- introduction: close associations with roots
How do micro-organisms influence trees?
All trees are affected by micro-organisms. Some of them, for instance:
What are ‘close associations with roots’?
Other kinds of micro-organisms in the soil have much closer relationships with trees, entering the roots and changing their structure.
Are they common?
Yes, close associations with woody plants appear to be widespread throughout the world. The great majority of tropical trees are thought to have at least one such association.
How important are they?
They are vital, especially on poor soils (C 62-C,D). Most of the micro-organisms closely associated with tropical trees play a particularly important part in:
NOTE: Many soil-improving trees and shrubs, which can reclaim degraded sites (D 22 in Manual 4), have at least two types of special associations with their roots.
What kinds of micro-organisms form close associations?
But aren't these the things that cause disease?
A few kinds of fungi and bacteria can cause disease (C 45);
Most of them just break down litter, and are not harmful; and
Some species form these special associations, with direct benefits for themselves and their host trees.
What do the useful micro-organisms gain?
But won't that reduce the growth of my trees?
Not necessarily. It is often shortage of nutrients that limits the growth of trees in the tropics, though many other factors may also be involved (D 10–16 in Manual 4). Except where young trees are heavily shaded or unhealthy, plenty of sugars can be available for this ‘trade-off’ with the fungus in exchange for nutrients.
The general principles are becoming clearer about close associations with roots (C 62), but so far only a limited amount of research has been done on specific details.
Are there names for these associations?
Mycorrhizas are close associations between fungi and tree roots (C 31);
Root nodules are close associations between bacteria and tree roots (C 32); and
Actinorhizas are close associations between actinomycetes and tree roots.
What are mycorrhizae like?
Mycorrhizal roots contain fungal tissue and have an altered structure. There are two different kinds:
The fungi forming both types of mycorrhiza also produce:
Can you see ectomycorrhizas quite easily?
Yes, they can usually be spotted with the naked eye or a hand lens. Ectomycorrhizal rootlets are often short and relatively, thick with blunt ends.
Most of the fungi involved form large fruiting bodies (‘mushrooms’) that are easily seen at certain times of year, though connection to a tree is difficult to establish.
Is it harder to see the arbuscular mycorrhizas?
Yes it is, for they cannot be seen with a hand lens, nor do the fungi involved form large fruiting bodies. However, infected roots are very common, and can be detected by using a simple staining technique and then looking at samples under a microscope.
How do mycorrhizas benefit the tree?
Through nutrients and probably water being passed to it from the fungus.
But surely a tiny fungus can't help the growth of a big tree!
Surprisingly it can, because:
In fact, these fungi may be so important that some trees cannot thrive without them (C 31, C 60), and mycorrhizas can even form a substantial part of the root systems of some tropical trees. Equally, many of the fungi cannot thrive without the trees.
Which nutrients are passed to tropical trees by mycorrhizas?
Many kinds of nutrients are probably transferred, but phosphorus is especially important. This is one of the three elements needed in large amounts by trees (C 14), but in tropical soils it is often scarce or locked away in an unavailable form.
How can mycorrhizal fungi utilise ‘unavailable’ nutrients?
What about root nodules?
These are modified, roundish bodies on the roots, a few mm in diameter, in which large numbers of bacteria occur. Such nodules are easily seen and are well known on the roots of leguminous crop plants like beans and groundnuts. They also occur on many (though not all) leguminous trees, particularly those in the families Mimosaceae and Papilionaceae (C 32, C 62-D).
Which nutrient are they mainly involved with?
Nitrogen - the nutrient required in largest quantities by plants (C 14), but often readily lost and not freely available in tropical soils.
How do the bacteria help?
Nitrogen gas forms about 80% of the atmosphere, but in this form it is unavailable to plants. However, bacteria belonging to the group called rhizobia are able to turn nitrogen gas into soluble nutrients. These nitrogen-fixing micro-organisms can therefore add to the total amount of nitrogen that is available.
Can trees get hold of this extra nitrogen easily?
Yes, by soluble nitrogen passing:
So you get a free fertiliser!
Yes, that's right. This is one of several reasons for planting mixtures of different species (D 30, D 53 in Manual 4). If nitrogen-fixers are included, the total amount of nitrogen that is cycling in the ecosystem is increased, which means that other trees and crops without root nodules may grow better, especially on poor soils.
Is that especially true of soil-improvers?
Most of these species, besides having nitrogen-fixing root nodules and mycorrhizas, are quick-growing colonisers that can become established on open or degraded land (D 22, D 32 in Manual 4).
Some examples with confirmed nitrogen-fixing ability are associations with Calliandra calothyrsus, Gliricidia sepium, Inga jinicuil, Mimosa scabrella, Leucaena leucocephala, Sesbania grandiflora and species of Acacia, Albizia and Erythrina.
What about actinorhizas?
These occur as root nodules on the roots of soil-improving trees such as Alnus, Casuarina, Allocasuarina and Coriaria. The micro-organisms belong to a kind called Frankia, and like rhizobia they help the tree to acquire nitrogen.
Research has shown that effective formation of nodules can vary considerably from one provenance of a tree species to another (C 5).
- mycorrhizas: fungi plus tree roots
What have mycorrhizas got to do with tree nurseries?
A good nursery root system is one of the keys to growing and planting trees successfully (C 4, C 11, C 34). If the root systems of the planting stock are already mycorrhizal, the young trees are likely to establish better.
Which kinds of tropical trees produce ectomycorrhizas?
And which trees form arbuscular mycorrhizas?
Around 95% of all tropical forest trees are thought to do so. A few species of Acacia, Afzelia, Eucalyptus, Hopea and Intsia can even form both types of mycorrhiza.
Are there some groups of trees that don't form mycorrhizas?
It is thought that trees in the families Lecythidaceae, Proteaceae and Sapotaceae do not do so.
Does it matter which fungus forms the mycorrhiza?
It often seems to make quite a lot of difference, because particular species of fungi appear to be associated with certain groups of tree species. For example, ectomycorrhizas formed by the fungus Pisolithus tinctorius and the conifer Pinus caribaea have been studied by several workers (see Lapeyrie and Högberg, 1994 in C 62-C).
In ectomycorrhizas, different groups of fungi appear to be involved as the tree gets older.
Is there much variation within one kind of fungus?
Yes, there can be. Some strains of a fungus may form more effective mycorrhizas than others, just like some clones of trees make better growth (A 11 in Manual 1).
There is some evidence that a specific strain of a fungus might combine particularly well with a particular tree clone.
Can I tell which kinds of fungi are present?
Mats of threads can sometimes be seen amongst wet litter or in the topsoil, but these could belong to non-mycorrhizal fungi, and are too fragile to trace far.
Identifying the fungal species in the soil or the tree roots is possible, but it requires detailed microscopic study of roots, threads and spores, and sometimes the culture of individual fungi in the laboratory.
So it's not much use to me!
Yes, it could be.
Why is that?
Although identifying species may be difficult, it is relatively straightforward to:
But how can I make use of that?
How is infection with ectomycorrhizas detected and assessed?
By collecting samples of fine roots, and:
And what about arbuscular mycorrhizas?
Infection is checked by collecting samples of fine roots, treating them with a clearing agent that makes it possible to look through them, and using a coloured stain to show up the fungal cells. The proportion of roots containing fungus can then be estimated under a microscope.
What does one use to inoculate the root systems of trees?
There are four possible sources of mycorrhizal fungi, which use an ‘inoculum’ from:
Which source is the best?
Source (1) is a useful starting point if thriving stands of the tree species are available;
Source (2) is convenient if there is an existing nursery producing good plants;
Source (3), where available, is likely to prove the best way of obtaining ectomycorrhizas that can improve tree establishment, reliably and substantially.
Source (4) - simple methods have been developed for multiplying the quantities of inoculum for arbuscular mycorrhizas. These consist of growing crop plants such as cowpea, millet, sorghum or maize for a few months in a sterilised (C 44–45) and inoculated potting mix in large containers. These enriched soil cultures can then be added to soil mixes as required.
Note: national regulations and international certificates govern the import of soil and roots from one country to another, because of the dangers of transferring pests and diseases.
Inoculating soil with spores from the fruiting bodies of ectomycorrhizal fungi.
Isn't it hard to inoculate the young trees?
No. One just mixes a small amount of the inoculum into the topsoil of seed-beds (C 23) or with the soil mix used to fill seed trays or pots (C 6, C 42). Failing this, it is possible to put a little inoculum into the hole when a tree is planted in the field (Manual 5).
Which method is likely to be best?
Ideally, one would inoculate the germination or potting soil with a mixture of known strains of fungi. Then the young seedlings or rooted cuttings should form particularly effective mycorrhizas early in life, before they are exposed to the extra stresses of hardening (C 47), transport, planting, competition and browsing.
How would I know that inoculation had done anything?
By doing a small experiment (C 7, C 15) in which you potted up comparable plants in the
same soil mix, with and without a fungal inoculation of about 5% by volume.
Both sets would need labelling (C 54) and looking after carefully (C 40, C 48).
For the layout, you could keep the two treatments in similar conditions near to each other, though not so close that spores could easily be transferred by water splashing on to the soil.
Similarly, the containers could be raised clear of the ground to prevent seepage of water from pot to pot.
If the results were positive, you might do a more detailed experiment to compare several sources.
How big a difference can mycorrhizas make?
For ectomycorrhizas, introductions of pines, cypresses and other species into new areas have sometimes failed (C 60) without a first inoculation of nursery or forest soil from the country of origin, followed by transfer of infected soil from nursery to nursery. Increased growth of inoculated pines has also been found in several trials during the first 1–2 years after planting.
For arbuscular mycorrhizas, a suitable local fungus may more often be available, though inoculation can often still be beneficial. For example, in establishment trials with Terminalia prunioides in Kenya more than twice the number of plants given a mixed inoculation in the nursery survived through the first long dry season than in the uninoculated controls.
What other trees have been studied?
Among the kinds of trees that have responded to arbuscular mycorrhizal inoculation are:
Arbuscular mycorrhizal inoculation is standard nursery practice for large Citrus plantations in some countries, but more research will be needed before this stage can be reached with other tropical tree species.
When is inoculation especially needed?
It is likely to be most important when:
Why is this so?
In all these cases the right kinds of mycorrhizal fungi may be absent or in short supply. Their growth and spread are usually interrupted by the removal of many trees, which exposes and disturbs the soil. Other fungi, associated for instance with persistent weeds, may have become common.
Planting introduced colonisers (D 14, D 32 in Manual 4) could help local species to regenerate naturally (D 2 in Manual 4), by restoring numbers of arbuscular mycorrhizal fungi.
When would inoculation be less important?
Collecting wildings and soil.
How long can I keep the inoculum?
It is generally best to collect fresh material each time. However:
What should I do if I run into problems with mycorrhizas?
You might contact (C 53):
Do any nursery procedures affect mycorrhizas?
Restoring degraded land with a pure stand of a soil-improving tree, Acacia mangium.
- nitrogen-fixing root nodules
What are nitrogen-fixing nodules?
They are modifications to some of the fine roots of a tree, in which closely associated micro-organisms live. Generally these are bacteria called rhizobia, or sometimes actinomycetes called Frankia. Both of them have the ability to change atmospheric nitrogen into available nutrient form (C 30).
Aren't root nodules found mainly in annual crop plants?
They have been widely studied in legumes such as groundnuts and soybean, and in temperate zone grazing and crop plants. However, there are 18,000 or so leguminous species, most of which are trees, many of them tropical species.
Which kinds of trees have nodules?
Are those used in agroforestry nitrogen-fixers?
Many of them. For instance Egli and Kalinganire (1988) (C 61-D) list the following for Rwanda: Acacia albida, A. sieberiana, Albizia chinensis, A. lebbek, Calliandra calothyrsus, Leucaena leucocephala, Pithecellobium dulce and Prosopis chilensis in the Mimosaceae; Cajanus cajan, Erythrina abyssinica, Gliricidia sepium, Sesbania sesban and Tephrosia vogelii in the Papilionaceae; with Alnus acuminata and A. nepalensis nodulating with Frankia.
Which legume trees don't form nodules?
Amongst those so far studied, the following are thought not to do so:
Caesalpiniaceae: Bauhinia, Caesalpinia and some species of Senna (Cassia);
Mimosaceae: Adenanthera, and perhaps certain species of Acacia and Parkia;
Papilionaceae: Vatairea, Vataireopsis and Brazilian Pterocarpus.
So are leguminous trees specially important?
Yes, they are. The many kinds that form effective nodules play unique roles through:
Does the planting stock itself need root nodules?
Having nodules on the young leguminous trees is likely to make a considerable difference if:
Research suggests that both the nitrogen content and the growth rates of nursery trees can be increased by root nodules.
How about when the young trees are planted out?
Already having effective nodules should add to the likelihood of planting stock surviving and establishing well. Such trees might grow well even in poor sites, and could soon contribute to the growth and yields of other trees and crops.
Is it easy to tell whether a tree has nodules?
Often they can be seen without difficulty when nursery plants are potted up, or by checking the outside of the root ball. However, especially in the field, you could miss nodules even though the tree does form them, because sometimes they:
Do young trees need to be inoculated?
Like other bacteria, rhizobia are so small that they are spread by the smallest current of air. Some of them are therefore likely to be present in most soil, unless it has just been sterilised (C 44–45). However, inoculation can often be useful, because:
For instance, inoculating with rhizobia has been recommended for Albizia lebbek, Pithecellobium
dulce and Prosopis chilensis.
Adding certain strains of Frankia to potting mixes in the nursery trebled the wood production of Casuarina trees in the field (See Reddell et al., 1989 in C 62-D).
What is the best source of inoculum?
You could try:
How does one do the inoculation?
What about doing an experiment?
A pot plant experiment (C 15) could be very useful, comparing treatments such as:
You might use 15–20 pots for each treatment, giving a total of 60–80 pots. Keep these under similar growing conditions, but slightly apart from each other and raised off the ground to reduce the risk of spreading bacteria from one treatment to another. Avoid fertilisers or a very rich potting mix.
Do trees vary much within a species in nodule formation?
Yes, this seems to be likely. Considerable variation has been recorded between different provenances (C 5; and Manual 2); and also within a provenance. So, if clonal cuttings are available (A 11 in Manual 1), these might be the best experimental plants.
Can trees have root nodules and mycorrhizas?
Yes, some of them do (C 30); for example Acacia nilotica and Leucaena leucocephala. There is evidence that, without the mycorrhizas:
It is likely that the phosphorus collected by the mycorrhizas allows normal production and functioning of the nodules.
A (above) - bare soil in degraded farmland.
B (below) - soil restored 18 months after direct sowing of Leucaena.
- wastes, composts and fertilisers
Why do wastes have anything to do with tree nurseries?
Some wastes can be useful in various ways, for example by providing:
Waste water and rainfall can also be collected or encouraged to remain in the nursery soil (C 24) rather than quickly disappearing by evaporation or run-off.
What sort of old containers could be useful?
Which waste materials can go into soil mixes and rooting media?
What can be used to improve the nursery soil?
As well as the wastes just mentioned, you might improve the soil in transplant beds by:
Fresh sawdust might be used to build up a wet, low-lying part of the nursery (C 23).
Mulching a bed when plants are removed.
How should a mulch be made?
Various kinds of soil covering can be used (see Goor and Barney, 1968 in C 61-A; and Manual 5), including cut grass, other kinds of leaves, sawdust, bark chippings or pieces of black polythene sheet.
Which wastes are useful for shade and shelter?
Cut poles and palm fronds, banana leaves or tall grasses could be useful to put up low or high shade over young trees (C 41).
Pieces of strong clear or white polythene sheeting from the covers of furniture and bedding are often suitable for building poly-propagators (A 31 in Manual 1), or protecting delicate seedlings from rain and wind (C 41).
Might some wastes be harmful?
Yes, they could be. Here are some examples to avoid:
Is there anything that can be done about these things?
What are composts?
They are made by mixing together plant and perhaps animal wastes to rot, in a pile, a large container or a pit (say 1.5 m deep).
The heat given off by the micro-organisms which break down these organic wastes can make the compost hot enough to kill some weed seeds, spores and insect eggs.
How does one make a good compost?
It is important to:
More details can be found in GTZ (1976) and Napier and Robbins (1989) in sheet C 61-A).
When is a compost particularly valuable?
Being rich in nutrients and organic matter, well-made compost is an especially useful addition when:
For any nursery, compost is a simple way of replacing some of the continual outflow of nutrients in planting stock.
Does a mulch do that as well?
Yes, it is one of the reasons for using it. Besides adding organic matter and nutrients, mulching could also help in the nursery by:
Mulch is also very useful when young trees have just been planted out (Manual 5).
Wouldn't mulching encourage young seedlings to rot?
It might do if:
But it would be unlikely to when:
Wouldn't it be simpler just to add fertilisers?
Yes, this can be an easy way of adding nutrients; but most fertilisers:
But surely fertilisers might be needed to get good growth!
Yes, sometimes they certainly could be. However, it is important to realise that fertilisers can have several disadvantages, such as:
What would be good reasons to add fertilisers?
They could be added to:
a - compost
b - old sawdust
c - loamy topsoil
d - coarse sand
Which ways can they be applied?
Whichever method is used, the fertilisers should be balanced (C 14) and not too rich in nitrogen (C 34).
How much fertiliser should I add?
This depends a great deal on what is already being supplied to the young trees in the nursery soil or potting mix, in the water supply and by closely associated micro-organisms (C 30–32). “Little and often” is usually a good motto for applying fertilisers. The doses for tree nurseries are generally less than those recommended to increase the yields of agricultural crops. A rough guide to start with might be:
How do I find out what suits my conditions?
Further information on fertilisers can be found in Carter (1987), Goor and Barney (1968), GTZ (1976) and Pancel (1993) in C 61-A, and from local manuals. If you think that fertilisers might be beneficial in your nursery you could do an experiment (C 15), in which for instance you might compare adding 0, 10, 25 and 62.5 g NPK per m2.
Sample results: average gain in height of experimental trees after 6 weeks.
|NPK added to bed (g/m2)||0.(control)||10||25||62.5|
|Gain in height (weeks 0–6)||3.4||15.6||16.7||13.9|
|Significance of differences||***||n.s.||*|
(See sheets C 55 and C 69 for assessing and analysing your results)
When should I not use fertilisers?
Avoid adding them:
Are there some cheaper substitutes for commercial fertilisers?
Yes, there are several locally available sources, such as:
for nitrogen: the leaves, roots, twigs and small branches of nitrogen-fixing crop plants, bushes and soil-improving trees (C 32);
for phosphorus: bone meal, made by grinding up animal bones; and
for potassium: ash from recent wood or charcoal fires.
How would I use them?
For transplant beds, you could apply:
nitrogen as a compost dug in or mulch laid on the soil; by growing a green manure and digging it in; or by letting leaves of suitable shade trees or hedges fall on it;
phosphorus worked into the topsoil every year or two; and
potassium as a top dressing.
For potting mixtures, an appropriate NPK supplement could be added where needed.
- size of planting stock
How big should I grow my nursery trees?
This depends a great deal on:
Do I want the maximum possible growth?
Generally no, because very vigorous growth in the nursery usually means:
Occasionally yes, for example when:
For enrichment planting of woodland (D 24 in Manual 4) with trees such as Pterocarpus, success has been achieved in India by growing nursery trees rapidly in very rich soil in large containers, and planting them when 3–4 m tall.
Are there any general guide-lines?
Why should there be more roots than shoots?
Because when trees go to the planting site:
How can I control tree size in the nursery?
Primarily by choosing an appropriate sowing date for seeds (Manual 2) and the date of taking cuttings for rooting (Manual 1), so that the young trees should have grown to the desired size by the expected time of planting;
Can't I alter the size just before planting?
Well, this could be done, for instance by:
Isn't that a bit drastic?
Yes it is, particularly reducing the root system just then, for it is the key to successful establishment (C 4, C 11). Aim instead to control the way roots grow in the nursery rather than having to do a ‘hacking’ job at planting time.
Nevertheless, some commonly used types of planting stock do involve considerable pruning at the last minute (C 47).
What constitutes a good nursery root system?
One that is of a suitable size and type (C 4) to sustain the young tree:
Is it mainly a question of size?
Well, the number and length of the roots are two of the important points. If the root system is:
too small, it is unlikely to be able to provide enough water (C 13) or nutrients (C 14) for the young tree to survive, let alone grow well; but if it is
too large, the tree may be difficult to plant properly (Manual 5).
In general, the root system should be as big as can easily be planted well.
What else is important about the roots?
The type of roots present is as important as the size of the root system.
In the great majority of species, good planting stock needs:
Inoculation with closely associated micro-organisms (C 30–32) may also make a considerable difference to how well a root system gets established.
But how can I know what is going on?
Do I need to prune the roots back at this stage?
Sometimes no, for example if:
Sometimes yes, for instance when:
What else could I do if the roots aren't as I want them?
You might try different:
Won't all that mean a lot of work?
Not necessarily. As well as following the suggestions in manuals (C 61-A), you could:
It is worth a good deal of effort to find ways of growing really good planting stock.
How big should the shoot system be?
A sturdy, shortish shoot system may often be the best, even when there are weed problems in the planting site (Manual 5).
What about the leaves?
Aim for a potting mix and growing conditions that give plenty of moderate-sized, dark green leaves. A lot of nutrients in the potting soil (C 6, C 33) generally mean vigorous stem growth and large leaves (C 12), which may:
So big leaves are best avoided, except when:
And what about the buds and branches?
In some tree species, it is important to have a good bud at the tip of the leading shoot, because lateral buds and branches:
In many other cases, specific types of bud are unimportant because:
In species which grow by flushes, it is preferable for the buds to be inactive or growing slowly when the trees are planted out.
Does the main stem need to be straight?
Yes, this is generally best for:
It may not matter for:
How can I keep the leader straight?
In several ways, including:
What other things influence the balance between roots and shoots?
The internal control systems of the tree (C 14) are affected by: