Environmental Engineering Reference
In-Depth Information
on these topics for tropical forests undergoing restora-
tion is limited, in this chapter I focus on summarizing
results to date on restoring tropical forest plant com-
munities, note when relevant information is available
on other organisms and ecosystem functions and high-
light the need for research on specifi c topics.
these forms of regeneration is highly variable (dis-
cussed below).
If forest plants either arrive at or are present in the
site, then several factors may limit their establishment,
survival and growth. These include competition from
aggressive ruderal vegetation, stressful microclimatic
conditions, limited soil nutrients, high seed predation/
seedling herbivory and plant pathogens (Holl 2002;
Meli 2003), and their relative importance varies from
site to site.
A factor that has been shown to limit survival and
growth of forest species in many sites, particularly in
agricultural land, is aggressive existing vegetation that
was either planted or rapidly colonized after the land
was abandoned.
Exotic
pasture grasses (e.g.
Imperata
cyclindrica
,
Melinus minutifl ora, Pennisetum
spp. and
Saccharum spontaneum
) often form a monoculture in
previously grazed areas (e.g. Hooper
et al
. 2002 ; Kettle
2010). In other cases, dense ferns (e.g.
Dicranopteris
spp. and
Pteridium
spp.) or other ruderal vegetation
compete with forest seedlings (Cohen
et al
. 1995 ; Zim-
merman
et al
. 2007). This residual vegetation may
slow recovery by providing shelter for seed and seedling
predators; competing for soil moisture, nutrients and
light; increasing the probability of fi re; reducing seed
germination and emitting allelopathic chemicals
which limit seedling growth (reviewed in Holl 2002).
Stressful microclimatic conditions may also limit
seed germination, and seedling survival and growth,
particularly in seasonally dry forests (Vieira & Scariot
2006). Light levels and air and soil temperatures are
commonly much higher, and humidity and soil mois-
ture levels are much lower in agricultural lands com-
pared to forests. These microclimatic conditions may
reduce survival and growth, particularly of later suc-
cessional seedlings which are adapted to regenerate in
the forest understorey and light gaps. Moreover, drier
conditions in pastures, along with high grass biomass,
make them particularly susceptible to the spread of
fi re, which kills the seeds and seedlings of most forest
species (Janzen 2002; Nepstad
et al
. 2008 ). In contrast,
pasture grasses are generally well adapted to resprout
after fi re, so repeated fi res can lead to a transition to
savanna vegetation (i.e. grasslands with trees spaced at
a suffi cient distance that the canopy does not close) in
drier areas.
Next, in some sites, seedling growth is limited by soil
conditions. Much of the tropics are covered by oxisols
and ultisols, which have low nutrient levels and high
acidity, whereas some areas have more fertile, volcanic
9.2 FACTORS AFFECTING THE RATE
OF NATURAL RECOVERY
The rate of autogenic tropical forest recovery from
human disturbance is notoriously variable. In some
cases, tropical forest biomass and species composition
may recover within a couple of decades (Marín-Spiotta
et al
. 2008 ; Letcher & Chazdon 2009 ), whereas in
other cases the areas may remain in a state of arrested
succession due to highly degraded soils or competition
with aggressive ruderal species (Chazdon 2003; Lamb
et al
. 2005). In order to most effectively design tropical
forest restoration strategies, it is critical to understand
factors infl uencing the rate of natural tropical forest
recovery generally, and to identify which factors are
most important in the specifi c system being restored.
9.2.1 Ecological processes affecting
tropical forest regeneration
Tropical forest plants commonly colonize degraded
lands from seeds primarily dispersed from sources
outside the site. Alternatively, plants may establish
from three sources within the site - the
in situ
seed
bank, seedlings already present and established at the
time of land abandonment and/or resprouts from
stumps, roots or stems - if these modes of regeneration
remain after human disturbance. Many studies dem-
onstrate that
dispersal
of animal - dispersed seeds - the
primary form of dispersal in tropical rain forests - is
often extremely low in former agricultural lands
(reviewed in Holl 2002; Meli 2003). Moreover, most
seeds falling in abandoned agricultural land are from
species already present in the site or from a few small-
seeded pioneer species, and seed rain is usually concen-
trated under remnant trees. Therefore, tropical forest
recovery is commonly limited by a lack of
propagules
of forest species. In some sites, particularly those that
were logged or used for short-term or low-intensity
agriculture, there may be regeneration from resprout-
ing and seed banks within the site; but, the extent of
