W.R.S. Critchley and L.A. Bruijnzeel - UNESDOC …unesdoc.unesco.org/images/0010/001096/109608eo.pdf · As the process of colonisation by agriculture continues, so the more remote

PREFACE

This booklet and its companion volume (no. 7 in this series) deal with the environmental impacts of change in land use from undisturbed moist tropical forest. The two publications look at the most common types of interference by man to tropical forests: selective logging for timber, and clearing for agriculture or plantations, respectively. Collectively they give an overview, individually they cater for specific concerns. Each booklet is self-standing, together they are complementary. To avoid unnecessary repetition, the basic forest hydrological and ecological processes - which have been described in some detail in volume no. 7 - have been summarized only in the present document.

This volume addresses the changes that occur when forest is cleared to make way for alternative landuses. The most widespread and historic reason for disturbing tropical forests is to make land available for agriculture. In order to gain a tighter focus we have concentrated here on the change from forest to rainfed crop farming, with only passing reference to irrigated agriculture and development of pastures for raising livestock. Plantations also thrive in land previously under forest. We examine both plantation crops, such as coffee, rubber and oil palm, and forest plantations for the production of timber, pulp or fuelwood.

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

0 UNESCO 1996

Contents

1. Forests and land use change 1 2. Understanding the forest-environment interaction 3 3. Disturbing the balance 7 4. Where there once was forest: post-clearing systems of production IO 5. Changes: for better or for worse? 21 6. From nature to nurture: transition with care 36 Selected references 44 The International Hydrological Programme 46 MA9 Programme activities in the humid tropics 47

Netherlands IHP Committee _ vrije Universiteit

‘Some of the

environmental

disasters

which are hastily

‘attributed to

‘deforestation’

are essentially

natural phenomena’

1. FORESTS AND LAND USE CHANGE

Exploitation of tropical forests is nothing new. Forest products have been harvested since homo sapiens emerged as a species, and forests have been cleared to make way for agriculture over millenia. What is new, however, is the accelerated rate of exploitation in the last few decades. This has coincided with a growing popular awareness of the environment, and of the dangers associated with unchecked interference with nature. One result is that various calamities have been blamed on ‘deforestation’. Floods, landslides, increased erosion and sedimentation of reservoirs are often said to be the ‘inevitable’ consequence when forests are logged for timber, or when land is cleared for farming or plantations. Climate changes are also blamed by ‘green’ politicians on deforestation. In reality some of the environmental disasters which are hastily attributed to deforestation are essentially natural phenomena - but the damage caused tends to be more severe now, because there is more infrastructure and more people downstream than ever before. Nevertheless, it cannot be denied that a range of detrimental environmental changes can be caused by unwise modifications of tropical forests.

This booklet aims to put the picture into perspective with respect to the development of agricultural land and plantations from moist tropical forest. Of course the majority of existing farm land in the humid tropics was originally developed after clearing forests. In some areas this process occurred many centuries ago. Terraces in the Far East, for example, replaced forest over two thousand years ago, and continue to produce crops and support millions of families. In the case of plantations, practically all of these - with the exception of some reforestation programmes on degraded land - are sited on land which used to flourish with forest.

It is significant that the majority of the area currently undergoing deforestation in the humid tropics - estimated at about 155,000 km* per year at present - is being transformed into agricultural land. For instance, in Indonesia alone up to half a million hectares of forest were cleared annually in the outer islands to make way for settlers from the overcrowded island of Java in the trans- migration programme of the 1980’s. Similarly, over 15 million hectares of forest were burned down for the creation of pastures along the southern fringe of the Amazon rain forest block in Brazil during the late 1980’s. As well as official programmes there is often an inexorable ‘creep’ of quasi-legal settlers into the forests, exploiting land made accessible through logging roads.

1

As the process of colonisation by agriculture continues, so the more remote areas are increasingly exploited - areas often characterised by steep topography and especially vulnerable to erosion. Certainly there is widespread concern in the humid tropics about the dangers of farming on slopes which are commonly much steeper than statutory limits permit. Hillsides are being cleared everywhere as farmers seek more land to produce food for their families.

There are two main issues to be addressed in this booklet: first, what evidence do we have about the environmental impacts of clearing forest for agriculture and plantations? And second, what safeguards are needed during forest clearing, and in the development of alternative land use systems, to minimise damage to the environment? It is clear that we are not dealing exclusively here with physical sciences, but with social aspects also. These changes are taking place in some of the most densely populated areas in the world and it goes without saying that the human factor is deeply implicated in the events that are unfolding.

‘Hillsides are being

cleared everywhere

as farmers seek

more land to

produce food for

their families’

2. THE FOREST-ENVIRONMENT INTERACTION

In order to appreciate the environmental consequences of con- verting tropical forest to other land uses, it is necessary first to understand the environmental dynamics of the undisturbed forest itself. A virtually closed canopy, composed of a multitude of species, together with a forest floor covered by a thin layer of leaf litter and underlain by a highly permeable topsoil are characteristic of many moist tropical forests. The canopy and the nature of the soil are fundamental in determining how the forest be- haves with respect to hydrology, soil conservation and nutrient cycling. We will now look briefly at these systems and the processes involved. This then becomes the backdrop against which it is possible to highlight changes.

The forest hydrological cycle

By definition, moist tropical forests have developed in warm humid regions where there is sufficient annual rainfall to sustain evergreen broadleaved forest vegetation. In some areas there may be a short dry season: in others, rainfall is year-round. Rain falling on the forest canopy reaches the ground through three routes. A small proportion reaches the forest floor as direct throughfall, without touching leaves or stems, and a further small proportion flows down the tree trunks as stemflow. The majority of the rain however falls on the vegetation and then reaches the ground from the canopy as crown drip. The total amount of water reaching the forest floor from these sources is called net precipitation. It typically makes up 80-90 per cent of the incoming rain in most tropical forests. The remaining lo-20 per cent of the rain falling onto the forest never reaches the ground: it is intercepted by the canopy and is evaporated back into the atmosphere. The gross rainfall therefore comprises two elements: rain which reaches the forest floor and rain which is intercepted and evaporated back. The great majority of rainfall which reaches the forest floor infiltrates the soil through the leaf litter and top soil - which usually provides excellent protection against raindrop splash and surface runoff. Undisturbed forest soil has good structural properties and this not only helps infiltration, but also increases the water holding capacity of the soil.

Loss of water from the soil in a moist tropical forest is either upwards through transpiration from the canopy, or via drainage into the nearest stream.

3

The water use of closed canopy tropical forests is high, and a large proportion of the soil moisture (typically about 1,000 mm per year) is pumped back by the trees into the atmosphere. Soil moisture drains into the stream network by throughflow, the result of downward moving water meeting an impermeable layer of subsoil or bedrock and then being deflected laterally. Between rain events, the water drains slowly and steadily throughout the season, from the store of moisture in the soil. This process ac- counts for the baseflow of streams. During and shortly after rain, streamflow may increase rapidly. This is due to the quick flow of water through the soil traveling via a number of pathways that are mainly determined by the nature of the soil. The increase in streamflow above the baseflow is commonly referred to as storm- flow or quickflow, and at its maximum is called peakflow.

Figure 1.

‘The hydrological cycle for a fores ted ecosystem’

4

Erosion and sediment yield

‘Undisturbed natural

forest usually has

one of the lowest

erosion rates

of any form of land

use in the tropics,

but the key here

is the word

undisturbed’

Undisturbed natural forest usually has one of the lowest surface erosion rates of any form of land use in the humid tropics - but the key here is the word undisturbed. In forest which is left in its natural state, the low erosion rates can be attributed largely to the nature of the forest floor. Splash erosion is effectively pre- vented by eliminating the direct impact of raindrops on the soil surface by both litter and understory vegetation. In addition, the leaf litter, undergrowth and highly permeable topsoil all help keep surface runoff and thus erosion to a minimum. Furthermore, the high levels of organic matter in the topsoil help it to resist erosion. In disturbed forest - for example where timber has been harvested or litter has been removed for firewood and undergrowth for livestock fodder - the situation is fundamentally different, and erosion rates can be very high as a result.

While surface erosion levels are normally very low under undisturbed forest, this is not necessarily true for mass wasting processes. Granted, through the stabilising function of their roots, trees help to reduce the risk of shallow land slips on hillsides and riparian vegetation helps minimise bank erosion, but more deep- seatedforms of mass wasting, such as large landslides or mass- ive mudflows, may still occur in forests, especially where these are situated on steep hillsides in wet mountainous terrain. Such deep-seated landslides in natural forest are essentially a geological phenomenon, and, because they are often precipitated by extreme rainfall or seismic activity, they cannot simply be pre- vented by vegetation. As a result, overall sediment production from forested tropical catchments can be very high under certain geological conditions. More importantly perhaps, such high sediment yields are unlikely to be affected significantly by the activities of man, as these are dwarfed by the sheer magnitude of the geological and climatic forces at work in mountainous regions.

The forest nutrient cycle

Tropical forests can produce a spectacular amount of plant biomass, even when their soils are very low in fertility. The reason that these forests manage to sustain such wealth under poor conditions lies in their relatively ‘closed’ nutrient cycle. In other words, the plant nutrients entering the forest system (mainly in rain, dust and aerosols) are being cycled continuously between the canopy and the soil, with only small amounts leaking out of the system. The chief adaptation mechanism developed by the forests growing on extremely poor substrates is a surface root mat capable of ‘trapping’ most of the nutrients entering it.

5

Only where soils are sufficiently fertile can forests afford substantial leaching losses from their soils. Under these circum- stances, weathering of residual minerals still present in the subsoil or in the rock beneath may supply further nutrients to the system, provided that the bedrock is not too deep (less than, say, 5 metres from the surface) and within reach of the tree root network.

EXCHANGE COMPLEXES decomposition

nitrogen fixation in soil. rhizospharo and on roots

SOIL rock weathering

--me_ -- -_______ - --------- -\

1

nutrient losses in _ _--_____-__ -__--_

I \

‘. wat*rmovem*nts from forest ’

,’ ROCK -__- _____--_____ -

Figure 2. The nutrient cycle in moist tropical forest.

6

3. DISTURBING THE BALANCE

Logging of forests can either be selective or complete. In contrast to systems of partial logging, clear felling comprises the complete removal of the forest cover to make way for entirely different forms of land use. These include small scale agriculture, the plantation of beverage or industrial crops, reforestation for timber, pulp and fuelwood, or the development of pastures. In some areas, selective logging can be the precursor to intrusion by slash and burn farmers or more permanent squatters, who take advantage of the newly constructed access roads and gaps created in the forest during logging. Elsewhere, the productive potential of logged-over forest land may be deemed insufficient, after which it is converted to other forms of land use as part of official government programmes. Only in relatively rare cases is the forest to be felled still untouched.

The harvesting of merchantable timber during selective logging operations using tracked or wheeled tractors may leave up to 30 per cent of the surface area bare in the form of compacted access roads, haulage tracks and log landings. As a rule of thumb, the damage inflicted to the soil escalates with the size of the machinery and the intensity of the harvesting. The more machine passes, the greater the compaction of the soil surface, and in this respect wheeled vehicles which lose traction and smear the surface can be especially deleterious.

Of all the methods of clear felling, manual clearing (with animal haulage) tends to be the least damaging to the soil surface and simultaneously best for subsequent plant growth, mainly because it causes the least disturbance to the forest floor. Nevertheless this is an expensive and slow method which is not suitable when large areas need to be cleared. The damage increases rapidly when heavy machines are used, particularly when the tractors are equipped with root rakes for uprooting tree stumps. It goes without saying that the degree of surface disturbance during mechanized c/ear felling operations involving ground-based machinery (as opposed to skyline yarding systems) can be much higher than that associated with selective logging. Also, because the volume of slash left behind after forest clearing can be substantial, it is common practice to ‘windrow’ the slash into lines and set these on fire to facilitate future access. As we will see, these measures cause additional deterioration of the soil through the loss of precious nutrients that go literally ‘up in smoke’ during the burn and ‘down the drain’ via surface erosion.

7

-

Even in the case of a new land use which is considered to be an effective substitute for forest - a mature tea plantation for example - the period leading up to the establishment of the system can be highly damaging. Nevertheless, precautions can reduce the risk, and these are discussed in Chapter 6.

a

‘As a rule,

damage inflicted

to the soil

escalates with

the size of the

machine and the

in tens@ of

harvesting’

4. WHERE THERE ONCE WAS FOREST

Small scale agriculture

In the areas where evergreen tropical forest thrives, farming is equally favoured by year-round warm temperatures and abundant supplies of moisture. These are, by definition, humid zones, where - apart from a short dry season in some areas - moisture rarely limits crop growth. In some cases there may be adequate resources of surface water which make irrigation possible. But where irrigation is not viable, rainfed farming almost always benefits from enough rainfall to permit two cropping seasons each year.

With respect to agriculture, the focus of this booklet is on rainfed farming. This is for two reasons. First, much of the forest which is currently being felled in the more populous regions of the tropics is situated in the higher reaches of catchment areas, where irrigation development is often not feasible. Second, irrigated land presents much less of an erosion hazard than rainfed land. This is because irrigated land is levelled, and there is by neces- sity a high degree of water control. Both of these factors help ensure good soil conservation. In fact, belts of irrigated fields on the footslopes below rainfed hillsides can even trap sediment eroded higher up on the slopes.

Although shifting agriculture is not our direct concern here, because it does not involve the permanent replacement of forest, it needs to be mentioned in passing. Shifting agriculture was the progenitor of rainfed agriculture in humid tropical regions, and is still important in many areas today. Pockets of land are cleared from the forest, and these are then cropped for two or three years before being left to regenerate over a fallow period that would typically last at least 10 - 15 years. This system of farming used to be much maligned by tropical agriculture ‘experts’ in the early days. More recently, however, there has been a change in opinion. It is now agreed that shifting cultivation is an environ- mentally stable form of agriculture, as long as the vegetation is given sufficient time to recuperate and so restore the fertility of the soil. Unfortunately, the system is observed to break down in many places where the fallow period has become too short due to increased population pressure. The vegetation is then set on fire too frequently to allow a proper succession to develop. In such areas, the forest is gradually replaced by a largely unpro- ductive grassland.

10

‘Precious

nutrients go

up in smoke

during the

burning of

slash’

‘Farms

developed

without soil

conservation

measures

court disaster,

especially on

steep slopes’

11

Settled rainfed agriculture is fundamentally different from shifting agriculture. Forest, or at least patches within forests, must be permanently cleared to make way for this entirely new land use. Three categories of rainfed farming can be distinguished from the point of view of environmental impacts.

These are:

i. Farms without conservation measures:

Some farms are developed without soil conservation or water management measures such as terraces, contour grass strips or barrier hedges. Even on gentle slopes, this courts disaster in regions of high and intense rainfall. Such farms may be the result of illegal encroachment into forest land, where the squatters are reluctant to invest in conservation measures. Hillside farms without adequate conservation are also found on deep volcanic soils where vegetable growing is profitable and where farmers can purchase fertilisers at subsidised rates. In neither case is the farmer very concerned about erosion: in both cases there can be lasting damage either on-site or downstream, or both.

ii. Terraced farmland:

Commonly, bench terraces of various designs are found in farms where there used to be forest. This is particularly true of Asia and parts of the Andes where terracing practices date back up to 2,000 years or more. Terrace risers are constructed from either stone or earth, or a mixture of the two. Despite the high labour input involved in construction, modern-day soil conservation projects still promote bench terracing as the most versatile technique in controlling drainage and surface erosion. The flat terrace beds facilitate cultivation - and also permit irrigation where this is viable. Terraced agriculture is often associated with a high standard of land husbandry.

. . . III. Farmland with agroforestry systems:

Some of the best conserved, and most productive land in the humid tropics, is under intensive forms of traditional agroforestry. This land is usually terraced also where steep. Examples of indigenous agroforestry systems in Indonesia are ‘mixed gardens’ and the even more intensively cultivated ‘home gardens’. Modern agroforestry systems are in the process of being promoted all over the humid tropics with the aim of improving productivity while at the same time controlling erosion.

12

‘Terraced agriculture is often associated

with a high standard of land husbandry’

13

-. _-...^--

It is difficult to classify farms precisely into categories, as it is common to find a mixture of land uses, and a range in the standard of soil conservation measures even within a single farm. Yet, the divisions listed above help us to identify where the potential problems lie. Clearly there is a huge difference between, on the one hand, farmland with an agroforestry mixture of trees and annual crops which is carefully terraced against erosion, and on the other hand, land which is exposed to intense rainfall without any conservation measures - structural or agronomic.

Plantation crops

Just as annual crops thrive in the conditions by moist tropical forest, so do many plantation species. Indeed, several of these plantation crops are forest trees by origin: rubber and cacao for example come from the Amazon. Estates of beverage crops: tea, coffee and cacao, and of industrial crops: rubber and oil palm, now occupy large tracts of land which have been cleared from the forest. Whilst only a relatively small proportion of current forest clearing (ca. 15 per cent) involves the creation of agricultural plantations, the management of existing estates, and indeed the replanting of old stands, have important environmental implications. We will concentrate here on the five species already introduced, and on their role as plantation crops. First, some general remarks about plantations themselves.

In contrast to small scale farming in the tropics, a crop plantation is characterised by uniformity and central management. This facil- itates the development of physical infrastructure, - including roads, as well as soil conservation structures - especially those constructed by machinery. Because the plantation crop itself is perennial, and therefore a year-round ground cover can be maintained, there is every opportunity for establishing and maintaining a production system which combines high yields with effective soil conservation.

Nevertheless, there are also opportunities for environmental damage - notably during the critical period of conversion from forest to plantation and when crop stands have to be replaced at the end of their productive lives. Within their production cycle, it is in the early years that the stands are most vulnerable to erosion - that is before a complete ground cover is formed. Because plantation species are perennial, they usually root deeper than annual crops. On the other hand, they tend to use less water than the forest they replaced. The hydrological implications of this are discussed in Chapter 5.

14

Some characteristics of the main crops follow:

Tea (Camellia sinensis)

In its natural habitat in south-east Asia tea grows into an evergreen tree up to 10 metres tall. However, in the cultivated form it is pruned into low spreading bushes to allow hand picking. Typically, tea is cultivated in areas with abundant, and year- round rainfall. On the steeper slopes of the highland areas where it is usually grown, it is sometimes the practice to construct terraces before planting. It is characteristic of tea that, if well managed, it gives a very effective ground cover once the canopy closes. But poorly managed tea can lead to serious erosion. The plantations last for several decades before production declines. Of all the land use systems which replace moist tropical forest, tea is usually considered to be one of the most effective with respect to soil conservation.

Coffee (Coffea spp.)

Like tea, coffee can be grown either on smallholdings or on estates. Coffee does not form such a good canopy as tea, partially because of the wider spacing required for access between rows for picking and spraying. In order to maintain good ground cover to improve infiltration of rainfall and to reduce weed competition, it is necessary to mulch, especially around the plants themselves. Coffee is very susceptible to weed competition, and does not tolerate the planting of cover crops, which is a common practice with other plantation species. Terracing is often used, especially where coffee is grown by smallholders on hillsides.

Cacao (Theobroma cacao)

Cacao, which is grown for its product, cocoa, is a common plantation crop in the lowlands of West Africa, where it has replaced much of the natural forest. Cacao requires planting beneath shade trees which are either introduced or scattered remnants of the original forest. The main environmental problems now are those associated with the maintenance of mature estates. Mulch- ing is commonly practised around the trees to achieve moisture conservation, reduce runoff and suppress weeds. Where cacao is still being planted, there is a potential erosion problem until the canopy closes.

15

‘Well managed tea

gives an effective

ground cover once

the canopy closes’

‘Leguminous cover

crops reduce

erosion,

provide nitrogen

and suppress weeds’

16

Rubber (Hevea braziliensis)

Rubber is now a characteristic plantation crop of Malaysia, Indo- nesia, Sri Lanka and the humid regions of West Africa. In the establishment of rubber plantations it is the practice to clean- stump the area to avoid transfer of disease to the rubber trees. Needless to say, this increases the vulnerability to erosion in the stages prior to establishment of a good ground cover. Rubber requires a deep, relatively fertile soil and thrives best on flat land whose intrinsic erosion hazard is rather low. It is common to plant a legume cover crop between the rows of trees, such as Centrosema pubescens. Again, the cover crop reduces erosion and has the additional benefits of providing extra nitrogen and suppressing weeds.

Oil palm (Elaeis guineensis)

Like rubber, oil palm requires a climate with high temperatures and abundant, well distributed rainfall. Oil palm originated in West Africa but Malaysia is the world’s leading producer. Once again it is usual to plant cover crops between the trees. Both rubber and oil palm plantations need to be rehabilitated - entirely replanted - when productivity is reduced through old age (usually after several decades) or disease. This normally involves clearing and burning, and therefore opens the land up to the possibility of degradation before the new plantation is fully established.

Plantation forests

The dividing line between plantation crops and plantation forests is somewhat arbitrary. ‘Plantation forestry’ generally refers to tree species grown for timber or pulp - and other locally important industrial uses such as resin or tannin production. In some regions forest plantations are established as ‘protection’ forests in envir- onmentally vulnerable zones, such as steep headwater areas. Plantation forests are often larger in size than crop plantations: for example, the minimum viable area to support a pulp mill is in the order of 25,000 ha. Nevertheless, woodlots for fuelwood, or protection forests may be quite small in size. Plantation crops often demand better sites and tend to be more intensively managed than plantation forests. But there are many factors common to the two types of production system, including centralised management, and a planned layout of physical infrastructure.

17

The planting of artificial forests has burgeoned in the tropics over the last 40 years. This is explained in part by ambitious national programmes, such as those in India, Brazil, Malaysia and Indo- nesia. However, the amount of moist tropical forest being directly converted into forest plantations (about 10 per cent of the total in 1980) is currently decreasing. This is partially as a result of the increasing demand for agricultural land in those former forested areas. Much of the land that is presently under conversion to forest plantations is scrubland, or poorly productive vegetation such as the fire-climax grasslands of south-east Asia. Another significant recent trend is ‘social forestry’ where tree planting is encouraged on an individual (farm forestry) or community (village woodlots) basis to meet local needs, primarily for fuelwood and timber.

Plantation forests are essentially artificialforests: they are almost invariably uniform blocks of monocrop tree species, planted in an orderly manner. The economic advantages of such plantations over natural forests are clear: the species planted are much more productive. The three most commonly planted genera include Pinus (pines), Eucalyptus and Tectona (teak). Timber yields of 15 - 30 m3 per hectare per year are not uncommon in warm humid regions, far outstripping what could be expected from selective logging of natural stands. Maturity cycles can be as rapid as 7-8 years for pines, Acacias and eucalypts in certain situations, though teak and other precious hardwoods take at least 40 years to reach maturity. After harvesting it is usual to replant with seedlings - often with the same species - although some species coppice well after felling (some eucalypts, for example). Whilst coppicing reduces harvesting costs and soil surface disturbance, it has serious implications for soil water reserves. In their efforts to regain their above-ground biomass, the coppiced trees - whose root systems are left intact - consume far more water than either young seedlings or mature specimens.

As with plantation crops, there is an obvious potential for soil disturbance during the initial phase of land preparation and plantation establishment. The need to keep the young stands weed free varies between species but regular control of ground cover around the saplings to boost their growth can also expose the soil. This may also be a problem under the widely used taungya system where labour is paid for by allowing farmers to cultivate their annual crops between the young trees. During the phase of rapid growth until and shortly after canopy closure, the next significant environmental impact is likely to be in the form of gradually changing streamflow regimes.

‘Plantations present

the opportunity

to establish

a system which,

with careful

management,

can provide

an acceptable

alternative to

natural forest’

ia

Thinning closed-canopy stands does usually not cause much damage to the soil surface, but clear felling the mature plantation certainly does have this potential - depending on the type of machinery used and how well the operation is managed. Fire can also be a significant hazard in forest plantations, particularly in areas with a distinct dry season, and where large amounts of litter shed by deciduous trees constitute a readily available source of fuel.

As we will see in the final chapter, it is quite possible to avoid the havoc that is so often associated with the initial forest clearance. This holds even more for the transition to subsequent forest rotations which often no longer require the removal of large tree stumps. Also, the volume of slash lying around at this stage is much reduced and does not necessarily need windrowing and burning. Regardless of the stage of the plantation, one of the most important aspects of all is the presence of a well-developed ground cover. When the protective understory - and above all, the litter layer - is removed (for fodder, compost or fuel) and the soil is exposed to the rain dripping from the trees, the ‘forest’ becomes much more vulnerable to surface runoff and erosion, despite the presence of a full tree cover. It takes more than trees to make a forest!

19

‘When the litter layer is removed and the soil exposed to the erosive

force of crown drip, the surface becomes extra vulnerable to erosion,

despite the presence of a full tree cover: it takes more than trees to

make a forest’

20

5. CHANGES - FOR BElTER OR FOR WORSE?

With the felling of forest to make way for agriculture or plantations there are, inevitably, significant changes to the environmental behaviour of the land unit. But whether these changes are detrimental, and if so, how detrimental and for what duration, depends on how the forest is felled, and how the new production systems - and the associated infrastructure of roads and settlements - are developed and maintained. Although outside the scope of this review, it is also vital to consider the socio- economic costs and benefits when looking at the environmental effects of land use change. Population pressure often dictates that there simply is no alternative to clearing tracts of forest. So, the most relevant question then becomes: how can damage to the environment be limited?

As we have seen, the land is especially susceptible to damage during the period leading up to the establishment of the new production system. The following section concentrates on the changes in the hydrological, erosion and nutrient cycles associated with the establishment and maturation of the new land use. Precautions to reduce soil degradation are discussed separately.

Impacts on hydrology

It is clear to the observant eye that agricultural land and undisturbed forest behave differently during a heavy rainstorm. Even from well-terraced farms, runoff waters, discoloured with sediment, can be seen cascading along drains by the side of the road. Simultaneously, small streams draining well-maintained forests apparently carry less sediment - and water - during a storm. It is often said that streams have disappeared in the dry season as the result of deforestation. And conversely, in the wet season, damaging floods have proliferated. But is there a scientific basis for these observations?

It is not surprising that deforestation should lead to changes in hydrological response. As we have seen, the key to the hydrological behaviour of a tropical forest is the presence of the canopy and the forest floor, with its blanket of leaf litter and con- centration of roots. The forest canopy (through its interception of rainfall and its evaporative loss via transpiration) together with the litterlayer (through its effect on infiltration) are crucial in the hydrological cycle of the forest. Disturbance to either has highly significant implications.

21

Wherever the forest is replaced by annual cropping there are bound to be profound changes. For example, there is virtually no effective substitute for the forest floor with its protective leaf litter layer and myriads of tiny soil fauna whose activities maintain soil aggregate stability and water intake capacity. The beneficial effect of high organic matter contents and abundant fauna1 activity may linger for a year or two after forest clearing but exposure of the surface to the elements generally leads to a rapid reduction in infiltration capacity, particularly if fire is used during the clearing. Therefore, the result of conversion to agriculture is almost inevitably.that amounts of runoff increase. Farmers sometimes even encourage surface drainage to prevent waterlogging of crops. Indeed, if complete infiltration did occur in high rainfall regions, this could in some cases destabilise steep slopes with shallow soils that are no longer protected by a network of tree roots.

Often, infiltration-excess overland flow makes up less than one per cent of the incoming rainfall in undisturbed tropical forest but on agricultural fields with little or no conservation practices this figure may increase to as much as 30 per cent. An additional problem in smallholder agricultural zones is the considerable area which is permanently occupied by compacted surfaces including household compounds, tracks and roads.

Streamflow (mm/ma)

200

0’ ’ I I 1 1 1 I ! I I I I

I 2 3 4 5 6 7 8 9 10 11 12 Month

Period

- 1919-1943 - 1951L1972

22

Figure 3a.

Changes in seasonal distribution of stream flo w folio wing changes in land use. (a) Konto area, East Java, Indonesia, where intensive rainfed cropping and residential areas oc- cupying one-third of the basin produced significant increases in surface runoff.

Figure 3b.

Changes in seasonal distribution of

streamflow following changes in land use.

(b) Mbeya area, Tanzania, where

montane forest was replaced by subsistence

agriculture. Stable soil aggregates and low intensity rainfall

precluded widespread erosion

and a deterioration of the flow regime.

These surfaces can make a significant contribution to both overall runoff and sediment yield. The effect is compounded by an increase in effective rainfall: no longer are there trees to intercept rainfall and evaporate it directly back into the atmosphere. Neither of course are levels of forest water use maintained. Com- pared with undisturbed natural forest, therefore, the overall catchment water yield increases significantly under rainfed agricultural use (typically by 150 - 450 mm per year, depending on rainfall). Although in theory the extra amount of moisture available in the soil due to the reduction in evaporation should permit an increase in baseflow - given good surface management - dry season flows are often seen to decline because of deteriorated infiltration opportunities. As a result, the magnitude and frequency of high flows tend to rise wherever a pattern of pre- dominant subsurface drainage is replaced by a superficial one. However, one should bear in mind that, as storms increase in intensity and duration, and as one moves further downstream from the cleared portion of a catchment, the effect of forest clearing dwindles into insignificance as a causative factor in the floods that are inevitably generated by unusually large storms. In short: once the capacity of a soil to store water is exceeded by intense and persistent rains, the presence or absence of trees will have only a marginal effect on downstream flood levels.

Streamflow (mmimo)

1501 I

125 c

0’ ’ I I I I 1 I I I I 1 I

1 2 3 4 5 6 7 8 9 10 11 12 Month

- Forest -+-- Agriculture _

average values between 1958 and 1968;

23

‘Dry season flows

are often seen

to decline

as a result of

deteriorated

infiltration

opportunities’

24

When a natural forest is converted to a plantation, whether of crop or forest trees, the overall catchment water yield increases in the first instance after clearing because of the reduction in plant cover. Less vegetation means lower evapotranspiration and therefore more water available to contribute to catchment water yield. Initial increases in streamflow of 150 - 900 mm per year have been recorded after clear-felling tropical forest, depending on rainfall and the severity of soil disturbance. In most forest plantations it then takes two to three years for the increase in water yield to become less pronounced as a result of the uptake of water by the vigorously growing saplings and the recovering understory. During the next three to four years until canopy closure water yields continue to be reduced gradually until they approach the pre-clearing value. Therefore, the gain in water yield following the clearing of natural forest is only temporary. However, there are indications that eucalypts may use more water than the original forest once the trees manage to reach the groundwater table. Similarly, significant reductions in streamflow can be expected after coppicing trees, notably eucalypts. During the first two to three years after coppicing, the water uptake by the extremely rapidly growing trees (whose root systems are left intact) may be 50 - 75 per cent higher than that of mature un- coppiced individuals. Therefore, although coppicing is relatively benign in respect to soil surface disturbance, the hydrological costs are high.

Crop plantations almost always use less water than the original forest. For example, streamflow totals from areas converted to rubber or cocoa will be permanently higher than before, typically by 300 - 400 mm per year. Oil palm, on the other hand, eventually consumes as much water as a natural forest. Again, dry season flows may or may not be reduced once a plantation has established. This depends on the ‘trade-off’ between (i) the loss of infiltration opportunities associated with compacted roads and tracks or poor soil surface management, and (ii) the gain in soil moisture as a result of the more modest water use of the plantation. Needless to say, this is a mainly a question of planning and plantation husbandry.

Closed canopy plantations exhibit higher overall water use than rainfed annual crops. This is the combined result of the higher stature (leading to more interception during rain) and better developed root network (allowing the continued uptake of water from deeper layers when the top soil has dried out) of trees compared to annual crops. The other side of the coin -a potentially serious reduction in streamflow after afforesting degraded man- made or natural grasslands - cannot be stressed enough, particularly where a well-developed dry season prevails.

25

It is frequently suggested that replacement of natural forests by pastures or rainfed crops causes major changes in rainfall or even ‘desertification’. Despite all the rhetoric in popular and quasi-technical articles these claims are, in any case, unproven. Where changes in rainfall patterns have occurred, such as the persistent trend towards increased aridity in West Africa during the 1970’s and early 1980’s, these probably represent natural cycles in weather patterns that are mainly related to large-scale variations in the movement of ocean currents which affect sea- surface temperatures. Similarly, the widely touted claim that the Amazon rain forest block generates more than half of its own rainfall is open to question. Recent work has shown that the influx of rain-bringing moist air from the Atlantic Ocean is much more important than previously thought. As a result, any adverse climatic effects of a large-scale conversion of the forest to, say, pastures, are likely to be correspondingly smaller. Indeed, the most sophisticated computer simulations currently available for the prediction of the impacts of such a drastic change in land use suggest a maximum decline in rainfall of only about five percent. Because a ‘deforested’ landscape is more likely to consist of a mosaic of patches of regenerating forest and other land uses one can safely assume that the effect on rainfall will be even less.

‘Desuite all 1

.A

the rhetoric

in popular articles,

claims of greatly

diminished rainfall

after forest

conversion to

agriculture are

in any case

unproven’

“.-.---. I ~. _ _ _

There are specific situations, however, such as cloud belts on tropical mountains, where amounts of water reaching the soil surface are strongly influenced by the presence of trees. In these so-called ‘cloud forests’ a significant portion of the incoming precipitation is ‘stripped’ by the vegetation from low clouds and fog blown through the forest canopy. Contributions by such ‘fog stripping’ may reach several hundred mm per year under favour- able conditions and even represent the sole input of moisture during an otherwise dry season. Another important characteristic of cloud forests is their extremely low water use which is probably related to their often short stature. This phenomenon has essentially escaped explanation until now, although a host of hypotheses have been advanced over the years. Whatever the underlying causes of the low water use of tropical montane cloud forests, the net result is that, in combination with the extra water supplied by ‘fog stripping’, it is not uncommon for streamflow totals from such areas to be higher than measured amounts of incoming ‘ordinary’ rainfall. Because of this, catchment headwater areas covered with these intriguing forests should be protected if a steady supply of water to the adjacent lowlands is to be guaranteed.

‘Contributions by

fog stripping

in montane cloud

belts may reach

several hundreds of

mm per year’

27

However, the special hydrological functioning of cloud forests can be easily lost. In many tropical mountain areas they are being cleared for agricultural purposes. Needless to say, the replacement of trees by short crops marks the end of any contributions by ‘cloud stripping’ and may well result in diminished streamflow. The same effect may also be achieved in a more indirect manner when forest clearing on the lower slopes of a mountain may produce a warming up of the overlying atmosphere which, in turn, causes a lifting of the level of cloud condensation. This effect may be particularly important on smaller mountains where a relatively small rise in the level of the cloud base may already leave the mountain altogether cloud-free. On the other hand, planting tall exotics in deforested montane cloud belts might reverse the decline in water yield by re-instating fog drip contributions.

Impacts on erosion and sediment yield

The impact of land use change on erosion and sediment yield concerns us for two main reasons. First, surface erosion causes the loss of productive soil nutrients for crops or trees - this is an on-site effect. Second, both surface erosion and mass wasting may also cause problems downstream by silting up reservoirs or irrigation channels and damaging watercourses, land and prop- erty - these are all off-site effects.

Four main variables determine the likely effects of deforestation on erosion and sedimentation. Relief is a primary factor: the steeper the land, the more prone it is to erosion and the easier it is for eroded sediment to be transported downslope to the nearest stream. Current exploitation of moist tropical forest land tends to be carried out increasingly in areas of high relief which are intrinsically vulnerable to erosion and mass wasting and have hitherto been left alone. Rainfall is often high in these zones by definition. Soils are commonly poor and shallow in the areas of current conversion since the best soils in valley bottoms and on footslopes have been claimed from the forest long ago. Plant and soil cover is the final major variable: almost without exception new land use systems cannot match the characteristically complete canopy cover of undisturbed forest, although certain forest plantations and multi-storied agroforestry gardens come close. A brief consideration of these four major variables, relief, rainfall, soils and cover leads to the inescapable conclusion that any change from undisturbed forest tends to increase erosion rates unless specific conservation measures are taken. We will return to these in the final chapter.

28

Surface erosion in undisturbed tropical forest is generally less than 0.1 kg per square metre. However, this figure may easily rise by a factor of 50 - 100 in forest where the litter layer is removed to be used as fuel or cornposting material and where, in addition, the undergrowth is harvested for fodder. Where forest has been cleared for rainfed agriculture without proper soil conservation measures, even higher soil losses (up to 20 - 50 kg per square metre) have been observed under certain adverse conditions (high intense rainfall, steep slopes, erodible soils).

0,06 .-

f

Figure 4. Figure 4.

Surface erosion Surface erosion in a young forest in a young forest

plantation as a plantation as a function of soil function of soil

with litter and understorY ___------

+ + 20 20 33 33 40 40 50 50 60 60

surface conditions. surface conditions. Rainfall (mm) Rainfall (mm)

In humid tropical steeplands, bench terracing is the accepted standard conservation treatment for small scale rainfed farming nowadays. Under ideal conditions, erosion rates may be reduced by 90 - 95 per cent in this way. However, the effects of terracing on sediment production are not necessarily beneficial unless terraces of good quality are combined with conservation husbandry on the cultivated terrace beds and some form of terrace riser protection (grassed or stone walls). Failure to do so may well result in erosion rates that are as high as those associated with unprotected slopes. Interestingly, most of the sediment leaving the terrace units seems to derive from the riser walls rather than from the terrace beds and this has been largely overlooked in tropical watershed management programmes, or at least under- estimated. As discussed later, the maximum slope for construction of bench terraces is usually considered to be about 25 degrees gradient. Where land is cultivated on slopes above 35 degrees the danger of (terrace induced) landslips may become an even bigger threat than surface erosion without terracing.

29

.-.- .._ _-----~-.

‘Terraces

of good quality

need to be combined

with some form of

riser protection’

Although there are notable exceptions (such as teak plantations on heavy clay soils) both crop and forest plantations generally afford good protection against surface erosion once an adequate ground cover (as opposed to merely a closed tree canopy) has established. However, in the early stages of development when much of the cleared surface lies still bare, or at any stage if management is poor, erosion can be a serious problem. It goes without saying that the larger the degree of soil disturbance by heavy machinery and burning of logging debris during the clearing operation, the greater the erosion hazard and the slower the establishment of both trees and ground cover. In forest plantations natural regeneration of the former understory vegetation usually takes care of soil surface protection whereas in crop plantations a leguminous cover crop is normally introduced. Many tree species require clean weeding during their early stage of growth and this has led to the development of such cost-effective ways of tending young trees as the taungya system referred to earlier. Needless to say, such regular exposure of soil, whilst boosting tree productivity, presents an additional erosion hazard.

During these early years plantations on steep slopes with impermeable rock types are also especially vulnerable to landsliding after prolonged heavy rain, as was widespread across southern Thailand in late 1988. Fortunately, both erosion and landslip hazards tend to decrease rapidly as the plantations mature and build up a protective ground layer while at the same time deepening their stabilising root systems. As indicated already, there is another time when plantations are vulnerable to erosion. This is when they are ready to be harvested (forest plantations) or reach the end of their economic life (crop plantations). The replanting cycle of fast-growing softwood plantations is usually shorter than that of crop plantations. While disruption to the soil surface is more frequent in the former, the intensity of soil disturbance can be less. This is because the harvesting of timber often does not require the traumatic uprooting and removal of tree stumps as in the case of plantation crops like rubber. Where coppicing is practiced - and both eucalypts and teak are sometimes managed this way - then erosion rates are likely to be low, although the effect is achieved at the ex- pense of increased subsequent water uptake.

In certain situations, forest plantations are subject to two special problems not shared by farmland or crop plantations: these are (i) the removal of litter by villagers using the forest to gather fuel or fodder, and (ii) fires. Clearly, either of these eventualities can increase the erosion problem massively. The removal of litter exposes the soil surface to the erosive force of crown drip, and the aftermath of fire can be severe soil loss through runoff.

31

32

Impact on soil fertility

‘Nutrient inputs

from rainfall

may take

50 - 60 years

to make up for

the losses

incurred during

clearing’

Upon tropical forest clearing, nutrients are lost from the ecosystem in various ways. Whether such losses will impair future plant productivity depends mainly on the inherent fertility of the site, which is determined largely by geological and climatic factors. As a rule of thumb, however, it can be stated that the more infertile the soil, the more serious the effect. This is because forests on the poorer substrates are only able to thrive there on the basis of an intricate and relatively closed nutrient cycle which is suddenly disrupted upon clearing.

The first direct route of nutrient loss during conversion to other land uses is via the harvesting of stems. The potential importance of this loss is brought home immediately when one con- siders the amounts of nutrients that are incorporated in the stemwood and bark of tropical forest trees. For instance, leaving the most fertile sites aside for the moment, it has been found that between 10 and 75 per cent of all the calcium and phosphorus present in above- and below-ground biomass plus that available in the root zone of the soil is stored in the stems of rain forests. Similar figures have been obtained for such other key nutrients as potassium (20-80 per cent) and magnesium (20-65 per cent). Whilst only the commercially sized stems will be removed upon logging, the remaining biomass left on the ground is usually ‘windrowed’ and burned.

Contrary to common belief, not only do the more volatile constitu- ents like nitrogen and carbon go up in smoke during burning of slash. Depending on the intensity of the fire, between 25 and 80 per cent of all the calcium, potassium and phosphorus present in the slash may be lost in this way as well. Burning represents a second major pathway for nutrient loss upon forest clearing, therefore. To make matters worse, nutrients which remain in the ash are vulnerable to removal in runoff and by leaching. High- temperature burns may render topsoil temporarily water repellent, causing potentially dramatic increases in overland flow frequency and intensity and therefore surface erosion. Nutrients carried away in eroded sediment constitute the third kind of loss, although this does not necessarily mean that they will be lost from the ecosystem as a whole. Sediment that is eroded on the steep, higher parts of a slope may be redeposited where the gradient becomes less or be trapped behind barriers of slash. Therefore, only a fraction of the eroded material, and the nutrients contained in it, may reach the nearest stream and be carried away. A fourth process via which nutrients are lost upon forest conversion is through enhanced leaching. Not only is there a lot more water percolating through the soil now that both rainfall interception and

33

water uptake are diminished, but also the capacity of the new vegetation to take up nutrients is reduced, at least temporarily. The sum result is that substantial amounts of nutrients may be washed into the streams and lost for future productivity.

Clearly, the dangers of nutrient loss. are most acute during the actual period of transition but in due course a new equilibrium will be reached. Naturally, overall nutrient losses differ widely between locations as a result of differences in rainfall, site fertility, and the volume of harvested and burned biomass. Nevertheless, ‘ballpark’ estimates for the period required to compensate overall nutrient losses via nutrient inputs in rainfall and dust are typically in the order of 50 to 60 years.

With rainfed agriculture, the cultivation of annual crops can lead to further soil degradation - after a season or two of good crop yields - unless astute land husbandry practices are applied. Cult- ivation may cause acidification as cations become depleted, and aluminium becomes increasingly soluble as a result. The possibility of enhanced surface erosion rates is always there unless prevent- ative and remedial measures are undertaken. The potential nadir at the end of a descending spiral is land which is too poor to cultivate anything but a succession of continuous cassava crops.

Nevertheless the picture is not as gloomy as it may seem. There are a number of husbandry practices, including manuring, com- posting and planting of legumes which can help maintain soil fertility under cropping systems. Agroforestry has a particularly important role to play in this respect. Furthermore there is a direct incentive for farmers to maintain soil fertility: after all, fertile soil means healthy and profitable crops.

Where plantations replace forest, a new ‘forest floor’ is created eventually, given good surface management. In some crop plantations, leguminous cover crops are grown with a similar protective effect. Application of inorganic fertilisers is the rule for crop plantations and is becoming commonplace in forest plantations also. Nevertheless, prevention is always better than cure. Similarly, under the taungya method of establishment there may be a fertility spinoff for the emerging trees when fertilisers are used or legumes planted during the intercropping phase. Furthermore in plantations, the role of trees as a nutrient ‘pump’ is resurrected, bringing mineral elements from deeper layers into the mainstream nutrient cycle.

On balance, plantations present the opportunity to establish a system which, with careful management, can provide an acceptable alternative to the nutrient cycle of natural forest.

34

‘The effect of conservation farming is simultaneously to protect

the soil and to ensure better crop yields’

35

6. FROM NATURE TO NURTURE - TRANSITION WITH CARE

Throughout the previous sections we have stressed the potential for environmental damage through careless systems of clear felling and subsequent irresponsible management of the land. The purpose of this section is to demonstrate how damage can be limited - how changes in land use from forest to agriculture, or to plantations, can be achieved without causing lasting detriment to the on-site or downstream environment. The key is understanding what factors cause degradation, and how transition can be achieved through applying the principles of sound logging and, subsequently, careful land husbandry.

Benefits without Penalties

In the companion volume on the impacts of selective logging it was pointed out that well planned operations could significantly reduce the damage to soil and vegetation. Most of the guidelines that should be followed to minimise environmental havoc during logging are equally relevant to clear felling campaigns. Once again it can be stated that a// that is needed is to put into practice what is already known - but so far rarely implemented. A summary of these guidelines is given below.

Above all, prelogging planning is necessary. This involves an as- sessment of the forest, including the identification of sensitive areas to be avoided, for example where it is too wet or too steep to gain access to the trees. The infrastructural layout of access roads and skid trails is of paramount importance in minimising damage to the soil. Roads and major skid trails must be located on ridge crests wherever possible. This will not only help control surface erosion, but also the frequency and size of road-related landslides. As roads provide the most direct route for runoff and sediment to water courses, proper drainage facilities are a must and the number of stream crossings should be minimised.

Although manual and animal-based systems of extraction tread much more lightly on the forest, it is unrealistic to expect loggers to revert to these relatively slow and expensive methods. In- evitably, machines will be used, but once again there are certain guidelines which can keep damage to a minimum. For example, skyline yarding should be prefered to ground-based extraction of logs wherever this is economic. Uphill log extraction tends to divert runoff and sediment away from streams, in contrast to downhill extraction. Where machines are used, then the golden

36

rule is that the fewer passes the machine makes, the less the damage. Needless to say, it is necessary to use machines of an adequate size and capacity to eliminate loss of traction, but oversized equipment means unnecessary compaction of the soil. Root rake equipment is particularly damaging to the soil. If at all possible, it is preferable to leave stumps to rot in situ, unless this is considered to be a disease hazard to the following crop.

There is an understandable temptation to resort to burning the logging debris, in order to complete the job of clearing the land. As has been repeatedly mentioned in the foregoing chapters, fire is detrimental for a number of reasons and should be used judiciously. Its contribution to increased surface erosion is perhaps the most important - and this is closely followed by its negative impact on soil fertility. Rather than burning slash, it should be windrowed into bands across the contour, thereby acting as anti-erosion strips, or spread as a mulch if this is compatible with the subsequent intended land use. An additional advantage of non burning is the slow release of nutrients as the material gradually decomposes.

Retaining buffer strips along stream sides is another important safeguard during forest clearing. These riparian bands of natural vegetation help protect stream banks from disturbance. In the early stages after felling, buffer strips also help to filter sediment out of the inevitable runoff from the cleared surfaces.

‘As roads provide

the most direct

routes for

runoff and sediment

to water courses,

the number of

stream crossings

should be

minimised’

37

Land husbandry - production through conservation

Farming on slopes previously under forest cover does not necessarily mean uncontrolled erosion, ‘mining’ of soil nutrients or an end to dry season flows. In environmental terms the difference between good and poor farming practices is in fact greater than that between good farming and forest. The conventional conservation safeguards are engineering measures - bench terraces, controlled drainage ways and so on. But engineering alone cannot give adequate protection to the land. More fundamental is the principle of sound ‘land husbandry’ and the concept of ‘conservation through production’. Important husbandry practices include integration of livestock, contour farming, the use of manures, composts and mulches, strip cropping and intercropping with legumes. In a special category are biological barriers: ‘hedges’ of perennial grasses, such as Vetiver, and of woody species, such as Gliricidia, which, while not yet widely adopted by farmers, can help to check erosion over a range of slopes. These techniques are gaining increasing credibility as cheaper and more versatile alternatives to terracing in specific situations - for example on shallow soils and steep slopes. The effect of conservation farming techniques is simultaneously to protect the soil and to ensure better crop yields - good news for the farmer, and good news for the environment also.

38

Figure 5.

Recommended slope

gradient classes

for various commonly

applied soil

conservation measures.

Nevertheless, soil conservation structures still continue to form the recommended framework for rainfed farming on humid tropical hillsides. On the lowest slopes the recommendation is normally ploughing and planting along the contour line - supported by earth bunds, grass strips or barrier hedges (Figure 5). The bench terrace, in all its variations, is standard for slopes between about 7 and 25 degrees (12-42 per cent). Bench terraces are not suitable for all soil types however and require a soil depth of at least 50 cm. Neither are all bench terraces as effective as they should be: a common shortcoming is poor maintenance of the riser, which can itself become a significant source of sediment. Pro- grammes which promote stall feeding of livestock indirectly contribute to soil conservation by encouraging the planting of fodder grasses on terrace risers. The grass helps to protect the risers against rainsplash and slope failure while the manure from the livestock improves the soil of the terrace bed. The main erosion problem arises above the 25 degree gradient - which is the legal limit for cultivation in many countries, but is increasingly exceeded in practice. It is impractical to build terraces above this slope: the proximity of the risers and narrowness of the beds means that 50 per cent or less of the land is available for cultivation. Also, the risk of landslips is increased. But reality has to be faced as land pressure is mounting. Damage limitation is the keyword.

Contour cultivation

Contour (or graded) bunds or:

terraces

Gradient Maximum without structures

Maximum without terraces

15 - 250

Maximum for bench terraces

_-

25 - 450

Maximum for agroforestry

39

Agroforestry systems, that is a mixture of trees and annual crops, hold some of the best potential for soil conservation on steeper slopes. The barrier hedges mentioned above are but one example. There is a temptation for the scientist to attempt to create ‘improved’ forms of agroforestry with precision. Typically, this may involve multipurpose trees spaced equidistantly amongst annual crops, as for example in ‘alley cropping’. But there is a danger of creating recommendation straightjackets. Rigidity thus risks replacing one of the most attractive aspects of traditional agroforestry - the shrewdness with which land users have built up complex systems which fit the needs of their families as well as protecting the land beneath. The ‘home gardens’ of the Far East, for instance, are highly productive systems of intricate mixed cultivation which effectively mimic natural forest.

‘The traditional

home gardens of the

Far East are highly

productive systems

of intricate mixed

cultivation that

effectively mimic

natural forest’

-... -

As for the steepest slopes, suffice it to say that above about 30 degrees neither cultivation of crops nor agroforestry systems can be sustained without danger of greatly accelerated erosion and landsliding. Here, the only safe land use system is undisturbed forest, whether natural or planted.

Managing the estates

Plantation crops have long life cycles, and when well managed, afford continuous ground cover after the first few years. As such, they may provide ample opportunity for soil surface protection and hydrological stability. However, in order to ensure that plantations achieve maximum ‘environmental friendliness’ there are a number of basic cultural principles which must be followed.

As stated repeatedly, the period between planting and the development of a full ground cover is the most critical. To minimise erosion hazard during the period of bare soil, terraces may be constructed before planting commences, as is common practice on steep slopes for tea in India, for coffee in East Africa and for rubber in Malaysia. On slopes less than f7 degrees it may be adequate merely to plant along the contour, with barrier hedges or grass strips as support measures. A network of access tracks should be strategically planned within the estate, and both road and terrace drainage designed at non-erosive gradients.

Protection of the soil between the immature plants is achieved in a number of ways. For instance, in East Africa, tea is sometimes interplanted with a ‘nursery crop’ such as oats which not only protects the soil from rainsplash but also provides mulching material after harvest. In subsequent years the prunings from the tea bushes themselves provide the mulch. Coffee cannot tolerate competition from a nursery crop. Instead, Napier grass or a similar species is often grown to provide mulching material - either in a separate plot, or in contour strips between blocks of coffee. Leguminous cover crops are instrumental in protecting the soil between young plants in plantations of rubber, oil palm and cacao while simultaneously increasing soil nitrogen. These can be shrubs like lndigofera and Desmodium or creepers such as Pueraria, Dolichos or Centrosema. Alternatively, annual pulses - beans, cowpeas and so on - may be intercropped during the early years, after which the leaf litter produced by the matur- ing trees can take over. Nevertheless, additional mulching is often necessary, particularly around the bases of the trees where shading may hamper the development of a good ground cover or stemflow may wash away the litter. Soil fertility is usually further enhanced by regular fertiliser applications.

41

New trees for old

Many of the comments made about crop plantations also pertain to plantation forests. Here also we have a form of land use which provides good opportunities for protection of soil and water resources. Indeed, as noted earlier, some plantations are specifically used for this very purpose - as ‘protection forests’ in vulnerable upland areas. But again there is a world of difference between a badly maintained plantation, and one which is managed according to enlightened practices. The key is a thoroughly planned and implemented infrastructure for planting, stand maintenance, protection and harvesting.

In view of the critical nature of the transition and harvesting periods in terms of erosion and leaching hazards, spot clearing of planting holes may be a better solution than complete clearing - especially when the latter implies burning of slash and mech- anised cultivation of the soil in preparation for planting. The same applies to steeply sloping land. The potentially negative impacts of slash burning have been pointed out already. Where land for cropping is scarce, there is scope for strategic intercropping of young plantations with a cash crop legume (or for practising some other form of taungya) to improve surface cover, fix nitrogen and at the same time generate income for the rural poor.

As we have demonstrated repeatedly, the forest floor is the most important component of the forest ecosystem in terms of soil surface protection. As such, it should be guarded against disturbance at all times. While the selective collection of dead or fallen branches for domestic fuelwood may be acceptable, open access for fuel and fodder collection or free range grazing can promote soil compaction, runoff and erosion surprisingly quickly. In addition, opening up a forest plantation increases the risk of fire.

Thinning and pruning operations cause some damage to the understory, although it is normally negligible compared to the havoc that can be wrought during timber harvesting. This is especially true for short-rotation plantations with fast-growing species planted for the production of paper pulp, which may be clear felled every 5-I 0 years. However, where the initial planning of the access infrastructure has been carried out efficiently, harvesting will be greatly facilitated and damage to the soil correspondingly limited. Such operations should adhere to accepted standards of good practice, including minimising machine size for extraction, using skyline systems in truly steep terrain and in general work uphill and away from water courses.

42

Clear felling of forest plantations usually produces far less slash than natural forest and the need for windrowing and burning of slash will be correspondingly less. In this way, precious nutrients are conserved for future productivity. Soil nutrient reserves may be seriously depleted after the repeated removal of harvested produce, particularly in the case of fast-growing species planted for the production of paper pulp that are harvested every 5-10 years. Regular fertilisation will be required then if soil fertility is to be maintained. However, even fertile soils on alluvial or volcanic deposits may become depleted after a few rotations of nutrient demanding hardwood species. Conifers on the other hand have more modest nutrient requirements. One further way of minimising nutrient losses is to only harvest the stemwood and leave bark, branches and foliage on site to rot.

A final word should be devoted here to ‘social’ or ‘farm’ forestry. No longer is tree planting seen as the exclusive preserve of large companies, or of the state. No longer are large forest stands seen as the only way of producing wood. More international development efforts are being directed towards resource poor farmers to help them plant trees on their own land, and this is a cause which is worthy of support. Soil conservationists throughout the tropics are beginning to realise that it is often the small scale land users themselves who make the greatest difference to the health of the local environment. The rationale behind this is, of course, that when people appreciate the value of a resource, they are much more likely to conserve it effectively.

‘In environmental

terms,

the difference

between good and

poor farming Concluding remarks

practices

is enormous’ Returning to our original premise - that forest conversion does not necessarily spell disaster - we have attempted to demonstrate that environmental damage can be limited, and indeed that in many cases a new land use system does not need to be de- structive. Of course, the process of conversion in itself is highly disruptive. But an established and well managed plantation, for example, represents a new form of equilibrium, while significantly increasing the economic productivity of the land. Similarly, although small scale agriculture in the often steep humid tropical lands exposes the soil to an increased erosion hazard, there are a number of highly effective conservation practices at our dis- posal. Indeed, in environmental terms, the difference between good and poor farming practices is enormous.

Where forest has been converted, it is our duty to make sure that sound land husbandry is practised - acknowledging what is here today, rather than lamenting what has been lost forever.

43

SELECTED REFERENCES

In keeping with the style and format of this Series, no specific references to literature have been included within the main body of the text. The following books and articles comprise our prin- cipal sources of information, and form a basis for further reading.

Adams, P.W. & Andrus, C.W. 1991. Planning timber harvesting operations to reduce soil and water problems in humid tropical steeplands. Paper presented at the International Symposium on Forest Harvesting in South-east Asia, Singapore, June 1991.

Bruijnzeel, L.A. 1990. Hydrology of Moist Tropical Forests and Effects of Conversion: A State of Knowledge Review. UNESCO, Paris, and Free University, Amsterdam.

Bruijnzeel, L.A. 1995. Soil chemical and hydrochemical respons- es to tropical forest conversion: a hydrologist’s perspective. In: Soils of Tropical Forest Ecosystems (A. Schulte & D. Ruhiyat, eds.), Volume 3, pp. 5-47. Mulawar- man University Press, Samarinda, Indonesia.

Bruijnzeel, L.A. & Proctor, J. 1995. Hydrology and biogeochem- istry of tropical montane cloud forests: what do we really know? In: Tropical Montane Cloud Forests (L.S. Hamilton, J.O. Juvik & F.N. Scatena, eds.), Ecological Studies 110: 38-78, Springer, New York.

Carson, B. 1989. Soil conservation strategies for upland areas of Indonesia. Occasonal Paper no. 9: East-West Environ- ment and Policy Institute, Honolulu, Hawaii.

Cassells, D.S., Gilmour, D.A. & Bonell, M. 1984. Watershed forestry management practices in the tropical rainforest of N.E. Australia. In: Effects of Forestry Land Use on Erosion and Slope Stability (C.L. O’Loughlin & A.J. Pearce, eds.), pp. 289-298. IUFRO, Vienna.

Critchley, W.R.S. & Bruijnzeel, L.A. 1995. Terrace risers: erosion control or sediment source? In: Sustainable Reconstruct- ion of Highland and Headwater Regions (R. Singh & M.J. Haigh, eds.), pp. 529-541. Oxford/lBH Press, New Delhi.

Critchley W.R.S., Reij, C.P. & Willcocks, T.J. 1994. Indigenous soil and water conservation. A review of the state of knowledge and prospects for building on traditions. Land Degradation & Rehabilitation 5: 293-314.

Doolette, J.B. & Magrath, W.B. 1990. Watershed development in Asia. Strategies and technologies. World Bank Technical Paper no. 127. The World Bank, Washington, D.C.

44

----. __- ._.. ~.

Evans, J. 1992. Plantation Forestry in the Tropics. Second edition. Clarendon Press, Oxford.

FAO, 1989. Soil conservation for small farmers in the humid tropics. FA0 Soils Bulletin no. 60. FAO, Rome.

Hamilton, L.S. 1991. Tropical forests: identifying and clarifying issues. Unasylva 166 (42): 19-27.

Hudson, N.W. 1995. Soil Conservation. Third edition. Iowa State University Press, Ankeny, Iowa.

Jackson, I .J. 1977. Climate, Water and Agriculture in the Tropics. Longman, London.

Jordan, C.F. 1987. Amazon Rain Forests. Ecosystem Disturb- ance and Recovery. Springer, New York.

Lal, R. 1987. Tropical Ecology and Physical Edaphology. J. Wiley, New York.

Moldenhauer, W.C. & Hudson, N.W. 1988. Conservation Farming on Steep Lands. Soil & Water Conservation Society, Ankeny, Iowa.

Moldenhauer, W.C., Hudson, N.W., Sheng, T.C. & Lee, S-W. 1991. Development of Conservation Farming on Hill- slopes. Soil & Water Conservation Society, Ankeny, Iowa.

Nykvist, N., Grip, H., Sim, B.L., Malmer, A. & Wong, F.K. (1994). Nutrient losses in forest plantations in Sabah, Malaysia. Ambio 23: 21 O-21 5.

Opeke, L.K. 1982. Tropical Tree Crops. J. Wiley, New York. Pearce, A.J. & Hamilton, L.S. 1986. Water and Soil Conservation

Guidelines for Land-Use Planning. East-West Environment and Policy Institute, Honolulu, Hawaii.

Proctor, J. 1987. Nutrient cycling in primary and old secondary rain forests. Applied Geography 7: 135-I 52.

Sanchez, P.A. 1976. Properties and Management of Soils in the Tropics. J. Wiley, New York.

Sanchez, P.A. 1995. Science in agroforestry. Agroforestry Systems 30: 5-55.

Tiffen, M., Mortimore, M. & Gichuki, F. (1994). More People, Less Erosion. Environmental Recovery in Kenya. J. Wiley, New York.

Wiersum, K.F. 1984. Surface erosion under various tropical agroforestry systems. In: Effects of Forestry Land Use on Erosion and Slope Stability (ed. by C.L. O’Loughlin & A.J. Pearce), pp. 231-239. IUFRO, Vienna.

Wiersum, K.F. 1985. Effects of various vegetation layers in an Acacia auriculiformis forest plantation on surface erosion in Java, Indonesia. In: Soil Erosion and Conservation (S.A. El-Swaify, W.C. Moldenhauer & A. Lo, eds.). Soil Conservation Society of America, Ankeny, Iowa.

Young, A. 1989. Agroforestry for Soil Conservation. CAB Inter- national, Wallingford, U.K.

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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 earth’s population by the end of the present decade. In the 21st century, these nations will pass the developed countries in numbers of people. Such a population shift will alter existing international economic and geopolitical relationships. With this major change looming on the horizon, coupled with the need to treat the tropical resources wisely, the United Nations Educational, Scientific and Cultural Organization (UNESCO) and the United Nations Environment Programme (UNEP) joined with 22 other organizations in July 1989 to hold the International Colloquium on the Development of Hydrologic and Water Management Strategies in the Humid Tropics at Australia’s James Cook University. The International Hydrological Programme (IHP) of UNESCO was the lead organization.

The Colloquium developed strong evidence that the present situation, including the question of tropical forest depletion, was not only in need of serious consideration, but that the potential for vastly increased human impacts will be quite significant if they are not adequately considered now. The formal scientific text em- bodying 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 Re- view, was produced by the joint efforts of IHP’s Humid Tropics Programme, the National Committee for IHP of the Netherlands and the Vrije Universiteit of Amsterdam in October 1990.

The present popularized volume on the impacts of tropical forest conversion is one of several such publications having their origin in the Colloquium. Others dealt with the disappearance of tropical forests, the hydrology of small tropical islands, the water-related problems of large tropical cities, the role of women, the hydrological impacts of forest logging, groundwater, reservoirs,etc. Additional volumes are in preparation, including one on the hydrological and conservation aspects of tropical montane cloud forests.

Further information on any of these publications can be obtained from the International Hydrological Programme of the Division of Water Sciences within UNESCO (see back cover for address).

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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.

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About the authors:

William Critchley is a conservation agronomist with the Centre for Development Cooperation Services at the Vrije Universiteit, Amster- dam, The Netherlands. He began his career in Kenya, where he worked on a number of development projects from 1973 until his return to Europe in 1987. Subsequently he has concentrated on a mixture of consultancies, research and publications in his field of specialisation: the interface between plant production and resource conservation in developing countries of Africa and Asia.

Sampurno (LA.) Bruijnzeel is a lecturer in hydrology at the Faculty of Earth Sciences of the Vrije Universiteit with over two decades of expe- rience with forest hydrological research in the humid tropics, mainly in South-east Asia, the Pacific and the Caribbean. Since the mid-1980s he has published a number of comprehensive reviews of the literature on environmental impacts of tropical forest disturbance and conversion. His other scientific interests include the hydrology and nutrient economy of tropical montane cloud forests and fast-growing plantation forests, as well as erosion and sediment transport processes.

Photo credits:

L.A. Bruijnzeel:

W.R.S. Critchley: R. Klinge: R. Mieremet: G.A. Persoon: J. Rupke: M.J. Waterloo: K.F. Wiersum:

8, 9 top, 11 bottom, 13, 16 bottom, 19, 24 bottom, 26,27, 30 bottom, 35, 37, 40 16 top, 30 top, 32 top, 38 11 top cover, 32 below 2 24 top 9 bottom 20

Sources of remaining illustrations:

Figure 1:

Figure 2:

Figure 3:

Figure 4: Figure 5:

Adapted from I. Douglas (1977). Humid Landforms. MIT Press, Cambridge, Massachusetts. Redrawn from J. Proctor (1987). Applied Geography 7, p. 135. After L.A. Bruijnzeel (1993). /nternationa/Association of Hydrological Sciences Publication 216, p. 21. Adapted from K.F. Wiersum (1985), (see reference list). Adapted from H.C. Pereira (1989). Policy and Practice in the Management of Tropical Watersheds. Westview Press, Boulder, Colorado, p.159.

The authors thank Mr Frans Stevens for his invaluable assistance during times of computer crisis and Mr Henny Colenbrander for his continued support throughout the preparation of this document.

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Documents

W.R.S. Critchley and L.A. Bruijnzeel - UNESDOC …unesdoc.unesco.org/images/0010/001096/109608eo.pdf · As the process of colonisation by agriculture continues, so the more remote