PREFACE
This booklet and its companion volume (no. 7 in this series) deal with the
environmental impacts of change in land use from undisturbed moist tropical
forest. The two publications look at the most common types of interference by
man to tropical forests: selective logging for timber, and clearing for agriculture
or plantations, respectively. Collectively they give an overview, individually they
cater for specific concerns. Each booklet is self-standing, together they are
complementary.
To avoid unnecessary repetition, the basic forest hydrological
and ecological processes - which have been described in some detail in volume
no. 7 - have been summarized only in the present document.
This volume addresses the changes that occur when forest is cleared to make
way for alternative landuses. The most widespread and historic reason for disturbing tropical forests is to make land available for agriculture. In order to gain a
tighter focus we have concentrated here on the change from forest to rainfed
crop farming, with only passing reference to irrigated agriculture and development of pastures for raising livestock. Plantations also thrive in land previously
under forest. We examine both plantation crops, such as coffee, rubber and oil
palm, and forest plantations for the production of timber, pulp or fuelwood.
W.R.S. Critchley and L.A. Bruijnzeel
0 UNESCO
1996
Contents
Forests and land use change
1.
2.
Understanding the forest-environment
interaction
Disturbing the balance
3.
Where there once was forest: post-clearing systems of production
4.
Changes: for better or for worse?
5.
From nature to nurture: transition with care
6.
Selected references
The International Hydrological Programme
MA9 Programme activities in the humid tropics
Netherlands IHP
Committee _
vrije Universiteit
1
3
7
IO
21
36
44
46
47
1.
‘Some of the
environmental
disasters
which are hastily
‘attributed to
‘deforestation’
are essentially
natural phenomena’
FORESTS AND LAND USE CHANGE
Exploitation of tropical forests is nothing new. Forest products
have been harvested since homo sapiens emerged as a species,
and forests have been cleared to make way for agriculture over
millenia. What is new, however, is the accelerated rate of exploitation in the last few decades. This has coincided with a growing
popular awareness of the environment, and of the dangers associated with unchecked interference with nature. One result is
that various calamities have been blamed on ‘deforestation’.
Floods, landslides, increased erosion and sedimentation of reservoirs are often said to be the ‘inevitable’ consequence when
forests are logged for timber, or when land is cleared for farming
or plantations. Climate changes are also blamed by ‘green’
politicians on deforestation. In reality some of the environmental
disasters which are hastily attributed to deforestation are essentially natural phenomena - but the damage caused tends to
be more severe now, because there is more infrastructure and
more people downstream than ever before. Nevertheless, it cannot be denied that a range of detrimental environmental changes
can be caused by unwise modifications of tropical forests.
This booklet aims to put the picture into perspective with respect
to the development of agricultural land and plantations from moist
tropical forest. Of course the majority of existing farm land in the
humid tropics was originally developed after clearing forests. In
some areas this process occurred many centuries ago. Terraces
in the Far East, for example, replaced forest over two thousand
years ago, and continue to produce crops and support millions
of families. In the case of plantations, practically all of these with the exception of some reforestation programmes on degraded land - are sited on land which used to flourish with forest.
It is significant that the majority of the area currently undergoing
deforestation in the humid tropics - estimated at about 155,000
km* per year at present - is being transformed into agricultural
land. For instance, in Indonesia alone up to half a million hectares of forest were cleared annually in the outer islands to make
way for settlers from the overcrowded island of Java in the transmigration programme of the 1980’s. Similarly, over 15 million
hectares of forest were burned down for the creation of pastures
along the southern fringe of the Amazon rain forest block in
Brazil during the late 1980’s. As well as official programmes
there is often an inexorable ‘creep’ of quasi-legal settlers into the
forests, exploiting land made accessible through logging roads.
1
As the process of colonisation by agriculture continues, so the
more remote areas are increasingly exploited - areas often characterised by steep topography and especially vulnerable to
erosion. Certainly there is widespread concern in the humid
tropics about the dangers of farming on slopes which are commonly much steeper than statutory limits permit. Hillsides are
being cleared everywhere as farmers seek more land to produce
food for their families.
There are two main issues to be addressed in this booklet: first,
what evidence do we have about the environmental impacts of
clearing forest for agriculture and plantations? And second, what
safeguards are needed during forest clearing, and in the development of alternative land use systems, to minimise damage to the
environment? It is clear that we are not dealing exclusively here
with physical sciences, but with social aspects also. These
changes are taking place in some of the most densely populated
areas in the world and it goes without saying that the human
factor is deeply implicated in the events that are unfolding.
‘Hillsides are being
cleared everywhere
as farmers seek
more land to
produce food for
their families’
2.
THE FOREST-ENVIRONMENT
INTERACTION
In order to appreciate the environmental consequences of converting tropical forest to other land uses, it is necessary first to
understand the environmental dynamics of the undisturbed forest
itself. A virtually closed canopy, composed of a multitude of
species, together with a forest floor covered by a thin layer of
leaf litter and underlain by a highly permeable topsoil are characteristic of many moist tropical forests. The canopy and the nature
of the soil are fundamental in determining how the forest behaves with respect to hydrology, soil conservation and nutrient
cycling. We will now look briefly at these systems and the processes involved. This then becomes the backdrop against which
it is possible to highlight changes.
The forest
hydrological
cycle
By definition, moist tropical forests have developed in warm
humid regions where there is sufficient annual rainfall to sustain
evergreen broadleaved forest vegetation. In some areas there
may be a short dry season: in others, rainfall is year-round. Rain
falling on the forest canopy reaches the ground through three
routes. A small proportion reaches the forest floor as direct
throughfall, without touching leaves or stems, and a further small
proportion flows down the tree trunks as stemflow. The majority
of the rain however falls on the vegetation and then reaches the
ground from the canopy as crown drip. The total amount of water
reaching the forest floor from these sources is called net precipitation. It typically makes up 80-90 per cent of the incoming rain
in most tropical forests. The remaining lo-20 per cent of the rain
falling onto the forest never reaches the ground: it is intercepted
by the canopy and is evaporated back into the atmosphere. The
gross rainfall therefore comprises two elements: rain which
reaches the forest floor and rain which is intercepted and evaporated back. The great majority of rainfall which reaches the forest
floor infiltrates the soil through the leaf litter and top soil - which
usually provides excellent protection against raindrop splash and
surface runoff. Undisturbed forest soil has good structural
properties and this not only helps infiltration, but also increases
the water holding capacity of the soil.
Loss of water from the soil in a moist tropical forest is either
upwards through transpiration from the canopy, or via drainage
into the nearest stream.
3
The water use of closed canopy tropical forests is high, and a
large proportion of the soil moisture (typically about 1,000 mm
per year) is pumped back by the trees into the atmosphere. Soil
moisture drains into the stream network by throughflow, the result of downward moving water meeting an impermeable layer of
subsoil or bedrock and then being deflected laterally. Between
rain events, the water drains slowly and steadily throughout the
season, from the store of moisture in the soil. This process accounts for the baseflow of streams. During and shortly after rain,
streamflow may increase rapidly. This is due to the quick flow of
water through the soil traveling via a number of pathways that
are mainly determined by the nature of the soil. The increase in
streamflow above the baseflow is commonly referred to as stormflow or quickflow, and at its maximum is called peakflow.
Figure
1.
‘The
hydrological
cycle for a
fores ted
ecosystem’
4
Erosion
‘Undisturbed natural
forest usually has
one of the lowest
erosion rates
and sediment
yield
Undisturbed natural forest usually has one of the lowest surface
erosion rates of any form of land use in the humid tropics - but
the key here is the word undisturbed. In forest which is left in its
natural state, the low erosion rates can be attributed largely to
the nature of the forest floor. Splash erosion is effectively prevented by eliminating the direct impact of raindrops on the soil
surface by both litter and understory vegetation. In addition, the
leaf litter, undergrowth and highly permeable topsoil all help keep
surface runoff and thus erosion to a minimum. Furthermore, the
high levels of organic matter in the topsoil help it to resist
erosion. In disturbed forest - for example where timber has been
harvested or litter has been removed for firewood and undergrowth for livestock fodder - the situation is fundamentally different, and erosion rates can be very high as a result.
of any form of land
use in the tropics,
but the key here
is the word
undisturbed’
While surface erosion levels are normally very low under undisturbed forest, this is not necessarily true for mass wasting processes. Granted, through the stabilising function of their roots,
trees help to reduce the risk of shallow land slips on hillsides and
riparian vegetation helps minimise bank erosion, but more deepseatedforms of mass wasting, such as large landslides or massive mudflows, may still occur in forests, especially where these
are situated on steep hillsides in wet mountainous terrain. Such
deep-seated landslides in natural forest are essentially a geological phenomenon, and, because they are often precipitated by
extreme rainfall or seismic activity, they cannot simply be prevented by vegetation. As a result, overall sediment production
from forested tropical catchments can be very high under certain
geological conditions. More importantly perhaps, such high sediment yields are unlikely to be affected significantly by the activities of man, as these are dwarfed by the sheer magnitude of the
geological and climatic forces at work in mountainous regions.
The forest
nutrient
cycle
Tropical forests can produce a spectacular amount of plant biomass, even when their soils are very low in fertility. The reason
that these forests manage to sustain such wealth under poor
conditions lies in their relatively ‘closed’ nutrient cycle. In other
words, the plant nutrients entering the forest system (mainly in
rain, dust and aerosols) are being cycled continuously between
the canopy and the soil, with only small amounts leaking out of
the system. The chief adaptation mechanism developed by the
forests growing on extremely poor substrates is a surface root
mat capable of ‘trapping’ most of the nutrients entering it.
5
Only where soils are sufficiently fertile can forests afford substantial leaching losses from their soils. Under these circumstances, weathering of residual minerals still present in the subsoil or in the rock beneath may supply further nutrients to the
system, provided that the bedrock is not too deep (less than,
say, 5 metres from the surface) and within reach of the tree root
network.
EXCHANGE
COMPLEXES
decomposition
nitrogen
fixation
in soil.
rhizospharo
and on roots
--me_
rock
weathering
--
-_______
SOIL
- ---------
-\
1
ROCK
Figure 2.
\
‘.
nutrient
losses in
wat*rmovem*nts
from
-_______--_____
I
forest
-
,’
_ _--_____-__
’
The nutrient cycle in moist tropical forest.
6
-__--_
3.
DISTURBING
THE BALANCE
Logging of forests can either be selective or complete. In contrast to systems of partial logging, clear felling comprises the
complete removal of the forest cover to make way for entirely different forms of land use. These include small scale agriculture,
the plantation of beverage or industrial crops, reforestation for
timber, pulp and fuelwood, or the development of pastures. In
some areas, selective logging can be the precursor to intrusion
by slash and burn farmers or more permanent squatters, who
take advantage of the newly constructed access roads and gaps
created in the forest during logging. Elsewhere, the productive
potential of logged-over forest land may be deemed insufficient,
after which it is converted to other forms of land use as part of
official government programmes. Only in relatively rare cases is
the forest to be felled still untouched.
The harvesting of merchantable timber during selective logging
operations using tracked or wheeled tractors may leave up to 30
per cent of the surface area bare in the form of compacted access roads, haulage tracks and log landings. As a rule of thumb,
the damage inflicted to the soil escalates with the size of the
machinery and the intensity of the harvesting. The more machine
passes, the greater the compaction of the soil surface, and in
this respect wheeled vehicles which lose traction and smear the
surface can be especially deleterious.
Of all the methods of clear felling, manual clearing (with animal
haulage) tends to be the least damaging to the soil surface and
simultaneously best for subsequent plant growth, mainly because
it causes the least disturbance to the forest floor. Nevertheless
this is an expensive and slow method which is not suitable when
large areas need to be cleared. The damage increases rapidly
when heavy machines are used, particularly when the tractors
are equipped with root rakes for uprooting tree stumps. It goes
without saying that the degree of surface disturbance during
mechanized c/ear felling operations involving ground-based
machinery (as opposed to skyline yarding systems) can be much
higher than that associated with selective logging. Also, because
the volume of slash left behind after forest clearing can be substantial, it is common practice to ‘windrow’ the slash into lines
and set these on fire to facilitate future access. As we will see,
these measures cause additional deterioration of the soil through
the loss of precious nutrients that go literally ‘up in smoke’ during
the burn and ‘down the drain’ via surface erosion.
7
-
Even in the case of a new land use which is considered to be an
effective substitute for forest - a mature tea plantation for example - the period leading up to the establishment of the system
can be highly damaging. Nevertheless, precautions can reduce
the risk, and these are discussed in Chapter 6.
a
‘As a rule,
damage inflicted
to the soil
escalates with
the size of the
machine and the
in tens@ of
harvesting’
4.
WHERE THERE ONCE WAS FOREST
Small scale agriculture
In the areas where evergreen tropical forest thrives, farming is
equally favoured by year-round warm temperatures and abundant
supplies of moisture. These are, by definition, humid zones,
where - apart from a short dry season in some areas - moisture
rarely limits crop growth. In some cases there may be adequate
resources of surface water which make irrigation possible. But
where irrigation is not viable, rainfed farming almost always
benefits from enough rainfall to permit two cropping seasons
each year.
With respect to agriculture, the focus of this booklet is on rainfed
farming. This is for two reasons. First, much of the forest which
is currently being felled in the more populous regions of the tropics is situated in the higher reaches of catchment areas, where
ir- rigation development is often not feasible. Second, irrigated
land presents much less of an erosion hazard than rainfed land.
This is because irrigated land is levelled, and there is by necessity a high degree of water control. Both of these factors help
ensure good soil conservation. In fact, belts of irrigated fields on
the footslopes below rainfed hillsides can even trap sediment
eroded higher up on the slopes.
Although shifting agriculture is not our direct concern here, because it does not involve the permanent replacement of forest,
it needs to be mentioned in passing. Shifting agriculture was the
progenitor of rainfed agriculture in humid tropical regions, and is
still important in many areas today. Pockets of land are cleared
from the forest, and these are then cropped for two or three
years before being left to regenerate over a fallow period that
would typically last at least 10 - 15 years. This system of farming
used to be much maligned by tropical agriculture ‘experts’ in the
early days. More recently, however, there has been a change in
opinion. It is now agreed that shifting cultivation is an environmentally stable form of agriculture, as long as the vegetation is
given sufficient time to recuperate and so restore the fertility of
the soil. Unfortunately, the system is observed to break down in
many places where the fallow period has become too short due
to increased population pressure. The vegetation is then set on
fire too frequently to allow a proper succession to develop. In
such areas, the forest is gradually replaced by a largely unproductive grassland.
10
‘Precious
nutrients go
up in smoke
during the
burning of
slash’
‘Farms
developed
without soil
conservation
measures
court disaster,
especially on
steep slopes’
11
Settled rainfed agriculture is fundamentally different from shifting
agriculture. Forest, or at least patches within forests, must be
permanently cleared to make way for this entirely new land use.
Three categories of rainfed farming can be distinguished from the
point of view of environmental impacts.
These are:
i.
Farms without conservation
measures:
Some farms are developed without soil conservation or water
management measures such as terraces, contour grass strips or
barrier hedges. Even on gentle slopes, this courts disaster in
regions of high and intense rainfall. Such farms may be the result
of illegal encroachment into forest land, where the squatters are
reluctant to invest in conservation measures. Hillside farms without adequate conservation are also found on deep volcanic soils
where vegetable growing is profitable and where farmers can
purchase fertilisers at subsidised rates. In neither case is the
farmer very concerned about erosion: in both cases there can be
lasting damage either on-site or downstream, or both.
ii.
Terraced farmland:
Commonly, bench terraces of various designs are found in farms
where there used to be forest. This is particularly true of Asia
and parts of the Andes where terracing practices date back up
to 2,000 years or more. Terrace risers are constructed from
either stone or earth, or a mixture of the two. Despite the high
labour input involved in construction, modern-day soil conservation projects still promote bench terracing as the most versatile
technique in controlling drainage and surface erosion. The flat
terrace beds facilitate cultivation - and also permit irrigation
where this is viable. Terraced agriculture is often associated with
a high standard of land husbandry.
...
III.
Farmland
with agroforestry
systems:
Some of the best conserved, and most productive land in the
humid tropics, is under intensive forms of traditional agroforestry.
This land is usually terraced also where steep. Examples of indigenous agroforestry systems in Indonesia are ‘mixed gardens’
and the even more intensively cultivated ‘home gardens’. Modern
agroforestry systems are in the process of being promoted all
over the humid tropics with the aim of improving productivity
while at the same time controlling erosion.
12
‘Terraced agriculture is often associated
with a high standard of land husbandry’
13
-.
_-...^--
It is difficult to classify farms precisely into categories, as it is
common to find a mixture of land uses, and a range in the standard of soil conservation measures even within a single farm. Yet,
the divisions listed above help us to identify where the potential
problems lie. Clearly there is a huge difference between, on the
one hand, farmland with an agroforestry mixture of trees and annual crops which is carefully terraced against erosion, and on the
other hand, land which is exposed to intense rainfall without any
conservation measures - structural or agronomic.
Plantation
crops
Just as annual crops thrive in the conditions by moist tropical
forest, so do many plantation species. Indeed, several of these
plantation crops are forest trees by origin: rubber and cacao for
example come from the Amazon. Estates of beverage crops: tea,
coffee and cacao, and of industrial crops: rubber and oil palm,
now occupy large tracts of land which have been cleared from
the forest. Whilst only a relatively small proportion of current
forest clearing (ca. 15 per cent) involves the creation of agricultural plantations, the management of existing estates, and
indeed the replanting of old stands, have important environmental
implications. We will concentrate here on the five species already
introduced, and on their role as plantation crops. First, some
general remarks about plantations themselves.
In contrast to small scale farming in the tropics, a crop plantation
is characterised by uniformity and central management. This facilitates the development of physical infrastructure, - including
roads, as well as soil conservation structures - especially those
constructed by machinery. Because the plantation crop itself is
perennial, and therefore a year-round ground cover can be maintained, there is every opportunity for establishing and maintaining
a production system which combines high yields with effective
soil conservation.
Nevertheless, there are also opportunities for environmental
damage - notably during the critical period of conversion from
forest to plantation and when crop stands have to be replaced at
the end of their productive lives. Within their production cycle, it
is in the early years that the stands are most vulnerable to
erosion - that is before a complete ground cover is formed.
Because plantation species are perennial, they usually root
deeper than annual crops. On the other hand, they tend to use
less water than the forest they replaced. The hydrological implications of this are discussed in Chapter 5.
14
Some characteristics
of the main crops follow:
Tea (Camellia sinensis)
In its natural habitat in south-east Asia tea grows into an
evergreen tree up to 10 metres tall. However, in the cultivated
form it is pruned into low spreading bushes to allow hand picking.
Typically, tea is cultivated in areas with abundant, and yearround rainfall. On the steeper slopes of the highland areas where
it is usually grown, it is sometimes the practice to construct
terraces before planting. It is characteristic of tea that, if well
managed, it gives a very effective ground cover once the canopy
closes. But poorly managed tea can lead to serious erosion. The
plantations last for several decades before production declines.
Of all the land use systems which replace moist tropical forest,
tea is usually considered to be one of the most effective with
respect to soil conservation.
Coffee (Coffea spp.)
Like tea, coffee can be grown either on smallholdings or on
estates. Coffee does not form such a good canopy as tea, partially because of the wider spacing required for access between
rows for picking and spraying. In order to maintain good ground
cover to improve infiltration of rainfall and to reduce weed competition, it is necessary to mulch, especially around the plants
themselves. Coffee is very susceptible to weed competition, and
does not tolerate the planting of cover crops, which is a common
practice with other plantation species. Terracing is often used,
especially where coffee is grown by smallholders on hillsides.
Cacao (Theobroma
cacao)
Cacao, which is grown for its product, cocoa, is a common plantation crop in the lowlands of West Africa, where it has replaced
much of the natural forest. Cacao requires planting beneath
shade trees which are either introduced or scattered remnants of
the original forest. The main environmental problems now are
those associated with the maintenance of mature estates. Mulching is commonly practised around the trees to achieve moisture
conservation, reduce runoff and suppress weeds. Where cacao
is still being planted, there is a potential erosion problem until the
canopy closes.
15
‘Well managed tea
gives an effective
ground cover once
the canopy closes’
‘Leguminous cover
crops reduce
erosion,
provide nitrogen
and suppress weeds’
16
Rubber
(Hevea braziliensis)
Rubber is now a characteristic plantation crop of Malaysia, Indonesia, Sri Lanka and the humid regions of West Africa. In the
establishment of rubber plantations it is the practice to cleanstump the area to avoid transfer of disease to the rubber trees.
Needless to say, this increases the vulnerability to erosion in the
stages prior to establishment of a good ground cover. Rubber requires a deep, relatively fertile soil and thrives best on flat land
whose intrinsic erosion hazard is rather low. It is common to
plant a legume cover crop between the rows of trees, such as
Centrosema pubescens. Again, the cover crop reduces erosion
and has the additional benefits of providing extra nitrogen and
suppressing weeds.
Oil palm (Elaeis guineensis)
Like rubber, oil palm requires a climate with high temperatures
and abundant, well distributed rainfall. Oil palm originated in
West Africa but Malaysia is the world’s leading producer. Once
again it is usual to plant cover crops between the trees. Both
rubber and oil palm plantations need to be rehabilitated - entirely
replanted - when productivity is reduced through old age (usually
after several decades) or disease. This normally involves clearing
and burning, and therefore opens the land up to the possibility of
degradation before the new plantation is fully established.
Plantation
forests
The dividing line between plantation crops and plantation forests
is somewhat arbitrary. ‘Plantation forestry’ generally refers to tree
species grown for timber or pulp - and other locally important industrial uses such as resin or tannin production. In some regions
forest plantations are established as ‘protection’ forests in environmentally vulnerable zones, such as steep headwater areas.
Plantation forests are often larger in size than crop plantations:
for example, the minimum viable area to support a pulp mill is in
the order of 25,000 ha. Nevertheless, woodlots for fuelwood, or
protection forests may be quite small in size. Plantation crops
often demand better sites and tend to be more intensively managed than plantation forests. But there are many factors common
to the two types of production system, including centralised
management, and a planned layout of physical infrastructure.
17
The planting of artificial forests has burgeoned in the tropics over
the last 40 years. This is explained in part by ambitious national
programmes, such as those in India, Brazil, Malaysia and Indonesia. However, the amount of moist tropical forest being directly
converted into forest plantations (about 10 per cent of the total
in 1980) is currently decreasing. This is partially as a result of the
increasing demand for agricultural land in those former forested
areas. Much of the land that is presently under conversion to
forest plantations is scrubland, or poorly productive vegetation
such as the fire-climax grasslands of south-east Asia. Another
significant recent trend is ‘social forestry’ where tree planting is
encouraged on an individual (farm forestry) or community (village
woodlots) basis to meet local needs, primarily for fuelwood and
timber.
Plantation forests are essentially artificialforests: they are almost
invariably uniform blocks of monocrop tree species, planted in an
orderly manner. The economic advantages of such plantations
over natural forests are clear: the species planted are much
more productive. The three most commonly planted genera include Pinus (pines), Eucalyptus and Tectona (teak). Timber
yields of 15 - 30 m3 per hectare per year are not uncommon in
warm humid regions, far outstripping what could be expected
from selective logging of natural stands. Maturity cycles can be
as rapid as 7-8 years for pines, Acacias and eucalypts in certain
situations, though teak and other precious hardwoods take at
least 40 years to reach maturity. After harvesting it is usual to
replant with seedlings - often with the same species - although
some species coppice well after felling (some eucalypts, for
example). Whilst coppicing reduces harvesting costs and soil surface disturbance,
it has serious implications for soil water
reserves. In their efforts to regain their above-ground biomass,
the coppiced trees - whose root systems are left intact consume far more water than either young seedlings or mature
specimens.
As with plantation crops, there is an obvious potential for soil
disturbance during the initial phase of land preparation and plantation establishment. The need to keep the young stands weed
free varies between species but regular control of ground cover
around the saplings to boost their growth can also expose the
soil. This may also be a problem under the widely used taungya
system where labour is paid for by allowing farmers to cultivate
their annual crops between the young trees. During the phase of
rapid growth until and shortly after canopy closure, the next
significant environmental impact is likely to be in the form of
gradually changing streamflow regimes.
ia
‘Plantations present
the opportunity
to establish
a system which,
with careful
management,
can provide
an acceptable
alternative to
natural forest’
Thinning closed-canopy stands does usually not cause much
damage to the soil surface, but clear felling the mature plantation
certainly does have this potential - depending on the type of
machinery used and how well the operation is managed. Fire can
also be a significant hazard in forest plantations, particularly in
areas with a distinct dry season, and where large amounts of
litter shed by deciduous trees constitute a readily available
source of fuel.
As we will see in the final chapter, it is quite possible to avoid the
havoc that is so often associated with the initial forest clearance.
This holds even more for the transition to subsequent forest
rotations which often no longer require the removal of large tree
stumps. Also, the volume of slash lying around at this stage is
much reduced and does not necessarily need windrowing and
burning. Regardless of the stage of the plantation, one of the
most important aspects of all is the presence of a well-developed
ground cover. When the protective understory - and above all,
the litter layer - is removed (for fodder, compost or fuel) and the
soil is exposed to the rain dripping from the trees, the ‘forest’ becomes much more vulnerable to surface runoff and erosion,
despite the presence of a full tree cover. It takes more than trees
to make a forest!
19
‘When the litter layer is removed and the soil exposed to the erosive
force of crown drip, the surface becomes extra vulnerable to erosion,
despite the presence of a full tree cover: it takes more than trees to
make a forest’
20
5.
CHANGES
- FOR BElTER
OR FOR WORSE?
With the felling of forest to make way for agriculture or plantations there are, inevitably, significant changes to the environmental behaviour of the land unit. But whether these changes are
detrimental, and if so, how detrimental and for what duration,
depends on how the forest is felled, and how the new production
systems - and the associated infrastructure of roads and settlements - are developed and maintained. Although outside the
scope of this review, it is also vital to consider the socioeconomic costs and benefits when looking at the environmental
effects of land use change. Population pressure often dictates
that there simply is no alternative to clearing tracts of forest. So,
the most relevant question then becomes: how can damage to
the environment be limited?
As we have seen, the land is especially susceptible to damage
during the period leading up to the establishment of the new
production system. The following section concentrates on the
changes in the hydrological, erosion and nutrient cycles associated with the establishment and maturation of the new land use.
Precautions to reduce soil degradation are discussed separately.
Impacts
on hydrology
It is clear to the observant eye that agricultural land and undisturbed forest behave differently during a heavy rainstorm.
Even from well-terraced farms, runoff waters, discoloured with
sediment, can be seen cascading along drains by the side of the
road. Simultaneously, small streams draining well-maintained
forests apparently carry less sediment - and water - during a
storm. It is often said that streams have disappeared in the dry
season as the result of deforestation. And conversely, in the wet
season, damaging floods have proliferated. But is there a
scientific basis for these observations?
It is not surprising that deforestation should lead to changes in
hydrological response. As we have seen, the key to the hydrological behaviour of a tropical forest is the presence of the
canopy and the forest floor, with its blanket of leaf litter and concentration of roots. The forest canopy (through its interception of
rainfall and its evaporative loss via transpiration) together with
the litterlayer (through its effect on infiltration) are crucial in the
hydrological cycle of the forest. Disturbance to either has highly
significant implications.
21
Wherever the forest is replaced by annual cropping there are
bound to be profound changes. For example, there is virtually no
effective substitute for the forest floor with its protective leaf litter
layer and myriads of tiny soil fauna whose activities maintain soil
aggregate stability and water intake capacity. The beneficial
effect of high organic matter contents and abundant fauna1 activity may linger for a year or two after forest clearing but exposure of the surface to the elements generally leads to a rapid
reduction in infiltration capacity, particularly if fire is used during
the clearing. Therefore, the result of conversion to agriculture is
almost inevitably.that amounts of runoff increase. Farmers sometimes even encourage surface drainage to prevent waterlogging
of crops. Indeed, if complete infiltration did occur in high rainfall
regions, this could in some cases destabilise steep slopes with
shallow soils that are no longer protected by a network of tree
roots.
Often, infiltration-excess overland flow makes up less than one
per cent of the incoming rainfall in undisturbed tropical forest but
on agricultural fields with little or no conservation practices this
figure may increase to as much as 30 per cent. An additional
problem in smallholder agricultural zones is the considerable
area which is permanently occupied by compacted surfaces including household compounds, tracks and roads.
Streamflow
Figure 3a.
(mm/ma)
200
0’
’
I
I
I
1
1
1
I
!
I
I
I
I
2
3
4
5
6
7
8
9
10
11
12
Month
Period
-
1919-1943
-
22
1951L1972
Changes in seasonal
distribution of
stream flo w folio wing
changes in land use.
(a) Konto area, East
Java, Indonesia,
where intensive
rainfed cropping and
residential areas occupying one-third of
the basin produced
significant increases
in surface runoff.
These surfaces can make a significant contribution to both overall runoff and sediment yield. The effect is compounded by an
increase in effective rainfall: no longer are there trees to intercept
rainfall and evaporate it directly back into the atmosphere.
Neither of course are levels of forest water use maintained. Compared with undisturbed natural forest, therefore, the overall
catchment water yield increases significantly under rainfed agricultural use (typically by 150 - 450 mm per year, depending on
rainfall). Although in theory the extra amount of moisture available in the soil due to the reduction in evaporation should permit
an increase in baseflow - given good surface management - dry
season flows are often seen to decline because of deteriorated
infiltration opportunities. As a result, the magnitude and frequency of high flows tend to rise wherever a pattern of predominant subsurface drainage is replaced by a superficial one.
However, one should bear in mind that, as storms increase in
intensity and duration, and as one moves further downstream
from the cleared portion of a catchment, the effect of forest
clearing dwindles into insignificance as a causative factor in the
floods that are inevitably generated by unusually large storms. In
short: once the capacity of a soil to store water is exceeded by
intense and persistent rains, the presence or absence of trees
will have only a marginal effect on downstream flood levels.
Figure 3b.
Streamflow
(mmimo)
1501
Changes in seasonal
distribution of
streamflow following
changes in land use.
(b) Mbeya area,
Tanzania, where
montane forest was
replaced by
subsistence
agriculture. Stable
soil aggregates and
low intensity rainfall
precluded
widespread erosion
and a deterioration of
the flow regime.
I
125 c
0’
’
1
I
I
I
I
1
I
I
I
I
1
I
2
3
4
5
6
7
8
9
10
11
12
Month
average values between
Forest
-+-- Agriculture
1958 and 1968;
23
_
‘Dry season flows
are often seen
to decline
as a result of
deteriorated
infiltration
opportunities’
24
When a natural forest is converted to a plantation, whether of
crop or forest trees, the overall catchment water yield increases
in the first instance after clearing because of the reduction in
plant cover. Less vegetation means lower evapotranspiration and
therefore more water available to contribute to catchment water
yield. Initial increases in streamflow of 150 - 900 mm per year
have been recorded after clear-felling tropical forest, depending
on rainfall and the severity of soil disturbance. In most forest
plantations it then takes two to three years for the increase in
water yield to become less pronounced as a result of the uptake
of water by the vigorously growing saplings and the recovering
understory. During the next three to four years until canopy
closure water yields continue to be reduced gradually until they
approach the pre-clearing value. Therefore, the gain in water
yield following the clearing of natural forest is only temporary.
However, there are indications that eucalypts may use more
water than the original forest once the trees manage to reach the
groundwater table. Similarly, significant reductions in streamflow
can be expected after coppicing trees, notably eucalypts. During
the first two to three years after coppicing, the water uptake by
the extremely rapidly growing trees (whose root systems are left
intact) may be 50 - 75 per cent higher than that of mature uncoppiced individuals. Therefore, although coppicing is relatively
benign in respect to soil surface disturbance, the hydrological
costs are high.
Crop plantations almost always use less water than the original
forest. For example, streamflow totals from areas converted to
rubber or cocoa will be permanently higher than before, typically
by 300 - 400 mm per year. Oil palm, on the other hand,
eventually consumes as much water as a natural forest. Again,
dry season flows may or may not be reduced once a plantation
has established. This depends on the ‘trade-off’ between (i) the
loss of infiltration opportunities associated with compacted roads
and tracks or poor soil surface management, and (ii) the gain in
soil moisture as a result of the more modest water use of the
plantation. Needless to say, this is a mainly a question of
planning and plantation husbandry.
Closed canopy plantations exhibit higher overall water use than
rainfed annual crops. This is the combined result of the higher
stature (leading to more interception during rain) and better developed root network (allowing the continued uptake of water
from deeper layers when the top soil has dried out) of trees compared to annual crops. The other side of the coin -a potentially
serious reduction in streamflow after afforesting degraded manmade or natural grasslands - cannot be stressed enough, particularly where a well-developed dry season prevails.
25
It is frequently suggested that replacement of natural forests by
pastures or rainfed crops causes major changes in rainfall or
even ‘desertification’.
Despite all the rhetoric in popular and
quasi-technical articles these claims are, in any case, unproven.
Where changes in rainfall patterns have occurred, such as the
persistent trend towards increased aridity in West Africa during
the 1970’s and early 1980’s, these probably represent natural
cycles in weather patterns that are mainly related to large-scale
variations in the movement of ocean currents which affect seasurface temperatures. Similarly, the widely touted claim that the
Amazon rain forest block generates more than half of its own
rainfall is open to question. Recent work has shown that the
influx of rain-bringing moist air from the Atlantic Ocean is much
more important than previously thought. As a result, any adverse
climatic effects of a large-scale conversion of the forest to, say,
pastures, are likely to be correspondingly smaller. Indeed, the
most sophisticated computer simulations currently available for
the prediction of the impacts of such a drastic change in land use
suggest a maximum decline in rainfall of only about five percent.
Because a ‘deforested’ landscape is more likely to consist of a
mosaic of patches of regenerating forest and other land uses one
can safely assume that the effect on rainfall will be even less.
‘Desuite all
1
.A
the rhetoric
in popular articles,
claims of greatly
diminished rainfall
after forest
conversion to
agriculture are
in any case
unproven’
“.-.---. I
~.
___
There are specific situations, however, such as cloud belts on
tropical mountains, where amounts of water reaching the soil surface are strongly influenced by the presence of trees. In these
so-called ‘cloud forests’ a significant portion of the incoming
precipitation is ‘stripped’ by the vegetation from low clouds and
fog blown through the forest canopy. Contributions by such ‘fog
stripping’ may reach several hundred mm per year under favourable conditions and even represent the sole input of moisture
during an otherwise dry season. Another important characteristic
of cloud forests is their extremely low water use which is probably related to their often short stature. This phenomenon has
essentially escaped explanation until now, although a host of
hypotheses have been advanced over the years. Whatever the
underlying causes of the low water use of tropical montane cloud
forests, the net result is that, in combination with the extra water
supplied by ‘fog stripping’, it is not uncommon for streamflow
totals from such areas to be higher than measured amounts of
incoming ‘ordinary’ rainfall. Because of this, catchment headwater
areas covered with these intriguing forests should be protected
if a steady supply of water to the adjacent lowlands is to be
guaranteed.
‘Contributions
by
fog stripping
in montane cloud
belts may reach
several hundreds of
mm per year’
27
However, the special hydrological functioning of cloud forests can
be easily lost. In many tropical mountain areas they are being
cleared for agricultural purposes. Needless to say, the replacement of trees by short crops marks the end of any contributions
by ‘cloud stripping’ and may well result in diminished streamflow.
The same effect may also be achieved in a more indirect manner
when forest clearing on the lower slopes of a mountain may produce a warming up of the overlying atmosphere which, in turn,
causes a lifting of the level of cloud condensation. This effect
may be particularly important on smaller mountains where a relatively small rise in the level of the cloud base may already
leave the mountain altogether cloud-free. On the other hand,
planting tall exotics in deforested montane cloud belts might
reverse the decline in water yield by re-instating fog drip contributions.
Impacts
on erosion
and sediment
yield
The impact of land use change on erosion and sediment yield
concerns us for two main reasons. First, surface erosion causes
the loss of productive soil nutrients for crops or trees - this is an
on-site effect. Second, both surface erosion and mass wasting
may also cause problems downstream by silting up reservoirs or
irrigation channels and damaging watercourses, land and property - these are all off-site effects.
Four main variables determine the likely effects of deforestation
on erosion and sedimentation. Relief is a primary factor: the
steeper the land, the more prone it is to erosion and the easier
it is for eroded sediment to be transported downslope to the
nearest stream. Current exploitation of moist tropical forest land
tends to be carried out increasingly in areas of high relief which
are intrinsically vulnerable to erosion and mass wasting and have
hitherto been left alone. Rainfall is often high in these zones by
definition. Soils are commonly poor and shallow in the areas of
current conversion since the best soils in valley bottoms and on
footslopes have been claimed from the forest long ago. Plant and
soil cover is the final major variable: almost without exception
new land use systems cannot match the characteristically complete canopy cover of undisturbed forest, although certain forest
plantations and multi-storied agroforestry gardens come close. A
brief consideration of these four major variables, relief, rainfall,
soils and cover leads to the inescapable conclusion that any
change from undisturbed forest tends to increase erosion rates
unless specific conservation measures are taken. We will return
to these in the final chapter.
28
Surface erosion in undisturbed tropical forest is generally less
than 0.1 kg per square metre. However, this figure may easily
rise by a factor of 50 - 100 in forest where the litter layer is
removed to be used as fuel or cornposting material and where,
in addition, the undergrowth is harvested for fodder. Where forest
has been cleared for rainfed agriculture without proper soil conservation measures, even higher soil losses (up to 20 - 50 kg per
square metre) have been observed under certain adverse conditions (high intense rainfall, steep slopes, erodible soils).
0,06
.-
f
Figure 4.
Surface erosion
in a young forest
plantation as a
function of soil
with litter and understorY
___------
+
20
33
surface conditions.
40
50
60
Rainfall (mm)
In humid tropical steeplands, bench terracing is the accepted
standard conservation treatment for small scale rainfed farming
nowadays. Under ideal conditions, erosion rates may be reduced
by 90 - 95 per cent in this way. However, the effects of terracing
on sediment production are not necessarily beneficial unless terraces of good quality are combined with conservation husbandry
on the cultivated terrace beds and some form of terrace riser
protection (grassed or stone walls). Failure to do so may well
result in erosion rates that are as high as those associated with
unprotected slopes. Interestingly, most of the sediment leaving
the terrace units seems to derive from the riser walls rather than
from the terrace beds and this has been largely overlooked in
tropical watershed management programmes, or at least underestimated. As discussed later, the maximum slope for construction of bench terraces is usually considered to be about 25 degrees gradient. Where land is cultivated on slopes above 35
degrees the danger of (terrace induced) landslips may become
an even bigger threat than surface erosion without terracing.
29
.-.- .._
_-----~-.
‘Terraces
of good quality
need to be combined
with some form of
riser protection’
Although there are notable exceptions (such as teak plantations
on heavy clay soils) both crop and forest plantations generally
afford good protection against surface erosion once an adequate
ground cover (as opposed to merely a closed tree canopy) has
established. However, in the early stages of development when
much of the cleared surface lies still bare, or at any stage if
management is poor, erosion can be a serious problem. It goes
without saying that the larger the degree of soil disturbance by
heavy machinery and burning of logging debris during the clearing operation, the greater the erosion hazard and the slower the
establishment of both trees and ground cover. In forest plantations natural regeneration of the former understory vegetation
usually takes care of soil surface protection whereas in crop
plantations a leguminous cover crop is normally introduced. Many
tree species require clean weeding during their early stage of
growth and this has led to the development of such cost-effective
ways of tending young trees as the taungya system referred to
earlier. Needless to say, such regular exposure of soil, whilst
boosting tree productivity, presents an additional erosion hazard.
During these early years plantations on steep slopes with impermeable rock types are also especially vulnerable to landsliding after prolonged heavy rain, as was widespread across
southern Thailand in late 1988. Fortunately, both erosion and
landslip hazards tend to decrease rapidly as the plantations
mature and build up a protective ground layer while at the same
time deepening their stabilising root systems. As indicated
already, there is another time when plantations are vulnerable to
erosion. This is when they are ready to be harvested (forest
plantations) or reach the end of their economic life (crop
plantations). The replanting cycle of fast-growing
softwood
plantations is usually shorter than that of crop plantations. While
disruption to the soil surface is more frequent in the former, the
intensity of soil disturbance can be less. This is because the
harvesting of timber often does not require the traumatic uprooting and removal of tree stumps as in the case of plantation crops
like rubber. Where coppicing is practiced - and both eucalypts
and teak are sometimes managed this way - then erosion rates
are likely to be low, although the effect is achieved at the expense of increased subsequent water uptake.
In certain situations, forest plantations are subject to two special
problems not shared by farmland or crop plantations: these are
(i) the removal of litter by villagers using the forest to gather fuel
or fodder, and (ii) fires. Clearly, either of these eventualities can
increase the erosion problem massively. The removal of litter exposes the soil surface to the erosive force of crown drip, and the
aftermath of fire can be severe soil loss through runoff.
31
32
Impact on soil fertility
Upon tropical forest clearing, nutrients are lost from the ecosystem in various ways. Whether such losses will impair future
plant productivity depends mainly on the inherent fertility of the
site, which is determined largely by geological and climatic
factors. As a rule of thumb, however, it can be stated that the
more infertile the soil, the more serious the effect. This is
because forests on the poorer substrates are only able to thrive
there on the basis of an intricate and relatively closed nutrient
cycle which is suddenly disrupted upon clearing.
‘Nutrient inputs
from rainfall
may take
50 - 60 years
to make up for
the losses
incurred during
clearing’
The first direct route of nutrient loss during conversion to other
land uses is via the harvesting of stems. The potential importance of this loss is brought home immediately when one considers the amounts of nutrients that are incorporated in the stemwood and bark of tropical forest trees. For instance, leaving the
most fertile sites aside for the moment, it has been found that
between 10 and 75 per cent of all the calcium and phosphorus
present in above- and below-ground biomass plus that available
in the root zone of the soil is stored in the stems of rain forests.
Similar figures have been obtained for such other key nutrients
as potassium (20-80 per cent) and magnesium (20-65 per cent).
Whilst only the commercially sized stems will be removed upon
logging, the remaining biomass left on the ground is usually
‘windrowed’ and burned.
Contrary to common belief, not only do the more volatile constituents like nitrogen and carbon go up in smoke during burning of
slash. Depending on the intensity of the fire, between 25 and 80
per cent of all the calcium, potassium and phosphorus present in
the slash may be lost in this way as well. Burning represents a
second major pathway for nutrient loss upon forest clearing,
therefore. To make matters worse, nutrients which remain in the
ash are vulnerable to removal in runoff and by leaching. Hightemperature burns may render topsoil temporarily water repellent,
causing potentially dramatic increases in overland flow frequency
and intensity and therefore surface erosion. Nutrients carried
away in eroded sediment constitute the third kind of loss, although this does not necessarily mean that they will be lost from
the ecosystem as a whole. Sediment that is eroded on the steep,
higher parts of a slope may be redeposited where the gradient
becomes less or be trapped behind barriers of slash. Therefore,
only a fraction of the eroded material, and the nutrients contained
in it, may reach the nearest stream and be carried away. A fourth
process via which nutrients are lost upon forest conversion is
through enhanced leaching. Not only is there a lot more water
percolating through the soil now that both rainfall interception and
33
water uptake are diminished, but
vegetation to take up nutrients is
The sum result is that substantial
washed into the streams and lost
also the
reduced,
amounts
for future
capacity of the new
at least temporarily.
of nutrients may be
productivity.
Clearly, the dangers of nutrient loss. are most acute during the
actual period of transition but in due course a new equilibrium will
be reached. Naturally, overall nutrient losses differ widely between locations as a result of differences in rainfall, site fertility,
and the volume of harvested and burned biomass. Nevertheless,
‘ballpark’ estimates for the period required to compensate overall
nutrient losses via nutrient inputs in rainfall and dust are typically
in the order of 50 to 60 years.
With rainfed agriculture, the cultivation of annual crops can lead
to further soil degradation - after a season or two of good crop
yields - unless astute land husbandry practices are applied. Cultivation may cause acidification as cations become depleted, and
aluminium becomes increasingly soluble as a result. The possibility
of enhanced surface erosion rates is always there unless preventative and remedial measures are undertaken. The potential nadir
at the end of a descending spiral is land which is too poor to cultivate anything but a succession of continuous cassava crops.
Nevertheless the picture is not as gloomy as it may seem. There
are a number of husbandry practices, including manuring, composting and planting of legumes which can help maintain soil
fertility under cropping systems. Agroforestry has a particularly
important role to play in this respect. Furthermore there is a direct
incentive for farmers to maintain soil fertility: after all, fertile soil
means healthy and profitable crops.
Where plantations replace forest, a new ‘forest floor’ is created
eventually, given good surface management. In some crop plantations, leguminous cover crops are grown with a similar protective
effect. Application of inorganic fertilisers is the rule for crop plantations and is becoming commonplace in forest plantations also.
Nevertheless, prevention is always better than cure. Similarly,
under the taungya method of establishment there may be a fertility
spinoff for the emerging trees when fertilisers are used or legumes
planted during the intercropping phase. Furthermore in plantations,
the role of trees as a nutrient ‘pump’ is resurrected, bringing
mineral elements from deeper layers into the mainstream nutrient
cycle.
On balance, plantations present the opportunity to establish a
system which, with careful management, can provide an acceptable alternative to the nutrient cycle of natural forest.
34
‘The effect of conservation farming
is simultaneously to protect
the soil and to ensure better crop yields’
35
6. FROM NATURE TO NURTURE - TRANSITION
WITH CARE
Throughout the previous sections we have stressed the potential
for environmental damage through careless systems of clear felling and subsequent irresponsible management of the land. The
purpose of this section is to demonstrate how damage can be
limited - how changes in land use from forest to agriculture, or to
plantations, can be achieved without causing lasting detriment to
the on-site or downstream environment. The key is understanding what factors cause degradation, and how transition can be
achieved through applying the principles of sound logging and,
subsequently, careful land husbandry.
Benefits
without
Penalties
In the companion volume on the impacts of selective logging it
was pointed out that well planned operations could significantly
reduce the damage to soil and vegetation. Most of the guidelines
that should be followed to minimise environmental havoc during
logging are equally relevant to clear felling campaigns. Once
again it can be stated that a// that is needed is to put into
practice what is already known - but so far rarely implemented.
A summary of these guidelines is given below.
Above all, prelogging planning is necessary. This involves an assessment of the forest, including the identification of sensitive
areas to be avoided, for example where it is too wet or too steep
to gain access to the trees. The infrastructural layout of access
roads and skid trails is of paramount importance in minimising
damage to the soil. Roads and major skid trails must be located
on ridge crests wherever possible. This will not only help control
surface erosion, but also the frequency and size of road-related
landslides. As roads provide the most direct route for runoff and
sediment to water courses, proper drainage facilities are a must
and the number of stream crossings should be minimised.
Although manual and animal-based systems of extraction tread
much more lightly on the forest, it is unrealistic to expect loggers
to revert to these relatively slow and expensive methods. Inevitably, machines will be used, but once again there are certain
guidelines which can keep damage to a minimum. For example,
skyline yarding should be prefered to ground-based extraction of
logs wherever this is economic. Uphill log extraction tends to
divert runoff and sediment away from streams, in contrast to
downhill extraction. Where machines are used, then the golden
36
rule is that the fewer passes the machine makes, the less the
damage. Needless to say, it is necessary to use machines of an
adequate size and capacity to eliminate loss of traction, but
oversized equipment means unnecessary compaction of the soil.
Root rake equipment is particularly damaging to the soil. If at all
possible, it is preferable to leave stumps to rot in situ, unless this
is considered to be a disease hazard to the following crop.
There is an understandable temptation to resort to burning the
logging debris, in order to complete the job of clearing the land.
As has been repeatedly mentioned in the foregoing chapters, fire
is detrimental for a number of reasons and should be used
judiciously. Its contribution to increased surface erosion is
perhaps the most important - and this is closely followed by its
negative impact on soil fertility. Rather than burning slash, it
should be windrowed into bands across the contour, thereby
acting as anti-erosion strips, or spread as a mulch if this is
compatible with the subsequent intended land use. An additional
advantage of non burning is the slow release of nutrients as the
material gradually decomposes.
Retaining buffer strips along stream sides is another important
safeguard during forest clearing. These riparian bands of natural
vegetation help protect stream banks from disturbance. In the
early stages after felling, buffer strips also help to filter sediment
out of the inevitable runoff from the cleared surfaces.
‘As roads provide
the most direct
routes for
runoff and sediment
to water courses,
the number of
stream crossings
should be
minimised’
37
Land husbandry
- production
through
conservation
Farming on slopes previously under forest cover does not necessarily mean uncontrolled erosion, ‘mining’ of soil nutrients or an
end to dry season flows. In environmental terms the difference
between good and poor farming practices is in fact greater than
that between good farming and forest. The conventional conservation safeguards are engineering measures - bench terraces,
controlled drainage ways and so on. But engineering alone cannot give adequate protection to the land. More fundamental is the
principle of sound ‘land husbandry’ and the concept of ‘conservation through production’. Important husbandry practices include
integration of livestock, contour farming, the use of manures,
composts and mulches, strip cropping and intercropping with
legumes. In a special category are biological barriers: ‘hedges’
of perennial grasses, such as Vetiver, and of woody species,
such as Gliricidia, which, while not yet widely adopted by farmers, can help to check erosion over a range of slopes. These
techniques are gaining increasing credibility as cheaper and
more versatile alternatives to terracing in specific situations - for
example on shallow soils and steep slopes. The effect of conservation farming techniques is simultaneously to protect the soil
and to ensure better crop yields - good news for the farmer, and
good news for the environment also.
38
Figure 5.
Recommended
slope
gradient
classes
for various
commonly
applied
soil
conservation
measures.
Nevertheless, soil conservation structures still continue to form
the recommended framework for rainfed farming on humid tropical hillsides. On the lowest slopes the recommendation is normally
ploughing and planting along the contour line - supported by
earth bunds, grass strips or barrier hedges (Figure 5). The bench
terrace, in all its variations, is standard for slopes between about
7 and 25 degrees (12-42 per cent). Bench terraces are not suitable for all soil types however and require a soil depth of at least
50 cm. Neither are all bench terraces as effective as they should
be: a common shortcoming is poor maintenance of the riser,
which can itself become a significant source of sediment. Programmes which promote stall feeding of livestock indirectly contribute to soil conservation by encouraging the planting of fodder
grasses on terrace risers. The grass helps to protect the risers
against rainsplash and slope failure while the manure from the
livestock improves the soil of the terrace bed. The main erosion
problem arises above the 25 degree gradient - which is the legal
limit for cultivation in many countries, but is increasingly exceeded in practice. It is impractical to build terraces above this slope:
the proximity of the risers and narrowness of the beds means
that 50 per cent or less of the land is available for cultivation.
Also, the risk of landslips is increased. But reality has to be faced
as land pressure is mounting. Damage limitation is the keyword.
Contour
(or graded)
bunds
or:
Contour
cultivation
Gradient
Maximum
without
structures
terraces
Maximum
without
terraces
39
15 - 250
25 - 450
Maximum
for bench
terraces
_-
Maximum
for agroforestry
Agroforestry systems, that is a mixture of trees and annual crops,
hold some of the best potential for soil conservation on steeper
slopes. The barrier hedges mentioned above are but one example. There is a temptation for the scientist to attempt to create
‘improved’ forms of agroforestry with precision. Typically, this
may involve multipurpose trees spaced equidistantly amongst annual crops, as for example in ‘alley cropping’. But there is a
danger of creating recommendation straightjackets. Rigidity thus
risks replacing one of the most attractive aspects of traditional
agroforestry - the shrewdness with which land users have built
up complex systems which fit the needs of their families as well
as protecting the land beneath. The ‘home gardens’ of the Far
East, for instance, are highly productive systems of intricate
mixed cultivation which effectively mimic natural forest.
‘The traditional
home gardens of the
Far East are highly
productive systems
of intricate mixed
cultivation that
effectively mimic
natural forest’
-...-
As for the steepest slopes, suffice it to say that above about 30
degrees neither cultivation of crops nor agroforestry systems can
be sustained without danger of greatly accelerated erosion and
landsliding. Here, the only safe land use system is undisturbed
forest, whether natural or planted.
Managing
the estates
Plantation crops have long life cycles, and when well managed,
afford continuous ground cover after the first few years. As such,
they may provide ample opportunity for soil surface protection
and hydrological stability. However, in order to ensure that
plantations achieve maximum ‘environmental friendliness’ there
are a number of basic cultural principles which must be followed.
As stated repeatedly, the period between planting and the development of a full ground cover is the most critical. To minimise
erosion hazard during the period of bare soil, terraces may be
constructed before planting commences, as is common practice
on steep slopes for tea in India, for coffee in East Africa and for
rubber in Malaysia. On slopes less than f7 degrees it may be
adequate merely to plant along the contour, with barrier hedges
or grass strips as support measures. A network of access tracks
should be strategically planned within the estate, and both road
and terrace drainage designed at non-erosive gradients.
Protection of the soil between the immature plants is achieved in
a number of ways. For instance, in East Africa, tea is sometimes
interplanted with a ‘nursery crop’ such as oats which not only
protects the soil from rainsplash but also provides mulching
material after harvest. In subsequent years the prunings from the
tea bushes themselves provide the mulch. Coffee cannot tolerate
competition from a nursery crop. Instead, Napier grass or a
similar species is often grown to provide mulching material either in a separate plot, or in contour strips between blocks of
coffee. Leguminous cover crops are instrumental in protecting
the soil between young plants in plantations of rubber, oil palm
and cacao while simultaneously increasing soil nitrogen. These
can be shrubs like lndigofera and Desmodium or creepers such
as Pueraria, Dolichos or Centrosema. Alternatively,
annual
pulses - beans, cowpeas and so on - may be intercropped during
the early years, after which the leaf litter produced by the maturing trees can take over. Nevertheless, additional mulching is
often necessary, particularly around the bases of the trees where
shading may hamper the development of a good ground cover
or stemflow may wash away the litter. Soil fertility is usually
further enhanced by regular fertiliser applications.
41
New trees for old
Many of the comments made about crop plantations also pertain
to plantation forests. Here also we have a form of land use which
provides good opportunities for protection of soil and water resources. Indeed, as noted earlier, some plantations are specifically used for this very purpose - as ‘protection forests’ in vulnerable upland areas. But again there is a world of difference
between a badly maintained plantation, and one which is managed according to enlightened practices. The key is a thoroughly
planned and implemented infrastructure for planting, stand maintenance, protection and harvesting.
In view of the critical nature of the transition and harvesting
periods in terms of erosion and leaching hazards, spot clearing
of planting holes may be a better solution than complete clearing
- especially when the latter implies burning of slash and mechanised cultivation of the soil in preparation for planting. The same
applies to steeply sloping land. The potentially negative impacts
of slash burning have been pointed out already. Where land for
cropping is scarce, there is scope for strategic intercropping of
young plantations with a cash crop legume (or for practising
some other form of taungya) to improve surface cover, fix nitrogen and at the same time generate income for the rural poor.
As we have demonstrated repeatedly, the forest floor is the most
important component of the forest ecosystem in terms of soil surface protection. As such, it should be guarded against disturbance at all times. While the selective collection of dead or fallen
branches for domestic fuelwood may be acceptable, open access
for fuel and fodder collection or free range grazing can promote
soil compaction,
runoff and erosion surprisingly quickly. In
addition, opening up a forest plantation increases the risk of fire.
Thinning and pruning operations cause some damage to the
understory, although it is normally negligible compared to the
havoc that can be wrought during timber harvesting. This is
especially true for short-rotation plantations with fast-growing
species planted for the production of paper pulp, which may be
clear felled every 5-I 0 years. However, where the initial planning
of the access infrastructure has been carried out efficiently, harvesting will be greatly facilitated and damage to the soil correspondingly limited. Such operations should adhere to accepted
standards of good practice, including minimising machine size for
extraction, using skyline systems in truly steep terrain and in
general work uphill and away from water courses.
42
Clear felling of forest plantations usually produces far less slash
than natural forest and the need for windrowing and burning of
slash will be correspondingly less. In this way, precious nutrients
are conserved for future productivity. Soil nutrient reserves may
be seriously depleted after the repeated removal of harvested
produce, particularly in the case of fast-growing species planted
for the production of paper pulp that are harvested every 5-10
years. Regular fertilisation will be required then if soil fertility is
to be maintained. However, even fertile soils on alluvial or volcanic deposits may become depleted after a few rotations of
nutrient demanding hardwood species. Conifers on the other
hand have more modest nutrient requirements. One further way
of minimising nutrient losses is to only harvest the stemwood and
leave bark, branches and foliage on site to rot.
‘In environmental
terms,
the difference
A final word should be devoted here to ‘social’ or ‘farm’ forestry.
No longer is tree planting seen as the exclusive preserve of large
companies, or of the state. No longer are large forest stands
seen as the only way of producing wood. More international development efforts are being directed towards resource poor farmers to help them plant trees on their own land, and this is a
cause which is worthy of support. Soil conservationists throughout the tropics are beginning to realise that it is often the small
scale land users themselves who make the greatest difference
to the health of the local environment. The rationale behind this
is, of course, that when people appreciate the value of a resource, they are much more likely to conserve it effectively.
between good and
poor farming
Concluding
remarks
practices
is enormous’
Returning to our original premise - that forest conversion does
not necessarily spell disaster - we have attempted to demonstrate that environmental damage can be limited, and indeed that
in many cases a new land use system does not need to be destructive. Of course, the process of conversion in itself is highly
disruptive. But an established and well managed plantation, for
example, represents a new form of equilibrium, while significantly
increasing the economic productivity of the land. Similarly, although small scale agriculture in the often steep humid tropical
lands exposes the soil to an increased erosion hazard, there are
a number of highly effective conservation practices at our disposal. Indeed, in environmental terms, the difference between
good and poor farming practices is enormous.
Where forest has been converted, it is our duty to make sure that
sound land husbandry is practised - acknowledging what is here
today, rather than lamenting what has been lost forever.
43
SELECTED
REFERENCES
In keeping with the style and format of this Series, no specific
references to literature have been included within the main body
of the text. The following books and articles comprise our principal sources of information, and form a basis for further reading.
Adams, P.W. & Andrus, C.W. 1991. Planning timber harvesting
operations to reduce soil and water problems in humid
tropical steeplands. Paper presented at the International
Symposium on Forest Harvesting in South-east Asia,
Singapore, June 1991.
Bruijnzeel, L.A. 1990. Hydrology of Moist Tropical Forests and
Effects of Conversion: A State of Knowledge Review.
UNESCO, Paris, and Free University, Amsterdam.
Bruijnzeel, L.A. 1995. Soil chemical and hydrochemical responses to tropical forest conversion: a hydrologist’s perspective. In: Soils of Tropical Forest Ecosystems (A.
Schulte & D. Ruhiyat, eds.), Volume 3, pp. 5-47. Mulawarman University Press, Samarinda, Indonesia.
Bruijnzeel, L.A. & Proctor, J. 1995. Hydrology and biogeochemistry of tropical montane cloud forests: what do we really
know? In: Tropical Montane Cloud Forests (L.S. Hamilton,
J.O. Juvik & F.N. Scatena, eds.), Ecological Studies 110:
38-78, Springer, New York.
Carson, B. 1989. Soil conservation strategies for upland areas of
Indonesia. Occasonal Paper no. 9: East-West Environment and Policy Institute, Honolulu, Hawaii.
Cassells, D.S., Gilmour, D.A. & Bonell, M. 1984. Watershed forestry management practices in the tropical rainforest of
N.E. Australia. In: Effects of Forestry Land Use on Erosion
and Slope Stability (C.L. O’Loughlin & A.J. Pearce, eds.),
pp. 289-298. IUFRO, Vienna.
Critchley, W.R.S. & Bruijnzeel, L.A. 1995. Terrace risers: erosion
control or sediment source? In: Sustainable Reconstruction of Highland and Headwater Regions (R. Singh & M.J.
Haigh, eds.), pp. 529-541. Oxford/lBH Press, New Delhi.
Critchley W.R.S., Reij, C.P. & Willcocks, T.J. 1994. Indigenous
soil and water conservation. A review of the state of
knowledge and prospects for building on traditions. Land
Degradation & Rehabilitation 5: 293-314.
Doolette, J.B. & Magrath, W.B. 1990. Watershed development in
Asia. Strategies and technologies. World Bank Technical
Paper no. 127. The World Bank, Washington, D.C.
44
----. __-._..~.
Evans, J. 1992. Plantation Forestry in the Tropics. Second
edition. Clarendon Press, Oxford.
FAO, 1989. Soil conservation for small farmers in the humid
tropics. FA0 Soils Bulletin no. 60. FAO, Rome.
Hamilton, L.S. 1991. Tropical forests: identifying and clarifying
issues. Unasylva 166 (42): 19-27.
Hudson, N.W. 1995. Soil Conservation. Third edition. Iowa State
University Press, Ankeny, Iowa.
Jackson, I .J. 1977. Climate, Water and Agriculture in the Tropics.
Longman, London.
Jordan, C.F. 1987. Amazon Rain Forests. Ecosystem Disturbance and Recovery. Springer, New York.
Lal, R. 1987. Tropical Ecology and Physical Edaphology. J.
Wiley, New York.
Moldenhauer, W.C. & Hudson, N.W. 1988. Conservation Farming
on Steep Lands. Soil & Water Conservation
Society,
Ankeny, Iowa.
Moldenhauer, W.C., Hudson, N.W., Sheng, T.C. & Lee, S-W.
1991. Development of Conservation Farming on Hillslopes. Soil & Water Conservation Society, Ankeny, Iowa.
Nykvist, N., Grip, H., Sim, B.L., Malmer, A. & Wong, F.K. (1994).
Nutrient losses in forest plantations in Sabah, Malaysia.
Ambio 23: 21 O-21 5.
Opeke, L.K. 1982. Tropical Tree Crops. J. Wiley, New York.
Pearce, A.J. & Hamilton, L.S. 1986. Water and Soil Conservation
Guidelines for Land-Use Planning. East-West Environment
and Policy Institute, Honolulu, Hawaii.
Proctor, J. 1987. Nutrient cycling in primary and old secondary
rain forests. Applied Geography 7: 135-I 52.
Sanchez, P.A. 1976. Properties and Management of Soils in the
Tropics. J. Wiley, New York.
Sanchez, P.A. 1995. Science in agroforestry.
Agroforestry
Systems 30: 5-55.
Tiffen, M., Mortimore, M. & Gichuki, F. (1994). More People,
Less Erosion. Environmental Recovery in Kenya. J. Wiley,
New York.
Wiersum, K.F. 1984. Surface erosion under various tropical agroforestry systems. In: Effects of Forestry Land Use on
Erosion and Slope Stability (ed. by C.L. O’Loughlin & A.J.
Pearce), pp. 231-239. IUFRO, Vienna.
Wiersum, K.F. 1985. Effects of various vegetation layers in an
Acacia auriculiformis forest plantation on surface erosion
in Java, Indonesia. In: Soil Erosion and Conservation
(S.A. El-Swaify, W.C. Moldenhauer & A. Lo, eds.). Soil
Conservation Society of America, Ankeny, Iowa.
Young, A. 1989. Agroforestry for Soil Conservation. CAB International, Wallingford, U.K.
45
The International
Hydrological
Programme
The developing nations of the humid tropics of the world will
represent about one-third of the earth’s population by the end of
the present decade. In the 21st century, these nations will pass
the developed countries in numbers of people. Such a population
shift will alter existing international economic and geopolitical
relationships. With this major change looming on the horizon,
coupled with the need to treat the tropical resources wisely, the
United Nations Educational, Scientific and Cultural Organization
(UNESCO) and the United Nations Environment Programme
(UNEP) joined with 22 other organizations in July 1989 to hold
the International Colloquium on the Development of Hydrologic
and Water Management Strategies in the Humid Tropics at
Australia’s James Cook University. The International Hydrological
Programme (IHP) of UNESCO was the lead organization.
The Colloquium developed strong evidence that the present situation, including the question of tropical forest depletion, was not
only in need of serious consideration, but that the potential for
vastly increased human impacts will be quite significant if they
are not adequately considered now. The formal scientific text embodying the Colloquium papers and supplementary material was
published by Cambridge University Press in the summer of 1993
under the title Hydrology and Water Management in the Humid
Tropics, with M. Bonell, M.M. Hufschmidt and J.S. Gladwell as
editors. A related publication, entitled Hydrology of Moist Tropical
Forests and Effects of Conversion: A State of Knowledge Review, was produced by the joint efforts of IHP’s Humid Tropics
Programme, the National Committee for IHP of the Netherlands
and the Vrije Universiteit of Amsterdam in October 1990.
The present popularized volume on the impacts of tropical forest
conversion is one of several such publications having their origin
in the Colloquium. Others dealt with the disappearance of tropical
forests, the hydrology of small tropical islands, the water-related
problems of large tropical cities, the role of women, the hydrological impacts of forest logging, groundwater, reservoirs,etc.
Additional volumes are in preparation, including one on the
hydrological and conservation aspects of tropical montane cloud
forests.
Further information on any of these publications can be obtained
from the International Hydrological Programme of the Division of
Water Sciences within UNESCO (see back cover for address).
46
MAB Programme
activities
in the humid tropics
Improving the scientific understanding of natural and social
processes relating to man’s interactions with his environment,
providing information useful to decision-making on resource use,
promoting the conservation of genetic diversity as an integral part
of land management, enlisting and co-ordinating the efforts of
scientists, policy-makers and local people in problem-solving
ventures, mobilizing resources for field activities, strengthening
of regional co-operative frameworks. These are some of the
generic characteristics of the Man and the Biosphere Programme
(MAB) - one of the sister environmental programmes within
UNESCO.
MAB, launched in the early 197Os, is a nationally-based,
international programme of research, training, demonstration
and
information diffusion. The overall aim is to contribute to providing
the scientific basis and trained personnel needed to deal with
problems of rational utilization and conservation of resources and
along with the problems
of human
resource
systems,
settlements.
One of the international research themes of MAB is specifically
concerned with the ecology and use of the forested lands of the
humid tropics. Throughout the 1970s and 1980s a number of
field studies were carried out which concentrated
on the
ecological functioning of tropical rain forests, including the now
classical work at San Carlos de Rio Negro in Venezuela, Tai in
Cote d’lvoire, and Luquillo in Puerto Rico. As the body of
research results obtained by the numerous field studies grew
larger and larger, MAB began to disseminate these results to a
wider audience in the late 1980s by means of the MAB Book
Series which to date includes several volumes dedicated to the
ecology and management of tropical rain forests. Similarly, the
MAB Digest Series was launched in 1989 to disseminate
overviews of recent, ongoing and planned activities within MAB
in particular subject or target areas, proposals for new research
activities, as well as distillations of the substantive findings of the
MAB activities.
Information on the MAB programme and various MAB publications is available from the MAB Secretariat, Division of Ecological
Sciences, UNESCO.
47
About the authors:
William Critchley is a conservation agronomist with the Centre for
Development Cooperation Services at the Vrije Universiteit, Amsterdam, The Netherlands. He began his career in Kenya, where he
worked on a number of development projects from 1973 until his return
to Europe in 1987. Subsequently he has concentrated on a mixture of
consultancies, research and publications in his field of specialisation:
the interface between plant production and resource conservation in
developing countries of Africa and Asia.
Sampurno (LA.) Bruijnzeel is a lecturer in hydrology at the Faculty of
Earth Sciences of the Vrije Universiteit with over two decades of experience with forest hydrological research in the humid tropics, mainly in
South-east Asia, the Pacific and the Caribbean. Since the mid-1980s
he has published a number of comprehensive reviews of the literature
on environmental impacts of tropical forest disturbance and conversion.
His other scientific interests include the hydrology and nutrient
economy of tropical montane cloud forests and fast-growing plantation
forests, as well as erosion and sediment transport processes.
Photo credits:
L.A. Bruijnzeel:
W.R.S. Critchley:
R. Klinge:
R. Mieremet:
G.A. Persoon:
J. Rupke:
M.J. Waterloo:
K.F. Wiersum:
8, 9 top, 11 bottom, 13, 16 bottom, 19,
24 bottom, 26,27, 30 bottom, 35, 37, 40
16 top, 30 top, 32 top, 38
11 top
cover, 32 below
2
24 top
9 bottom
20
Sources of remaining illustrations:
Figure 1:
Figure 2:
Figure 3:
Figure 4:
Figure 5:
Adapted from I. Douglas (1977). Humid Landforms. MIT
Press, Cambridge, Massachusetts.
Redrawn from J. Proctor (1987). Applied Geography 7,
p. 135.
After L.A. Bruijnzeel (1993). /nternationa/Association
of
Hydrological Sciences Publication 216, p. 21.
Adapted from K.F. Wiersum (1985), (see reference list).
Adapted from H.C. Pereira (1989). Policy and Practice
in the Management of Tropical Watersheds. Westview
Press, Boulder, Colorado, p.159.
The authors thank Mr Frans Stevens for his invaluable assistance
during times of computer crisis and Mr Henny Colenbrander for his
continued support throughout the preparation of this document.
48