Introduction | B 1 |
- why grow tropical trees? |
Why are more tropical trees needed?
Because so many have been lost, and too few have replaced them (C 1 in Manual 3). In many sites, natural regeneration of the trees (D 2 in Manual 4) contains relatively few of the local tree species, while in degraded land it can be insufficient to cover the ground.
Does it matter much?
Yes it does! Trees are crucially important for most people in the tropics, because they:
How do trees do that?
The presence of trees is vital in most tropical landscapes, forming the basis for sustained yield management (D 5).
I thought that development involved clearing away the natural vegetation.
Humans have always made openings in forest and savanna for houses, paths, crops and so on. More recently in our history, much larger clearings have been made for towns and cities, roads and airports, mining and factories, plantations and pastureland.
However, the idea that development means that most or all the natural vegetation should be cleared may turn out to be one of the most dangerous fallacies of the 20th century.
Surely that's an exaggeration!
No, I don't think so; for many people's livelihoods and even their chances of survival do actually depend on trees.
Is that really so?
Yes it is, for without them more and more tropical land is becoming degraded (D 22); and
yet
With appropriate numbers of suitable trees, many kinds of land-use can become sustainable.
But it is no use trying to turn the clock back, is it?
No; although people can learn quite a lot from the ways their ancestors devised for managing
the land.
To-day a dynamic shift is needed in ideas and policies, because of the combinations of:
What would need to alter for there to be more trees?
Well, a lot of things are already changing, including:
Can tree planting really help local communities that lack resources?
That is the 64,000 dollar question, isn't it?
I believe it can make all the difference, even for people with little or no money to spare.
What would be needed for that to happen?
Wouldn't it all cost too much, though?
People and small communities can plant trees effectively without having to spend much money, provided that they are enthusiastic and supported. Larger agencies do need to allocate finance to tree-planting programmes, but these could prove a great deal cheaper than:
Is a lot of information needed to plant trees successfully?
Most of the ideas and techniques are straightforward, but information is usually required, as:
Are ideas like those behind this series of manuals?
Yes, and of the videos planned to accompany them, as with Manual 1 and Video 1.
Translations of these are available in Spanish and Bahasa Malaysia, while other languages
are planned. Some users have already been stimulated to write about individual species and
local conditions.
Examples of further sources of information are given in sheets B 51–52.
Are there some other general guide-lines?
B 2 | |
- propagation by seed |
How does raising trees from seeds differ from vegetative propagation?
Both techniques have the same general aim: growing good trees for planting; but
propagation by seeds uses the natural organs of dispersal (B 12), and produces planting
stock that is generally quite variable; whereas
vegetative propagation utilises the ability of parts of plants to regenerate missing organs,
and can give young trees that are much more uniform (A 3, A 11 in Manual 1).
Which method is best?
This depends a great deal upon the species. Since ‘domestication’ of most kinds of trees is at an early stage, or has not started (B 20; C 5 in Manual 3), it is usually sensible to:
What are the advantages of seeds?
For instance, they can often be:
How about rooted cuttings?
Amongst their advantages are:
And what about other methods of vegetative propagation?
Various types of grafting (A 3 in Manual 1) have been widely used for propagating fruit trees such as mango. Recently, a simple approach graft method has been developed in south India for tamarind, the multipurpose species (D 40 in Manual 4). Potted plants of the rootstock are brought close to a selected clonal stockplant, and shoots of similar size grafted together. After 45 days the clonal scion has become attached to the rootstock, and can be separated from the stockplant.
Grafting selected parent trees is also used when making seed orchards (B 23).
What disadvantages are there with seeds?
In some tree species, the seeds may be:
Aren't there also some disadvantages with rooted cuttings?
Yes, shoots for leafy cuttings are:
If poor techniques are used, rooted cuttings may also:
NOTE: use the recommended 10–30 clones for large plantings (A 11).
What are the main points about good seed germination environments?
What should I do about a species I haven't grown before?
For the commonly grown species, you could ask for advice from other growers, look for local sources of information about seed collection, handling and nursery techniques, or consult publications from other countries (B 51–52).
For indigenous trees, however, there may well be no information available. If you obtain some fruits or seeds, the first thing to do is to decide whether to try and store some seed.
How should I make that decision?
If the fruits are fleshy and the seeds are moist (B 33), then it is usually best to sow them all, because they may not retain viability (B 13) if stored. You could for instance divide them into similar batches, and do a small trial (B 48) comparing different germination media or environments, perhaps in trays and seed beds, including methods successful with other species. Use this approach also if the seed-lot is small.
If the seeds are dry when collected, you could try germinating some, and storing the rest in conditions that are favourable for other dry seeds. This gives you more flexibility, as you can:
What if I only get a very few plants?
You might grow most of them on (in large pots or in the nursery soil) as stockplants (A 23– 24), taking cuttings from them to multiply the numbers.
Are there any other ways of growing trees that I could try?
Yes, there are several, including:
B 3 | |
- overcoming seed problems |
Are there a lot of difficulties when propagating from seeds?
Various problems can certainly occur (B 50), but usually few of them are troublesome provided that the seed sources, nursery handling and germination conditions are good.
What sort of problems might come up?
See sheets C 3 and C 60 in Manual 3 for:
But I thought that germinating seeds was easy!
Well, that's what it can often be. These are just the main areas to be aware of, in case they
should apply in your case.
It is important to take them into account, since any one of them might hinder or stop
production of sufficient, good planting stock, even when you had dealt with all the others.
Are some of these problems insuperable?
A combination of organisational problems can sometimes stand in the way of success, but the risks of accidents can be greatly reduced and solutions found to most biological and nursery problems.
Aren't there some kinds of trees that are hard to raise from seed?
Yes, a number of species can be difficult, but you could try:
What sorts of problems might occur with seed supply?
How about difficulties concerning suitable genetic origins?
Linked with the problems listed in (A) and (B) above, are:
Doesn't all this make genetic improvement impossible?
No; although domestication only by seed is certainly slower and more difficult than in short-lived plants. Combining it with vegetative propagation is quicker and easier.
Is it possible to be unintentionally going backwards?
Yes, indeed! For instance, if seed is collected from:
In such cases, inferior, stunted trees are more likely than improved, thriving stands.
Which kinds of stress can damage germinating tree seedlings?
The commonest kinds (C 40–41 in Manual 3) are to do with water, light levels, temperature, nutrients and mechanical breakage.
Too little and too much watering, or high transpiration rate. |
When would young seedlings suffer water stress?
How about light levels?
Stress can happen through young seedlings being grown:
Can the temperature be too hot or too cold?
Yes; the best germination temperatures are generally between 20 C and 30 C.
What about nutrients?
Stress could be caused if the germination medium had:
Certain trees, including tropical pines, may suffer nutrient stress if the germination medium contains no inoculum of a suitable fungus to form mycorrhizas (C 30–31), as these are essential for the trees' nutrition.
And how about mechanical breakage?
This can be caused by such things as wind, heavy raindrops, falling twigs, large leaves, flowers or fruits, or by animals of various kinds.
Are there some general guide-lines about avoiding problems?
But shouldn't trees be treated ‘rough’ if they are to survive?
No! - this is a misunderstanding that often leads to unnecessary problems.
Young nursery trees need to be given favourable environments and then gradually
accustomed to less sheltered growing conditions.
B 4 | |
- direct sowing |
What does direct sowing of seeds mean?
That the seeds (or sometimes the fruits containing them) are sown directly in the place where they are to grow, instead of being raised in a tree nursery and then transplanted.
Isn't that much simpler than having a nursery?
Yes it is; but direct sowing is usually only successful with relatively few kinds of tree.
Why is that?
For most species, tree nurseries are needed because:
But every sort of tree regenerates itself naturally, doesn't it!
Yes it does - under natural conditions or when human disturbance has been slight. However,
of all the many seeds a tree produces during its reproductive phase, only a handful ever grow
into mature trees.
Although replacing one tree by another is enough to perpetuate both the species and the
ecosystem (D 10), it is achieved through the failure of enormous numbers of seeds.
So even a million seeds sown directly might still not produce a good stand of trees!
Exactly! Losses are so high because there are such a lot of:
What kinds of tree can be sown directly with reasonable success?
Those which are colonisers (D 14), rapidly establishing themselves in gaps and small clearings. These species usually produce large quantities of seeds, starting from an early age and continuing regularly (B 10–11).
Direct sowing has been suggested for the following multipurpose species (D 40): Albizia lebbek, Anacardium occidentale, Artocarpus heterophyllus, Azadirachta indica, Cassia siamea, Dalbergia sissoo, Derris indica, Erythrina poeppigiana, Gliricidia maculata, Moringa oleifera, Pithecellobium dulce, Prosopis juliflora, Samanea saman, Sesbania grandiflora, Syzygium cumini and Tamarindus indicus (Anonymous, 1997 in B 52).
Are there some well-tested examples?
But doesn't Leucaena have ‘hard’ seeds?
Yes it does. For best results a suitable pre-sowing treatment (B 13, B 34) should be applied, although some of them may well germinate without it.
Are there any kinds of tree that might get out of hand?
Very occasionally, an introduced species can turn into a troublesome ‘weed’, as for example
Melaleuca quiquenervia in southern Florida.
However, in most cases extra useful trees are welcome, and any unwanted plants can be cut
back.
The edge of a small gap in the forest |
How should I choose just where to sow directly?
How should the ground be prepared for direct sowing?
Ground cover: remove any vegetation (especially troublesome weeds) covering the ground surface, except:
Soil cultivation (D 64): you could choose between:
Try to avoid unnecessary soil cultivation, as it increases the loss of soil by erosion (D 23).
This is especially the case when the soil is cultivated with machinery.
When should I do direct sowing?
In the same season as the sowing of farm crops, when the soil is already moist from early rains.
And how should it be done?
What problems might I have?
Very light or winged seeds. To stop these blowing away, choose a windless day, sow close to the
ground surface, cover at once and then tread lightly over the area.
Larger seeds. It may be easier to make individual small holes with a stick or hand trowel.
Palatable seeds. Seeds that are attractive to birds may need to be quickly covered with extra
soil, sand or mulch. If insect pests are a problem, you could consider pre-treating the seeds
with a copper salt.
Seeds with a tendency to rot. Avoid sowing during very wet weather, and if necessary dust the
seeds with a suitable fungicide (C 45 in Manual 3).
How about looking after the seedlings?
If possible, visit the planting sites from time to time, and check whether watering (if feasible),
weeding or other work is needed.
Later on, the trees might need thinning if some are growing too close to each other.