006). Recovery also tends
to be faster in relatively warmer and lower elevation
areas, which generally favour more rapid growth
(Zarin et al . 2001). Finally, differences in soil fertility
across sites can strongly infl uence plant growth rates
and forest recovery (Moran et al . 2000 ; Zarin et al .
2001 ).
The relative importance of different factors limiting
forest recovery is also infl uenced by past land use
Figure 9.1 Factors affecting colonization and establishment of forest plants in cleared lands in the tropics.
(Modifi ed from Holl 2007 .)
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