Impacto Ambiental de La Deforestación

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L.A. Bruijnzeel and W.R.S. Critchley

0 UNESCO 1994

Contents

1. Introduction

2. Logging systems

3. Impact of logging on vegetation and soil

4. Impact on streamflow

5. Impact on erosion and sedimentation

6. Impact on the forest nutrient budget

7. How can logging be improved?

8. Costs and benefits

Selected references

The International Hydrological Programme

MAB Programme activities in the humid tropics

3ifgRih

TROPENBOS

Netherlands IHP

Committee

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5

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12

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vrije Universiteit


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1. INTRODUCTION

Logging

operations have

burgeoned in the

last decades as

mechanisation has

enabled the

exploitation of

previously

inaccessible areas

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 eco-

system 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 mechanis-

ation has permitted the exploitation of previously inaccessible

areas, and at an ever-quickening pace.

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 vulnera-

ble river sources, at the same time providing for (limited) recreat-

ional 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 estab-

lished, 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 earths

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 commit-

ment to the cause of good land husbandry after forest removal.

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No wonder, therefore, that there is increased scope for and inter-

est 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 worlds tropical forests are being

managed in a fully sustainable manner, the silver lining is that

relatively simple precautions can lead to substantial improve-

ments. 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 be-

come 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 dis-

asters, 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 log-

ging. This is then followed by a set of guidelines for simple,

improved logging practices which are known to keep environment-

al damage to a minimum. In this way the interests of both envir-

onmentalists 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 de-

monstrated in our conclusions, ecological benefits and economic

returns need not be mutually exclusive.

There is room for

use without

abuse:

mankind can reap

benefits from this

rich resource while

maintaining its value

for the future

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Many

environmental

disasters, such as

floods and

droughts, are often

hastily blamed on

logging

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4

Mechanisation has

largely replaced

traditional methods

of logging


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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 re-

planting. As an example, even in the very species-rich forests of

Malaysia, harvesting of all marketable trees would on average re-

sult 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 re-

moved 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 con-

sists of an irregular mosaic of almost undisturbed cover, pock-

marked 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,

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the so-called rotation period. This may be as long as 60-80 years,

equivalent to the lifespan of man. Because monocyclic logging re-

moves 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 re-

generation of shade-loving species, which are often those with the

greater commercial value. By mimicing the natural cycle of tree-

death, 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.

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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 con-

ditions and the size of the trees in some forests permit logs to be

extracted using elephants or even manpower, mechanized tract-

ion using rubber-tyred or tracked vehicles are commonplace now-

adays. 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 ex-

tracted during each operation. This is only partially balanced by

the longer periods allowed for the forest to recover under mono-

cyclic 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 mono-

cyclic logging this figure may increase to 40-60 . As lowland

forests are becoming more and more depleted, loggers are turn-

ing 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, IOW-

intensity logging there is a threshold of damage which is unavoi-

dable.


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Figure 1.

High-lead yarding.

Figure 2. Skyline logging.

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 con-

sequences of soil disturbance for seedling survival. F inally, 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 inter-

cepted 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.

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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 manage-

ment practices as well as the economics of the respective extract-

ion methods will be discussed in the final sections.

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Typically,

f

or

tree which is

every

hwd,

a second is destroyed

and a third is

damaged beyond

recovery


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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


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4. IMPACT ON STREAMFLOW

Of all the environmental effects associated with logging, it is

probably the hydrological changes which are the most misunder-

stood. 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 wis-

dom. 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 season-

ality 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 major ity 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 struct-

ure, 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 over-

land flow and erosion. Indeed, infiltration excess overland flow is

a relatively rare phenomenon in most tropical forests.

Of all the

environmental

effects associated

with logging,

it is probably

the hydrological

changes which are

the most

misunderstood

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I Crown drip/

Figure 3.

The hydrological cycle for a forested ecosystem.

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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-satur-

ated conditions. Additionally, such saturation overland flow (SOF)

can be observed during and after intense rainfall where an impe-

ding 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 hydro-

logical 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 trans-

piration 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.


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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

I I I I

I

I

0 20 40

60 60 too 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.

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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

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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 atmo-

spheric 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. 1.1.

270 290

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.

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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 ex-

treme 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 monit-

ored 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 oper-

ation 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 ex-

traction of about 33 of the commercial stocking was carried out

according to environmentally-friendly guidelines (the supervised

treatment, which involved closely specified prescriptions for lay-

out, 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).

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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 regener-

ating logged-over forests may well stabilise after 5-10 years.

Whilst the relative increase in water yield recorded after unsuper-

vised 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 evapo-

rated back into the atmosphere could achieve such an effect,

particularly in view of the fact that the reduction in evapotranspira-

tion tails off within a decade after logging. Climatic changes in the

tropics in relation to land-use dynamics are only poorly under-

stood 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

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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 log-

ging.

Quicker response

We have shown that one impact of logging is, generally, to in-

crease 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 soils 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 dis-

charges 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

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The repeated

passage of heavy

vehicles and logs

over the skid tracks

has dramatic

consequences for the

water intake capacity

of the soil I

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regard for the environment: where road and track lay-out has

been poorly planned and constructed, and where heavy equip-

ment 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 extract-

ion procedures, soil disturbance and thus overland flow and sheet

erosion are minimal, particularly when only small volumes of tim-

ber 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 stat-

istically 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 exploit-

ation. True as these may be, the experimental results at least

provide strong evidence that effects of timber harvesting oper-

ations 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


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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 lakes 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 mans activities in this

respect? Should upstream logging receive the blame for accel-

erated reservoir siltation and raised river beds as is so often

claimed, or is an increasing amount of forest clearance for agri-

culture 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


26/52

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, part-

icularly 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 un-

necessarily, hampering forest regeneration and future productivity.

Heavy erosion is

commonly observed

on poorly drained

roads - especially

where erodible

subsoil is exposed

24

- .-. ---

-


27/52

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 observ-

ations 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 reserv-

oirs 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


28/52

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 sedi-

ment yields in areas with initially low sediment production may

increase by between two and ten times as a result of road con-

struction 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 construct-

ed, 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 re-

growth occurs rapidly, sediment temporarily stored in the catch-

ment (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


29/52

f

orn

Vegetation

recovering on

2er skid tracks

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.

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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 separ-

ated 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

--


31/52

EXCHANGE

COMPLEXES

--mm_

---__

SOIL

rock

weathering

-__---

-----__

-T

-.

nutrient losses in

,,--------------- --

\

. water movemantr from forrrt ,

Ro& - -_______-_-_-_-- -

Figure 6.

The nutrient cycle in moist tropical forest.

29


32/52

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 doc-

umented 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.

Inputs and outputs

of nutrients are

often very limited

and nutrients

are in a continuous

state of circulation

through the system

Nutrient losses

It is clear that any interference with forests growing on nutrient-

poor 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, increas-

ed 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 bio-

mass. 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


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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 recolo-

nisation of the skid tracks. Little is known about the accompa-

nying 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 accel-

erated because more net rainfall reaches the soil through the

gaps in the forest, while less water is pumped back by evapo-

transpiration. 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


34/52

Once an area is

opened up by

logging roads there

is often an influx of

land-hungry people

32


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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 oper-

ation, 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-and-

burn types of agriculture once the area has become opened up

by the logging roads. Finally, the logging debris left on site con-

stitutes 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


36/52

Sustainability and safety limits

Taking these various losses into account, what then are the chan-

ces 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? Unfortuna-

tely, 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 in-

dications 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. How-

ever, it is almost certain that natural inputs of nutrients to the

richly stocked dipterocarp forests of South-east Asia are not suf-

ficient 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.

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

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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 collab-

oratively 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 charact-

eristics as well as forests differ widely, and each situation should

be examined in sufficient detail to allow the identification of part-

icularly 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 tech-

nique. This process may be facilitated by the use of a Geographi-

cal 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.

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60

50

40

30

20

IO

6

S

L

3

2

1

i

I

I

\

I

\

u

\

\

Track-type machine

\

\

\

\

b..

.

-.

s

---_ 0

14 moisture

21 moisture

I -

t

Rubber-tyred machine

I

I I I I I I I I

I 1 I I I I

0 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.

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Road infrastructure

The considerable potential of the haulage roads and skid trails to

contribute to environmental damage has been stressed in pre-

vious 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 land-

slides. 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 essent-

ial, 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. Add-

itional advantages of reduced tyre pressure include a decline in

the rutting of road surfaces and therefore erosion, and an ex-

tended 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.

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40/52

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 tim-

ber 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

skid tracks shou

located on ridge

crests wherever

possible

for

Id be

38


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If alternatives like skyline techniques (Figure 2) are considered

uneconomical and the use of wheeled or tracked vehicles un-

avoidable, 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 (prefera-

bly 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 ad-

verse impacts of logging on streamflow is to concentrate activities

away from drainage channels that could quickly transport sedi-

ment 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 ripari-

an 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


42/52

Adequate drainage

facilities need to be

designed I

Streamside buffer

strips are one of the

simplest and most

effective

measures It


43/52

Queensland and Surinam: where improved logging evolved

Improved logging practices were developed in northern Queens-

land, 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 Queens-

land 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 manage-

ment system, which aims to harvest timber economically within a

polycyclic framework. The central objective is to minimise environ-

mental 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 direction-

al 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 refer-

ences listed.

The next chapter demonstrates clearly that profitability and dama-

ge limitation can go hand-in-hand. Long-term production pro-

spects are best served by protecting the environment. While the

methodologies are now available for environmentally sound log-

ging, 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.

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42


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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 profit-

ability. 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 scien-

tific and environmental circles, and the general tendency of

traditional economists to ignore the costs associated with potenti-

ally adverse off-site consequences of logging (such as increased

flooding and stream sedimentation), it is difficult to attach a com-

plete 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 dama-

ge can indeed be economically profitable. For example, it was

established that, within the framework of the Queensland poly-

cyclic 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 sedi-

mentation in Queensland were not quantified in economic terms,

there is little doubt that the savings in downstream water treat-

ment costs did more than offset any increases in logging costs.

However, and perhaps somewhat as a surprise to some, improv-

ed 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 tech-

niques, due to savings brought about by increased efficiency. To

this should be added the substantial reductions in damage inflict-

ed 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.

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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 sys-

terns, 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

Benefits outweigh costs

forests

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 pro-

blems or improved future forest productivity. Nevertheless, few

can disagree that improved selective logging practices are the key

to sustained production from moist tropical forests.

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SELECTED REFERENCES

In keeping with the sty/e and format of this Series, no specific refer-

ences 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 Col-

lege of Wales, Bangor, U.K.

Adams, P.W. & Andrus, C.W. 1991. Planning timber harvesting oper-

ations to reduce soil and water problems in humid tropic steep-

lands. 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 Mal-

aysian Forester 18: 11 -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 pro-

duction: 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. OLoughlin

& 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. Ph ilosophical Transactions of the

Royal Society B 335: 397-406.

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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 refer-

ence to soil and stream protection in tropical rainforest situat-

ions. FAD Watershed Management Guide no.1, pp. 223-235,

FAO, Rome.

Gomez-Pompa, A. et al. 1990. Rain Forest Regeneration and Manage-

ment. 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 Silvi-

culture 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 logged-

over 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 rain-

forests. 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.

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The International Hydrological Programme

The developing nations of the humid tropics of the world will

represent about one-third of the earths 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

Australias James Cook University. The International Hydrological

Programme (IHP) of UNESCO was the lead organization.

The Colloquium developed strong evidence that the present situ-

ation, 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 variabi-

lity 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 produ-

ced by the joint efforts of IHPs Humid Tropics Programme, the

National Committee for IHP of the Netherlands and the Vrije Uni-

versiteit 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

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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 mans 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 manageme

Documents

Impacto Ambiental de La Deforestación