- introduction to ecosystems
What is an ecosystem?
All the non-living and the living components that underlie and form a particular biological
Examples include tropical forest (D 24), savanna (D 25) and mangrove (D 26) ecosystems.
Which are the non-living parts?
- all the various features of the climate, acting continuously or occasionally (D 11);
- the type of terrain and its altitude above sea-level (D 12); and
- the physical and chemical features of the rocks, soil, water and air.
How about the living components?
These consist of all the:
- green plants that grow in or near the area (D 14);
- micro-organisms that exist in its soil, water and air (D 13);
- animals which live in or visit the site (D 15), including humans who often affect it
directly and indirectly (D 16).
Isn't this just a list?
In a way, yes, since one of the important thoughts about ecosystems is to avoid ignoring other
parts when concentrating on one.
But in another way, no, because the basic idea is how each component interacts with the
other parts of a self-controlling system.
Aren't there a lot of such interactions?
Yes, there must be thousands of them, which makes the study of ecosystems quite complicated.
However, some relationships between components occur regularly, and are easier to follow.
- the average annual rainfall and its distribution through the year clearly play a large
role in determining the type of ecosystem found (D 11);
- green plants are the ‘producers’ in nearly all ecosystems, using the energy of sunlight
to fix carbon dioxide as sugars and other kinds of organic matter (D 13–14);
- humans, other animals and most micro-organisms are ‘consumers’, depending directly
or indirectly on green plants for their food (D 15–16).
Does that mean we depend on plants?
Yes it does. In each ecosystem they are at the base of food chains, providing the food supply
of all the other organisms, including ourselves. Food chains work like this:
- There is a maximum amount of sunlight that the producers can capture in a particular
site, however tightly packed their leaves;
- Particular aspects of climate, soil or human activity may mean that considerably less
than this is actually produced;
- Some of the organic matter formed is:
- used up by the green plants themselves as they grow;
- built into stems, leaves, roots, flowers and seeds; and
- The organic matter in 3 b and c supports all the consumers, including the:
- ‘herbivores’ which eat plant food;
- ‘carnivores’ that eat animal food;
- ‘decomposers’ which are organisms that break down dead organic matter into
simpler substances (D 13); and
- ‘omnivores’ that eat a wide range of different foods.
Are consumers always harmful to green plants?
Very often they eat parts of the producers, and a few may cause diseases. But there is also
collaboration (D 15). For example:
- Micro-organisms such as some species of fungi and bacteria often form close associations
with tree roots (D 32);
- Bees, butterflies, moths and some birds pollinate the flowers of many trees, allowing
fertile seeds to develop (Manual 2);
- Some ants may protect trees from attack by pests of other insects.
In addition, carnivores (and sometimes parasites) help to keep the ecosystem in balance.
How do ecosystems control themselves?
- If the green plants in an area produce more organic matter, a larger number of consumer
can be supported; but
- If consumers multiply rapidly, their numbers will tend to be decreased again by:
- running short of food;
- more carnivores surviving and eating them; and/or
- contaminating their environment with waste products.
What has all this got to do with planting trees?
Growing trees successfully is helped by appreciating how biological systems work. For instance:
- in natural ecosystems, a balance is generally maintained between the numbers of
producers and consumers;
- in ecosystems managed to grow plants for food or materials, the aim is to have
high yields without these being heavily consumed by other organisms, or overexploited
- in ecosystems managed to produce meat or fish, the aim is to maintain the number
of consumers within the carrying capacity of the green plants supporting them.
Aren't most people more interested in man-made vegetation?
Perhaps, but that does not mean that the natural systems in an area can be ignored, for they
may give valuable information on:
- how the trees that ‘create’ the ecosystem perpetuate themselves;
- what the carrying capacity of the site is likely to be;
- how far the original vegetation can be modified without the system breaking down;
- what kinds of use can be sustainable, maintaining yields in the future.
But surely economic matters must be paramount!
Important, yes, but only if the full economic values and benefits are considered, in the light
of ecological principles.
Of course, the person growing trees will often need to think about particular products, and
weigh up costs and prices.
But often a narrow, short-term view of economics has prevailed, with the longer-term consequences
of actions ignored.
Unless ecologically sustainable methods are used, the natural resource that could have
gone on producing in perpetuity will become degraded or lost entirely. And that is clearly bad
But doesn't the natural vegetation need clearing away?
Only for a few purposes. It is generally best to retain some tree cover:
- if a stand of trees is to be grown;
- when growing many kinds of farm crops; and
- for raising domesticated animals.
Why is this?
Because the existing trees can allow many of the features and functions of the natural ecosystem
to be continued (D 2), including:
- producing many items that people need (D 33–40);
- protecting the soil from overheating, rapid erosion and loss of fertility (D 1, D 13–14,
- helping to maintain open woodland conditions for the establishment and growth of plants
(D 21, D 50–51);
- giving shelter and food for domesticated animals (D 3, D 15, D 34).
But I shall want to plant my own trees!
Yes, indeed - that is what this Manual is concerned with. But often:
- trees will establish and grow better under some cover, rather than in completely
cleared land (D 50);
- mixtures of different species (D 30, D 53) may be more likely to continue to thrive
than pure stands, and keep more options open;
- a storeyed system can be appropriate, with foliage at more than one height (D 3,
How much of the original vegetation should I leave?
As well as your own aims as a grower, this will depend on:
- the type of ecosystem;
- its previous history;
- the kind of trees to be planted (D 30–42); and
- the growing system that is used (D 50–55).
As a very rough guide, one might aim to leave 40–60% of the original cover (D 3, D 51). More
shade might be appropriate when planting shade-bearing species, and less over light-demanding
trees and farm crops.
What is a growing system?
The way in which planted and natural trees are grown, particularly the planting pattern that
is chosen (D 54). For instance trees might be planted in lines or small groups, scattered at
wide spacing over the whole area or put in larger blocks (D 55).
Unless there is a lot of local experience, trials comparing different amounts and ways of opening
up and planting are often worthwhile (D 6, D 29, D 55, D 67).
Can studying ecosystems provide some practical guide-lines?
Yes it can, and these are scattered through the Manual. Some general guide-lines are that:
- natural ecosystems:
- capture energy efficiently and produce a lot of organic matter;
- recycle most mineral nutrients, rather than losing them (D 13);
- provide living space for many organisms, some of them essential for others to
- preserve a stable balance in the numbers of any one organism;
- regenerate themselves naturally when gaps form (D 2, D 54);
- repair themselves after natural disturbances (D 11).
- managed ecosystems:
- are subject to the same principles, with any of the components affecting how well
the trees grow;
- are liable to breakdown if the conditions are altered beyond a critical point; and
- are capable of sustained production of items useful to humans, without these
natural resources being destroyed in the process.
- average climate and extreme conditions
Does climate determine where trees can grow?
It is one of the main features of the environment influencing:
- where trees grow and regenerate themselves naturally;
- where planted trees are likely to survive and grow to maturity.
But other factors play a part too, such as the soil type (D 12), availability of nutrients (D 13),
competition and mutual assistance (D 14), browsing by animals (D 15) and the effects of
humans (D 16).
How many different effects of climate are there?
The main ones are:
- Rainfall and the humidity of the air;
- Natural disturbances, small and large.
Which aspects of climate influence trees the most?
All of them can affect the survival and growth of trees, through both:
- the average and the usual range of the environmental factor; and
- the extreme conditions occasionally experienced.
What amount of rainfall suits trees?
These Manuals are concerned with two major types of vegetation. As a general rule, tropical
trees grow in:
- Closed forests if:
- there is an average of well over 100 mm of rain a month, and
- months with an average of less than 50 mm usually occur only once or twice a
- there is less rainfall than this, and/or
- a more seasonal climate.
Savannas are typically dominated by grassy vegetation (D 25), but trees also grow scattered
through it, along stream lines or in patches of relatively closed woodland.
Can the site make a difference, even with the same rainfall?
Yes it can. Forests can grow with lower or less reliable rainfall:
- when the terrain (D 12) means that there is more groundwater, for instance near streams,
river and lakes, in valley bottoms and swampy depressions, on seasonally-flooded plains
or in some artesian basins;
- where they receive extra moisture from mist and/or dew;
- if it is regularly cool and cloudy;
- when the soil type (D 12) retains rainwater longer; or
- if they can form deep root systems.
On the other hand, closed forest requires higher or more regular rainfall:
- when it is typically sunny and hot from about 10 am to 4 pm;
- if periods with much less humid air occur during the drier season;
- if it is often windy; or
- when terrain or soil type allow rainwater to be lost quickly from the site.
But not all kinds of trees are the same!
No, they are not. For example, some species of Acacia, Adansonia, Prosopis and Terminalia can
withstand drier conditions, while particularly humid conditions are needed for the establishment
of many dipterocarp seedlings.
What about exceptional droughts?
Unusually severe drought, especially if bright sun and drying winds occur together with lack
of rain, can kill newly planted trees and natural seedlings unless they can be watered. Older
trees can sometimes be affected too, though not usually in drought-tolerant species. Drought
may also greatly increase the risk of fire (D 66).
Does the temperature matter much?
Tropical trees generally grow where no month has an average below 18°C. However, some
species can survive where night temperatures sometimes drop as low as 12–15°C.
Can cool temperatures damage tropical trees?
Most tropical tree species would be quickly killed by frost, except those growing high up in
mountains. Many are also seriously affected by cool temperatures of about 10°C or below,
which can damage root tissue especially, so that water uptake is interrupted.
What about high temperatures?
As the temperature rises between 20°C and 35°C, trees may grow faster, but the drying power
of the air becomes increasingly greater, tending to put them under more and more water stress
However, some kinds of trees can survive in sites where the mid-day air temperature regularly
goes over 40°C.
Are seasonal differences in temperature important?
Not as much as in arid areas and in the temperate and boreal zones. However, tropical trees
often respond to quite small changes in temperature (Manual 3).
In addition, the diurnal fluctuation of temperature (the difference between the day-time
maximum and the night-time minimum) can affect germination (Manual 2) and shoot growth.
It is usually greater:
- at certain times of year;
- amongst exposed crowns, compared to within a stand;
- when trees have been cut down; and
- in mountains.
Is it just the air temperature that matters?
No, because the temperature of the:
- ground surface and topsoil in cleared areas can be lower than the air temperature
during still nights, and much higher during sunny days. High temperatures can damage
young trees, root systems of older trees and important decomposers like earthworms
and micro-organisms (D 13);
- inner bark can rise sharply when the trunks and aerial roots of trees are suddenly
exposed to direct sunlight. This can cause damage or death, but the risks are less when
the canopy is not opened so much (D 51);
- leaves can be a few degrees higher or lower than the air temperature.
How does light affect trees?
Light is also a key part of the climate, and affects trees in many different ways, including:
- the total amount of light energy that reaches the leaves sets the upper limit on
how much photosynthesis (manufacture of sugars) a tree can do, and how much organic
matter a stand can produce (D 10, D 14);
- the quality of light, for example full sunlight, sun-flecks or shade light, can alter the
type of growth made (A 24 in Manual 1; Manual 3);
- the day-length (photoperiod) can have large effects on all aspects of shoot growth,
and can interact with the effects of night temperatures (Manual 3).
What does the amount of light mean in practice?
- For the individual tree: if most or all of its leaves are in heavy shade, a tree may be
producing less sugars than it is using. If this continues for several weeks or months, it
will stop growing, run out of stored reserves and then die. Shaded saplings within
natural forests may often receive as little as 1% of full sunlight. This might allow them
just to survive, making little new growth unless a tree or large branch falls, making a
gap in the canopy.
- For a stand of trees: the total amount of light energy reaching the canopy during a
year is one of the factors that determine how great the average growth of the stand
could be, and so influences yields and rotations (D 24, D 26). It is affected by latitude,
altitude, topography and climate, and by changes in the weather such as cloudiness or
Can the light be too bright?
In some cases, especially with extensive opening of the canopy (D 51), and for:
- shade-bearing species (D 14);
- newly planted trees (Manual 5);
- leafy cuttings that have rooted but not yet been ‘weaned’ or hardened (A 54 in Manual
Some tree species are better able to tolerate the heating effect of bright sunlight, and control
the water loss it tends to cause. Similarly, some mountain species can tolerate the higher levels
of ultra-violet B radiation found there.
How does light quality matter?
As sunlight passes through the canopy, some parts (colours) are absorbed by the leaves, with
the result that shade light is of a different quality than sunlight. This can affect establishment
(Manual 5) by altering leaf size, branching and the growth rate of shoots (Manual 3). It can
also change internode length and the rooting ability of cuttings (A 24 and A 42 in Manual 1).
Surely the day-length doesn't vary much in the tropics!
Not as much as it does farther from the Equator, but there is a variation of more than 30
minutes at 5°, more than an hour at 10° and 2 hours at 17°. In a series of experiments, over
80% of 20 species of tropical trees and shrubs were shown to be sensitive to changes in day-length,
generally growing more when the days were longer (Manual 3).
What about climatic disturbances?
Storms are rare in some tropical regions but a regular feature of others. If your site is subject
to strong winds, you could think about:
- opening up the canopy less (D 51), perhaps in small groups or narrow strips (D 54) at
right angles to the usual wind direction;
- planting shelter-belts (D 22, D 41);
- choosing tree species that are known to be wind-firm (and perhaps salt-tolerant if the
site is close to the sea).
How about severe storms?
Extreme conditions that occur rarely, but cause a lot of damage, include:
- typhoons and hurricanes, that can uproot or break every tree;
- hailstorms, that can strip off the leaves.
Are there other kinds of disturbance?
Yes, stands of trees can be affected by forces that are sometimes linked indirectly to climate.
- Fires can occur naturally, started for example by lightning, and in drier areas are a
regular feature of the environment. They are also frequently started by humans
(D 16). Many savanna species such as Piliostigma thonningii and various shrubs, Pinus,
and troublesome weeds like Eupatorium (Chromolaena) odoratum and Imperata cylindrica
are fire-tolerant. Such species often have thick bark or the ability to resprout rapidly
from ground level.
It is important to think about whether to use fire or not, and how to control it, before
planting (D 63), and when protecting trees (D 66 and Manual 5).
- Prolonged flooding is a normal feature of mangrove woodland (D 26), freshwater
swamps, and seasonally flooded forest and savanna. The species which grow there
naturally are adapted to survive. But if flooding occurs on normally freely-drained
sites, most tree species can be damaged or killed because their root systems are not
adapted to survive in waterlogged soil with low oxygen levels.
- Other events, such as volcanic explosions, mudslides, stone avalanches, earthquakes,
tidal waves, etc, can also have destructive effects on trees.
Can anything be done about such damage?
Only in some cases, for example:
- avoiding clearing trees on hill tops (D 12) and steep slopes (D 23);
- using terracing with tree planting on slopes (D 65);
- cutting drainage channels to carry excess water away quickly (D 65).
Is the climate changing?
It changes naturally over long periods of time, but scientists are becoming increasingly worried
that the scale of human activities may now be altering the Earth's climate more rapidly. For
example, burning very large quantities of oil, gas and coal, together with widespread cutting
down of trees, increases the proportion of carbon dioxide in the air.
Does this matter?
A little more carbon dioxide might even increase the amount of organic matter produced, but
the steady rise measured in the last 50 years may be increasing the average temperature of the
Earth as a whole. Taken together with other changes, it is predicted that over the next 30–50
- average temperatures may increase by a few degrees in some areas, which could be
important where tree survival is already marginal;
- rainfall patterns anywhere may be altered, including perhaps the reliability of seasonal
- sea-levels may rise by a few metres, flooding a lot of low-lying human settlements,
farmland and vegetation; and
- the balance of ecosystems (D 10) may be disturbed in other ways.
Can anything be done?
Yes, although there will be a delay before it has an effect. Useful actions include:
- using energy sources that do not produce carbon dioxide, such as the wind, waves,
tides, solar energy and many sets of ‘run-of-the-river’ hydro-electric generators;
- making smaller, more efficient engines for vehicles;
- reducing wasteful use of fuels containing carbon;
- decreasing the cutting down of trees; and
- planting more trees.
How would more trees help?
- Perhaps globally, by ‘locking away’ a lot of carbon for many decades, but also altering
the reflectivity of the Earth. If an area is replanted with trees and then managed
sustainably, carbon will continually be removed from the atmosphere and stored in the
stems and roots of the stand, and in the organic matter of the soil.
- Probably regionally, by altering the movement of water into the atmosphere, and
influencing clouds and rainfall patterns.
- Definitely locally, by helping to create conditions in which young trees can easily
become established, and other important organisms thrive.
- variety of terrain and soils
How does the terrain influence trees?
The general geography of the site can affect trees in many ways, both directly and indirectly.
Important factors are:
- the topography, including the steepness of slopes;
- the altitude above sea-level; and
- the rock and soil types present.
What is the topography?
It describes the type of landscape that has been produced by mountain building, the action of
rivers, the weathering of rocks, etc. Examples are:
- flat plains;
- gently undulating terrain; and
- a sharply dissected plateau.
The conditions for tree growth vary considerably between types of landscape, and often within
How does that affect tree planting?
- By influencing soil and moisture conditions, so that individual planting sites can be
very different from each other (D 20), even before human activities are taken into
account (D 16, D 21–29);
- By making particular tree species appropriate or unsuitable (D 30–42), even before the
purpose for which they are to be grown is considered;
- By affecting the choice of a suitable growing system (D 50–55); and
- By altering the conditions for preparing the ground (D 60–67).
Are there some general rules for different planting sites?
- Tops of hills and ridges are particularly liable:
- to lose soil and nutrients through erosion by water and wind;
- to be subject to drought because of extra exposure to sun and wind, coupled with
very free drainage of water.
Main slopes are the commonest planting site, and:
- show a tendency for soil particles to move down the slope;
- are generally well-drained.
Valley bottoms, estuaries and other hollows usually:
- receive inputs of water, soil particles and nutrients from higher up; but
- are poorly drained, with water accumulating, temporarily or all the year round. In
mangrove woodland sites (D 26), they are also affected by the presence of salt.
Why does the steepness of the slope matter?
The steeper the slope, the greater the tendency for rapid erosion of soil, as the rainwater runs
quickly off into lower ground. NOTE: The amount of erosion is much greater still when
steep slopes are cleared of trees (D 23).
A minor effect is that steep east-facing slopes receive morning sun, while west-facing slopes are
sunny in the afternoon.
How does the altitude make a difference?
Average temperatures fall by about 1°C for every 165 m above sea-level, so trees growing at
1000 m will experience average temperatures around 6°C lower than at sea-level.
Trees in mountainous areas may also receive more ultraviolet-B radiation from the sun, and
experience greater diurnal variation of temperature (D 11), and often increased cloudiness or
What does that mean in practice?
Different species, and different varieties of the same species, tend to be well adapted to the
conditions where they originate. This suggests concentrating on local sources of seedlings and
rooted cuttings, from a similar altitude. It may often be best to use only limited numbers of
introduced trees (D 31) until they have been thoroughly tested.
How about different kinds of soil?
Forest and savanna soils show overall differences, and even within areas with a similar climate
there are many kinds of soil.
The names they have been given are quite confusing (D 70), but the important differences
between them depend particularly on the:
- type of ‘parent’ rock from which they were originally derived;
- period of time that they have been forming, and their depth;
- position they occur within the topography, including the water regimes that affect
- amount and type of vegetation covering them.
What else makes individual tropical soils vary?
Many factors, including:
- how much of each size of soil particle they contain;
- whether they are compacted or not;
- how freely water drains through them;
- whether they are well or poorly aerated;
- the degree of acidity (or sometimes alkalinity) they show;
- the amounts of mineral nutrients and trace elements available in them;
- how much organic matter they contain;
- what micro-organisms, earthworms and other decomposers are present (D 13).
NOTE: Tropical soils can vary considerably over distances as short as 10–20 metres.
How big are soil particles?
- Stones: more than 20 mm in diameter;
- Gravel: 2 – 20 mm;
- Sand: 0.06 – 2.0 mm;
- Silt: 0.002 – 0.06 mm; and
- Clay: less than 0.002 mm.
Soils usually consist of mixtures of particles of different sizes.
What mixtures are common?
The larger components tend to be absent in:
- old soils, subjected to weathering of the parent material for many thousands of years,
reducing everything to smaller pieces;
- sandy soils, sometimes derived from sandstone rocks, but often formed through rivers,
the sea or winds sorting out and depositing particles of that particular size in one
- silty or clayey soils, formed by settling of mud in lakes or river estuaries.
Volcanic soils may contain particles of any size, and are usually more fertile.
How do tropical soils get compacted?
For many different reasons, for example if:
- they consist entirely of smaller particles that are packed close together;
- nodules or hardened layers such as laterite have formed because of chemical changes
in the soil;
- bulldozers or other heavy vehicles have run to and fro over them (D 60, D 62);
- cattle have trampled the soil repeatedly, especially in wet weather;
- a large tree previously grew on that spot; or
- the vegetation has been cleared away (D 22).
On the other hand, they tend to be less compacted when:
- larger particles create more air spaces within the soil; or
- there are many earthworms, ants and/or termites in the soil.
What about soil drainage?
Most tree species cannot stand very dry or very wet conditions, because:
- Periodically dry soils can impose severe drought stress, particularly on young, newly
planted trees, and on older trees with large exposed crowns. The important fine roots
and fungal threads can be killed.
- Soils that are periodically or permanently flooded can also cause drought stress, because
water filling the air cavities in the soil can deprive the roots of oxygen. This may
also kill useful soil animals.
Aren't there trees that can stand such conditions?
Yes there are. Provided there is enough rain (D 11), there are trees that can grow on any soil,
- on rocky cliffs, many species of Ficus and Hildegardia barteri can establish in cracks
and grow surprisingly large;
- in permanently swampy soils, a number of trees can thrive, including palms such as
Copernicia, Mauritia, Nipa and Raphia. Some of these have specially adapted ‘breathing’
roots that extend above the wet ground and allow oxygen to reach the submerged
- in temporarily flooded forest and savanna, there are many tree species that can
tolerate the conditions.
Only a few species can tolerate a wide range of different types of soil drainage.
Is soil drainage or rainfall more important?
These two factors interact in helping to determine whether trees survive. For example, trees
can cope with less rain when they are:
- growing in a gully or the bottom of a valley;
- along the edge of a stream or river;
- near a spring.
And they need more rainfall when they grow:
- on ridges;
- on other very freely drained soils;
- on windy sites;
- in areas sometimes subject to periods with drier air.
How much does soil aeration matter?
It is very important in determining which trees can grow.
Reduced soil aeration often occurs through poor soil drainage, because of:
- low-lying or flat topography;
- small soil particles;
- soil compaction.
It may also tend to occur because much of the available oxygen has been used up by roots, soil
micro-organisms and larger animals living in the soil.
What governs the acidity of the soil?
- The kind of parent rock from which the soil originally came;
- Soil drainage and aeration;
- The kinds of leaves falling on it (D 13);
- Additions from the atmosphere, including any pollution; and
- Amounts of certain fertilisers applied.
Acidity (sometimes called soil reaction) is measured on a scale known as pH, in which a pH of
5 is ten times more acid than a pH of 6.
Does this make a difference to tree growth?
Yes it can do. For instance:
- Somewhat acid topsoils (pH between about 5.0 and 6.0) are common in the tropics,
and this suits most trees and soil organisms. Phosphorus in the soil is often insoluble,
- Alkaline or basic soils (pH above 7) are usually fertile, but are less frequent in the
tropics. Those with a pH over 8 are often difficult for tree growth, with only a few
- Very acid soils also restrict the number of species that can grow well, partly because
aluminium in solution can be present in toxic quantities.
How about the other ways in which soils vary?
Variation in soil fertility is the most important. Interactions between trees, mineral nutrients,
organic matter and soil organisms are described in sheet D 13.
What damages the soil?
- Extensive or complete clearance of vegetation (D 50–51, D 60);
- Kinds of farming that keep the soil exposed to bright sun, heavy rainfall and wind
- Compaction by heavy vehicles (D 62) or herds of animals (D 3);
- Repeated burning of forest vegetation (D 63);
- Planting of a single species where many grew before (D 30, D 53);
- Pollution (D 16).
How can I maintain a good soil?
- Keep and/or plant enough trees as cover for the soil (D 2, D 60);
- Do any felling by hand or chain-saw, rather than bulldozer (D 62);
- Extract logs with animals, or cut them up on site (D 62);
- Mix several species together (D 30, 53).
Suppose my soil has already been degraded?
- Consider terracing or draining (D 65) to control water flow and erosion (D 22–23);
- Plant soil improvers (D 32), shade or shelter trees (D 41);
- Protect land from burning (D 63, D 66) for at least 15 years;
- Use trees that have been inoculated with useful micro-organisms, if available (D 32 and
- Put mulch around exposed small plants (Manual 5), or dig in green manure or compost.
- litter and the cycling of nutrients
What is litter?
- dying leaves, twigs, dead branches, shed bark and fallen trunks;
- dead flowers, uneaten fruits and unsuccessful seeds; and
- waste droppings, cast off skins and dead bodies of animals,
that reach the surface of the soil.
Why is this important?
Because it is continually:
- supplying organic matter to the soil, maintaining its structure;
- providing food for important soil organisms;
- recycling many of the mineral nutrients that the plants have taken up from the soil;
- protecting the soil surface and topsoil, by acting as a natural mulch.
What happens to it?
It is constantly being broken down into simpler substances, releasing nutrients which are then
absorbed again by the fine roots and fungal strands in the topsoil.
How is it broken down?
By decomposers, organisms that specialise in breaking down litter. It is:
- eaten up by small animals, particularly earthworms, insects, mites and protozoa;
- decayed with digestive substances produced by bacteria and fungi.
But won't all these bugs attack my trees?
Most of them only break down dead organic matter. However, there are a few that act as pests
or cause disease, and here trees may need some protection (Manuals 3 and 5).
Doesn't all this breaking down of litter go on by itself?
Yes it does in undisturbed forest and savanna. But soil fertility is often lost if many of the trees
and shrubs have been cut down, because:
- less litter is produced, and it may be broken down more rapidly;
- it can also be easily blown or washed away;
- unprotected topsoil is a less suitable environment for fine roots (D 50) and for many of
the important soil organisms;
- the recapture of nutrients becomes less efficient, so that they may be lost from the site.
What nutrients are important for trees?
- the chief essential nutrients, required in substantial quantities, are:
- nitrogen (N), which forms part of substances such as proteins, including the
enzymes that build up and break down organic matter;
- phosphorus (P), occurring in nucleic acids and substances involved in photosynthesis;
- potassium (K), important in enzyme activity and for flower and fruit formation.
- other essential nutrients, needed in moderate amounts, are:
- calcium (Ca), used in joining plant cell walls together;
- magnesium (Mg), part of the green pigment chlorophyll;
- sulphur (S), part of many proteins, including enzymes.
- the micro-nutrients (trace elements), required in small quantities:
(7–12) iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu) and molybdenum
(13) cobalt (Co) may be needed for nitrogen-fixation in leguminous plants (D 32).
What about carbon?
This comes from carbon dioxide in the air, which is ‘fixed’ and made into sugars by photosynthesis
in green plants, using the energy of sunlight.
Some of the large quantities of this organic matter is used as trees grow, but all animals and
most micro-organisms also depend on green plants for their food and energy supplies (D 10–11). All living organisms release carbon dioxide to the air as they break down organic carbon
in respiration, and obtain energy and chemical building blocks for growth and reproduction.
Are any other elements needed?
Yes, oxygen and hydrogen from water are needed in very large quantities. They are essential
parts of nearly all organic substances, and also form water, which is vital for life.
Oxygen is released to the air in photosynthesis, and is used by nearly all living organisms in
Do nutrients only come from the litter?
No, they can be added to the topsoil from the:
- weathering of any stones there;
- subsoil or parent rock by the deeper roots;
- decay of larger roots;
- regular replacement of fine roots in the ‘root mat’ just below the surface;
- death of organisms living in the soil;
- ‘fixing’ of N and ‘release’ of P by some micro-organisms (D 32);
- dust and substances dissolved in the rain, especially after lightning, and when there is
How can nutrients be lost from the site?
The ecosystem can lose nutrients by:
- run-off in rainwater on the surface, especially after soil compaction (D 12) or burning
(D 11, D 16, D 63);
- leaching to deep layers, out of the reach of roots;
- washing or blowing away of the smaller soil particles (D 12), which often hold
- degradation of favourable soil structure in exposed soils (D 22, D 60);
- wholesale erosion of soil (D 23);
- harvesting of crops and wood, collecting fruits and forage, hunting and fishing; and
- the activity of de-nitrifying micro-organisms.
NOTE: Trees have the effect of reducing 1–5 above, and their root systems are also
very efficient at recapturing nutrients before they are lost.
But people must use the things that trees produce!
Yes, certainly. But it is important to think about the ‘balance-sheet’ of gains and losses. Even
if only one nutrient is in short supply, leaves may turn a yellowish or blotchy green, and tree
and crop yields are likely to decrease.
Can I do anything to reduce the loss of nutrients?
There are a lot of things that you can do. For instance:
- keep enough of the existing trees to protect the soil (D 2);
- plant species that quickly cover the ground (D 14);
- include some deeper rooting trees;
- avoid over-grazing (D 34);
- saw up trees on site (D 62), leaving branches and bark behind;
- return crop wastes, fruit skins and animal/human waste products to the site.
How can the nutrient supply be increased?
Both by adding nutrients and by releasing those which are locked up. You could:
- plant soil-improvers (D 32), and perhaps cover crops;
- put much around the young trees (Manual 5);
- allow suitable domesticated animals to graze or browse (D 34), and leave their droppings;
- if possible, inoculate trees with mycorrhizal fungi or nodule-forming bacteria before
planting them (Manual 3), or bring in some soil from undisturbed sites where the
particular species is thriving;
- add animal manure or compost, or apply fertilisers or trace elements;
- if the soil is strongly acid (D 12), apply ground limestone to increase the pH, add Ca
and let nutrients be more easily cycled.
Isn't it easiest just to use fertilisers?
This may be so if one or more of the nutrients are in very short supply. But:
- using fertilisers can be expensive;
- adding too much of one nutrient can upset the absorbing of another;
- too much fertiliser, leached into the groundwater or washed into rivers and lakes, can
contaminate drinking water supplies and upset fishing by stimulating a thick scum of
algae to form.
There are often enough nutrients, if the site is managed to conserve them. The natural breakdown
of litter usually provides a steady, moderate supply, whereas adding fertilisers often gives
a lot of nutrients for a short time.
What about slow-release fertilisers?
These are better, though at present they are more expensive still. However, bone meal (groundup
bones) is a cheaper slow-release source of P, which is often in short supply or unavailable in
many tropical soils.
- interactions with other plants
How do plants fit into the ecosystem?
Green plants are the ‘producers’ of organic matter from simple substances, making energy
from sunlight available for their own growth and reproduction, but also providing food for all
the animals and most decomposers in the ecosystem (D 10–11, D 13).
Are all plants green?
Almost all of them contain the green pigment chlorophyll, even when other colours are present
in their cells.
A very few are completely parasitic on other plants, and are not green.
NOTE: Santalum album (sandalwood) is a partial parasite, taking some sugars and nutrients
from the root systems of other species of tree. Similar transfers probably occur when the
root systems of trees such as Aucoumea klaineana become naturally grafted together.
Do the trees in tropical ecosystems vary much?
Yes, most tropical forests and savannas contain trees of many species, and of several different
ages and sizes. For example, rain forests on well-drained sites typically have scattered large
emergent trees, a more continuous middle tree layer and some smaller saplings and seedlings
below, often concentrated in gaps where a big tree or branch fell. In the understorey there may
also be woody plants that never grow particularly large.
Are there exceptions to this great variability?
Yes, the trees are often more uniform in size and age, with fewer species present, in:
- ecosystems on difficult sites, such as:
- steep rocky slopes, where for example Hildegardia barteri may predominate in
- swamps and temporarily flooded forest, which in Mexico may consist of pure
- peat swamp forests, often dominated in Borneo by even-aged Shorea albida;
- cloud forests in dry areas, often with predominant Scalesia paniculata in some
of the Galápagos Islands;
- salty, tidal mudflats, which throughout the tropics are colonised by a few
species of mangroves (D 26); and in
secondary forest that has recolonised after a destructive storm or fire (D 11), or after
being cleared by humans and later abandoned (D 16, D 22).
What differences do tropical trees show?
A great many; including different:
- plant groups, such as broadleaved trees, palms and conifers;
- ecological stages at which they thrive; for instance:
- colonisers (pioneers) - tree species that are good at establishing themselves quickly
after natural disturbances (D 11), and in areas cleared or burnt by humans; for
example Cecropia, Macaranga, Musanga, Ochroma, Pinus and Trema;
- most other species, which survive best under some shade or in small gaps (D 50–51);
- growth responses to the environment (Manual 3).
There can also be quite large differences within a tree species, which is why genetic selection
and domestication are so important (Manuals 1 and 2).
How will my trees interact with other plants?
In a lot of different ways, both negative and positive, such as:
- occupying above-ground space, and casting shade on each other;
- competing for rooting space, water and nutrients;
- perhaps producing chemical substances that influence the growth of other plants; and
- by affecting the numbers of soil organisms, herbivores, pollinators, etc (D 15).
However, tropical trees also help each other to thrive.
But surely plants mostly compete with each other!
Yes, they generally do. Except on very difficult or degraded sites, plants form a closed community,
- natural regeneration (D 2) within gaps in the stand is generally sufficient to maintain
the various species present, even though most natural seedlings fail; and
- it is difficult for planted trees to become established unless the canopy is partly opened
Is it extra light that is needed by planted trees?
Yes, this is one of the main reasons. If it is very shady, shoot growth may either be soft and
weak; or slow down and stop altogether.
On the other hand, some overhead shade is favourable for the growth and successful establishment
of many tree species (D 1, D 50).
In deciding how much opening up to do (D 51), you need to know whether young stages of the
various trees and crop plants to be grown are ‘light-demanders’ or ‘shade-bearers’.
Does opening up alter the conditions in other ways as well?
Yes, both positively and negatively, for instance by:
- disturbing the soil, which can provide rooting space for new trees, but also could make
erosion more rapid;
- increasing the diurnal fluctuation of temperature (D 11);
- stimulating a more rapid breakdown of litter and organic matter, and release of nutrients
- allowing grasses to become dominant over trees (D 25), or choking weeds to gain a
How would other plants help my trees to grow well?
Especially by providing some overhead shade that helps keep the soil protected (D 1, D 23,
D 60). They may also:
- keep down weeds and strong grass growth;
- drop different kinds of litter on to the soil surface (D 13);
- alter the soil conditions through growth of roots, and exudation from them of various
- provide food for animals (D 15, D 34), whose droppings return nutrients and organic
matter to the soil.
Some tree species add nitrogen to the soil because they have root nodules, and many probably
make phosphorus available through mycorrhizas (D 32).
Are there plants that go particularly well together?
- Individual pairs, such as a leguminous (D 32) and a timber tree (D 36);
- Cover crops and plantation species, such as Pueraria under young rubber trees, or
shade trees over beans and squash in farmland (D 21);
- Shade trees and underplanted trees, in 2- or 3- storey woodland (D 52).
A general guideline is that suitable mixtures of tree species are more likely than pure stands
to continue to thrive and produce useful materials and other benefits in perpetuity (D 30,
Should some mixtures be avoided?
- Gonostylus bancanus with Shorea albida;
- Ochroma lagopus with Cecropia obtusifolia;
- Trees amongst bamboos that form spreading clumps;
- When one species is or may become a troublesome weed;
- Where the branching or rooting habit of one species interferes with that of others;
- Where one is an alternative host for a pest or disease (Manual 5);
- If one produces chemical substances that damage the other.
What sort of chemicals?
Dissolved substances or vapours that inhibit germination, root elongation or shoot growth
nearby. For instance, the shrubby weed Lantana camara produces essential oils that inhibit the
growth of trees, other weeds and even its own seedlings.
Many trees produce a range of different chemicals, particularly in their leaves and bark, that
are poorly digested by, or poisonous to, most browsing animals (D 15, D 34) or boring insects.
Some of these are used as medicines for humans or domesticated animals (D 33–34) or as
poisons for hunting or fishing (D 39). In most cases, it is not yet known what effects these
chemicals may have upon young trees.
What is the best way to get maximum production from a site?
This will depend on:
- understanding enough about the productivity of the local natural ecosystem to develop
a managed system with high yields, but without harvesting more than can be replaced
(D 4, D 24, D 26, D 51); and also on
- why you are growing the trees (D 30 – 40).
Supposing I particularly want forage?
Work out what is the best:
- cutting system for the tree species involved (D 34), for example:
- pruning of small shoots regularly (as when plucking tea);
- lopping of the top of the tree and strong branches less frequently, so that the new
growth can still be reached easily;
- coppicing periodically, with new sprouts near the ground; or
- pollarding, where the shoots are out of reach of many animals;
- spacing between the trees that allows enough room for the shoots to develop and be
collected or browsed, without wasted space; and
- cutting cycle, that gives a high, sustained yield. This could vary from a few weeks to
several years, depending on the cutting system, the tree species and the site. If the best
cutting cycle is not known, you might do some simple field trials (D 6, D 29).
Does the same hold for firewood?
The general principles are the same - to find how to produce numerous suitable stems, and
how often to cut them so that the average annual yields remain high.
Firewood (including charcoal) is often produced in coppice systems, with a cutting cycle of
about 2–10 years, depending on the rate of growth (D 35). It may sometimes be worth reducing
the number of sprouts to 1–4 per stump, to obtain larger and straighter lengths.
Is it different when growing trees for timber?
The basic ideas are similar, but this time the trees will usually stand for a longer time (D 36).
With a rotation of 30–75 years and a cutting cycle of 15–25 years, there will be several opportunities
of channelling growth into straight, single-stemmed trees of desirable species (D 24,
D 26, D 62).
One might also, for instance:
- not poison acceptable natural trees, but regard them as part of the crop (D 2);
- supplement natural regeneration with enrichment planting (D 2), perhaps with genetically
superior trees (Manuals 1 and 2);
- use mulch, and weed as necessary (Manual 5);
- thin the stand later on, to favour the best stems (D 51, D 62).
Won't animals just destroy my trees?
All animals depend on food from plants (D 10, D 14), but under natural conditions they
seldom destroy their resource.
In managed ecosystems:
- certain kinds of animals may severely damage or kill trees;
- many herbivores eat some leaves and fruits, but do little harm;
- other animals are wholly beneficial.
Which kinds of animals are most harmful?
Damage can sometimes happen with:
- Browsers, either wild or unchecked domesticated animals. For example:
- a single elephant eats around 300 kg of foliage a day, and will also break down
branches and young trees;
- some kinds of monkeys cause repeated branching by eating young expanding buds;
- antelopes, deer, porcupines, mice, etc may gnaw off bark near the base of saplings
and pole-stage trees, damaging or killing them;
- Sucking insects such as bugs and aphids may harm young trees if they are present in
large numbers, or if they transfer virus diseases;
- Boring insects can damage the bark, twigs, shoot tips or leaves;
- Munching insects such as caterpillars can eat all the leaves on a young tree, while
locusts can strip an entire area.
But I can't keep all these animals off my trees!
No, it is usually best to think about how to cope with the main problems, and to maintain
diversity (D 30, D 53) so that some of the natural ‘checks and balances’ are retained. Problem
areas could include:
- goats, because they will eat almost anything (D 27, D 34);
- herds of cattle, especially on slopes (D 23);
- large groups of wild animals, since they can do a lot of damage in a short time;
- swarms of insects, which can sometimes multiply very rapidly.
Which parts of the tree are most important to protect?
- The leading shoot, in order to maintain vigorous growth and a straight main stem;
- A new flush of foliage, because reserves of energy and nutrients will have been depleted
in producing it; and
- The root system, because if it is weakened the whole tree may die.
How long is protection needed?
Young, newly planted trees are often more liable to attack by insect pests, and are more seriously
affected by browsing. So concentrate on trying to protect your trees during the first year
or two, extending it if you can until they are well established (Manual 5).
In agroforestry (D 3), you could use temporary fencing or suspend grazing entirely while the
young trees are starting to grow.
What about damage by humans?
This is covered on sheets D 16 and D 66.
How can animals help trees thrive?
In many different ways, including:
- earthworms and soil micro-organisms acting as decomposers, breaking down dead
organic matter and mixing it into the soil (D 13);
- animal droppings returning nutrients to the soil (D 34);
- ants and termites moving soil particles and litter around (D 13);
- insects and birds pollinating flowers which:
- maintains genetic diversity by continually mixing the features from different parent
- produces fertile seed (Manual 2);
- various animals distributing the fruits and seeds, so that natural seedlings may spring
- birds and some kinds of ants keeping down the numbers of plant-sucking bugs that
might become pests;
- domesticated animals keeping down weed growth (D 3).
Can I encourage useful wild animals?
This might be helpful, as long as their numbers do not get too high. For instance:
- wild carnivores that keep herbivore populations from over-multiplying might be protected
from hunting or disturbance;
- useful pollinators such as bees or insect-eating birds could be encouraged by including
other plants that provide them with nesting sites, or food sources at different times
Having some dense patches of woodland in an area is important to the survival of
many groups of useful animals.
What about fish?
In the same way that meat, milk, skins, etc from land animals are derived from green plants,
fish harvests from rivers, lakes and the sea depend on:
- organic matter produced by large numbers of minute algae in the water;
- organic matter derived from land plants; and
- fishing being done on a sustained yield basis.
Fish stocks in tidal estuaries can often be largely dependent upon organic matter from, and
feeding areas in, nearby mangrove woodland (D 26).
How much do human activities affect trees?
All over the world, humans have altered natural ecosystems, especially by cutting down trees,
burning vegetation, planting crops and grazing their flocks of animals.
With our mechanised tools and heavy vehicles, humans have more influence on ecosystems
nowadays than any other living organisms.
Does this matter?
The balance between being harmful or helpful depends on whether:
- people have knowledge about local habitats, and relevant experience from other areas;
- scientific understanding of the particular ecosystem is available;
- human populations are above or below the carrying capacity (D 10);
- social and political structures favour short-term measures or longer-term management;
- actions are carefully thought out beforehand (D 5);
- unexpected accidents (D 4) alter the planned procedures;
- the natural system is greatly or little altered (D 2);
- fire is used or not (D 63);
- natural disturbances or unusual climatic events (D 11) coincide with the removal of
Have the tropics been much affected?
Over the centuries, extensive deforestation has occurred, first around the Mediterranean Sea,
then in the Temperate Zones. Now in the tropics, more than a million tropical trees are being
cut down and not replaced each day.
As many as that?
Yes, and the lack of trees usually leads to rapid and severe soil degradation (D 22), so that the
land can end up completely unproductive.
Some regions still have substantial numbers of trees left, while other parts are becoming increasingly
Would it be best just to leave the natural ecosystems alone?
Where possible, it is a good idea to conserve certain areas with little disturbance, for example
- breeding sites for wild animals (D 15), or sources of local seed (Manual 2);
- nature reserves for scientific study, education and recreation (D 28);
- areas of cultural or religious significance (D 42).
Some human societies live by hunting and gathering alone, and need large areas of natural vegetation
which they may not greatly change.
But for most people, the really important questions are about how best to manage tropical
Well, what is the best way?
Sustainably - so that it continues to produce many useful materials and other benefits in
But what does that involve in practice?
In many tropical sites it means:
- Retaining some of the natural trees (D 2);
- Only opening the canopy moderately (D 51);
- Restoring the cover over the soil in cleared areas by tree planting (D 21–22);
- Protecting the soil from exposure (D 23, D 60);
- Improving its structure and fertility (D 12, D 32);
- Avoiding large single-storey blocks of one species (D 30, D 53).
Isn't this too much to ask of people?
No, because sustainable management is required if future generations of children are to survive
without being forced to migrate.
But doesn't clearing the land lead to economic development?
Some land does need to be cleared to make roads, towns and buildings. But, except where the
country can live by trade, the people involved in urban and industrial activities have to be
supported by those working in agriculture and forestry on the remainder of the land, and
The mistaken idea that “clearing = development” comes from temperate-zone misconceptions
about tropical ecosystems and the meaning of ‘development’. It is clearly not ‘economic’ to
pursue policies that degrade a site so that it is no longer productive (D 10).
Isn't more intensive agriculture the answer?
On certain more fertile sites, intensive farming can be practised continuously; but
On most tropical land, attempts to increase yields can lead to soil degradation unless trees
are present (D 3, D 21–23).
Do I need to plant if there are still plenty of natural trees?
Sometimes no, when there are plenty of natural seedlings of useful species;
Often yes, because:
- the numbers of natural seedlings may be too low (D 1–2); or
- desired species may be lacking.
A mixture of natural and planted trees often gives you more options (D 50–54).
What about fire?
This is a natural feature of savanna and dry forest (D 11), but very rarely of untouched humid
Fires are also frequently started by humans:
- when preparing land for planting crops or trees;
- to obtain palatable new growth in pastures (D 34);
- when hunting;
- for cooking; and
- by accidental spread of small fires (D 4).
Whether to use fire or not, and how to control it, are important decisions before planting
(D 63) and when protecting trees (D 66 and Manual 5).
What other effects have humans had?
- Many positive things, such as:
- learning about ecosystems;
- building up detailed knowledge of local plants and their usefulness;
- developing and exchanging improved varieties of cereals and clones of other food
- recording and sharing information on management; but also
- A lot of negative actions; for instance:
- causing greatly increased erosion and degradation of soil (D 12, D 22–23);
- hastening local or total extinction of some plant and animal species;
- carrying out negative genetic selection of trees;
- producing various kinds of pollution; and probably
- unintentionally causing climate change.
But won't species have become extinct anyway?
Many species have died out over very long periods of time, and new forms have evolved.
But humans have greatly hastened the rate at which species are lost, partly by farming and
hunting, but mainly through changing vegetation and habitats on such a large scale.
Does it matter?
- some animals and plants can play key roles in the ecosystem (D 10); and
- many may have the potential to provide useful products such as medicines (D 33).
Haven't people tried to improve trees?
Yes, with a few species. But little has been done until recently with most of tropical tree.
Very often the best specimens are taken, leaving inferior ones as the only parents for future
generations of that species.
This is why positive genetic selection and domestication is so important (Manuals 1 and 2),
and also conservation of rare and threatened species.
How about pollution?
Waste gases, smoke particles, liquids and solids produced by human activities have often been
released into the air, rivers and oceans, or dumped on the land. If they are not quickly broken
down to simple, harmless, substances, some of them can be quite dangerous. For example,
factories may emit, intentionally or by accident:
- large amounts of smoke, causing ‘smog’ in windless weather, reducing yields by
blocking-out sunlight and depositing dirt and acid particles on leaves and soil;
- toxic fumes and dissolved substances into the atmosphere and rivers, risking human
injuries and deaths;
- high-, medium- and low-level radioactive wastes that remain dangerous for decades,
centuries or many thousands of years. Any extra nuclear radiation is a potential hazard
for humans and domestic animals, especially if the substances accumulate in their
What have trees got to do with this?
- Trees may filter out large amounts of pollution from the atmosphere (D 28); and
- Some species can tolerate metal pollutants in the soil (D 26). The trunks of old trees
can also provide a record of when some radioactive pollutants have spread.
How does removing trees affect the climate?
Widespread clearing of trees is likely to lead:
- locally, to a less suitable environment for growth and establishment of young trees
(D 23, D 50);
- regionally, to altered amounts and patterns of rainfall, including more severe droughts
- globally, to appreciable alterations in climate and sea-level (D 11).
Replanting trees can help to counteract these problems, besides making sense in
many other ways (D 1).