L.A. Bruijnzeel and W.R.S. Critchley
0 UNESCO
1994
Contents
1.
2.
3.
4.
5.
6.
7.
8.
Introduction
Logging systems
Impact of logging on vegetation and soil
Impact on streamflow
Impact on erosion and sedimentation
Impact on the forest nutrient budget
How can logging be improved?
Costs and benefits
Selected references
The International Hydrological Programme
MAB Programme activities in the humid tropics
1
5
7
12
23
28
35
43
45
47
48
3ifgRih
TROPENBOS
Netherlands
Committee
IHP
vrije Universiteit
1. INTRODUCTION
Explorers and naturalists have long been fascinated by moist
tropical forests. It is not difficult to see why. The mass of luxuriant
vegetation and rich diversity of living species represents an ecosystem that is unrivalled on earth. Such forests provide not just a
magnificent spectacle and a sanctuary for an incredible array of
plants and wildlife but also protection for fragile soils against
erosion and degradation by the torrential rainfall that sustains
these very forests. But alongside the explorers and naturalists
came timber merchants. The latter, too, developed a keen interest
in the forests, but for very different reasons: the vast volumes of
potentially harvestable timber. Logging operations in tropical
forest areas have burgeoned in the last decades as mechanisation has permitted the exploitation of previously inaccessible
areas, and at an ever-quickening pace.
“Logging
operations have
burgeoned in the
last decades as
mechanisation has
enabled the
exploitation of
previously
inaccessible areas”
Land use alternatives
There are three main alternatives for land use in areas of moist
tropical forest. The first is to protect the forest completely and
prohibit logging or other man-made disturbances. Forests can
thus be maintained in their natural state for protection of vulnerable river sources, at the same time providing for (limited) recreational activities. However, most tropical forests are concentrated in
poorer countries which cannot afford the luxury of locking up vast
portions of their forests in the form of inviolate reserves. The
second option, and at the other extreme, is to clear the forest and
use the land for an alternative production function. For example,
plantation crops such as rubber, cocoa or oil palm may be established, or the forest may be converted into agricultural fields or
pastures, Clearance for settlements, roads and mines also fall
into this category. However, whilst reality says that the earth’s
natural resources are there to be used, common sense equally
dictates that these must be managed sensibly and sustainably for
the benefit of future generations. Forest conversion, therefore,
can only be successful if it is accompanied by soil and water
control measures which, in addition, must be applied with a rigour
that matches the erosivity of tropical rainfall. The widespread
occurrence of hydrologically and ecologically disrupted land in the
humid tropics, however, is a sad reminder of our lack of commitment to the cause of good land husbandry after forest removal.
1
No wonder, therefore, that there is increased scope for and interest in the third option, the management of forest for continued
production of both timber and other commodities by means of
some form of selective logging. Whilst it is ominous that currently
only a tiny proportion of the world’s tropical forests are being
managed in a fully sustainable manner, the silver lining is that
relatively simple precautions can lead to substantial improvements. Indeed, the main tenet of this document is that there is
room for use without abuse: mankind can reap benefits from this
rich resource while maintaining its value for the future.
Exploitation or conservation ?
The booming exploitation of tropical forests in recent years has
been matched by growing prophecies of environmental doom. In
both tropical and temperate countries conservationists have become increasingly concerned with the welfare of the indigenous
peoples living in tropical forests as well as with the environmental
consequences of forest destruction at various levels of scale,
ranging from the local silting up of streams to changes in the
global climate. As the environmentalists have become more and
more vociferous about their views of the impending catastrophes
accompanying deforestation, a battle between ‘exploiters’ and
‘conservationists’ has resulted. However, the struggle has been
emotive, short of hard factual evidence and often divorced from
the cold light of day. On the one hand, many environmental disasters, such as floods, droughts or massive landsliding, are often
hastily blamed on ‘deforestation’ without taking into account
climatic variability or geological instability. On the other hand, it is
an undeniable fact that ruthless logging operations in many parts
of the tropics have wrought environmental havoc: ‘environmental
friendliness’ is rarely the main concern of commercial companies.
This booklet aims to put the picture into perspective with respect
to different logging practices. We begin with an analysis of what is
actually known about the various environmental impacts of logging. This is then followed by a set of guidelines for simple,
improved logging practices which are known to keep environmental damage to a minimum. In this way the interests of both environmentalists and all that they stand for, and those of the timber
companies and governments of the countries fortunate enough to
have such magnificent forests, may be reconciled. As will be demonstrated in our conclusions, ecological benefits and economic
returns need not be mutually exclusive.
2
“There is room for
use without
abuse:
mankind can reap
benefits from this
rich resource while
maintaining its value
for the future”
“Many
environmental
disasters, such as
floods and
droughts, are often
hastily blamed on
logging ”
3
Mechanisation has
largely replaced
traditional methods
of logging
4
2. LOGGING SYSTEMS
There is a widespread misconception about the proportion of a
given forest stand that is felled in logging operations in moist
tropical forests. It is commonly thought that, as in temperate zone
forestry, the entire forest is felled and extracted for timber, after
which replanting may (or may not) follow. In reality, the species
composition of most tropical forests is so diverse that only a small
proportion of the trees is suitable for exploitation under current
marketing conditions. Natural regeneration takes the place of replanting. As an example, even in the very species-rich forests of
Malaysia, harvesting of all marketable trees would on average result in the removal of only about 15 trees per hectare of forest,
leaving a stump every 25 m or so if the trees had been evenly
spaced. In the case of the much poorer forests covering much of
the Amazonian Basin and common in Africa also, a typical count
would be as low as about eight commercially attractive trees removed per hectare. Whilst the impact of removing so few trees
may seem to be limited it should not be forgotten that such
exploitable trees are often large emergents, which may attain
heights of 35-50 m and have crowns up to 15 m in diameter.
When these giants crash they destroy a considerable part of the
lower stories of the forest. In addition, some of the biggest trees
may be hollow and this may become apparent only after the tree
has been felled. As such, more trees will sometimes be felled
than actually extracted and this of course tends to increase the
damage to the remaining stand.
After the marketable trees have been removed, the forest consists of an irregular mosaic of almost undisturbed cover, pockmarked with patches that have been disturbed to varying degrees.
There are different forms of logging, depending on the intensity of
the timber harvest, and the interval between logging operations.
Two main forms are usually distinguished. These are termed
monocyclic and polycyclic logging.
Systems of logging
Monocyclic logging represents the removal of up to 100% of the
commercially valuable stocking from a forest at relatively long
intervals. The interval between harvesting operations is typically
equal to the maturation period of the main species of trees felled,
5
the so-called rotation period. This may be as long as 60-80 years,
equivalent to the lifespan of man. Because monocyclic logging removes not only mature but also semi-mature trees, a relatively
large proportion of the forest may be affected. The volume of
timber removed during monocyclic operations may be as high as
120 m3/ha in certain South-east Asian forests (home to the prized
dipterocarps), although more commonly the harvested volumes
tend to converge around a value of about 60 m3/ha. The result of
such intense logging is the creation of relatively large gaps in the
canopy. This has the effect of stimulating light-loving species in
the regrowth. As will be shown, the potential for damage to both
soil and remaining trees through monocyclic logging is relatively
high. This is indeed often the case in practice.
Polycydic logging is the selective removal of only the largest
individuals of desirable species. The objective is to wait for a
sufficient number of trees to reach maturity, and then to remove
these alone. Compared with monocyclic logging, fewer trees and
a lower volume of timber is harvested, but the intervals between
harvests are shorter. In some polycyclic systems, such as the
‘CELOS’ system developed for Surinam, or the ‘Tebang Pilih’
system advocated in Indonesia, this interval may be as short as
20-25 years. Volumes of wood removed are typically 20-30 m3/ha
per coupe. Whilst forest disturbance occurs more frequently than
under monocyclic cutting regimes, the amount of damage caused
to the overall forest is, theoretically, considerably less for each
operation due to the smaller amounts of timber being extracted.
Indeed, it is against the interests of loggers to damage immature
trees because these constitute their next harvest. An important
characteristic of polycylic logging is that the gaps formed are
smaller than under a monocyclic regime, and this favours the regeneration of shade-loving species, which are often those with the
greater commercial value. By mimicing the natural cycle of treedeath, gap formation and establishment of seedlings, carefully
conducted polycyclic logging alters the forest less, and comes
closest to a scientific way of sustaining the forest while utilising its
products. However, in practice, this level of care is not always
reached and the cumulative damage which may be inflicted to the
forest has often been such that polycyclic logging systems have
been thought to be unsustainable in the past. Nevertheless, most
tropical rain forests are logged nowadays under some form of
polycyclic system, for better or for worse.
6
3. IMPACT ON VEGETATION AND SOIL
Logging operations of any type inevitably cause disturbance to
the soil surface and to the vegetation which remains. Primary
access roads are cut into the forest and connected by secondary
tracks to regularly spaced ‘log landings’ or ‘timber decks’ cleared
for the temporary storage of logs that have been extracted from
the forest along so-called ‘skid tracks’. Whilst topographic conditions and the size of the trees in some forests permit logs to be
extracted using elephants or even manpower, mechanized traction using rubber-tyred or tracked vehicles are commonplace nowadays. Needless to say, such heavy machinery demolishes all
that stands in its path, adding to the damage already done by the
falling trees themselves. As already indicated, monocyclic logging
inevitably causes more disturbance to the forest canopy and the
soil surface than polycyclic systems, because more trees are extracted during each operation. This is only partially balanced by
the longer periods allowed for the forest to recover under monocyclic regimes.
Typically, for every tree which is logged, a second is destroyed
and a third is damaged beyond recovery. Under unimproved,
standard management practices, polycyclic logging may cause
damage to 15-35% of the remaining trees, whereas under monocyclic logging this figure may increase to 40-60%. As lowland
forests are becoming more and more depleted, loggers are turning their attention to forests growing in (much) steeper terrain
where the use of tracked vehicles is often less practical. Under
such conditions the use of a ‘high-lead’ yarding system, where
one end of the log is attached to a high cable and the other end
is dragged along the ground or swings about (Figure l), has been
shown to be particularly damaging to the vegetation along and
surrounding the cable lines. This system has indeed been banned
in some countries recently. By contrast, the use of a skyline
corridor system (Figure 2) or helicopters, where the logs are
hauled to the landings without making contact with the ground,
produces relatively little damage to the remaining stand, most of
which is caused by the felling of the trees rather than by the
extraction process. Even under the most carefully executed, IOWintensity logging there is a threshold of damage which is unavoidable.
Figure 1.
High-lead yarding.
Figure 2.
The forest floor
The forest floor is especially vulnerable to damage. Most of the
roots are concentrated in the top 30 centimetres of the soil. In
forests growing on the most infertile substrates, the roots form a
distinct superficial ‘root mat’. Damage to the soil by machinery
causes disturbance to the root mat, and this in turn impedes
nutrient uptake by the trees and, therefore, their growth. Similarly,
the forest floor constitutes the seed bank from which new trees
are recruited and it takes little imagination to picture the consequences of soil disturbance for seedling survival. Finally, in the
undisturbed situation the litter on the soil surface plays a vital role
in preventing splash erosion from the rain drops falling from the
canopy. Contrary to the popular belief, the canopies of tropical
rain forests do not break the power of the incoming rain but rather
tend to increase it. This is due to the fact that rain drops intercepted by the trees coalesce to form a film of water on the leaves
from which drops then fall that are up to twice as large as the
original rain drops. Because ground vegetation in undisturbed
tropical forests is often rather sparse due to the low levels of light
at the forest floor it is the task of the litter layer to absorb the
enhanced erosive power of the rain drops falling from the canopy.
Needless to say, its removal exposes the bare soil to the impact
of the rain, with enhanced overland flow, surface erosion and the
loss of precious topsoil nutrients as the result. This, in turn, has
obvious consequences for the emergence of seedlings and the
regeneration of the forest.
8
Skyline logging.
In summary, the forest floor is extremely important in maintaining
proper ecosystem functioning and should therefore be disturbed
as little as possible during timber harvesting operations.
Damage and disturbance
Depending on the extraction system that is used, logging leads to
varying degrees of soil and litter disturbance. In the case of
skyline or helicopter logging, the disturbance will be minimal and
only a few per cent of the area will suffer exposure of the soil.
Similarly, if the logs are hauled to the landings on sleds which are
pushed or dragged by a group of men along a ‘railway’ of jungle
poles laid out according to a falling gradient, disturbance to the
soil is negligible as well. However, where heavy machinery rubber tyred or tracked vehicles - is employed, up to 30% of the
surface may be laid bare in the form of roads, log landings and
skidder tracks. The high-lead yarding technique illustrated in
Figure 1. is sometimes claimed to be less damaging to the soil
than yarding by tractor, but experimental evidence suggests the
reverse. The repeated passage of heavy vehicles and logs over
the extraction tracks generally has dramatic consequences for the
porosity and water intake capacity of the soil, which are both
significantly reduced. The effects have been demonstrated to be
worse for wheeled vehicles, particularly when used on wet, clayey
soils. Unfortunately, there is growing evidence that severely
compacted roads and tracks on clayey substrates have seldomly
recovered to their original water intake capacity, even after more
than a decade. A high degree of compaction not only hampers
seedling establishment and root penetration but also causes leaf
litter from the surrounding vegetation to be washed off the surface
during rain, thus preventing the build-up of a new organic layer.
Similar problems are often encountered when subsoils become
exposed by bulldozing during road building. Due to its low organic
matter content and poor aggregate stability subsoil material in
many areas is particularly vulnerable to the impact of rainfall and
needs rigorous protection if erosion is to be avoided.
The picture then is rather gloomy: it must be concluded that most
current selective logging practices generally cause considerable
damage to both vegetation and the soil surface - and the damage
inflicted is much more than could be expected by natural tree-falls
during heavy rain storms or due to landslides. The better news is
that this damage can be reduced significantly when improved
practices are adopted. Various suggestions for improved management practices as well as the economics of the respective extraction methods will be discussed in the final sections.
9
” Typically,
f or
every
tree which is
hwd,
a second is destroyed
and a third is
damaged beyond
recovery ”
“Where heavy
machinery is
employed,
up to 30% of the
surface may be laid
bare in the form of
roads, log landings
and skidder tracks ”
4.
IMPACT ON STREAMFLOW
Of all the environmental effects associated with logging, it is
probably the hydrological changes which are the most misunderstood. ‘Deforestation’ and logging are held up to be the culprits
when there are floods or landslides after extreme rain, and fingers
are pointed whenever droughts occur and streams dry up. What
started off as genuine concerns have become exaggerated, and
are now repeated so often that myth has become accepted wisdom. As we have seen in the previous section, injudicious logging
causes widespread damage to the soil and remaining vegetation.
So too is the hydrological cycle negatively affected. However, this
does not always need to be the case, and a balanced, objective
analysis is required based on the facts available.
The Hydrological
Cycle
The natural hydrological cycle in moist tropical forest is illustrated
in Figure 3. By definition, moist tropical forests are found where
annual rainfall is high - at least 1.5 metres per year - and seasonality is limited to a dry season of about three months maximum.
Rain falling on the forest ecosystem reaches the ground via three
routes. Between 5% and 25% of the rain reaches the forest floor
as ‘direct throughfall’, falling through openings in the canopy
without touching leaves or stems. A further small proportion
(usually i-2%) flows down the tree trunks as ‘stemflow’. The
remainder of the rain strikes the vegetation. The majority of this
then reaches the ground as drip from the canopy, but a significant
proportion (typically lo-25%) 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 total amount of water
reaching the forest floor in throughfall, drip and stemflow is
usually referred to as ‘net precipitation’.
Undisturbed forest soils are usually rich in organic matter and
show abundant fauna1 activity which helps to maintain soil structure, porosity ‘and infiltration rates. Therefore, upon reaching the
forest floor, the great majority of the rain infiltrates the soil via
the leaf litter - which provides excellent protection against overland flow and erosion. Indeed, ‘infiltration excess overland flow’ is
a relatively rare phenomenon in most tropical forests.
12
“Of all the
environmental
effects associated
with logging,
it is probably
the hydrological
changes which are
the most
misunderstood”
I Crown
drip/
Figure 3.
The hydrological cycle for a forested ecosystem.
13
However, surface runoff may also be caused by rain falling onto
an already saturated soil in certain situations. This typically occurs
in hillside hollows or concave footslopes near the stream where
subsurface flow tends to converge and so maintains near-saturated conditions. Additionally, such ‘saturation overland flow’ (SOF)
can be observed during and after intense rainfall where an impeding layer or solid bedrock is found close to the surface. As will
be shown later, it is of the utmost importance to stay clear of
such SOF ‘hot spots’ during logging operations if adverse hydrological effects are to be kept to a minimum.
Further losses of water from the soil in moist tropical forest are
either upwards through uptake by the trees for subsequent transpiration from the canopy, or downwards through drainage into the
stream network. The amount of water that is consumed by closed
canopy tropical forest is high, typically about 1000 millimetres per
year as long as no severe soil water deficits are experienced. In
this way, a large proportion of the soil moisture is pumped back
into the atmosphere by the trees. The remaining 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. Such water drains
slowly and steadily into the drainage network from the reservoir of
moisture in the soil. This process accounts for the ‘baseflow’ of
streams. In seasonal climates, baseflow reaches a minimum in
the dry season and this is referred to as ‘dry season flow’.
After rainfall, streamflow increases. This increase comes both
from the rapid throughflow of water already present in the soil
before the start of the rain, and from contributions by saturation
overland flow in a narrow zone around the stream channel (in
most situations) or by infiltration excess overland flow during
periods of extreme rainfall. The increase in streamflow above the
baseflow is often called ‘stormflow’ or ‘quickflow’. The highest rate
of stormflow is commonly referred to as ‘peakflow’, and this rate
may be reached during a rainfall event or as late as a number of
days afterwards, depending on catchment characteristics and
wetness, as well as on the duration, intensity and quantity of the
rainfall event itself (Figure 4).
01
0
I
20
I
40
I
60
I
60
I
too
I
120
Hours since start of rain
Flgure 4.
Catchment response to rainfall as a function of topography:
high peaks in runoff, that follow the rainfall pattern, are produced 6y
(saturated) overland flow in streamhead hollows and other concavities,
whereas straight slopes with relatively deep soils tend to discharge their
water via delayed subsurface throughflow.
The total volume of water produced as streamflow from a given
catchment area or ‘watershed’ over a given period of time is
called ‘water yield’. As we have seen, streamflow can be broadly
divided into two components: baseflow and stormflow. Logging
affects both and the following sections explain the reasons for
these changes.
15
“When a gap is
formed, evaporation
is reduced, at least
temporarily ‘I
“There is strong
evidence that
effects of selective
timber harvesting
operations on peak
discharges can be
kept small by a
reasonable amount
of care”
16
Impact of logging on water yield
As indicated earlier, logging creates a mosaic of gaps in the
forest canopy. Whilst the size and number of the gaps depend on
the intensity of the logging, they invariably occupy a greater total
area than gaps which are formed by natural tree mortality and
landsliding. When a gap is formed, evapotranspiration is reduced,
at least temporarily. The large trees which used to pump the bulk
of soil moisture back into the atmosphere have been removed. In
addition, more rainfall reaches the soil in a newly created gap
because less rain is intercepted by the still immature canopy.
Both factors tend to increase the levels of soil moisture in these
gaps, despite higher soil temperatures and an increased atmospheric evaporative demand in the clearings. The larger a gap,
the more this effect becomes apparent, and the longer it lasts
(Figure 5).
250
I.
1.
270
”
1.1.
290
310
Day of the experiment
Soil water depletion patterns as observed during the dry
season in undisturbed forest, in six-year-old regrowth, and in freshly maae
large (50 x 50 m) and small (10 x 50 m) gaps in Costa Rica.
17
As long as soil disturbance during logging is limited - affecting,
say, less than 10% of the area - the extra net rainfall and reduced
evapotranspiration will be translated automatically into increased
water yield, particularly through increased baseflow. However,
where logging operations are crude and soil disturbance high, the
gain in soil moisture due to decreased evapotranspiration will still
tend to increase overall water yield, but a larger proportion of it
will be in the form of rapid throughflow and overland flow, thus
increasing the stormflow component of streamflow. In more extreme situations, baseflow during prolonged dry spells may even
be reduced after logging. This may occur when soil compaction
by machinery has become so widespread that during wet periods
the water storage capacity of the soil is no longer replenished by
infiltrating rainfall. The resulting lack of water stored in the soil
subsequently shows up as reduced baseflow, despite the reduced
uptake of water by the remaining forest.
Experimental evidence
Quantitative evidence for these effects is far from comprehensive,
however, and some of it must be treated with caution. Forests
and catchments are diverse and rainfall patterns are often erratic.
Even where streamflow emanating from forests is carefully monitored before and after logging, the variation in rainfall between
years complicates analysis. Arguably the best experimental
approach is to use the ‘paired catchment’ methodology, where
similar, forested catchments in the same area are simultaneously
monitored. The two are intercalibrated over a number of years to
account for climatic variations before one is logged while the
other is left undisturbed. Results are now available from a few
such studies for moist tropical forests, and although some showed
little or no effect on water yield after light (polycyclic) logging, one
of the most comprehensive studies, a monocyclic logging operation using crawler tractors and winch lorries at Bukit Berembun
in Peninsular Malaysia, confirmed that water yield was indeed
increased by the logging, with the majority of the increase coming
through increased baseflow. This was found to be true for both
‘supervised’ and ‘unsupervised’ logging practices. Where the extraction of about 33% of the commercial stocking was carried out
according to ‘environmentally-friendly’ guidelines (the ‘supervised’
treatment, which involved closely specified prescriptions for layout, gradients and drainage of tractor tracks), overall water yield
was increased by 40% compared with pre-logging conditions.
Total water yield was increased by about 70% upon harvesting
40% of the stocking in the adjacent catchment by means of
generally practised methods (the ‘unsupervised’ treatment).
18
There is no solid published information on the time span which is
required for post-logging levels of water yield to diminish to their
original value. It is to be expected that different logging intensities
will result in different recovery periods. At Berembun, there was
no significant decline in the net gain of (base)flow with time over
a period of four years after logging, suggesting that water use by
the regrowth in the gaps created by logging remained below that
of the original stock for at least this period of time. However,
there are indications from Borneo that the water use of regenerating logged-over forests may well stabilise after 5-10 years.
Whilst the relative increase in water yield recorded after ‘unsupervised’ timber extraction at Bukit Berembun (70%) may seem high
at first sight, the absolute increase in flow (ca. 160 millimetres per
year) becomes far more modest (about 10%) when expressed as
a percentage of the evapotranspiration of the undisturbed forest
(ca. 1450 mm). Forest exploitation is often accused of causing
substantial reductions in rainfall, but it is difficult to see how such
a relatively small drop in the amount of moisture which is evaporated back into the atmosphere could achieve such an effect,
particularly in view of the fact that the reduction in evapotranspiration tails off within a decade after logging. Climatic changes in the
tropics in relation to land-use dynamics are only poorly understood at present, but it is almost certain that such changes are
hardly effected by logging operations.
“It is often
conveniently
forgotten that floods
are a natural
hazard in areas with
heavy rainfall ”
19
Impact on catchment response to rainfall
Popular wisdom holds that logging causes floods, and that floods
are damaging. At the same time, it is often conveniently forgotten
that floods are a natural hazard in areas with heavy rainfall. A
‘flood’ is defined by the maximum rate of streamflow over a given
duration. The magnitude of the peak discharge determines the
maximum height of the water in the stream channel, and has
therefore direct implications for downstream land users. It is
important to know, therefore, what the evidence is for increased
peak flows (and stormflow volumes) as a result of selective logging.
Quicker response
We have shown that one impact of logging is, generally, to increase overall water yield from catchments, with higher gains in
yield corresponding with larger volumes of timber removed. As
long as most of the rain is able to infiltrate, the soil becomes
wetter compared with pre-logging conditions. When additional rain
falls, this has the effect of ‘pushing’ the stored moisture more
rapidly into the drainage network. Therefore, the ‘concentration
time’ for the catchment is reduced: that is, more water reaches
the stream network more quickly during and immediately after a
storm, rather than filling the soil’s storage capacity and then being
released slowly over a later period as baseflow. In addition,
contributions by saturation overland flow also tend to increase
with increasing catchment wetness. It has been demonstrated
that, in the case of complete forest removal, mean peak discharges are on average enhanced by about 50% due to the effect
of increased wetness of the soil alone: that is, without additional
contributions by overland flow due to soil disturbance. Because
reductions in vegetation cover usually associated with logging are
much smaller, the effects on peak discharges related to changes
in soil moisture status will be typically (much) less than 10%.
However, as indicated earlier with respect to damage to the
vegetation and soil, the extra rainfall reaching the ground in the
newly created gaps does not necessarily infiltrate as efficiently as
before. This is clearly the case for roads, landings and skid
tracks, where the protective litter layer has been removed and the
surface has become compacted. Runoff rates from such surfaces
can be very high (up to 70% of incident rainfall), particularly in the
case of thoroughly compacted clay soils. Therefore, in recently
logged-over forests, infiltration excess overland flow usually
becomes more common. It may become particularly widespread
in areas where logging operations are implemented without
20
“The repeated
passage of heavy
vehicles and logs
over the skid tracks
has dramatic
consequences for the
water intake capacity
of the soil ‘I
21
regard for the environment: where road and track lay-out has
been poorly planned and constructed, and where heavy equipment has been allowed to roam the forest (‘timber cruising’),
meanwhile compacting the earth and creating larger gaps than
necessary by wanton destruction of vegetation.
Clearly these effects are linked to the method of logging used - as
well as to the physical characteristics of the catchment itself.
Again, in the case of skyline, helicopter, or animal-based extraction procedures, soil disturbance and thus overland flow and sheet
erosion are minimal, particularly when only small volumes of timber are harvested. The opposite is true for operations removing
large volumes of wood, using wheeled skidders, crawler tractors,
or a high-lead yarding system in the case of steep terrain. As we
have seen, typically IO-30% of the soil may become more or less
seriously disturbed during such intensive monocyclic operations.
Similarly, flat terrain underlain by highly permeable sandy soils
tends to be far less vulnerable in this respect than steeper areas
with clayey soils of low permeability.
Peak flows: what changes?
The experimental evidence with respect to the effect of logging on
the magnitude of peak flows is, again, rather limited. Whilst peak
discharges in all experiments reported to date have shown to be
increased as a result of logging, the increases were rarely statistically significant. It could be argued that these experiments may
have produced biased results because the loggers, being aware
of the hydrological monitoring programme and its objectives, may
have tended to behave more carefully than they might have done
otherwise. There are also numerous anecdotal accounts of greatly
altered streamflow regimes following commercial forest exploitation. True as these may be, the experimental results at least
provide strong evidence that effects of timber harvesting operations on peak discharges, and thus downstream flooding, can be
kept small by the application of a reasonable amount of care
during the exercise. As will be discussed in more detail in Chapter
7, the lay-out of the extraction network is of particular importance
in this respect.
22
5. IMPACT ON EROSION AND SEDIMENTATION
In response to the increased demands for water by a rapidly
growing population, and the often deteriorated flow regimes of
rivers, engineers in the humid and seasonal tropics are relying
more and more on the establishment of large structural works
such as river dams. Ideally, the lakes created behind these dams
act as storage reservoirs to diminish floods during the rainy
season and to provide water for irrigation and other purposes
during rainless periods. In addition, they may be used for the
generation of hydro-power, as well as for recreation.
However, there are countless examples of reservoirs silting up in
the humid tropics well before their ‘design life’ has been reached.
The reservoirs unintentionally act as sediment traps, capturing
river bedload of rocks and stones, and also filtering out the sand
and silt particles carried in suspension by the rivers feeding the
lake. These products of erosion accumulate at the points of entry
into the reservoir, gradually fanning out as the process continues,
and so reduce the lake’s storage capacity. To what extent are the
volumes of sand and stones carried by tropical rivers a natural
phenomenon, and what is the role of man’s activities in this
respect? Should upstream logging receive the blame for accelerated reservoir siltation and raised river beds as is so often
claimed, or is an increasing amount of forest clearance for agriculture in the upland areas the culprit?
There is no doubt that both erosion and sediment yields increase
after selective logging. This is not surprising, as undisturbed
natural forest has one of the lowest surface erosion rates of any
form of land use in the humid tropics. As we have seen, this is
due especially to the nature of the forest floor - the leaf litter and
root mat - which resist erosion. However, before discussing the
factors which influence the level of erosion and sediment yields
after forest exploitation, we will briefly look at the processes
involved.
Processes of erosion
There are several different forms of soil erosion. Splash erosion is
the process by which soil particles are detached and moved by
the impact of raindrops splashing onto the soil surface.
23
The eroded particles - which may only have been moved a few
centimetres - are then vulnerable to further transport downslope
by overland flow. The tell-tale signs of such ‘sheet erosion’ are a
series of ‘steps’ a centimetre or so high running across the slope,
or the accumulation of litter and soil behind small obstructions
such as tree roots, rocks, etc. As indicated earlier, both splash
and sheet erosion are of little importance in undisturbed forest
conditions, but they may well produce substantial amounts of
sediment after the soil is bared, particularly after compaction.
Once this stage is reached, topographic irregularities often lead to
the concentration of overland flow in micro-channels termed ‘rills’.
If the process continues long enough, these rills may deepen and
widen into gullies.
Both rills and gullies are often observed on poorly sited skidder
and tractor tracks and along badly drained feeder roads, particularly where erodible subsoil material has become exposed by
bulldozing. Intensive rilling and gullying is a sure sign that large
volumes of soil material have been removed from the site unnecessarily, hampering forest regeneration and future productivity.
Heavy erosion is
commonly observed
on poorly drained
roads - especially
where erodible
subsoil is exposed
24
- .-.
---
-
‘Mass wasting’ is another mechanism of sediment supply to
streams, especially common in steep areas where rainfall is high.
Landslips and riverbank erosion fall into this category, and are
often a natural hazard, although the intensity of, particularly,
shallow landslips (say, less than a metre deep) has been shown
to increase after tree removal due to the loss of the stability
imparted by the tree roots. Similarly, riverbank erosion may
increase after logging, as a result of scouring by the increased
peak flows which we have seen to be associated with poorly
executed logging operations.
However, it is important to recognise that not all eroded material
is delivered directly into the stream network. Particles are often
stored temporarily (or permanently) lower down the slope. This is
especially true for splash and sheet erosion, and explains why it
is impossible to predict catchment sediment losses from observations of erosion made on small runoff plots alone. On the other
hand, gully erosion, large landslides and river bank erosion tend
to deliver sediment quickly and directly into the stream. But even
here there is storage: just because sediment reaches the stream
network, that does not mean it will appear in downstream reservoirs overnight. It may take an exceptional peak flow to flush the
stream bed and wash the sediment into its final resting place.
Building roads in
midslope locations
displaces
unnecessarily large
amounts of material and is highly
damaging to slope
stability
Difficulties in quantification
The extent of the increase in erosion and sediment yield after
logging in moist tropical forests is poorly quantified, and more
work on this topic is urgently required. However, putting numbers
on these processes is difficult for several reasons. High on the list
of complicating factors are the differences between individual
catchments. Amounts of sediment carried by rivers draining fully
forested tropical catchments may easily vary by a factor of 20-30,
depending on topography and soil erodibility. Furthermore the
erosion and sediment delivery rates for a given catchment may
differ enormously from year to year due to variability of rainfall:
this effect has to be isolated from the impact caused by logging.
No single study has, so far, achieved an accurate picture of the
true extent of erosion and sediment delivery for logged forests.
Nevertheless, the evidence available to date suggests that sediment yields in areas with initially low sediment production may
increase by between two and ten times as a result of road construction alone - depending on the siting and extent of the road
network - and this may then increase to twenty times the original
amount from undisturbed forest as a result of log extraction by
means of tractors or skidders. Roads and compacted tracks often
form a lasting source of runoff and sediment to the streams and
where the extraction network has been poorly sited or constructed, a return to pre-logging sediment concentrations is never likely
to occur. Where stream sediment loads used to be low, increased
sediment concentrations after timber or mineral exploitation may
ultimately affect the composition of the fish population in the
streams and therefore directly affect the diet of forest dwellers.
Recovery after logging
Some general reduction of stream sediment loads does of course
take place as the forest regenerates, but the rate of this recovery
is the least documented of all. Although there is some evidence
that erosion on former skid tracks may be halved after a few
years where recolonisation is successful, rilling and gullying of
steep, compacted tracks may last much longer. Even where regrowth occurs rapidly, sediment temporarily stored in the catchment (in hillside depressions or at the foot of the slopes) will
continue to find its way to the streams, and this tends to keep the
sediment yield values high for a number of years after logging.
There is some evidence to suggest that annual sediment yields
are reduced to about twice the original rates after two to five
years, depending on the amount and intensity of rainfall.
26
Needless to say, all this points to the paramount importance of
minimising ground disturbance during harvesting operations.
Catchment sediment yields in the warm temperate zone have
been shown to remain virtually unaffected by skyline logging,
whereas manual extraction of timber in lowland rain forest in
Borneo did not increase stream sediment loads either. Even
where tractors are used, however, the application of a series of
simple precautions (discussed in more detail in Chapter 7) may
already lead to two- to fourfold reductions in catchment sediment
yields compared with those produced by ‘unsupervised’ logging
operations.
Vegetation
recovering on
f orn 2er skid tracks
27
6. IMPACT ON THE FOREST NUTRIENT BUDGET
It is widely assumed that the majority of the plant nutrients within
a moist tropical forest are held in the vegetation, and that such
forests generally grow on poor or very poor soils. One popular
opinion is, therefore, that repeated removal of trees from the
forest, without any return of nutrients by man, will rapidly lead to
the creation of a ‘nutrient desert’. Again, fact needs to be separated from fiction. Before discussing the effects of selective logging
on soil fertility and the forest nutrient budget, let us consider the
various gains and losses of nutrients to and from the undisturbed
ecosystem (Figure 6).
The forest nutrient cycle
Nutrients enter the forest both from above, and from below. Rain
falling on the forest contains plant foods, while both dust and
aerosols deposit nutrients on the forest canopy during rainless
periods as well. Below the ground (and to some extent above)
atmospheric nitrogen is fixed from the air by micro-organisms.
Weathering of rock beneath the soil may provide further nutrients
to the biotic portion of the system where the fresh bedrock is
situated close enough to the surface to be within reach of the tree
rootlets. There is a steady flow of nutrients from the canopy (the
above-ground plant community) to the forest floor by litterfall, but
also by throughfall of rain and stemflow which wash nutrients
down from the trees. The forest floor is carpeted with litter - dead
and decaying leaves, branches and other debris, including entire
trees which have been downed as a result of heavy rain, wind
gusts or landslides. This organic matter decomposes to release
plant nutrients which may be taken up by the plants for growth,
thus completing the cycle. Losses of nutrients in an undisturbed
forest occur primarily through leaching from the soil in drainage
water, and to a lesser extent via surface erosion. In seasonal
climates, forest fires may play a role as well. Some nitrogen is
lost from the forest floor by denitrification processes, and a
portion of the phosphorus in the system can effectively be lost to
plants through fixation by organic and mineral compounds.
28
--
EXCHANGE
COMPLEXES
--mm_ ---__
-__---
rock
weathering
-----__
SOIL
-.
-T
Ro&
Figure 6.
\.
nutrient
losses in
water movemantr
from
,,--------------forrrt
‘- -_______-_-_-_--
,’
-’
The nutrient cycle in moist tropical forest.
29
--
Undisturbed tropical forests generally produce a spectacular
amount of plant growth - in the region of 300-500 tonnes (dry
weight) per hectare of above-ground standing biomass - even on
exceedingly infertile substrates. How do these forests manage to
sustain such wealth under such poor conditions? The answer
apparently lies in their relatively ‘closed’ nutrient cycle. Inputs and
outputs of nutrients are usually very limited in the case of forests
growing on the most infertile soils, and nutrients are therefore in a
continuous state of circulation through the system. In other words,
they are used again and again without being lost from the system.
According to this view, the concentration of fine roots at and just
below the forest floor plays a key role in the process of nutrient
uptake. The concept of a relatively closed nutrient cycle remains
important, even though recent soil surveys in hitherto poorly documented rain forest areas have shown that the areal extent of
really poor soils is only half of that commonly assumed, and that
fine tree rootlets in these soils can be found many metres below
the surface.
Nutrient losses
It is clear that any interference with forests growing on nutrientpoor soils could potentially have a serious effect on the nutrient
budget, by disturbing the natural cycle. As will be shown below,
this impact can indeed be serious where logging is carried out at
a high intensity, and in an uncontrolled manner. Where this is the
case, future productivity will be impaired as a result. Four aspects
need to be considered in this respect. These are: loss of nutrients
in harvested timber, erosion and redistribution of topsoil, increased leaching, and forest fires.
Logging removes a proportion of the above-ground nutrients held
in the timber. The critical factor here is how much is removed,
and how often. Where intervals are long, and timber extraction
light, as in certain polycyclic regimes of selective logging, the
associated losses of nutrients will be very small - typically about
24% of the total amount stored in the above-ground living biomass. On the other hand, there are grounds to justify fears that
heavy logging (extracting, say, more than 100 m3 of timber per
hectare as is not uncommon in the rich forests of South-east
Asia) may lead to losses of as much as 15% of the total stock of
nutrients in the biomass.
The second kind of nutrient loss concerns the nutrients contained
in the extra soil eroded from the disturbed parts of the forest. We
have already seen how uncontrolled logging can lead to high soil
loss, particularly from poorly sited and compacted skid tracks. A
closely related phenomenon is the redistribution of topsoil material
30
“Inputs and outputs
of nutrients are
often very limited
and nutrients
are in a continuous
state of circulation
through the system ”
Amounts of
nutrients removed in
harvested. timber
may be substantial
when earth is moved aside to construct roads, landings and
tracks. Although this may not lead to a net loss of nutrients from
the area as a whole, it has obvious implications for the recolonisation of the skid tracks. Little is known about the accompanying floristic dynamics.
Thirdly, leaching losses tend to occur more readily after logging.
Where the rootmat and surface litter are extensively disturbed,
there is no longer the same filtering effect which helps to capture
nutrients washed down from the canopy. Leaching is also accelerated because more net rainfall reaches the soil through the
gaps in the forest, while less water is ‘pumped back’ by evapotranspiration. Moreover, the sudden addition of large amounts of
organic matter in the form of logging debris left to rot on the
forest floor and the (at least temporarily) reduced capacity of the
new vegetation in the gaps to utilise the available nutrients, both
add to the problem. The sum result is increased drainage into the
stream network, and more leaching losses of soluble nutrients.
31
“Once an area is
opened up by
logging roads there
is often an influx of
land-hungry people ”
32
Evidence points to a threshold gap size in the canopy (probably
located somewhere between 200-500 m*) above which leaching
increases significantly. Sandy soils are more vulnerable in this
respect than more clayey soils, reflecting their different water and
nutrient retention characteristics.
The final potential cause of nutrient loss after logging is fire.
Selective logging does not include burning as a deliberate operation, but it sometimes occurs as an unwelcomed side-effect. The
availability of discarded logs provides an opportunity for local
charcoal makers to start small-scale operations within the forest
concession. Smouldering charcoal heaps can blaze out of control,
and cause at least localised forest fires. More important perhaps
is the influx of land-hungry people who start practising slash-andburn types of agriculture once the area has become opened up
by the logging roads. Finally, the logging debris left on site constitutes a potential hazard in that it provides large volumes of fuel
if a forest fire occurs. Whatever the cause of these fires, post-fire
leaching and erosion greatly add to the direct losses of nutrients.
33
Sustainability and safety limits
Taking these various losses into account, what then are the chances for sustained site fertility in relation to selective logging? Will
natural contributions of nutrients from atmospheric and geological
sources be sufficient to compensate for these losses? Unfortunately, information which would allow the accurate computation of
‘safety limits’ for timber offtake and harvest intervals for different
situations is still far from complete.
The general picture is that damage can, and does occur to the
nutrient cycle; but this is mainly associated with heavy logging,
and poorly planned operations in areas with infertile soils. The
evidence available suggests that overall nutrients reserves will
hardly be depleted where modest amounts of timber (say, 20 m3
per hectare) are harvested at intervals of about 20 years, even on
poor soils with little or no nutrient input by rock weathering. The
relatively small losses in timber and extra leaching are roughly
compensated by atmospheric additions. Similarly, there are indications that volumes of 60 m3 of timber may be harvested per
hectare of forest about every 60 years as part of a monocyclic
system, without serious depletion of soil “nutrient reserves. However, it is almost certain that natural inputs of nutrients to the
richly stocked dipterocarp forests of South-east Asia are not sufficient to sustain the harvesting intensities of 100 m3 per hectare
and above that occur in some parts of the region. There is a need
for further research in this respect.
34
“It is almost certain
that natural inputs
of nutrients to the
richly stocked
forests of
South-east Asia are
not sufficient
to sustain harvesting
intensities of
100 m3 per hectare
and above”
7. HOW CAN LOGGING BE IMPROVED?
‘All that is needed,
is to put into
practice what is
already known - but
so far rarely
implemented ”
There is a growing body of evidence that at least half of the
damage caused by selective logging to vegetation and the soil
can be avoided by careful execution of well-planned harvesting
operations. All that is needed, is to put into practice what is
a/ready known - but so far rare/y implemented. As we will see in
the next chapter, the extra cost of ‘environmental-friendly’ logging
is modest, yet the benefits are considerable. What follows now is
a checklist of the requirements for improved logging, both the
organisational framework and the technical measures.
Pre-logging planning: contracts and conditions
The basic prerequisite for improved logging is planning before
operations begin. Pre-logging planning must be carried out collaboratively between the relevant Government department and the
logging company. The end-product is a contract agreed by the
two parties. The contract not only specifies the agreed timber
harvest quota, but also the conditions which must be adhered to.
The actual planning involves an assessment of the catchment to
be logged, and the preparation of an inventory of its particular
requirements for environmental safeguards. Catchment characteristics as well as forests differ widely, and each situation should
be examined in sufficient detail to allow the identification of particularly sensitive areas. Generally, these will include riparian
zones and other wet spots that are not only likely to produce
saturation overland flow during rain, but are also particularly
vulnerable to soil compaction, Other sensitive categories are
steep slopes that are prone to landsliding, very shallow soils, etc.
Apart from the delineation of sensitive areas, the logging contract
may also need to include the prohibition of log extraction during
very wet periods to minimise the risk of soil compaction (Figure
7). The next step is to combine the information on the positions of
the trees to be harvested with the characteristics of the terrain to
derive the most economical, yet least damaging extraction technique. This process may be facilitated by the use of a Geographical Information System. Most importantly, the various conditions
specified in the agreement should be monitored and supervised
by the responsible government agency. Finally, there should be
provision for penalties where conditions are not met.
35
60
50
40
u
30
\
\
\
Track-type machine
\\
20
\
‘b..
14 % moisture
‘.
-.
‘s
---_
0
IO
21 % moisture
6’
S
i
I
I
I
\
L
I
-
Rubber-tyred
t
machine
3
2
1
0
I
I
I
I
I
I
I
I
I
I
1
I
I
I
I
2
4
b
8
10
12
I4
16
16
20
22
24
26
28
30
Number of vehicle passes
Figure 7.
The impact of rubber-tyred and tracked skidding machinery
on the water intake capacity of the soil as a function of
the number of vehicle passes. Note the contrast in impact
between the two types of machines, and the effect of soil
wetness.
36
Road infrastructure
The considerable potential of the haulage roads and skid trails to
contribute to environmental damage has been stressed in previous chapters. Even if no other measures are introduced, the
careful planning of this infrastructure can help enormously to
minimise damage, both on-site (erosion) and off-site (stream
sedimentation). Roads and major skid tracks should be located on
ridge crests wherever possible. This will not only minimise surface
erosion, but also the frequency and size of road-related landslides. This has its price, however, because in steep terrain some
of the most productive parts of the forest stand are often found on
ridges. Where sloping terrain cannot be avoided, maximum road
gradients need to be specified and adequate drainage facilities
designed. As roads generally provide the most direct routes for
runoff and sediment to water courses, the number of stream
crossings should be minimised (Figure 8). Where crossings are
necessary, they should be located and constructed in such a way
as to minimise sediment contributions to the stream. Whilst proper
planning and construction of the haulage road network is essential, subsequent maintenance is equally important. This aspect is
often overlooked or inadequately addressed. The recent finding
that the application of reduced tyre pressure not only reduces
vehicle operational and maintenance costs but also expenditure
on road surfacing and maintenance, is good news therefore. Additional advantages of reduced tyre pressure include a decline in
the ‘rutting’ of road surfaces and therefore erosion, and an extended haul season due to improved traction.
Figure 8.
Uphill log extraction
tends to divert
runoff and sediment
away from streams,
in contrast to
downhill extraction.
37
Much of what has been said about roads also holds for skid
tracks, and the proper planning of their location and drainage
should be one of the key elements of any logging agreement. As
illustrated in Figure 8, uphill extraction of timber is to be preferred
to downhill extraction, because the former tends to divert the flow
of runoff and sediment away from the streams. Needless to say,
this will greatly reduce downstream flooding and sedimentation
hazards. Similarly, log landings should be located in such a way
as to minimise contributions of runoff and sediment to streams.
Finally, they may need to be ‘ripped’ after completion of the
operation to promote their recolonisation.
Mechanisation
It is unrealistic to expect logging companies to revert to the
original damage-limiting methods of manual or animal-based timber extraction, but it is certainly possible to demand the minimum
use of heavy equipment. Usually, the bigger the machine, the
more damage is caused through destruction of vegetation and
compaction of surfaces, particularly where soils are clayey or wet.
“Roads and mq for
skid tracks shou Id be
located on ridge
crests wherever
possible ”
38
If alternatives like skyline techniques (Figure 2) are considered
uneconomical and the use of wheeled or tracked vehicles unavoidable, then at least overall machine size should be restricted
where possible. In addition, the use of winch rope systems should
be encouraged to avoid heavy machinery having to gain access
to every individual log. Logs may then be winched uphill (preferably with the leading end lifted off the ground to prevent it from
ploughing into the soil) by the machine on the ridge. This, of
course has the added advantage that the area occupied by skid
tracks will not be unnecessarily large. Other measures which help
to reduce the areal extent or intensity of soil disturbance include:
(i) the felling of trees in the direction of the nearest skid track
(which reduces the hauling distance; a herringbone pattern often
appears to be the most economical); (ii) the combined haulage of
several logs at the same time (reducing the number of vehicle
passes; however, this is only practical in moderately flat terrain
and the advantage may be offset by the need to employ heavier
equipment); and (iii) the use of tracked rather than rubber-tyred
vehicles (Figure 7), particularly in steeper terrain. The benefits of
reduced tyre pressure has been commented upon already.
Buffer strips
One of the simplest, but most effective, measures to reduce adverse impacts of logging on streamflow is to concentrate activities
away from drainage channels that could quickly transport sediment downstream during wet weather. This may even include
some valleys that carry no water during extended dry spells. The
resulting bands of natural vegetation along the streamside, which
are left untouched, are usually referred to as ‘streamside buffer
strips’, or simply as ‘buffer strips’. The stream banks are thus
protected from disturbance, thereby reducing bank erosion. The
buffer strips also help to filter out eroded material from overland
flow, and therefore sediment losses are reduced. In addition, they
help to moderate extremes in stream water temperatures, which
is important for the conservation of the aquatic ecosystem. Last
but not least, they may also act as miniature reserves of genetic
material within the logged forest and as a refuge for tree-dwelling
animals during logging operations. Desirable widths for the buffer
strips will vary widely, depending on local terrain, soils, and
stream conditions. Experience indicates that IO-40 m wide riparian strips are effective in protecting water quality in most streams.
Buffer strips are not a panacea, however, and detailed harvest
planning may reveal potential problems or trade-offs in their use.
For example, runoff and sediment problems may still occur if
roads and landings become poorly located through avoiding buffer
areas, or if newly exposed streamside trees crash during storms.
39
“Adequate drainage
facilities need to be
designed ‘I
“Streamside buffer
strips are one of the
simplest and most
effective
measures It
Queensland and Surinam: where improved logging evolved
Improved logging practices were developed in northern Queensland, Australia, over a quarter of a century ago as part of a
polycyclic system, and were widely implemented there until
logging was prohibited when the rain forest in the area was
declared a World Heritage Site in 1988. Few forested areas in the
tropics experience the quantity and intensity of rainfall of this
region, and therefore any measures which work well in Queensland are likely to prove effective elsewhere. Key elements in the
Queensland system comprise pre-logging planning, buffer strips,
controlled drainage and suspension of the operation during wet
periods. Indeed, much of what has been said in the previous
paragraphs is based on the Queensland experience.
“Enough is
currently known to
make interim
recommendations...
...the next step is
for Governments to
act decisively ”
More recently, a specific system of damage-controlled logging has
been formulated for rain forests in Surinam. This ‘CELOS’ system
of harvesting is a sub-component of a broader forest management system, which aims to harvest timber economically within a
polycyclic framework. The central objective is to minimise environmental damage while maintaining the forest in as “natural” a state
as possible. The system is appropriate for similar forests in the
Amazon basin. Typical extraction rates are 20-30m3 of timber per
hectare at intervals of 20-25 years. Major elements of the CELOS
harvesting method are pre-logging planning, followed by directional felling, the establishment of a complete skid trail network before
logging, and winch extraction of logs (as opposed to ‘timber
cruising’. Further information on the experience from Queensland
and Surinam can be found found in several of the literature references listed.
The next chapter demonstrates clearly that profitability and damage limitation can go hand-in-hand. Long-term production prospects are best served by protecting the environment. While the
methodologies are now available for environmentally sound logging, the level of implementation is disappointingly low. Even
when put into practice, follow-up and supervision have often
proved inadequate. The majority of forests are still harvested
according to the methods which best suit the short-term profit
motives of logging companies - while the environmental issues
are sidelined. This means uncontrolled logging, and, usually,
considerable damage to soil and vegetation. Although further
research will sharpen our knowledge about the various processes,
enough is currently known to make interim recommendations.
Therefore, education and training for policy-makers, forestry staff
and, particularly, loggers and operators of machinery are the most
vital keys in the overall process. However this all needs to be set
in the context of appropriate institutional and policy frameworks at
national level. The next step is for Governments to act decisively.
41
42
8. COSTS AND BENEFITS
Protecting the environment is an attractive concept - but what
price must be paid to achieve it? Provision of improved planning
and infrastructural works incurs extra expenditure, and both buffer
strips and prohibited zones represent loss of harvestable area. In
addition, avoiding wet periods of the year means ‘downtime’ for
contractors. What are the increases in costs and what, precisely,
are the benefits in the case of improved selective logging? After
all, commercial logging companies generally react to hard profitability. To change their habits, they need to be convinced that
‘improved’ practices are not a drain on their pockets...
Improved efficiency - better profits
“Improved practices
are not necessarily
more expensive than
conventional logging
operations ”
Because of the economic ‘shyness’ that is so prevalent in scientific and environmental circles, and the general tendency of
traditional economists to ignore the costs associated with potentially adverse off-site consequences of logging (such as increased
flooding and stream sedimentation), it is difficult to attach a complete economic picture to ‘improved’ practices. However, there is
a steadily growing body of evidence showing that the combination
of improved logging efficiency and reduced environmental damage can indeed be economically profitable. For example, it was
established that, within the framework of the Queensland polycyclic system, application of controlled logging practices raised
the average cost of logs delivered to the mill by as little as 3%.
Whilst the demonstrated off-site benefits like reduced stream sedimentation in Queensland were not quantified in economic terms,
there is little doubt that the savings in downstream water treatment costs did more than offset any increases in logging costs.
However, and perhaps somewhat as a surprise to some, improved logging practices are not necessarily more expensive than
conventional operations. For example, application of the CELOS
harvesting system in Surinam reduced the overall costs of timber
extraction by 16-31% compared to conventional logging techniques, due to savings brought about by increased efficiency. To
this should be added the substantial reductions in damage inflicted to the remaining stand (up to 40%) that may be obtained by
well-planned and well-conducted operations. Similarly positive
results have been obtained by various studies in East Malaysia.
43
Needless to say, this will enhance future forest productivity, and
thus profitability.
Savings achievable
The timber extraction network occupies a key position in many
respects. As we have seen in the previous chapters, a substantial
proportion of downstream flooding and sedimentation problems
are related to poorly planned roads and skid tracks. As such, any
reductions in the area occupied by such impervious surfaces as
well as improvements in their lay-out can be expected - and have
been shown - to bring about substantial reductions in adverse
downstream effects, and thus off-site costs. Giving that stream
sediment loads associated with improved logging practices are
typically 2550% of those that would have been recorded in the
absence of prevention measures, a first idea of the associated
savings in water treatment costs can be obtained. Depending on
the volume of raw water that needs to be treated, such costs can
easily run into thousands of dollars per day.
Building all-weather roads in remote terrain in the humid tropics is
generally a very costly affair. As such, any reductions in the
length of the extraction system quickly become economically
attractive, particularly in steep terrain, Under such conditions the
use of skyline yarding systems, which may need only one-third of
the roads and tracks required by ground-based extraction systerns, provide an economically viable, yet highly environmentally
friendly, alternative, even before off-site benefits are taken into
account.
“Improved
selective logging
practices are the key
to sustained
production from
moist tropical
forests ”
Benefits outweigh costs
Evidently the careful planning of operations leads to increased
efficiency of logging as well as environmental protection. Indeed,
the experience gained to date strongly suggests that the benefits
of improved logging practices outweigh the costs incurred, not
only in terms of short-term, on-site benefits but especially when
both on-site and off-site factors are considered in the longer term.
The evidence may not be complete at present, particularly with
respect to the costing of reduced downstream hydrological problems or improved future forest productivity. Nevertheless, few
can disagree that improved selective logging practices are the key
to sustained production from moist tropical forests.
44
SELECTED REFERENCES
In keeping with the sty/e and format of this Series, no specific references to literature have been included within the main body of the text.
However, the following books and articles comprise our principal
sources of information, and form a basis for further reading.
Abdul Rahim Nik, 1990. Effects of Selective Logging Methods on
Streamflow Parameters in Berembun Watershed, Peninsular
Malaysia. PhD Thesis, Department of Forestry, University College of Wales, Bangor, U.K.
Adams, P.W. & Andrus, C.W. 1991. Planning timber harvesting operations to reduce soil and water problems in humid tropic steeplands. Paper presented at the International Symposium on
Forest Harvesting in South-east Asia, Singapore, June 1991.
Appanah, S. & Putz, F. 1984. Climber abundance in virgin dipterocarp
forest and the effect of pre-felling climber cutting on logging
damage. The Malaysian Forester 47: 335-342.
Blakeney, K.J. 1992. Cable and helicopter logging for reduced damage.
Paper presented at the international Symposium on Harvesting
and Silviculture for Sustainable Forestry in the Tropics, Kuala
Lumpur, October 1992.
Brown, G.S. 1955. Timber extraction methods in N. Borneo. The Malaysian Forester 18: 11 l-1 32.
Bruijnzeel, L.A. 1990. Hydrology of Moist Tropical Forests and Effects
of Conversion: A State of Know/edge Review. UNESCO, Paris,
and Free University, Amsterdam.
Bruijnzeel, L.A. 1992. Managing tropical forestry watersheds for production: where contradictory theory and practice co-exist. In:
Wise Management of Tropical Forests 7992 (ed. by F.R. Miller &
K.L. Adam), pp. 37-75. Oxford Forestry Institute, Oxford.
Burgess, P.F. 1971. The effect of logging on hill dipterocarp forests.
Malayan Nature Journal 24: 231-237.
Cassells, D.S. et a/, 1984. Watershed forestry management practices in
the tropical rainforest of N.E. Australia. In: Effects of Forestry
Land Use on Erosion and Slope Stability (ed. by C.L. O’Loughlin
& A.J. Pearce), pp. 289-298. IUFRO, Vienna.
Clinnick, P.F. 1985. Buffer strip management in forest operations: A
review. Australian Forestry 48: 34-45.
De Graaf, N.R. 1986. A Silvicultural System for Natural Regeneration of
Tropical Rain Forest in Suriname. Pudoc, Wageningen, the
Netherlands.
Dykstra, D.P. & Heinrich, R. 1992. Sustaining tropical forests through
environmentally sound harvesting practices. Unasylva 169: 9-l 5.
Douglas, I. et a/. 1992. The impact of selective commercial logging on
stream hydrology, chemistry and sediment loads in the Ulu
Segama rain forest, Sabah. Philosophical Transactions of the
Royal Society B 335: 397-406.
45
Gillman, G.P. et al. 1985. The effect on some soil chemical properties
of the selective logging of a north Queensland rainforest. Forest
Ecology and Management 12: 195214.
Gilmour, D.A. 1977. Logging and the environment, with particular reference to soil and stream protection in tropical rainforest situations. FAD Watershed Management Guide no.1, pp. 223-235,
FAO, Rome.
Gomez-Pompa, A. et al. 1990. Rain Forest Regeneration and Management. Man and the Biosphere Series Volume 6, UNESCO, Paris
& Parthenon Publishing Group, Carnfotth, U.K.
Hendrison, J. 1990. Damage-controlled Logging in Managed Tropical
Rain Forest in Suriname. Pudoc, Wageningen, the Netherlands.
ITT0 Tropical Forest Management Update, 1991-l 994. Various issues.
ANUTECH Pty Ltd., Canberra.
Jonkers, W.B.J. 1987. Vegetation Structure, Logging Damage and Silviculture in a Tropical Rain Forest. Pudoc, Wageningen, the
Netherlands.
Kamaruzaman Jusoff, 1991. Effect of tracked and rubber-tyred logging
machines on soil physical properties of the Berkelah Forest
Reserve, Malaysia. Pertanika 14: 1-l 1.
Ludwig, Ft. 1992. Cable crane yarding: an economical and ecologically
sustainable system for commercial timber harvesting in loggedover rain forests of the Philippines. Paper presented at the
International Symposium on Harvesting and Silviculture for
Sustainable Forestry in the Tropics, Kuala Lumpur, Oct. 1992.
Malmer, A. & Grip, H. 1990. Soil disturbance and loss of infiltrability
caused by mechanized and manual extraction of tropical rain
forest in Sabah, Malaysia. Forest Ecology and Management 38:
l-12.
Marn, H.M. & Jonkers, W.B. 1981. Logging damage in tropical high
forest. Working Paper no. 5. FAO/UNDP Forestry Development
Project Sarawak, Kuching, 15 pp.
Pearce, A.J. & Hamilton, L.S. 1986. Water and Soil Conservation
Guidelines for Land-use Planning. Report of a seminar-workshop
held at FTC Gympie, Queensland, Australia.
Poels, R.L.H. 1987. Soils, Water and Nutrients in a Forest Ecosystem in
Suriname. Pudoc, Wageningen, the Netherlands.
Proctor, J. 1987. Nutrient cycling in primary and old secondary rainforests. Applied Geography 7: 135-l 52.
Van der Plas, MC. & Bruijnzeel, L.A. 1993. Impact of mechanized
selective logging of rain forest on topsoil infiltrability in the Upper
Segama area, Sabah, Malaysia. International Association of
Hydrological Sciences Publication no. 216: 203-211.
Whitmore, T.C. 1990. An introduction to Tropical Rain Forests.
Clarendon Press, Oxford.
Zulkifli Yusopp, 1989. Effects of selective logging methods on dissolved
nutrient exports in Berembun watershed, Peninsular Malaysia.
Paper presented at the Regional Seminar on Tropical Forest
Hydrology, Kuala Lumpur, September 1989.
46
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. It was noted that although the
general characteristic of the humid regions is an abundance of
water, this very abundance - and the spatial and temporal variability of its distribution - is one of the leading contributors to the
difficulties.
An executive summary of the Colloquium was released shortly
after it was held, whereas 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
exploitation 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, etc. Additional
47
volumes are in preparation, including the companion volume to
the present one - Environmental Impacts of Tropical Forest
Conversion to Other Land Uses.
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).
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
resource systems, along with the problems of human 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.
48
About the authors:
Sampumo (LA.) Bruijnzeel is a lecturer in environmental hydrology at
the Faculty of Earth Sciences of the Vrije Universiteit in Amsterdam, the
Netherlands, with almost two decades of experience with forest hydrological research in the humid tropics, mainly in South-east Asia and the
Pacific. Since the mid-1980s he has published a number of comprehensive reviews of the literature on environmental impacts of tropical forest
disturbance and conversion to other land uses. His other scientific
interests include the hydrology and nutrient economy of both tropical
montane cloud forests and fast-growing plantation forests.
William Critchley is a conservation agronomist with the Centre for
Development Cooperation Services at the Vrije Universiteit, Amsterdam.
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.
Photo credits:
L.A. Bruijnzeel: cover, 3 (above), 11, 15, 17, 19, 27, 31, 32, 40
Th.B.A. Burghouts: 25, 40 (below)
D.S. Cassells: 38
A. Malmer: 4, 21
M.J. Waterloo: 33
T.C. Whitmore: 24, 42
K.F. Wiersum: 3 (below)
Sources of remaining Illustrations:
Figures 1,2:
Figure 3:
Figure 4:
Figure 5:
Figure 6:
Figure 7:
Figure 8:
A.R. Farid & I. Shamsudin in A. Sarre (1992). /TO
Tropical Forest Management Update no. 2 (6), p.3.
Adapted from I. Douglas (1977). Humid Landforms. MIT
Press, Cambridge, Massachusetts.
Adapted from T. Dunne (1978). Hi//slope Hydrology. J.
Wiley & Sons, New York.
Adapted from G.G. Parker (1985). The Effect of Disturbance on Water and Solute Budgets of Hillslope Tropical
Rainforest in Northeastern Costa Rica. PhD Thesis,
University of Georgia, Athens, Ga., U.S.A.
Redrawn from J. Proctor (1987). Applied Geography 7,~.
135.
Adapted from J. Kamaruzaman (1991). Pertanika 14, p.
274.
Redrawn from D.A. Gilmour (1977). FA0 Watershed
Management Guide no. 1, p. 231.
The authors thank Dr T.C. Whitmore for his constructive comments.