Huxley Agroforestry Ch 2

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2The Nature of Agroforestry

INTRODUCTION

What is agroforestry?

Agroforestry is the ar t and science of growing

woody an d non-woody plants together o n the same

unit o f land for a range of benefits. There ri many

ways in which this can be done. In some form or

another, it has been happening since mankind gave

up hunting an d gathering. In the tropics, agro

forestry is essentially for land occupiers, so that

'services' as well as products are important.

Agroforestry, as it is no w called (Lundgren, 1982),

usually possesses all of the following features :

• multiple plant components, at least on e of which

must be a woody perennial

• a high level of interaction (economic an d bio

physical) between the woody and non-woody

components

• usually multiple products, often of different cat

egories (e.g., food , fodder, fuelwood)

• at least one service function (shelter, shade, soil

amelioration, ' convenience', etc.)

and, in tropical low input situations frequently also

• a dependence on the use an d manipulation of

plant biomass, especially b y optimising the use of

plant residues

Even if we apply these criteria it is sometimes

difficu It to distinguish agroforestry from some

forms of agriculture an d forestry, shade trees in

plantation crops , fo r example, or 'corridor farming'

an d practices that promote the use of non-Wood

products in forests and woodlands. Furthermore,

ho w is it to be separated from s o me aspects

of 'social forestry ' or 'community forestr

(Wickremasinghe, 1996)? I f pastoralists manipula

or enrich the woody plants i n certain areas in ord

to foster young stock at certain times, is this ' rang

management' or agroforestry? When it is difficu

to know exactly what is meant by a term, as in th

case, it usually implies that it means somewhat di

ferent things to people with different background

Below are some definitions of agroforestry used b

ICRAF at different times:

"AgroforestlY refers to those landuse practices in whic

woody perennials (trees, shrubs, woody vines, bamboo

palms) are grown in association with agricultur

crops or pastures, sometimes with livestock or othanimals (e.g., insects such as bees, fish), and in whic

there are both ecological and economic interactio

tw n the woody plants and the other components .

"Agroforestry landllse is the deliberate inter-

sequential cropping of woody and non-woody pla

components (sometimes with animals) in order

generate multiple products and 'services'. There a

both ecological and economic interactions between th

plant components."

"Agroforestry is a dynamic , ecologically based , natur

resources management system that, through the i

tegration of trees in fannland and rangeland, diversifi

and sustains production for increased social, econom

and environmental benefits for landusers at all levels

Nevertheless , despite its flaws, th e nam

'agroforestry ' has served to direct th e attention

the technical an d scientific community to the nee

to understand more about the possibilities a nd lim

tations of growing woody an d non-woody plan

on t he same piece o f land. As farmers do no t dis ti



The Nature of Agroforestry

guish themselves as ' agriculturists', 'foresters' or

' agroforesters' it is unlikely that they will care much

what the practices ar e called, as long as the advice

they receive is technically sound and takes into

account their social and economic circumstances.

Agroforestry is

essentially a form of mUltiplecropping, but there are many successful forms of

landuse that involve growing just one crop a t time

(monocultures). In Western countries the develop

ment of biological and physical sciences occurred

concurrently with the rise of industry an d urbanisa

tion. T he first has provided the opportunity and th e

second the incentive fo r specialisation in farming

practices. Th e result was that in Europe an d North

meric traditional mixed farming, and the man

agement of natural forests, moved rapidly towards

extensive monocultures from the middle of the last

century onwards.

Monocultures have proved to be extremely pro

ductive with the aid o f inputs such as mechanisation

for tillage and harvesting, fertilisers, irrigatioJil, and

th e application of chemicals for the control o f pests,

diseases and weeds. All these can most effectively

be used in a single crop that ca n be managed effi

ciently so as to exploit its value fully. Large gains

in productivity have been achieved by breeding

cultivars that can take advantage of this type of

system. Fo r example, cereals have been 'miniatur-

ised' an d their harvest index increased by reducing

unwanted biomass. There are many reasons fo r

continuing to encourage productive monocultures

where they ar e sustainable an d environmentally

friendly.

In temperate regions the trend towards spe

cialisation in landuse practices led to a separation

of t he applied sciences that supported them. So the

disciplines of agriculture, forestry, horticulture, e tc .

emerged. A consequence has bee n tha t applied

research disciplines related to landuse have become

separate entities. Fo r example, in th e 1970s it was

seen as an innovation when participants from these

separate disciplines actually contributed together

at th e same scientific meeting (e.g., Luckwill an d

Cutting, 1970). A feature of agroforestry is that, of

necessity , it involves inputs from a very wide range

of disciplines.

Although, in practice, the social and economic

wellbeing of farmers an d their families will nearly

always determine whether or no t an y particular

agroforestry practice is adopted (Raintree, 1991),

it is no t usually the predominant factor used

categorise one; this mainly depends o n th e techno

o y being used. However, it is very important th

socio-economic considerations should no t be le

ou t of the earliest considerations of the suitabili

or otherwise of any proposals. We should cosider, for example , requirements for labour an

skills. Some forms of agroforestry (e.g., hedgero

intercropping) require more of both than othe

(e.g., simple boundary planting or fallow enrich

ment). Some demand a high initial input (perhap

partly in cash) in order to provide the trees, and

plant an d protect them . Making it clear what th

outputs ar e (e.g., cash, shelter, energy) will he

designate th e economic focus of the activity,

well as what products a nd services can be expecte

Certainly, differen t k ind s of agroforestry w

involve different levels of social input (labou

management, etc.) and have different econom

consequences (for food supply, cash flow, etc.).

Although individual agroforestry practices m

be technically suitable for on e or more ecozone

the design and management of actual system

(i.e., functional units) that are eventually adopte

may well change both between ecozones an

because of farmer preferences as socio-econom

constructs change. Also, the 'richer' the enviro

ment the greater the choice of landuse practic

that ca n successfully be undertaken (Fig. 13

below). This applies equally to agroforestry, and

cannot be assumed that because a particular for

of agroforestry (e.g., alley cropping, better terme

hedgerow intercropping) succeeds in one place

will do so in a different environmental situation.

IS THERE AN'AGROFORESTRY UNIT'?

The importance of scale

Th e rudimentary elements in most agroforest

practices consist of a woody perennial, an agricu

tural crop species (or some pasture) and man (an

there may b e d omes ticated animals or no

(Huxley, 1983a). Simple as this scheme may seem

it c an be combined in a great many ways, an d the

can involve very disparate scales of both time an

space. Th e spatial scales involve both horizont

an d vertical dimensions, and this 'verticality' nee



10 Tropical Agroforestry

to be understood with regard to how it can affect

the availability of environmental resources, their

'ca pture' by the plants and their utilisation (see

Chapter 9). In agroforestry, therefore, we have

often to consider spatial scales that differ widely

from the distance between component plants (their

environmental interactions), the plot, the farm, andlarger areas such as the watershed. Then there is

also a need to look at processes, or events, that

happen over very short time periods (e.g., hours

or days for sharI-term competitive effects), over

periods of weeks within seasons (Ii tterfa11), over

several seasons (the accumulation of soil organic

matter), or over decades or even hundreds o f years

('s ustainability').

An issue that arises from the complex structure

of woody/non-woody plant mixtures, both above

and below ground , is the extent to which vertical

an d lateral transfers of environmental resources

occur, and how 10 evaluate the way in w i h

th e plant components in any system, particularly

the trees, ar e accessing environmental resources.

Farms can present a puzzling array of spatial

patterns (see Chapter 22). Possibilities to exploit

vertical and lateral opportunities to capture

environmental resources (light, water and nutri

ents) represent an important way in which

agroforestry may enhance productivity, bu t know

ing from where the resources ar e used up, or how

they are being moved around within a plot, between plots, or within the farm or landscape as

a whole, is crucial to ou r understanding o f how any

system is working. Th e acquisition of 'extra'

resources (e.g., water and nutrients) by scattered

trees or hedgelines is at the expense of the

surrounding area , so that it is merely a redistribu

tion of resources within the environment as a

whole.

ARRANGEMENTS AND

INTERACTIONS

Agroforestry is a generic term and the various

forms that it can take (agroforestry practices,

sometimes called technologies) can be classified

into various groups (see Table 2.1) based on the

arrangement of their components in space and time

an d their outputs in terms of both products and

'services' (e.g., soil and water conservation, micro

climate improvement, demarcation of boundaries,

etc.). Thus there is agrisilviculture (trees wit

crops) , silvopastoral practices (trees with pastur

an d livestock), agrosilvopastoral practices (crops

pasture , animals and trees) , and various other tree

based practices such as woodlots, boundary plan

tings, shelterbelts, etc. From a functional point o

view it is sometimes useful to indicate whether thwoody plants ar e grown in zones (lines, plots

blocks) or ar e more widely scattered among th

crops or in th e pastures.

Th e concept of 'simultaneous' or 'sequentia

use of the land is important; the latter avoids an

direct competition between trees an d crops, bu

there can be economic implications with regard t

labour inputs required to clear land after tre

growing. There are, in fact , some instances wher

woody species can have adverse effects on an

subsequent plantings, i.e ., possible allelopathi

effects on follow-on crops or on subsequent tre

establishment. Fo r example, it is difficult to cro

after black wattle (Acacia mearnsii) in East Africa

an d maize sown after Sesbania sesban at ICRAF

Field Station, Machakos, was observed to establis

poorly.

In semi-arid regions a 'bare fallow' is sometime

undertaken in orcler to store enough water for

succeeding crop (as in some North African coun

tries). Usually, however, the land becomes covere

with weeds or, if the fallow is long enough, wit

grasses , bushes and trees (a 'bush fallow'). Somform of continuous rather than intermittent lan

occupancy is usually desirable if soil erosion is no

to occur. Taungya, which is the practice of growin

crops during the first year or so of a long-term

commercial forestry plantation, is ' listed as

sequential practice although , strictly speaking, it

simultaneous - cropping ceases as soon as an

effective competition with the trees begins to occur

This is really the key to us understanding simulta

neous practices. In agricultural intercropping ther

ar e a whole range of ways in which two or mor

crop components can be associated with on e an

other an d grow together during a single croppin

season (see Figs. 2.1 and 8.4). In agroforestry

because they occupy the land together , there is a

opportunity for trees and crops to compete with o

facilitate one another (see Chapter 13). T he actua

period over which they do interact, however, wi

depend on when tbey ar e actually growing. Th

very varied behavioural patterns of growth an



The Nature of Agrofo restry

Table 2.1 Agroforestry practices.

Trees with crops (agrisilvicuJture)

Rotated in time (sequen tial practices):

• Shifti ng cultivation (sometimes with enrichment of the woody components)

• Improved tre e fallow• T aungy a (i.e., cropping during the establishme nt phase of commercial forest tree plantations)

Spatia lly mixed (simultaneous practices):

• Trees on cropped land• Multiple use of trees in crop plantations

• Mixed multistorey tree and crop arrangem e nts (e.g., tropical home gardens)

Spatially zoned (simultaneous practices):

• Hedgerow intercropping (alley cropping) , co ntour hedging o r barrier planting and other types of linear tr ee

plantings• Boundary plan ting for various products, or l v fencing• Strip planting in forests or timber plant ations (corridor farming)

• Windbreaks, tree-e nriched windstrips, shelterbelts and wild animal habitat plantings

Trees with grass and animals (silvopastoral) (Sim ultaneous practices)

Spatially mixed:

• Trees planted on rangelands, in permanen t pastures and grass and grass-legume leys for leaf and/or pod fodder

(some times for edi ble flowers additionally)

• Tree-crop plantations with past ure s

Spatially zoned:• Alley farming (sometimes with 'energy' gardens of shrubs and grass mixtures)

• Live fencing

• Boundary plantings, mainly for fodd er

Managed tree plots

• Fodder bank s using woody spe cies• Fuelwood lots• Mixed orchards (especially if for several pr oducts, e.g., fruits and honey)

Other combinations

• Agrosilvopastoral (crops, pasture, animals and trees)• Trees can a lso be grown , o r nutured, to provide products from insects such as bees (honey), lac insects (shellac

varnish) , silkworms (silk), and so on

• Fish farming also sometimes utilises leafy tree fodder

flowering found in tropical woody pl ants a re impor

tant in this respect (see Chapter 17). Merely considering that they are occupying the same land is

not the same as describing periods of functional

activity.

In simultaneous systems, practices can be organ

ised according to whether they are mixed or zonal.

Again, this has some functional implications.

Where trees or shrubs are scattered in cropland or

in pasture, the amount of tree-crop interface will

be greater ttwn if they are arranged in lines, strips

or zones (i.e., narrow plots) (see Cha pter 12). Fur

thermore in mixtures it can be important to desig

trees are in dense or sparse mixtures. This will, o

viously affect the ratio of products , as well as moerating interactions between components. Wheth

trees/sh rubs are scattered or in lines also, clear

has management implications.

A regular, zonal arrangement does not nec

sa rily mean that there will be no taU trees. F

example, Paulownia spp. a re usually planted in ro

between strips of wheat in China , bu t Cord

alliodora may be grown scattered coffee in Co

Rica . I n a number of zonal systems, e.g., hedgero

intercropping, the woody component may be ke

quite small; barrier planting o f hedges on slopes f



12 Tropical Agrofor estry

Fig . 2.1 Pepper (Piper nigrum) is a profitable crop forsmall farmers . This farmer was encouraged and helpedby the Upper Mahaweli Watershed Project, Sri Lanka, tocreate a small, highly intensive home garden of 0.5 ha onwhich he supports himself, his wife and two children.There are over 30 plant species being grown, stall-fedcattle , a silkworm enterprise and a methane plant.

Chapter 6), and 'feed gardens' to produce animal

fodder is another (see Chapter 5). In a home garden,

where 'verticality' is being fully exploited, aU th eplants a re usually mixed and scattered, each occupy

ing its own niche (Fig. 2.1), unlike monocrops (Fig.

2.2).

Obviously, the arrangement o f plant components

in a mixture affects the intimacy with which the

components interact and, as competition can occur

only between closely associated plants requiring

the same scarce environmental resources (light,

water, nutrients), the extent and nature of the tree-

crop interfaces will influence how al1Y system

functions. It is n ot an easy task to Ul avel how trees

and crops ar e interacting with one another within

and between seasons. Nor must we forget the

conflicts that c an arise for inputs (labour, cash , and

materials ), which must balance the specific demand

for the time and amount of various outputs that any

system may potentially be able to Supply. There

need to be convincing arguments that growing trees

with crops will confer more benefits than growing

them alone or st quentially

THE SYSTEM

So far we have been discussing 'practices' but in

later chapters, we will often be considering how

'systems' function. To be absolutely distinct, any

complete system needs to be distinguishable fromits surrounding environment by either physical or

conceptual boundaries. Because different opera

tors frequently investigate dissimilar aspects of the

system, it is especially important to describe its

limits accurately, but this may not always be easy

omet mes the purpose or usage of a system i

enough to distinguish it from others, e.g., a single

farm on which a family subsists. However the

boundaries of farms and / or households

become blurred. Some members of the family may

work elsewhere for part of their time, and bring

resources into or remove them from the home

farm. The farm animals may sometimes be kept a

home, or they may be grazed elseWhere, perhaps on

common land , or lent out, and so On. Economist

think of these aspects as making a system 'porous'

Systems with a common purpose may b

grouped, for example, as subsistence systems o

cash-generating systems. From a biophysical stand

point such systems mayor may not have much i

common, but their economic and, perhaps, socia

characteristics will be readily distinguishable. How

ever we may wish to view a practice Or an individua

system it is important that it is described clearl

and, for any particular system, that the nature o

its boundaries, sub-systems and components be d e

fined unequivocally, In agroforestry we may ofte

find that economists, sociologists and biophysica

scientists need to start investigating at differen

scale levels (e.g., see Chapter 25 on Sustainability

they may also each wish to define a system usindiffereilt boundaries.



The Nature of Agroforestry

Fig. 2.2 Plantation of Cupressulusitanica , Kenya. This sectional vieshows a rough, well-coupled canopClose spacing ensures that light inteception is maximal, but there is no plaground cover. High planting densiensures that these timber trees grostraight and that lower branches seprune.

The terms 'practice' and 'system' are often used

indiscriminately, but the re is an important differ

ence between them. 'Practice' is better applied as a

form of classification, whereas 'system' has func-

tional connotations and so is more properly used

to describe specific examples of practices. Thus,

any reference to a landuse practice is to a general

an d recognisable way of using land (e.g., plantati on

forestry, shifting cultivation, hedgerow intercrop

ping for fodder production and soil fertility im

provement, and so on). To investigate any specific

practice it will usually be necessary to examine

individual systems (e.g., specific farm examples) so

as to examine the way plant components function

or interact, or in order to understand the specific

economic or social attributes that apply to that

particular case. (A 'system' is an arrangement of

components (o r sub-systems) that process inputs

into outputs. Each system consists of boundaries,

components, interactions between components,

inputs an d outputs.)

Practices may be grouped under major headings

(e.g., multistorey tree gardens) with several subor

dinate groups (home gardens, village forest gar

dens , spice gardens) which are similar in general

form an d arrangement but different in some social

context, or in the outputs required, etc. (see Table

2.1). By giving a name to a practice it is important

no t to set restrictions on how the farmer may s ee it

being applied. There are many adaptations that can

be made to fit local circumstances. Hedgerow

intercropping defines a spatial arrangement of

plant components and a method of management

(i e pruned hedges) but this approach can be uti-

lised for very different purposes: fodder productio

an d / or soil fertility improvement (when it might b

called alley cropping) or for soil and water conse

vation on slopes (when it would be known as ba

r planting). Each of these will have contoure

hedgerows, but the exact form of layout and man

agement could be quite distinctive, and the system

may look, and function, rather differently. Detaile

discussions and accounts of agroforestry practice

can be found in Nair (1989, 1993) and many of th

books referred to in 'Recommended Reading'

the en d of this Section.

Th e concept of hedgerow intercropping can b

amalgamated with that o f a rotational practice so a

to combine a simultaneous and a sequential prac

tice on the same piece of land if this is divided int

two. Th e whole plot is planted with trees at hedge

row spacings, but, at anyone time, only half o

the land is maintained under a hedgerow system

the rest is left as a fallow. When the farmer fee

that it is appropriate, the hedges are left to grow

out, cropping stops and a bush fallow phase take

over. Crops are then sown in the area that had bee

under bush fallow, an d this is accomplished b

cutting back the lines of trees and forming the

into hedgerows. Obviously this is suitable on

where a farmer can afford to have a part o f the lan

under fallow and where suitable tree species ar

used (Fig. 2.3; Huxley , 1986).

Systems may contain distinguishable sub-system

or, for convenience, they can be thought of as ope

ating in this way, according to how we want to vie

them. Fo r example, if the farm depends on agro

forestry enterprises these can be considered as con



14

Fig. 2.3 Rotational hedgerow inlercropping demonstration (Machakos , Kenya).

An appropriate woody species is plantedin rows with close in-row spacing , and

the plot is divided into two. Hedgerowintercropping is carried out on one half,and the other is left as a 'fallow' tree plot.When the farmer decides, the trees arecoppiced to allow hedges to form andcrops are sown. The area previously

used for hedgerow intercropping thenbecomes a tree fallow.

Tropical Agroforestry

tributing to farm sub-systems providing basic

needs: such as the 'food sub-system', the n lgy

sub-system', an d those primarily generating cash

income, shelter , raw materials , and so on. This is

particularly useful in analysing a nd describing farm

constraints, as is done in rapid rural appraisal

schemes (e.g., ICRAF's 'Diagnosis an d Design ';

Raintree , 1987a,b). Individual plant components

might be contributing to more than one of such sub

systems, of course.An agroforestry system is, therefore, identifiable

by a coherent and unique set of circumstances. It

could be a single specific local example of a prac

tice , characterised by environment, the plant spe

cies and arrangement, management applications

and the particular set of social and economic cir

cumstances that apply. Or, we can use the term for

a cluster o f almost identical such units, considering

each as a sub-system if this'was useful. F or example,

if we wanted to contrast some functional aspects of,

say, Chagga (Tanzania) home gardens (Fernandeset al., 1984) with Kandy (Sri Lanka) home gardens

(Ranasinghe and Newman, 1993), we could think of

the typical system for each of these and compare

them.

There ar e many kinds of interactions to be con

sidered in agroforestry, some instigated a t manage

ment level. Fo r example, there may be transfers of

plant material by the farmer as when tree prunings

ar e fed to livestock kept elsewhere. Or prunings

from trees or hedges can be moved so as to provide

leafy mulch for crops. Economic transfers or inte

actions may include the use of cash income fro

the crops to purchase and establish trees, the sa

of timber or fuel wood in order to purchase fe

tilisers for the crops or pastures, o r the bene fit

having more labour available for weeding crops

peak periods because a home supply of fuelwoo

alleviates the need to spend time collecting woo

from distant sites. Ecological and economic facto

may interact, e.g., less labour needed for weedinbecause of fewer weeds under trees, and so on .

Usually it is only possible to generalise abo

practices, e .g., ' most home gardens have been foun

to be sustainable' (Torquebiau , 1992); specific stat

ments can only be made of agroforestry s ystems.

FINDING OUT ABOUT

AGROFORESTRY

Th e speed at which interest and involvement

agroforestry developed during the 1980s was extrordinary. It was clear that everyone was lookin

an d hoping for answers to the emerging landu

problems. Many new agroforestry enthusias

seemed prepared to take chances that solution

would emerge and a range of hypotheses about th

benefits of agroforestry were p ut forward ; for som

evidence was available, but not for all (see Preface

However, haste and hope were really unnecessary

Agroforestry has a sound and valid base fro

which to evaluate any situation: on the one han



The Nature of Agroforestry 15

from those field practitioners already familiar with

growing trees with crops or pastures; on the other,

from existing scientific evidence. Th e first could be

addressed through field appraisal techniques such

as ICRAF's 'Diagnosis and Design' methodology;

the second demanded its own evaluation in order to

thoroughly explore and consider available scientificknowledge. In the early days, collecting knowledge

from the field was more difficult than one might

think, because of a lack of infrastructure to support

agroforestry. In most countries the subject fell

under different government ministries or parastatal

organisations, an d there was a need to create a

minimum level of awareness and collaboration

before multidisciplinary field teams could work

together. Inputs from scientific disciplines were

slow to have an impact, partly due to the initial

scepticism about the latest fad from those working

on more basic aspects (they did not believe in

'magic' trees!), a nd partly because funding for work

other than very applied field testing was slow to

come. Without this two-pronged approach, how

ever, there is no sound basis from which to promul

gate existing agroforestry practices, or design

improved ones.

It is interesting to take a moment to consider

exactly what supportive scientific work is available.

Ecologists, for example, have long been studying

natural associations of woody and non-woody plants

and, with physiologists, have developed an understanding of how environmental resources are cap

tured and shared in mixed plant communities. Soil

scientists know about the processes whereby plant

residues can improve soil fertility; and they can

quantify these. Foresters have obtained consider

able information about the l ong-term effects of trees

on soils, and we can compare this with what agricul

turists have found in croplands. nformationabout

secondary forest products has long been available,

and tropical agriculturists are conversant with many

tree-based systems such as Acacia senegal gum gardens i n the Sudan, mixed cropping under coconuts,

an d many others. Weed scientists know much about

competition between crop and weed species, and o f

woody weeds in rangelands. An d there is also a

considerable body of information about the effects

of, mainly, legume groundcovers in a whole range of

plantation crops, as well as much useful information

on the effects of mulching. Woody agricultural spe

cies such as tea, coffee, cocoa, oil palm and rubber

have been very well studied, and so offer relevant

examples of th e effects of tree and soil management

on productivity. Th e reports of scientific investiga

tions on these provide extremely rich sources of

information that ca n underpin any further work with

multipurpose tree species. The behaviour of tea

under different forms of training an d plucking, forexample, is particularly relevant to the management

of trees being grown for leafy fodder. The work on

pruning of coffee can, similarly, provide us with an

invaluable example which, together with the ex

tensive literature on th e p ru ning o f temperate

fruits, establishes a useful set of guidelines for the

management of any woody plant for fruits / seeds

(see Chapter 19). Finally, range ecologists have long

studied the interactions between animals, grass an d

shrubs.

Is there enough information?

11£ such a wealth of information is available why do

we need more? First of all , although the plant

plant an d plant-environment processes involved

in growing woody and non-woody plants together

are well established, the actual extent to which

they occur in any particular agroforestry situa tion

still needs to be established. Then again, agro

forestry mixtures involve many interfaces between

th e plant components, t he outcome of which will b e

highly site-dependent, an d so require testing in th efield. Finally, although many of th e sources

of information I have mentioned previously pro

vide a sound scientific foundation on which to

base progress in agroforestry, the actual questions

addressed in ecology, forestry, horticulture, etc.

have not gone far enough in many instances. Fo r

example, we find that although ecologists have

studied competition in considerable depth, they do

not have aJJ t he answers we require. This is because

they are usually concerned with which species

ultimately 'win ' , and which 'lose'. In agroforestrywe do not want winners or losers - we need all our

plant components to survive and flourish (Fig. 2.4).

Thus, although ecological research is often highly

relevant, and will point us in the right direction, the

results to hand may not be immediately what

we want, or go far enough to answer specific

agroforestry questions.

Intercropping research in agriculture is another

source of information an d relevant ideas that,



16 TropicaL Agrojoreslry

Fig. 2.4 Part of an early experiment to compare maize ·grown with Sesbania sesban simultaneously and at different spacings (Makoka, Malawi). Clearly, there isnot enough water in this system to supply both plantcomponents.

similarly, still needs development for ou r purposes.

Th e concepts of environmental resource capture

and resource utilisation (see Chapter 9) have arisenfrom studies on crop mixtures, as an extension

and collaboration of original work on monocul

tures. All this material is certainly immediately

relevant to agroforestry. However, because in

agroforestry we are dealing with a mixture of

plants of diverse stature and of varying lengths of

life cycle, intercropping concepts derived from

agriculture again h av e to be modified a nd

extended.

Despite these rich sources of information and

understanding about woody plants, multiple crop

ping and soil and environmental issues, there are

some essential topics that remain largely unex

plored. Fo r example, ou r knowledge of rooting

patterns and root behaviour in woody/non-woody

plant mixtures is inadequate (see Chapter 18). Here

the task is to define more clearly what questions

need to be answered and then to mount sufficient

effort to obtain the necessary information. On e of

the myths about trees is that they are always deeper

rooting than associated plant types, but this is no t

necessarily tru e (Fig. 2.5). Again, the importance of

Fig. 2.5 End view of a three-line windbreak of n(Azadirachta indica) in the Majjia Valley, Niger. Esure of trees by this riverbed (now dry) shows thattrees are mainly shallow rooting.

mycorrhizal associations for enhancing nutr

capture, especially on poor sites, is undisputed,

very little is still known about this for mana

multiple cropping systems containing 'resid

woody plants. Also little is known about optimi

pest management in agroforestry systems; certaperennial woody vegetation can provide a home

a variety of pest and disease organisms, and th

ar e many gaps in ou r knowledge of the pest spe

associated with multipurpose trees and the ecol

of pests an d their predators in agroforestry circ

stances. These are just a few examples of the gap

information, as we shall see.

Then, also, there are the numerous areas wh

a reappraisal of existing knowledge is requi

Thinking more clearly about the advantages

disadvantages of fast-growing trees is one of th

(Fig. 2.5; see Chapter 13). Only recently h

concerted attempts been made to explain, m

fully, how to apply to agroforestry certain conce

that are well-established and well-tried in agri

ture, e.g., soil organic matter dynamics (Sanc

et a 1985), or environmental resource capt

considerations (Ong and Huxley, 1996). Even

general thesis that trees are always 'good' ha

be disclaimed; on sloping land, and in combinat

with overgrazing, scattered trees can help prom

severe soil erosion through gulleying (Fig. 2.6).



The Nature of Agr oforestry

Fig. 2.6 For example, trees can caus

erosion. On this slope (upper part to lef

previous overgrazing has removed mosof the grass cover, and then water flowing downslope has been channelled between the trees. causing gullies.

Th e complexities found in intercropping woody

and non-woody plants make it imperative to know

not only the convenient disposition of plant

types from a management point of view, but also

how the various plant components will function and

interact (and the animals, too, if included). In

the long term the plant system will affect the site

and, especially, the characteristics of the soil.

Relating form and function is, therefore, an

important issue when selecting plants for agroforestry, and one which even now has no t been

adeg uately addressed in agroforestry research

(Huxley, 1996).

Computer modelling may seem an esoteric activ

ity when there is, clearly, so much investigation to

be done in the field and so much site-moderated

variability to understand . However, i t is specifically

because the number of agroforestry practices is

large, and the differences in the characteristics of

individual systems are limitless, that it is vital that

th e science of agroforestry is directed at establishing a basis of understanding about function that

ca n support field activities under whatever circum

stances they might occur, and help to predict

their outcome. Th e alternative is an infinity of at

tempts based on trial-and-error. It is in such cir

cumstances that computer modelling can ultimately

become a powerful practical tool for decision

making once a sound basis of scientific fact has

been established and verified and validated in the

field (see Chapter 27).

TREES FOR ALL REASONSI

To be able to use woody plants wisely in agro

forestry it is important to appreciate exactly wha

the woody habit entails (see Chapter 7) , and t

have a clear idea about how specific woody specie

can best be used. Th e woody components i

agroforestry have come to be known as ' multipur

pose trees ' (MPTs). MPTs are woody perennial

(trees, shrubs, woody vines, palms, bamboos) thaar e purposefully grown to provide more than on

significant contribution to the production and/o

service functions of the landuse systems they oc

cupy (see Chapter 15). To some extent this term

a misnomer because any tree species c an be grow

in this way. It is true that some MPT species can

no t help but provide a number of products (e.g

palms), and a few will readily provide a s upply o

different products over t he sa me season if reg uire

to do so (e.g., Leucaena leucocephala , grown fo

leafy fodder and some fuelwood). However, to expect any plant to maximise parts that are competin

internally for resources with others that are to b

harvested also is unfeasible, so that we need t

evaluate, in the farm context, whether to grow tre e

for multiple products or separately (Huxley, 199

in press) . What is important is that many, if no

most, of the so-called MPTs have the potential t

provide several products a nd can be managed to d

this as an d when the need arises (e.g., the neem

trees in Fig. 2.5). In the same way, they can b



18 Tropical Agroforestry

ananged and managed to fulfil more than one kind

of service. Because the cost to the farmer of replac

ing one tree species with another will be high,

should requirements change with time, there is

considerable value in growing a species that is a

flexible provider, an d most of the MPTs used in

agroforestry today have this characteristic. Theterm ' multipurpose' really applies to the way in

which we wish to use trees, etc.; such species are n ot

necessarily a particular kind of woody plant!

CONCLUSIONS

We can see that agroforestry is really not a new

discovery at all, even for scientists, although they

have overlooked it until recently. Much of what

we need to know in order to supply a scientific basis

for advice on how to practise agroforestry success

fully has already been studied to a degree in many

other disciplines. Certainly, existing knowledge

needs extending and elaborating, but, hopefully,

progress can rapidly be achieved. We must temper

enthusiasm about what agroforestry might achieve

with a considered view of its limitations derived

from a sound knowledge of how agroforestry

systems function and what may constrain farmers

who want to use it. Agroforestry is obviously a part

of the whole spectrum of using land sensibl

with potentials for increasing both productivi

and sustainability, but these potentials must not b

taken for granted; many aspects remain to be pr

perly explored, as we shall see later in this book.

Agroforestry is scientifically stimulating fo

those involved because there is so often a need resolve technical conflicts. For example, we mu

conserve land whilst yet attempting to make it mo

productive. Indeed, it is this trade-off between pro

ductivity and sustainability that lies at the heart

the matter (see Chapters 24 and 25) , and for whic

some of the claims made for agroforestry have y

to be tested. Better forms of land use (i.e., mo

productive and more sustainable) will be acheive

only by improving o ur understanding o·f how to u

all the scientific and indigenous knowledge ava

able. Furthermore , improvements in land manag

ment are likely to come about only through

multidisciplinary approach. Working in this inte

disciplinary way presents both a challenge and a

opportunity. It encourages us to appreciate an

accommodate other points of view, and to co

tribute to a holistic solution that is more like

to support an d sustain the wellbeing of the peop

whose existence and livelihood depend, cruciall

on good land management.

Huxley Agroforestry Ch 2

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