Environmental Engineering Reference
In-Depth Information
9.1
INTRODUCTION
agricultural uses, this chapter focuses on the restora-
tion of such lands.
Despite ongoing pressures to clear tropical forests,
there is substantial interest in restoring these forests,
and tropical forest cover is increasing in certain regions
(Asner
et al
. 2009). The motivation for restoring tropi-
cal forests comes from an interest in enhancing or
restoring the delivery of
ecosystem services
(e.g.
sequestering carbon, minimizing erosion and improv-
ing water quality, maintaining hydrological cycling
and harbouring biodiversity) and maintenance of
natural capital
(Aronson
et al
. 2007a ). In particular,
there is increasing awareness that tropical forest clear-
ing is the cause of approximately 15% of carbon emis-
sions globally (van der Werf
et al
. 2009), so any effort
to reduce carbon emissions must slow deforestation
and forest degradation, and increase carbon stocks in
degraded and cleared forests, a set of measures known
collectively as
REDD
+. Moreover, there is a pressing
need to restore forests to reduce erosion into nearby
waterways and to conserve the high diversity of tropi-
cal forest organisms. In certain areas of the tropics,
agricultural land is being abandoned and undergoing
secondary succession due to changes in commodity
prices and rural - urban migration (Mar í n - Spiotta
et al
.
2008 ; Letcher & Chazdon 2009 ). Together these
changes offer opportunities to restore tropical forests,
which necessarily must be balanced with the need to
sustain human
livelihoods
in these regions.
Given the limited funds available for restoring tropi-
cal forest, it is critical to assess what type of
interven-
tion
is needed in order to select the most effective forest
restoration strategy (Lamb
et al
. 2005 ; Holl & Aide
2011). In this chapter, I fi rst discuss factors limiting
tropical forest
recovery
. I then briefl y outline an
approach for selecting an appropriate methodology
from the different strategies used to facilitate tropical
forest recovery and describe these methodologies in
detail. This framework is broadly applicable to a range
of tropical forest types, and I draw on examples from
many of them. I conclude the chapter with thoughts
about how to pay for tropical forest restoration and
important considerations for tropical forest restoration
in the future.
A common assumption of many restoration efforts
is that if the full complement of plant species is restored
this will result in the colonization of other groups
of species (e.g. fauna and microbes), as well as the
recovery of desired ecosystem functions (e.g. nutrient
cycling and erosion control). Given that our knowledge
Tropical forests are the most diverse ecosystems on
Earth. These forests are found within 23.5° N or S of
the equator in Asia, Oceania, Africa and Central and
South America, in areas with a mean monthly
minimum temperature of >18°C and a difference of
<5°C between the warmest and coldest months. There
are many tropical forest types that range along both
moisture and elevation gradients (Holdridge
et al
.
1971). Rain forests are typically categorized as areas
receiving >1500 mm of rain with > 100 mm of rain in
each month, whereas seasonal tropical forests gener-
ally have 3- to 6-month dry seasons when rainfall is
<50 mm per month, and dry forests receive < 1500 mm
rainfall annually (Whitmore 1998). Tropical forests
are found from sea level to beyond 3000 m elevation,
and are categorized as lowland (< 800 - 1000 m), pre-
montane (∼ 1000 - − 1500 m) and montane ( > 1500 -
3000 m), although the transitions between these
altitudinal belts vary regionally. Tropical forests are
found on well-drained soils, as well as on seasonally or
permanently fl ooded soils (riparian forest or fresh-
water swamps) and along coasts (mangroves). This
chapter focuses on nonwetland tropical forests.
Over half the tropical moist forest biome has been
cleared over the last several decades (Asner
et al
.
2009), and much of the remaining forest is affected by
fragmentation, selective logging and hunting. While
the rate of tropical deforestation has slowed in some
countries, it continues at a rapid pace in others (Asner
et al
. 2009). The causes of deforestation are complex
and interrelated, and their relative importance varies
by region (Geist & Lambin 2002). In Latin America,
much of the forest has been cleared for pasture land
and commercial agriculture, such as soybeans, sugar
cane and coffee. In Africa, forest land is often cleared
or degraded to supply fi rewood and/or arable land for
subsistence agriculture; recently there has been
increasing concern about the extensive bushmeat
hunting and how the paucity of large mammals
within some forests will alter plant-animal interac-
tions, such as herbivory and seed dispersal, and in turn
forest dynamics (Wright 2003). In Asia, much of the
forest has been cleared for timber and/or agriculture
with the land area in industrial-scale agriculture
increasing in recent years. Smaller areas of tropical
forests have been highly impacted by mining and oil
drilling. As the majority of forest clearing has been
carried out for a mix of logging and different types of
