- introduction to diverse conditions
Do planting sites vary a lot?
Yes; there are very large differences between them. This means that:
- the general conditions under which the trees are to grow and become established can
differ greatly (D 10–16); and so
- the choice of suitable tree species will not depend only on what they are to be grown
for (D 30–42).
How do planting sites differ?
- Latitude, climate and liability to stressful conditions (D 11);
- Altitude, slope and topography (D 12);
- Physical components and broad chemical constituents of the soil (D 12);
- Soil structure, organic matter and nutrients (D 12–13);
- Soil water status (D 13);
- Presence and abundance of soil organisms (D 13);
- Microclimate and the amount of overhead shade (D 11, D 14);
- Types of other plants and degree of root competition (D 14, D 63–64);
- Accessibility to animals, especially herbivores (D 15);
- Liability to diseases and insect pests (Manual 5);
- Risk of damage by fire (D 66); and especially
- Any previous use of the land by humans (D 16).
But I can't do much about all these things!
The first three are basic features of the site, that are more or less fixed.
But you could for instance think about whether your preferred tree species:
- are able to stand a long dry season, or irregular rainfall (D 11);
- might be resistant to regular or occasional low temperatures;
- could stand the strong ultra-violet B radiation in mountains;
- would tolerate exposure to strong winds;
- were suited to growing on steep slopes (D 12, D 23).
Can the other aspects be modified?
Some of them can, and they could all be taken into account in deciding:
- which groups of species to plant (D 30–42);
- what type of planting stock would be suitable (Manual 3);
- which growing system to use (D 50–54); and
- how to prepare the ground (D 60–65).
What needs doing first?
- Think about each of these points, well ahead of the planting time; and
- Make a working plan (D 4), discussing it with others (D 5) to give your trees a good
chance of successful establishment and growth.
Can I do anything to improve the soil?
Planting trees can often do so, and there are usually several other things that can be done, such
- retaining some existing trees to protect the soil (D 2, D 51);
- including soil-improvers (D 32) or quick-growing shade trees (D 41) among the species
- using mulch (Manual 5) to protect the soil, increase its organic matter and improve its
structure (D 13);
- considering terracing on steep slopes to decrease soil loss and retain water (D 65);
- cutting drainage channels to lower the usual or exceptional water level in the soil (D 65);
- using suitable domestic animals in an agroforestry system to add nutrients and keep
down weeds (D 3, D 15, D 21).
What if the soil is a difficult one?
There are usually only a few tree species that can thrive under extreme conditions, such as:
- Degraded sites (D 22), where colonisers (D 14) that are soil-improvers (D 32) can often
start to restore them;
- Strongly alkaline soils (D 12), for which it might be sensible to concentrate on tolerant
local trees or shrubs (D 31); and
- Muddy soils flooded by sea-water, where mangrove woodland can be managed
sustainably (D 26).
How about the microclimate?
This means the actual environmental conditions experienced by a tree at a particular place.
A favourable microclimate for establishing trees is one that has:
- some rooting space in the soil (D 14, D 50); and
- enough overhead shade to maintain ‘open woodland’ conditions (D 51).
What does that mean in practice?
- the young trees are not exposed to full sun (D 11) when first planted;
- the air and the soil around them remain reasonably humid;
- they are protected from excessive wind;
- the soil is secured against rapid erosion (D 1, D 23);
- air and soil temperatures are moderate (D 11);
- conditions for soil organisms are favourable (D 13, D 60);
- root competition from other plants is not excessive (D 14);
- there are no severe shading or toxic effects from other plants.
How about protecting my trees from animals?
Whatever you do, some foliage on the new trees will be eaten by herbivores (D 15). Protection
is chiefly needed against:
- herds of the larger domesticated or wild animals (D 3, D 21, D 34);
- swarms of insect pests (Manuals 3 and 5).
Don't forget that some animals could be beneficial or even essential to your tree crop (D15).
What about fire?
Since this can undo all your efforts, you could:
- consider alternatives to burning the site (D 63);
- realise that dry grass or large dry leaves could assist fire to spread into or among your
planted trees (D 66).
In sites where fire is regular (D 11), choose groups of species that can tolerate ‘light’ fires, and
will soon re-grow from coppice shoots.
How does the previous use of the site matter?
Large differences exist between sites that, for instance, are:
- largely untouched natural vegetation or well-established secondary regrowth, where
the option exists to enrich the existing trees (D 24), with only moderate opening of the
canopy (D 51, D 54);
- under single species plantations, which may not be as successful as expected (D 2),
and could be converted into mixed stands (D 30, D 53), perhaps with more than one
storey (D 52); or are
- already greatly opened up, where the aim might be to restore forest or savanna
(D 24–26), or open woodland conditions for agroforestry (D 3, D 21–23).
What about the reasons I want the trees for?
The condition of the site and your needs may interact. For example, if:
- the land is bare (D 22), hardy colonising trees may be planted in order to increase the
options later on;
- the soil needs improving (D 21–22, D 25), it might make sense to add mulch or leave
farm soils untilled for some time (D 60, D 64);
- little natural forage is available (D 34), animals might need to be kept off the site for
some time, while the planted trees establish (D 3);
- fodder or fuel is to be cut regularly (D 34–35), too frequent opening of the canopy might
prevent the trees from protecting the soil, especially on slopes (D 23);
- timber trees are being grown for a relatively long time (D 36), you could plant them at a
wide enough spacing (D 54) to have shorter-lived trees, crops and/or animals between
them (D 3).
But common to all planting sites is the need to create conditions where the new trees have a
good chance of surviving (D 50).
Are there some key thoughts?
Whatever your particular conditions:
- choose growing systems (D 50–55) and methods of preparation (D 60–67) that encourage
favourable microclimates for tree growth and establishment; and
- select mixtures of tree species (D 30–31, D 53) that can improve soil fertility while
providing the range of products and other benefits that are needed (D 1, D 32–42).
Won't trees be in the way on my farm?
Not necessarily. It is important to have some trees on most kinds of farmland. This is because
many tree species can:
- protect the soil (D 60);
- improve soil structure and fertility (D 13);
- yield many useful products themselves (D 33–40), including items for sale and for use
on the farm;
- be used as boundaries;
- provide shade for animals and for shade-tolerant crop plants;
- improve living conditions for people (D 28, D 41–42).
In addition, leaving farmland under tree fallows for several years may be done (D 3), in order
to allow the soil to recover some fertility.
But won't trees reduce my crop yields?
This would happen if the crop plants were under heavy shade or suffered intense root
competition for water and/or nutrients from unsuitable trees (D 3).
But having a moderate number of appropriate tree species can:
- let you make several different harvests, perhaps at different times of the year (D 3);
- allow cropping to continue for a longer period, without yields declining greatly;
- sometimes give crop yields as high or higher than without trees;
- help restore farmland that has become degraded (D 22, D 32).
Are there any guidelines?
The key questions to ask are: what would the total yields plus protection of the soil be:
- with trees or without trees?
- over the short, medium and long term?
How do trees protect and improve the soil?
- By greatly decreasing soil erosion both by wind and water, particularly on slopes
(D 12, D 23, D 41);
- By keeping the surface soil cooler and moister (D 11–12) and adding organic matter,
and so making conditions more suitable for root growth and for beneficial soil
organisms (D 13, D 50);
- By aiding the capture and recycling of nutrients (D 13), and sometimes by adding extra
nitrogen (D 32).
(For more details, see D 1, D 3, D 22, D 30–32, D 70–71).
What products could trees on my farm provide?
A very large number, including items for sale as well as for use by your own family, such as:
- diverse fruits, seeds and fresh leaves that can provide important proteins, minerals and
vitamins for a healthy diet (D 33);
- fodder and browse for domesticated animals (D 34);
- fuel for cooking and for boiling water (D 35);
- poles for use in building houses, sheds, fences, gates, etc (D 38–39);
- wood for tools and household utensils (D 39);
- leaves and litter (D 13) to use as mulch (D 65 and Manual 5).
How about trees for boundaries?
These can be very useful, for instance in:
- marking the boundary between two farms in a clear and long-lasting way;
- making a close-planted, thorny hedge to keep animals in or out;
- giving shade along a road (D 41) or stabilising river banks;
- inserting a fire-break (a line of fire-resistant trees to check the spread of fire), amongst
trees at risk of being burnt (D 66); and
- surrounding specific plots or trees in a field trial (D 55).
How can trees benefit farm animals?
The two can often help each other (D 15),
with the trees:
- providing forage for the animals (D 34);
- giving them some shade and shelter from the wind (D 41);
while the animals:
- return nutrients to the soil, which can then be taken up again by the trees (D 3, D 13);
- keep down weeds (D 14, Manual 5).
Note: watch out that the leaves are not poisonous (D 66), and that damage by the animals to
the trees or soil is not too great (D 3, D 34).
What about improved conditions for people?
This can happen in many ways, such as:
- allowing more varied and nutritious diets (D 33);
- providing a wide range of different medicines (D 33);
- giving shade from the sun and shelter from the wind (D 41);
- checking the spread of dust and some industrial pollutants (D 16);
- acting as a focus for education and for social life (D 5, D 27, D 42).
Have all these methods with trees on farms been well tested?
Unfortunately not, although:
- traditional farming techniques generally retained trees during cropping and used tree
fallows, and were often successful when there were fewer people;
- mixed cropping or grazing under trees is practised sustainably in some regions to-day;
- much new information is becoming available about agroforestry and multipurpose tree
species (D 3, D 40, D 71).
But I can't afford to risk my crop yields with new methods!
Many tropical smallholders and larger growers are caught in a trap because:
- their crop yields are going down because the unprotected soil is losing fertility and
being washed away;
- there is less fresh land available for farming; and
- there are always more mouths to feed.
So what can I do then?
This is a very difficult situation to be in. Because today's cropping often reduces tomorrow's
yields, farming systems that used to work well now have to be re-thought. Among key points
are the need to:
- mix local experience with scientific knowledge about managed ecosystems (D 10–16);
- consider carefully which areas might be suitable for more intensive farming, for tree
crops and for woodland;
- retain some existing trees where they are present (D 2);
- plant a diversity of useful trees (D 30, D 53) in appropriate patterns (D 54);
- protect the soil (D 60), by mulching, terracing (D 65), etc;
- use a broad range of existing crop varieties while trying out new ones on a small scale.
Can I use trees if my main crop is a cereal?
Most kinds of cereals are light-demanding plants (D 14), so any overhead shade must be quite
light. However, you could grow the cereals:
- while the trees are leafless - for example under Faidherbia (Acacia) albida (D 40);
- with trees in lines along farm boundaries, the front edge of terraces (D 65) or steep
watercourses (D 23), where they can be particularly effective in retaining soil, nutrients
and water that would otherwise be lost;
- in an alley-cropping system, between lines of Dactyladenia (Acioa), Leucaena, Senna
(Cassia) siamea or other nitrogen-fixing shrubs which are cut back frequently and used
as a much.
In some agroforestry systems, the trees or shrubs can produce up to 5 tonnes of mulch and
green manure each year, reducing the need for expensive fertilisers.
What about new varieties of food crops?
Hybrid cereal varieties are being promoted, which may have the potential for giving higher
yields, but these:
- often require expensive inputs of fertilisers and/or pesticides;
- may not allow farmers to select and keep back seed for the next crop;
- could fail altogether in unfavourable seasons.
What if I am growing a mixture of crop plants?
Individual trees can usually be successfully added. You could think about:
- what kind of products and soil protection you need most (D 30–40);
- which tree and shrub species would fit best into your existing farming system, perhaps
including some soil-improvers (D 32) and multipurpose trees (D 40);
- how best to arrange the trees to protect the soil (D 54), without giving too much shade
or taking up too much water and nutrients.
Can I improve a tree fallow?
Yes, by adding planted trees to those that come up naturally (D 2). You could, for instance,
plant a mixture of soil-improvers (D 32), fruit-trees (D 33) and shade trees (D 41) to increase
your choices when cropping starts again.
Because wild animals are often attracted to them, young tree fallows are used as managed
hunting areas in some parts of the tropics.
How can trees help farms on abandoned grazing land?
Treat this as degraded land (D 22), keeping all existing shrubs and trees except weeds, and
plant soil improvers (D 32) or shade-trees (D 41) in lines or at wide spacing (D 54). Acacia
auriculiformis and A. mangium often colonise abandoned grassland, if there are seed trees nearby.
If trees or shrubs are pruned, keep the branches as a mulch.
What about abandoned plantations?
Open the canopy moderately at first (D 51) retaining denser cover near streams and on steep
slopes (D 23). Underplant with shade-tolerant crops and trees.
Are there some general guide-lines for trees in farms?
- Wherever possible, aim to farm amongst the trees rather than removing the trees in order
- Use mixtures, rather than just one tree species (D 30, D 53).
- Carry out informal trials (D 6) of tree species, planting patterns, cutting cycles, etc.
- reclaiming degraded land
What is degraded land?
Ground on which plants can no longer produce more than a small percentage of the potential
amount of organic matter for that site (D 11).
Why has it become degraded?
Conditions for plant growth have become much less favourable (D 12–14, D 23, D 50), for
- most fine soil particles in the topsoil have been washed or blown away;
- there are fewer nutrients available;
- the soil structure is less favourable, for example by becoming compacted;
- the soil microclimate is now unfavourable for most living organisms; and/or
- all the soil has been eroded away.
What causes this?
The commonest reasons are because the site has been:
- completely cleared of trees;
- over-grazed or over-browsed (D 34);
- cropped continuously without inputs;
- repeatedly burnt (D 63);
- planted with a single species (D 30, D 53); or
- heavily polluted (D 16).
Are the problems the same for humid and drier areas?
In general, many of the same points apply throughout the tropics (and to a lesser extent in
cooler parts of the world). Compared with moist forests, however, dry forest and savanna areas
(D 25) differ in that:
- the soil is somewhat more exposed to sun, wind and rain; and
- fire is a regular, natural feature (D 11, D 16).
Although soil and nutrients tend to accumulate in freshwater swamp forest and in mangrove
woodland (D 12, D 26), these can also become degraded if they are overcut.
Does soil degradation happen quickly?
Yes, many tropical soils are quite fragile, so that their condition can change markedly in a matter
Natural disturbances (D 11) such as a single heavy downpour, sudden rise in river level or
severe windstorm can remove a lot of unprotected soil within a few hours.
How much soil?
When tropical land is completely cleared, the power of rain and wind to remove soil is not just
increased a little, nor is it doubled or trebled.
For savanna it is about a hundred times greater.
For dense tropical forest it is around a thousand times more (D 23, D 70).
Why then has land been allowed to become degraded?
For many reasons, including:
- lack of understanding of tropical ecosystems (D 10–16, D 60) and so introducing
inappropriate techniques (D 16);
- felling trees for timber without replacing them (D 24, D 36);
- over-emphasis on large-scale, single-storey, pure plantations (D 2, D 30, D 50–54);
- increasing human populations (D 3, D 5);
- social problems related to land tenure (D 4);
- short-term economic attitudes (D 1) and misunderstandings about ‘development’
Is it worth while bothering with degraded land?
Emphatically yes, because:
- Over 20% of tropical land is already moderately or strongly degraded;
- land is often in short supply;
- there are several ways of helping to reclaim degraded land; and
- it can often be restored relatively quickly.
Note: degraded land would have to lie idle for longer, if for example:
- most of the soil has been washed away (D 23); or
- it is heavily polluted with toxic chemicals (D 16).
How can it be restored?
- Completely cleared: plant various species of trees, including soil-improvers (D 32);
and also perhaps cover crops or suitable grasses.
- Over-grazed or over-browsed: carry fodder for farm stock (D 34), use pollarding and
put up fencing or grow hedges (D 39) to protect foliage from grazing by them or by
wild animals (D 15, D 21). Encourage regrowth of existing trees from coppice shoots,
and plant additional trees.
- Over-cropped: leave under tree fallow or plant cover crops and soil-improving trees
and shrubs. Then plan how to adjust towards more diverse cropping and the returning
of crop wastes, etc to the site (D 3).
- Frequently burnt: plant fire-breaks (D 21, D 66) and remove dry grass and large dry
leaves from the boundaries of the area, to protect it from fires. Use cutting of
undergrowth, mulching or cultivation instead of burning (D 63–64).
- Single tree species present (D 20, though see exceptions in D 53):
- if trees cover the land, try to introduce other species and different sizes of tree in
small groups or lines (D 53–54), or by thinning and underplanting (D 51–52);
- if not covered with trees, treat as for (1) above.
What about reclaiming land where the soil is salty?
There are several alternatives, including;
- Restoring mangrove woodland (D 26) by natural regeneration (D 2) and/or planting;
- Introducing other useful salt-tolerant plants such as Salvadora persica and Prosopis
- Building mud walls to keep the tide from bringing in fresh salt (D 65).
Can I use animals to help restore land?
It may be best to keep them off the land in the early stages of reclaiming an area, because they
may eat or damage the young, colonising trees, or dig into and churn up the soil (D 34).
Later on, they might be used to keep down weeds, and if fodder was carried to them, some
nutrients will be added to the site in their urine and droppings.
Well anyway, isn't it better to stop land getting degraded?
Yes, it is! Aim to maintain soils in a similar condition to how they are under natural vegetation.
What can I do to stop the soil becoming degraded?
Here are some hints:
- Keep enough of the naturally established trees that may be present (D 2);
- Plant mixtures of different species of trees, shrubs and/or crops (D 30, D 53) including
soil improvers (D 32);
- Try and put back nutrients into the site (D 13) from crop and animal residues.
You might also:
- Recall your ancestors' understanding of the relationship between people and the soil
(D 5, D 42);
- Learn more about how ecosystems work (D 10–16);
- Keep the best of the traditional methods, but also try out new species (D 30–31) and
techniques, on a small scale at first (D 6).
What does soil degradation mean for tropical people?
It is one of the most critical of the many problems facing them, because it:
- represents the loss of many benefits and products;
- places a greater pressure on land that is still fertile, tending to degrade that as well;
- introduces the fear of being forced to move;
- may be linked with other serious problems, such as drought and war.
Losing the use of a site can happen through forgetting the community's cultural understanding
of how to look after the land (D 42).
Then what's to be done?
- Spread the realisation of how big and urgent the problem is (D 1);
- Exchange knowledge and experience about the value of trees (D 4–6);
- Reclaim degraded land by planting soil-improvers (D 12, D 32);
- Modify systems so that less land becomes degraded (D 2, D 50–55);
- Develop various kinds of sustainable use (D 5).
Why do slopes need special attention?
Because the steeper the slope:
- the more the fine soil particles, nutrients and litter are liable to be washed or blown
- the greater the risk of the soil as a whole being eroded.
Are the fine particles particularly useful?
Yes indeed. They are the most important part of the soil (D 12–13) because:
- by adding extensive internal surfaces, they help to give soil a good structure for roots
and fungal strands to grow in;
- they tend to retain water which is readily available to the trees;
- nutrients are often attached to their surfaces.
Fine soil particles are often moved around, for example by ants and termites, or are eaten by
earthworms which return them to the soil surface.
Aren't there plenty of nutrients in the soil?
Only in a few sites. Most tropical soils tend to be short of essential nutrients, so tree growth
can be restricted.
What about the litter?
This is also important, because it:
- helps protect the topsoil (D 1, D 12, D 20);
- provides food for the decomposers (D 10, D 13);
- returns nutrients to the soil (D 13).
But won't these things get washed and blown away anyway?
Yes they will be, but at a much slower rate when the land is covered by trees, undisturbed
grasses or a thick mulch.
NOTE: fine particles on bare ground are far more likely to be washed away than under the natural
vegetation (D 22).
Isn't that just on steep slopes?
It is certainly greater on steep slopes, but it also happens when the terrain is gently sloping
(D 12). Only on flat land, in the bottoms of valleys, in lakes and at the mouths of rivers will
the fine particles be deposited instead of eroded.
Is that why swampy land can be more fertile?
Yes, the continuing supply of fine particles, water and nutrients often keeps low-lying land
fertile (D 12). However, serious problems may be caused by too much incoming:
- water, causing flooding which can:
- make soils unsuitable for trees or farm crops unless there are deep drains (D 65);
- perhaps drown farm animals;
- soil particles, which can lead to thick deposits of mud, silt, sand, gravel or shingle on
top of the existing soil and any crops, or in reservoirs; and
- nutrients, which may become too concentrated in streams and lakes (D 13).
But what about all the rest of the land?
All of it is liable to very rapid erosion and soil degradation if it is exposed (D 22).
And what about wholesale loss of the soil?
Under a natural vegetation cover, this happens only rarely, for example on ridge-tops, from
cliffs and in gorges, or as ‘stone-avalanches’ or mud-slides in high mountains.
But when most trees are removed, these destructive disturbances become much more common,
and deep gullies can be cut very rapidly by rushing water, or the entire soil washed off a cleared
patch of hillside.
What can be done about all this loss of soil?
Here are some guide-lines:
- Understand the scale and importance of the wastage;
- Adapt land-use systems to retain more of the features of the natural ecosystem (D 10–16, D 50–55);
- Retain and perhaps ‘liberate’ existing trees wherever possible (D 2, D 24, D 51);
- Plant trees on each kind of site (D 20–21, D 24–28), especially in and above farmland,
along water-courses and wherever you see soil beginning to be washed away;
- Reclaim land that has already become degraded (D 22, D 32), and consider terracing
with planted trees stabilising the front edge (D 65);
- Encourage much more research on trees and soil stability (D 5–6).
How do trees help?
In a lot of different ways. For instance, rainfall may cause much less erosion under a closed
canopy of trees because:
- the force with which the raindrops hit the ground can be decreased when their speed
and/or their size are reduced by first hitting the lower branches, foliage and litter;
- the rain reaches the soil over a longer time, some of it dripping off or trickling down
the stem after the storm has passed;
- the quantity of rainfall reaching the ground is less since some of it evaporates directly
off the crowns.
What else do trees do?
- Their shade allows roots to grow and branch repeatedly in the topsoil;
- Their root systems hold the soil together, particularly if several species are present;
- Nutrients are more likely to be recaptured, rather than lost from the ecosystem (D 13).
What should I avoid on steeper slopes?
- Opening up the canopy too strongly (D 51, D 60). On a steep slope, the fall of a single
tree can expose more soil than on flatter ground (D 12).
- Tilling the soil (D 64), except where it is protected by terracing (D 65) or mulch
- Overgrazing, especially by animals that break up the soil surface (D 34).
- Frequent burning, which can degrade sites and kill undergrowth (D 63).
- Damaging the soil with heavy vehicles (D 60–62), or by many animals or people
walking (D 28).
Are there species that are especially useful?
Yes. Besides the introduced colonisers like Gliricidia that establish quickly (D 32), think about
local trees and shrubs that grow naturally on similar sites (D 31). You could also:
- concentrate on species that yield fruits, medicines, gums, etc (D 33, D 37), without
the tree itself being cut down;
- choose species that form a canopy quickly;
- plant ground cover plants to help the trees establish.
Should certain trees be avoided on slopes?
Yes, you could avoid:
- species with large leaves from which large drops of water fall; hitting the soil with extra
- those tree species that lose their leaves for a long time (Manual 3);
- trees for timber (D 36) and poles (D 38) that need to be very straight; and
- short rotation stands for fuel and forage that are cut frequently (D 34–35).
Can terracing help?
Yes, it has been widely used for centuries on steep slopes in the tropics, in order to retain soil,
litter and nutrients and sometimes water on the flatter parts.
A newer system of terracing uses rows of shrubs planted along the contour (D 65), instead of
- enriching logged forest
Isn't logged forest usually converted to farmland?
This often happens, for example because:
- logging roads provide easier access than forest paths (D 61);
- the rights to the land are disputed (D 4);
- farmland is in short supply (D 5); and
- the forest soil may be more fertile than other available farmland.
But it would usually be far better if part was kept as forest, and some trees were also left on
each farm (D 21).
Why should some forest be kept?
- the soil is well protected against the loss of fine particles, litter and nutrients (D 12–13, D 23);
- trees for timber and raw materials (D 36–37) can be harvested again after a time, and
these yields continue into the future (D 62);
- collection of fruits, medicines, forage, firewood, poles, etc, and hunting can continue
(D 30, D 33–35, D 38–39);
- there is a reservoir of tree seeds and genetic diversity for the future (Manuals 1 and 2);
- some forested land remains for cultural, religious, educational and research needs
(D 16, D 42), and for future use.
But people need fresh, fertile land to farm on!
Yes, but they also need to keep forest, both to supply products and also for farming later on.
A mixture of small farms, fallow land and forest could be the best pattern to aim for (D 54).
In each case, soil fertility ought to be maintained and improved rather than degraded, so that
people's future needs are not subordinated to those of the present because of pressure on the
land (D 5).
How can that be done?
By making both farming and forestry sustainable.
What does that really mean?
That what is taken out now does not prevent similar yields continuing in perpetuity (D 4, D 14).
Which parts should be kept under forest?
Here are some guide-lines:
- Keep forest in parts that contain promising immature trees.
- Retain forest on ridges and steeper slopes (D 23).
- Don't remove all the most accessible forest, keep some patches (D 54) near at hand.
- Aim for at least 20% of the total area under forest.
Liberation thinning, directional felling and group planting.
Can anything be done to favour good immature trees?
Yes, a number of things, including:
- adopting liberation thinning in place of selective logging (D 62);
- planning the felling and extraction of big trees to reduce damage to smaller ones;
- sawing logs up within the forest, perhaps with a portable sawmill, and then extracting
- cutting, ring-barking or poisoning trees of poor form that overtop promising trees or
saplings, or desirable seedlings;
- cutting climbers or stranglers like some species of Ficus.
But selective logging brings in money we badly need!
So it may, but what is the true cost?
A more effective alternative is to select which tree to cut in order to favour a continuing supply
of good trees from the stand.
Liberation thinning has been tested in S.E. Asia and S. America, and is one of many systems
by which foresters have assisted the natural regeneration of forests (D 2).
Do they work?
Some of them have been shown to increase the growth of saplings and immature trees under
carefully controlled conditions. Because the stand as a whole may also grow faster, they are now
beginning to be used commercially for sustainable forestry.
In other cases, it has proved difficult in practice to avoid degrading the forest resource when
logging (D 62).
Is that why logged forests need enriching?
Yes, it is one of the reasons. Unless there are plenty of thriving younger trees, enrichment
planting may be needed because:
- many trees have been damaged by felling and extraction;
- there are few younger trees of valuable species present;
- the forest contains sizeable clearings;
- the soil has been compacted by vehicles;
- additional tree species are to be included; or
- the genetic quality of the existing trees needs improvement.
How is enrichment planting done?
Before it becomes too difficult to enter the logged area, the following will be needed:
- an inventory, to estimate what species, numbers and sizes of tree the logged forest
- a sketch-map of the area, showing where there are clearings, patches of saplings,
thriving immature trees, poor quality forest, steep slopes, etc.
These provide a basis on which to choose where enrichment planting is required, and where
farming might be done or small plantations established.
What's the best way to put in the trees?
See sheets D 50 and D 54. There are advantages if planting is done in:
- Lines, because they can be marked out on a compass bearing, and the trees easily found
- Groups, since if they are sited in gaps then one or two trees from each small group are
more likely to survive and grow tall. (This is how forests regenerate naturally after a
tree fall - see D 14.)
Won't I need to open up the canopy for them to survive?
Sometimes no, if:
- the planting area is already fairly open;
- poorer existing trees are to be poisoned;
- shade-bearing species are to be planted, with opening up done later;
- evergreen species are being planted under species that lose their leaves for long periods;
- the planting area is on a steep slope.
- in parts where no trees were logged;
- in dense patches of poorly formed trees;
- where there are thickets of undergrowth;
- where there are many woody climbers shading the area.
It is usually necessary also to cut some of the undergrowth (D 63) when marking lines for
planting (Manual 5).
Which kinds of trees are most suitable?
This varies a great deal from region to region, and also depends on what products are especially
wanted. Examples might include:
- Under moderately heavy shade - shade-bearing species of the dipterocarp, legume or
mahogany families (Dipterocarpaceae, Leguminosae, Meliaceae); and other species that
are locally important, such as Calophyllum, Guaiacum or Triplochiton.
- Under moderate shade - more light-demanding species of Casuarina, Terminalia, etc.
- Open clearings - colonising species or soil-improvers (D 32) such as Acacia, Cassia,
Gliricidia or Ochroma. These might be grown as small, short-rotation plantations or to
restore degraded land (D 22), where they might be underplanted later with other trees
How soon can a forest be logged again?
The appropriate cutting cycle will depend for example upon:
- the latitude, altitude, climate and soil type (D 10–12);
- the growth rates of the trees (Manual 3);
- the rotation or number of years for species to reach the desired size (D 26);
- how much damage was done at the previous logging (D 62);
- how well any liberation thinning and enrichment planting were done;
- how fully stocked the forest is with maturing trees;
- whether there has been any illegal cutting (D 4, D 66).
Cutting cycles for tropical forests often vary from 15 to 30 years. If they are too short, the forest
is likely to become impoverished. If they are too long, the growth rates of the maturing trees
may have slowed right down. Rotations for individual tree species range from 3 – 100 years.
What exactly is savanna?
Any lowland tropical vegetation that is generally dominated by perennial grasses and
interspersed with small trees and shrubs.
How many trees and shrubs?
This can vary a lot; for example between:
- patches of savanna woodland with some grasses and similar plants;
- open woodland with more grasses;
- scattered trees across a grassy area; or
- grasses without trees except in depressions and along streams.
What are the main features of savanna environments?
- a substantial dry season (D 11), in which grassy vegetation often dies back to ground
level, annual plants die and many trees become leafless;
- a somewhat greater natural exposure of the soil to sun, wind and rain than for forest,
and often a tendency to greater fertility; and
- liability to regular fires.
Does savanna have a sharp boundary with forest?
Sometimes yes, especially where:
- there is a sudden change in soil type;
- a large-scale natural disturbance occurred some time ago (D 11); or
- human activity has substantially altered the conditions (D 16).
Often no, with a broad transition zone in which patches of trees become less frequent as one
moves towards the savanna. Here the competition between woody and grassy plants is finely
balanced and can go either way, depending on minor local variations.
Are all grassy areas in the tropics savannas?
Not really. Large areas of what used to be forests have been changed by human activities into
grassland with few or no trees. This has happened to many dry forests, but also sometimes to
those in humid climates.
How did that happen?
Mainly because of increased cutting, burning and cultivation by people, and browsing by
domesticated and wild animals.
Without trees, the forest soil usually becomes degraded (D 22), and the site may be colonised
by grasses or troublesome weeds.
What can I do about it?
- Make a plan (D 4) which avoids overstressing wooded sites;
- Plant suitable trees (D 32) to help restore degraded areas (D 22).
What about the boundary between savanna and semi-desert?
This boundary is strongly controlled by the low amounts and unpredictable timing of rainfall,
and has also been altered by humans.
Savanna is a closed ecosystem, with plant shoots normally filling all the above-ground space,
unlike semi-desert which is an open ecosystem.
Is savanna tree planting easier or harder than in the forest?
In some respects it may be easier in savanna, because:
- preparing the site for planting (D 60–67) can be simpler;
- soils may be more fertile; and
- the rate of soil degradation after exposure may be less.
But successful establishment can be more difficult than in forest areas, because:
- microclimates for newly planted trees are often more stressful (D 50);
- their limited root systems have to support the trees right through the first dry season;
- rainfall is often more unpredictable (D 11);
- there may be little else for herbivores to eat when the grasses have died back (D 15);
- the risk of the young trees being burnt is greater (D 66).
What can I do to help my trees establish?
- Shade/shelter: any existing trees could be kept (D 2) to provide some shade from the
sun and shelter from the wind, or shelter belts could be established first (D 41).
- Choice of species: certain tree species, such as Acacia senegal, Adansonia digitata and
Azadirachta indica, develop very deep root systems that can still take up water even
when the upper few metres are very dry. Some grasses and shrubs are good at stabilising
bare patches of soil.
- Planting stock:
- the trees need to have really good root systems (Manual 3);
- if shoots have recently been cut back or the leaves trimmed, trees may be more
likely to survive (Manual 5);
- trees that have been hardened in the nursery, but not subjected to great stress before
planting, are also less likely to die (Manual 5);
- there is some evidence that plants with root systems that have been inoculated in
the nursery with a suitable mycorrhizal fungus (D 32) establish better in semi-arid
conditions, for instance Terminalia spinosa in northern Kenya (Manual 3);
- Watering and mulch, especially during the first year, could be well worthwhile in some
situations (Manual 5).
What tree species are best?
Indigenous species: look out for local species (D 31) that:
- thrive in areas similar to the planting site;
- are good at protecting the soil and improving fertility (D 32);
- provide several useful products (D 33–40).
Introduced species: you might also include some of these, such as Faidherbia (Acacia) albida,
Eucalyptus camaldulensis or species of Prosopis.
Can crops be grown with trees?
This is often a good idea (D 3, D 21), because:
- it helps to cover the ground while the trees are still small;
- light shade from the trees can help some crops;
- watering, mulching or fertilisers for the crop also help the trees;
- some tree species make more nutrients available for the crops (D 32);
- if crops replace tussocks of grass, the fire risk may be reduced.
It may be best to avoid:
- a lot of soil cultivation (D 64), which could increase the loss of soil (D 12), especially
on slopes (D 23);
- trees that cast too heavy a shade, or extract too much of the water needed by the crop
plants, especially in drier parts of the savanna.
How about animals?
They usually need to be excluded from tree planting areas until they are well established,
- most domesticated animals will damage trees (D 3, D 21), while goats can quickly
destroy them completely (D 27, D 34);
- many wild animals are herbivores (D 15), and some of them could also damage or kill
At a later stage, your trees could provide valuable fodder in the dry season, that would be
carried for animals to eat elsewhere (D 34). Browsing the trees might be sustainable later on,
provided that the quantities eaten are kept well below the amount of available organic matter
that the grasses and trees are together able to produce (D 10–11, D 14).
What other trees might be useful in savanna country?
Those that provide:
- poles for sale (D 38);
- food (D 33), especially during the ‘hungry season’ while crops are growing;
- shade and shelter for crops (D 21), domesticated animals (D 3) and human activities
(D 41), and also to reduce weeds (D 14).
- maintaining mangrove woodland
Why do mangroves need maintaining?
- allow salty, regularly flooded land to continue to be covered by a productive, self-regenerating
- support large fish populations nearby (sometimes also shellfish);
- produce a lot of wood, including fuel for drying and cooking fish.
But, in spite of this, they have often been overcut or otherwise degraded.
How do mangroves survive in salty water?
Some, including Rhizophora, are able to restrict the entry of salt to the roots. Others, such as
Avicennia, excrete salt through glands in the leaves.
The root systems of mangroves are also able to grow in an environment that is short of oxygen
(D 12–13). The peg-roots of Avicennia, and perhaps also the stilt-roots of Rhizophora, allow
exchange of gases above the water level.
Do mangroves vary much in different parts of the tropics?
Remarkably little. Avicennia and Rhizophora are found throughout the tropics.
How can mangroves affect fish?
There are several different ‘producers’ in the food chains (D 10) which support the fish living
where rivers meet the sea, including:
- algae (microscopic green plants) in the water, and larger ones attached to the mangrove
- parts of trees and other plants, washed down from further up the rivers; and
- the mangrove trees growing nearby.
Without the mangroves, there would only be enough food and nutrients for much smaller
populations of fish.
What else does mangrove woodland do?
The trees have several other effects, including:
- colonising mud-flats;
- providing a place where silt brought down by the rivers can settle;
- reducing the force of waves and tides; and therefore
- forming stable habitats for fish and various other animals to live, feed and breed.
Do mangroves do anything else?
Yes, by slowing the flow of fresh water into the sea, they can also limit the penetration of salty
or brackish water:
- upstream in the rivers; and
- into the groundwater that lies beneath unflooded land.
Mangrove woodlands are also efficient at capturing many industrial pollutants (D 16). Their
soils can form insoluble compounds with some toxic heavy metals, and contain bacteria that
can break down excess nitrogen from sewage, etc.
What about wood production?
Mangroves can grow in extensive stands, with trees sometimes up to 25 m tall, though in other
places they are shorter. The rate at which they produce organic matter is often high, and the
wood is used in large quantities by local inhabitants for drying, smoking and cooking fish
(D 35), drying salt and for various building purposes (D 38).
Don't young trees come up by themselves?
Yes they are usually produced in large numbers whenever a gap occurs.
Avicennia will also send up coppice shoots from cut stumps.
Well, what's the problem, then?
If only a few trees are cut, mangrove woodland readily regenerates itself, even in areas where
there is a fair amount of depositing or washing away of the mud.
However, when larger numbers of trees are cut repeatedly, or substantial areas are clearcut,
natural regeneration may fail to replace a stand, because:
- fewer seeds and seedlings are produced;
- water movement is more active, discouraging the establishment of young trees;
- changed water levels have made conditions less suitable for mangroves;
- weeds, such as the large fern Achrostichum in S.E. Asia, have choked the young trees.
What happens then?
Sites can become degraded (D 22) and of little use.
Is planting difficult?
No, mangroves are amongst the simplest of tree species to plant. Nurseries are not generally
needed, as the already germinated seeds can be collected under fruiting trees of Avicennia and
transferred directly to the planting site (Manual 2). The same can be done with Rhizophora,
where the seedlings develop on the trees and are quite large when they fall.
Vegetative propagation has also been successfully tried using air layers (‘pre-rooted cuttings’),
and this could be useful for genetic selection of superior trees (Manual 1).
How deep should they be planted?
Rhizophora - push the seedlings into the mud “as deep as your finger”.
Other mangroves - press the seeds or germinated seeds gently into the mud, without burying
them. Establishment is most likely if the site is not flooded for a week after planting.
Do most of them survive after planting?
It would be reasonable to plant at least twice as many as you need, to allow for losses. These
can be greater than 50% when:
- the climate is more arid;
- the mud is being eroded or deposited more rapidly than usual;
- water levels had previously been altered, for example to grow rice. Here it may be
necessary to break down the old mud walls to allow more water movement; or
- crabs ‘ring-bark’ the one-year old trees.
Which mangrove species is best?
Some species are particularly good at colonising fresh mud-flats, while others are commonest
on well-stabilised land that is flooded less often, or where it is less salty.
It is generally best to use local species, and it is important to plant them in similar sites to those
where they thrive naturally.
Should I plant a mixture?
Yes, but this is often done as a set of moderate sized, pure, single-storey stands (D 52–54). Even
when there is a preferred species, it is best to maintain several others (D 30, D 53).
Do I need to open up the canopy?
Mangroves are quite light-demanding (D 14, D 51), but if the area has recently been cut there
will be plenty of light, and also decreased root competition.
Natural regeneration is usually best if narrow strips or small groups are felled (D 54) - avoid
large clearings and repeated lopping. As with other species, it is normal for many more seeds
to be produced than will establish successfully.
Can I combine natural regeneration and planting?
- If there is plenty of natural regeneration, planting is unnecessary.
- If there is no natural regeneration, planting may be essential.
- When there is patchy natural regeneration, plant up the parts that are lacking in young
How can I manage mangroves sustainably?
The most sensible method is to:
- visit all parts of the area, and make an inventory of what is there;
- divide up the whole area into manageable patches, each consisting of trees in a similar
- estimate how long a rotation will be needed to regenerate a cleared patch into mature
trees (D 24);
- group sets of patches that are not touching each other into coupes, one to be harvested
in each year of the rotation;
- start off with patches that are likely to regenerate naturally;
- later, inspect the natural regeneration, and plant up any parts where the natural
seedlings are few;
- also plant any sections of nearby patches that have become degraded.
What problems might I find?
Problem 1. All the best patches are far from where the wood is needed.
Problem 2. The year's coupe doesn't provide enough wood.
Problem 3. There isn't enough planting stock of the main species.
Problem 4. Few young plants survive.
How can I tackle the problems?
Problem 1 - explain the reasons why sustained management makes sense (D 5, D 10); improve
access (D 61) to more distant stands without risk of over-exploitation; include one or two nearer
Problem 2 - try and avoid clear-cutting two year's coupes in a single year; consider felling or
lopping a few large scattered trees to make up the quantity; regenerate extra parts as well as
the original coupe.
Problem 3 - collect seeds (Avicennia, etc) and seedlings (Rhizophora) from farther afield, using
boats, nets, etc; look for good trees that are fruiting well (Manual 2), and plan to collect under
them at the best time.
Problem 4 - cut a larger number of smaller areas to make up the year's coupe; check species
are suitable for the site; alter planting time in relation to tides and rainfall; plant sooner after
Are there any other valuable plants in mangrove swamps?
In S.E.Asia, one of the most useful is the Nipa palm (Nipa fruticans), which grows especially
along the edges of tidal rivers and produces edible seeds (D 33), a sugary sap (D 37) and leaves
that are used for thatching (D 38).
- community tree planting
Why should ordinary people bother about tree planting?
For many reasons, particularly because trees:
- protect the soil and help to keep it fertile (D 10–14);
- produce many items that people use each day (D 1, D 30–40);
- allow sustainable yields of food and materials (D 3, D 21–24, D 33);
- provide a more pleasant human environment (D 41–42);
- have been cut much faster than they have been replaced (D 16); and
- can often help restore land that has been degraded (D 22, D 32).
Tree planting is now urgently needed by most tropical communities.
Can't just a few people get on with it?
Sometimes yes, if there is no conflict over land-use, the work is well-staffed and other problems
can be overcome (D 4);
Usually no, because the work would not be successful for one or more of these reasons:
- the roles of trees are not fully appreciated;
- there is an expectation that most trees come up by themselves;
- unsuitable planting stock might be used;
- any planted trees need protection from fires lit by other people, and from their goats,
- it is supposed that official bodies will do any tree planting;
- it would be too big an undertaking for a few individuals;
- there are conflicting views on how the land should be used;
- joint decisions have not been taken.
How can everybody be involved?
This is discussed in sheet D 5, but will differ from place to place.
What's the best way of organising it?
This will vary considerably according to the circumstances, but may include:
- linking tree planting to an existing community project, such as maintaining the water
- holding adequate discussions, and making sure that decisions take all relevant views
- sorting out potential and actual conflicts, for example between grazing and tree planting
- having a key person who co-ordinates the discussion and gets the work going (D 4);
- arranging equitably the work of preparing the ground, planting and care of the trees;
- sharing out the produce fairly, perhaps through a co-operative, rather than allowing
only a few people to benefit.
Where should it be done?
- on a site that is generally agreed;
- on land not currently in active use by individuals;
- preferably not too far from people's houses;
- where the chosen species are likely to grow well.
Which species are best?
Sets of local species, or well-tested introduced ones (D 31), that are most likely to provide the
benefits and products that are needed.
What growing system should be used?
- If there are many trees on the site, one might perhaps choose:
- group or line enrichment (D 54), with valuable species (D 36–40) supplementing
existing trees and natural regeneration (D 2); or
- agroforestry (D 3) with crops in a 2- or 3-storeyed system (D 52).
- If there are only a few trees, examples could include:
- several, small, pure firewood plots of different species (D 35, D 53);
- planting trees for forage (D 34) or multipurpose species (D 40);
- renewal of trees protecting water catchment areas or the edges of rivers; or
- agroforestry with grazing animals and additional replacement trees, protected by
fencing (D 39) until well established.
- If the site is bare and degraded, work could be concentrated on:
- establishing soil-improvers (D 22, D 32); and
- terracing steep slopes (D 23, D 65).
When should planting be done?
When rainy weather is expected (Manual 5). If people are too busy then, an alternative would
be to plant carefully from pots, and to water twice weekly until the rains start.
What problems might there be?
There can be many, often interacting with each other, such as:
- lack of suitable plants of the chosen species (Manuals 1–3);
- unusual weather occurring around the time of planting (Manual 5);
- inappropriate choice of species (D 30–42) or growing system (D 50–55), resulting in
rapid erosion (D 22–23);
- unresolved problems interrupting the work (D 4);
- loss of confidence because the first tree plantings had not succeeded (D 5).
Are there special advantages in community tree planting?
Yes, there are several, including:
- there could be less conflict over land-use than if the planting is done by someone else,
and it can fit in better with local needs;
- agreed schemes may provide legal access to collect tree products, in place of disputed
- when many people are interested, there is likely to be more care taken of the trees,
especially protection from fire (D 66) and destruction by animals (D 34);
- if schools are involved, it adds practical application to theoretical education, and helps
spread knowledge, interest and experience. When individual children tend specific
trees, these are more likely to thrive.
Are trees best planted in community schemes?
They are one way of tackling tree planting, but not a universal answer to the problems (D 4).
There are some situations where community schemes have failed, but others in which they have
succeeded, even with incoming groups of people.
The cost of appointing a community education worker to plan tree planting schemes could be
recovered many times over through increased yields of produce for sale and for local use, and
through soil protection and improvement of fertility.
What about existing woodland?
It is very useful if this can be included in the overall scheme, so that the community can:
- still collect produce while the planted trees are growing; and
- work towards sustainable yields, planning the regeneration of each piece of land with
appropriate rotations and cutting cycles (D 24–26).
- planting in towns, parks and reserves
Why plant trees in towns?
Because this can make them a healthier and more pleasant place to live.
How do trees make towns healthier?
Especially by acting as a filter that can clean the air by trapping large quantities of dust, smoke
and soot (D 16).
Does it matter whether a town looks nice?
Yes, it can make quite a big difference, both to the inhabitants and to visitors. In a large city,
trees can turn what would be a ‘concrete jungle’ into a reasonable human environment.
What about villages?
Besides pleasant living conditions, trees are needed to provide:
- shade and sometimes shelter (D 41);
- suitable places for markets and for meetings (D 5);
- cultural reminders and memories of the ancestors (D 42);
- clearcut boundaries (D 21, D 54).
But won't trees just encourage snakes and mosquitoes?
Snakes are usually attracted to open areas if there is a source of food. Trees do not encourage
the breeding of mosquitoes, but you could choose species (D 41) that are unlikely to provide
shelter for poisonous snakes, scorpions or pugnacious ants, and not plant trees too close to each
Which species are best?
See sheet D 41.
What else is important?
- Look out for patches of wasteland and small corners where trees could be planted,
without later having to be cut down to make way for buildings;
- Think about the best spot to plant shade trees for roadsides, courtyards and near
buildings, setting them back a little so that:
- they give shade where it is needed;
- they are unlikely to drop heavy branches or fruits on to roofs or vehicles;
- their root systems will not disturb foundations or drains.
Supposing space is limited?
- Ornamental shrubs can be planted in a nursery into large tubs, with adequate drainage
at the base, and batches of these can be taken in sequence to the site;
- The growing of house plants can be encouraged.
Why bother about parks?
Because they are important for several reasons, including:
- collecting and studying particular kinds of trees and shrubs;
- helping to conserve rare species and let them become known;
- educating people about the importance of plants;
- providing an attractive and shady place for recreation, events, etc;
- exchanging information, seeds, etc with other centres.
Sometimes, parks are the only places in big cities where trees can be found.
What is different about tree planting in parks?
- Only a few specimens of each kind are generally grown;
- Unusual introduced species (D 31) may be tried out to see if they will survive;
- More open planting patterns (D 54) may be used that allow the tree to be clearly seen,
which might not be the case within a closed stand;
- It may be possible to give special attention to each tree.
What about ornamental trees?
Decorative trees and shrubs (D 41) play a key role in making parks attractive. But landscape
architecture is even more important, so that a changing series of large and small, densely planted
and more open patches is seen, as in the Lestari concept in Indonesia.
Do some parks have special aims?
Yes, for instance:
- Botanical Gardens, which grow large numbers of different kinds, sometimes
concentrating on collecting particular groups of trees;
- Wildlife Sanctuaries, where interesting animals and plants can be seen in semi-natural
- Nature Reserves, that are large enough to allow sizeable populations of trees and large
herbivorous and carnivorous animals to be conserved.
How are parks used for education?
In many ways, such as:
- Spreading an understanding of how diverse trees are;
- Showing the links between different plants, animals and humans (D 14–16);
- Demonstrating the differences between built-up, bare and tree-covered land (D 22–23);
- Encouraging schools to undertake tree planting (D 27) and study projects;
- Providing research facilities for students and lecturers (D 6, D 29);
- Permitting the use of plants and animals for study;
- Helping the transfer of seeds and information from one country to another.
How can they help in conservation?
Partly by education, and partly through actually conserving endangered plant and animal
species, for example by:
- keeping an area free of cutting, burning, hunting or building;
- providing a place where planted trees can be well protected, especially when new
introductions or difficult local species are chosen (D 31);
- multiplying plants vegetatively, by rooting cuttings or micropropagation (Manual 1),
so that diverse clones can be replanted where a species has become rare or locally
- collecting seeds and pollen for distribution and storage (Manual 2);
- breeding animals in captivity, followed where possible by releasing young ones into the
But aren't there too many tree species to conserve them all?
Yes, they can't all be grown in parks, but a lot can be done if some larger areas are protected
as reserves. Then the parks can work with a smaller number of particularly endangered tree
Would any trees need to be planted in reserves?
Although the main aim is usually to allow trees to regenerate naturally (D 2), some planting of
local species may, for instance, be needed because:
- some clearing was done before the reserve was established;
- a very large old tree has fallen and weeds might take over the gap;
- there are not enough trees of a rare species for its population to be large enough to
- extra trees are needed as food plants for endangered animals;
- lines of trees are needed on the margins as fire or wind breaks.
- field experiments and demonstrations
How should I choose the area for a field experiment?
This depends firstly on whether it is for:
- an informal trial, to try techniques out locally;
- a formal experiment, investigating, for instance, how trees grow on contrasting parts
of a planting site (D 20–28) or under different growing systems (D 50–54); or
- demonstration plots to show people how trees can be grown well.
What is needed for an informal trial?
- Any accessible piece of ground (D 61) that is available;
- A site whose previous use and present condition are appropriate for the particular trial;
- Ownership that can allow the trial to continue and be assessed.
Does the same apply to an experiment?
Yes it does. In addition:
- extra care is needed in finding a site that is reasonably uniform;
- the area has to be large enough to take all the experimental Blocks and plots, plus any
extra land needed between and around them (D 55);
- its shape should allow a sensible layout of Blocks and plots.
How about demonstration plantings?
It is important to have these where the main differences can be easily seen, even by travellers,
passers-by and people on a short visit.
Remember the old saying, “Once seen is better than ten times told”.
What are the first steps in choosing a site?
- Discussing possible sites with people who know the area well;
- Studying any maps or remote sensing plans that are available;
- Walking the ground, if necessary by cutting inspection lines.
What sort of sites are unsuitable?
Unless they form part of the study, avoid;
- steep or rocky ground (D 12);
- boggy land or swamps;
- very exposed sites (D 11);
- land with very variable soil, present vegetation or previous use;
- areas infested with choking weeds (D 14);
- places subject to frequent damage by herbivores (D 15), pests or diseases;
- situations with a high risk of fire or vandalism (D 66).
Does choosing the site determine the layout of the experiment?
To some extent. For example, if the only planting areas are small gaps in the existing vegetation
(D 2, D 54), or the boundary lines between farms (D 21), the layout needs to be adapted to
But wherever possible try to find sites that allow room for the most suitable layout (D 55).
What are most experiments concerned with?
Field trials with trees commonly investigate:
- plants of different genetic origin (Manuals 1 and 2);
- various types of planting stock (Manual 3);
- contrasting sites, modifications to the conditions or methods of site preparation (This
- alternative ways of encouraging successful establishment (Manual 5).
How are different genetic origins compared?
Usually the aim is either to find out which:
- species of tree (or shrub) might be most appropriate for the type of site (D 20–29), and
the purpose for which the trees are to be grown (D 30–42);
- provenance (origin) of rooted cuttings or seedlings would be most suitable (Manuals 1
- clones or progenies perform well in the area; or which
- mixtures of (1) – (3) may be most useful (D 30, D 53).
Sets of trees that are well-known may be taken as ‘controls’, and a few others tried out in
comparison with them (D 55).
What about kinds of planting stock?
You might for example compare:
- different types and extent of root systems; or
- tall and short stems.
Or you could investigate:
- how much foliage plants are carrying;
- the amount of nutrients they have been given in the nursery;
- whether they have become inoculated with mycorrhizal fungi or nodulating bacteria
(D 32, Manual 3);
- what degree of stress they have been put under.
How can I compare different sites?
This can be more difficult than (A) and (B) above, because:
- the treatments may need to be applied to sizeable plots (D 55);
- the edges of the plots tend to be different from the central parts;
- it is difficult to arrange for such plots to be comparable with each other; and
- all the treated and control plots really need to be replicated, so each occurs at least three
times (D 55).
What might I test?
Here are some examples:
- Contrasting sites:
- ridge-tops against mid-slopes or valleys (D 12);
degraded compared with more fertile land (D 22).
- Modifications to conditions:
- fenced or unfenced plots (D 34);
pruned compared with unpruned overstorey trees (D 3, D 52).
- Methods of site preparation:
- moderate or considerable opening of the canopy (D 51);
- burnt compared with unburnt plots (D 63).
What experiments could help successful establishment?
Survival, growth and establishment are key questions in most experiments. Here are a few
examples where establishment might be especially studied:
- Planting in various patterns (D 54);
- Planting on different dates (Manual 5);
- Use of topsoil or fertilisers in the planting hole;
- Mulching or watering.
But no-one could possibly do so many field trials!
No, indeed not. But these examples may give you an idea of:
- ways of increasing yields without degrading the site;
- what changes to current practice might be helpful;
- how to tackle biological problems that may come up with your trees;
- how not to overlook a key point.
Moreover, a main aim of these Manuals is to explain to growers of trees many of the general
points that scientists, practical researchers and other growers have already discovered.