- introduction: juvenility and maturity
What are the various stages in sexual reproduction by trees?
That's quite a long and complicated process, isn't it?
Yes it is. Yet it regularly results in:
In addition, sexual reproduction means that the genetic diversity within each species is mixed around and re-sorted in each succeeding generation (B 20).
Is that important?
Yes it is, because it means that:
Why aren't the seedlings just like the parents?
How do those longer-term changes happen?
Adaptation and evolution occur because:
This process of natural selection amongst varying individuals was proposed by Alfred Russell Wallace and Charles Darwin to replace the previous theory that every species was created much as they are to-day.
Do all kinds of trees flower?
Nearly all species of broadleaved trees and palms produce flowers, except for a small number
that reproduce only vegetatively.
Most coniferous trees, such as pines, Agathis, Araucaria, Callitris and Cupressus form cones. The ovules and seeds of Dacrydium and Podocarpus are borne singly.
Some species produce separate male and female trees, while others have flowers in which either the male or the female parts are sterile.
Examples of flowers and cones.
Are flowers and cones generally easy to see?
Sometimes they are so striking that they can hardly be missed;
Commonly it is fruiting which is more obvious than flowering; and
Often the flowers can be missed, because they may be:
Why can't young trees flower?
In most species they cannot because they are still ‘young’, or juvenile. During this period, survival and vegetative growth (C 10–15 in Manual 3) predominate.
How long does the juvenile period last?
This stage, during which flowering is unlikely or impossible, can vary from 6 months to 40–50 years, and is often about 15–30 years.
For instance Cola lizae in Gabon is reported to have to exceed 15 cm in diameter at breast height before it can flower, while Swietenia macrophylla in Bolivian rain forest needs to reach 70 cm.
But how can I tell whether my trees are juvenile or mature?
If they have started forming considerable numbers of flowers, they are mature; but
If they are producing only vegetative shoots, it is difficult to know, since they could either be juvenile; or alternatively mature but not flowering because:
Supposing I just take seeds from those trees that are flowering anyway?
This is what people often do, but there can be a big drawback-one has given up on selecting the genetic sources most suitable for one's purposes (B 3, B 20).
What's wrong with relying on those that Nature selects?
With a more or less natural stand, it might not matter too much, provided seeds were taken for local use (B 21) from a sufficient number of parent trees (B 22); but
When collecting from disturbed stands or planted trees, several important problems can occur, including:
But what if I want my trees to produce a lot of flowers and fruits?
Then it is more likely to be sensible to select parent trees that flower early.
If, on the other hand, you want to select for vegetative characters, those parents may well be unsuitable, because their inherent tendency may be to divert both energy and growing shoot tips (C 10, C 12 in Manual 3) towards reproduction.
Maturity is hastened by having well spaced trees with plenty of sunlight, deep soil and weed-free ground.
Can I do anything to make trees flower sooner?
Yes; if possible grow the desired parent trees as rapidly as possible, and allow plenty of sunlight to reach the crowns. You could also try a specific flower-inducing treatment (B 14) - some of these can also hasten the onset of maturity.
Will plenty of fertiliser make a tree flower?
Generally no, for there is some evidence that adding a lot of nutrients promotes vegetative growth rather than reproductive activity.
However, a good nutrient supply and growing conditions in the nursery (C 7, C 33 in Manual 3) may help a tree to grow big enough to flower more quickly.
Could I propagate desirable genetic kinds vegetatively?
Yes, this can very often be done, either by:
- from flower initiation to pollination
Where do tropical trees form their flowers?
Frequently towards the tips of vigorous branches in the crown of the tree;
Often on smaller side branches there;
Sometimes from clusters of buds in the bark of the main stem and larger branches, as in Cacao (cocoa), Durio (durian), Nephelium (rambutan) and Parmentiera (candle tree);
Occasionally at the tips of the main stem and replacement leading shoots, causing repeated forking, as in Tectona (teak).
How often does flowering happen?
This varies a great deal. Once they are mature, trees may flower:
But isn't there usually a flowering season?
Yes, flowering is generally more common in certain months, even when the climate is rather uniform. In the seasonal tropics, a peak in the flowering of stands of indigenous species tends to become more marked as the dry season intensifies.
Sometimes evergreen trees tend to show more flowering at the end of the dry season, whereas most deciduous species which lose their leaves for a month or more may flower during the dry season.
When does the process of forming flowers actually start?
The initiation of flowers starts in specific shoot tips either:
Type (A) - a few weeks before they can be easily seen; or
Type (B) - 2–10 months earlier than that.
Type (A) is found for instance in free-flowering woody ornamentals like Bougainvillea and
Hibiscus. The flowers are initiated and expand without delay, along with the new leaves (C
10, C 12 in Manual 3);
Type (B) is probably more common. Here the flower ‘initials’ remain very small within inactive buds for some time before they grow larger and open. These flower buds may or may not also contain foliage leaves
So which are the months when these changes happen?
The time at which flower initiation occurs is still unknown for the great majority of tropical trees. This hampers research on stimulating flower formation (B 14).
Is there a lot of variation in the numbers of flowers produced?
Yes, this is very typical, except sometimes in clonal stands, because of interactions between:
When individual trees form large numbers of flower buds, their diameter or height growth may be less that year. However, large numbers of flower buds are often shed before they open.
Do all the trees of a particular species flower together?
Sometimes the timing of flowering can be very precise, with many trees of a species flowering
Often it is more variable and unpredictable. At times one tree may flower while others of the same species nearby may remain entirely vegetative, and it is not uncommon to see flowering on one side of a tree while the rest of the crown has none.
If provenances (B 21), for example of pines, are grown far from the latitudes or altitudes of their origin, pollen shedding and female receptivity may not be synchronised.
Is the period of flowering generally short?
Yes it is. For instance, in six species of Shorea in Malaysia some flowers were open for 15–25 days, but the main flowering periods were only 5 to 11 days long. This means that:
Other kinds of tree produce a succession of flowers over a period of weeks or months, which makes it easier to observe flowering and do any pollinations.
How long do individual flowers remain receptive?
In many trees, they only last for 24 hours or less; but
In other species, unpollinated flowers may remain receptive for several days.
One can usually recognise that a flower or cone is receptive because:
How does the pollen reach the female parts?
Why do these animals bother to transfer pollen?
Many flowers contain nectar, a sugary ‘reward’ that the animal pollinator is seeking, and some of them may collect pollen. Pollination usually happens as a side-effect of finding food.
Does each tree have a specific pollinator?
Trees can be:
The time of flower opening (often early evening or early morning) quite often coincides with the activity of a specific pollinator.
Can I do anything to encourage pollination?
With large trees, it is usually very difficult trying to work from long ladders.
With smaller trees, you might consider:
To avoid killing insect pollinators, avoid using chemicals nearby against weeds or pests (C 44–45 in Manual 3).
If receptive flowers are pollinated, will I definitely get good seed?
There is a better chance that you will, after this important step, but there can still be several kinds of losses (see B 3, B 12–13, B 24).
Are there always two parent trees?
In most species the seed comes from cross-pollination of one tree by another (B 23). Exceptions are:
Isn't self-pollination bound to be more likely to happen?
It would be, except that selfing is reduced in most tree species by one or more of the following processes:
Don't all these things cause a lot of problems?
Well, in many cases reproductive activity just happens by itself, without any trouble. However, in some species flowering problems can certainly make it difficult to:
- fruit and seed formation
There are a lot of different kinds of fruits, aren't there?
Yes; the main distinction is into:
Similarly, the ripe female cones of coniferous species can dry out or remain fleshy.
What makes a tree set fruits?
Pollination while the female parts are receptive (B 11) is usually essential; and then
Fertilisation (B 10) is generally the next trigger for the setting of fruits and the development of the seeds inside.
However, most of the flowers on a tree may not set fruit, for one reason or another. For instance, only an average of 1–10% of the flowers on tropical leguminous trees are thought to set fruit, and the proportion is even lower in mango.
Is there anything I could do about that?
Drought at this point can sometimes reduce fruit set even further, so you could consider watering selected trees during flowering and fruiting, if that is possible.
Don't a few trees set fruits without pollination?
Occasionally, as in seedless varieties of Citrus, individual clones have been selected and propagated in which fruiting is not dependent on fertilisation.
After fruit set, what happens next?
Usually some fruits increase in size while many others may drop off. Inside the developing
fruits the embryos within the seeds are taking shape, forming a root tip, shoot tip and cotyledons
Fully-grown fruits vary greatly in size, from species where they weigh only a few grams to those of several kilograms, which can be dangerous if they fall on you unexpectedly.
That's when they have ripened?
Yes. After growing for a period varying from a few weeks to many months:
The fruits and seeds are now ready for dispersal.
How do they get distributed, under natural conditions?
What is the best time to collect fruits and seeds for the tree nursery?
Keep a close watch on the parent trees (B 21–22) which you have chosen for collection.
In each case, it is a question of striking a balance between collecting (B 31):
too early, when seeds might not yet be quite ripe, while those shed early from the tree could be the empty ones; and
too late, when the fruits may have been eaten or the seeds all shed.
And how should they be handled?
See sheet B 32. For fleshy fruits, the seeds usually need to be separated from the fruit tissue
and washed clean. It may sometimes be possible to keep the fruits for a few days, but they
usually go bad quite quickly.
For most dry fruits and female cones, open them up if necessary, and then shake or sieve them.
For dry fruits which do not open, treat the fruit as if it were a seed.
How about storing the seeds?
Not all seeds can be stored! (see sheet B 13, B 33). For instance:
Most dry seeds (and dry fruits) can be stored, for a shorter or longer period, in suitable conditions (B 33).
Are there some useful guide-lines about handling fruits and seeds?
Remember that the seeds are alive! The likelihood of good germination may be decreased if they are subjected to stress, for example by being:
Why does the natural reproduction of trees involve such a lot of wastage?
In one sense it does seem like waste, because a lot of energy, materials and growing points are
used up: and yet only a tiny fraction of the ovules formed, and still fewer of the pollen grains,
ever produce a growing seedling; but
In another way maybe it isn't, since sexual reproduction usually results in the perpetuation of the species, through all the difficulties of pollination, herbivores eating the fruits and seedlings, and competition from other trees.
Can I reduce the wastage when growing trees for their fruits?
Yes, to some extent, for instance by:
And what if my trees are for forage, firewood or timber?
Then you want reproductive activity only when seeds are needed, so you might for example:
- seed viability and dormancy
If my seeds don't come up, how do I know what's the reason?
That is one of the tantalising questions about seeds, since:
if they germinate quickly and well, there's no problem; but
if they don't, this could be for several quite different reasons. They might be:
Why would seeds be empty?
Does ‘viable’ just mean the opposite of ‘dead’?
Yes, for most practical purposes; although sometimes embryos could still be alive but their food reserves might now be insufficient for germination.
Living seeds can lose viability because they:
What might make seeds I have sown quiescent?
Can you explain what is meant by seed dormancy?
Live seeds are said to be dormant when few or none of them germinate within a reasonable time when placed in good germinating conditions (B 40).
What stops dormant seeds from germinating?
There can be many different barriers to germination, including:
Type 1 - hard coats around the fruit or seed, which prevent the entry of water and/or oxygen;
Type 2 - fruits or seeds which contain inhibitors of germination;
Type 3 - seeds that have a requirement for light; and
Type 4 - seeds that have special temperature requirements.
Why does this happen?
Under natural conditions, competition between all the different species is intense (D 14 in Manual 4). In those with seed dormancy, germination will tend to:
Both A and B tend to increase the chance of natural seedlings surviving, but in the tree nursery dormant seeds usually require an appropriate pre-sowing treatment (B 34) for rapid, full germination.
How can I get ‘hard’ seeds to germinate?
By making the hard coat penetrable without killing the seed. Techniques include:
What about seeds that contain inhibitors?
These are more difficult to identify than hard seeds, except in a well-equipped laboratory.
However, ‘breaking’ the dormancy by decreasing the level of inhibitors can easily be done:
for dry fruits, by storing them dry at temperatures of 25–40°C for a few weeks or months;
for fleshy fruits, by:
How about seeds that need light?
Breaking the dormancy of light-requiring seeds is very simple:
It isn't easy to give special temperatures, is it?
Well, the most common requirement is for a hot day and a cool night. So you could try and increase the diurnal fluctuation of temperature (D 11) by:
As soon as the seedlings begin to emerge, remove the polythene and put up the usual shading, or move the seed trays to a shadier place;
Do most kinds of trees have dormant seeds?
No. Many tropical forest trees have seeds that germinate promptly, for example cocoa, dipterocarps, eucalypts, Lovoa, Pinus and Triplochiton.
Which species do show seed dormancy?
Type 1 - many leguminous species and teak;
Type 2 - for example oil-palm, Azadirachta and Gmelina;
Type 3 - many colonisers, including species of Cecropia, Harungana, Milicia (Chlorophora), Macaranga, Musanga, Piper, Trema, and probably Nauclea and Terminalia;
Type 4 - Didymopanax, Ochroma lagopus, Heliocarpus donnell-smithii, Phytolacca icosandra and probably other colonisers.
Do seeds ever have more than one kind of dormancy?
Yes, sometimes. For instance Trema can require light and a fluctuating temperature, while Ochroma lagopus has a hard coat as well as needing fluctuating temperatures.
Can dormant seeds be stored longer than non-dormant ones?
Sometimes yes: for example a seed-lot with hard seed coats will usually keep best if they
have not yet been pre-treated; but
Sometimes no, for a seed-lot that contains rotting pieces of fruit will probably go mouldy and the seeds die. (See B 30–34 for more information on seed handling.)
- stimulation of flowering
Won't natural flowering provide all the seeds I need to collect?
Sometimes it may do so; but often it does not, because for instance:
What is the chief advantage of being able to stimulate flowering?
The most important point is that then you can select which parent trees contribute to the seeds you are using (B 22–23), instead of having to rely on anything that is available, or on buying seed (B 24).
Maturity is hastened by having well spaced trees with plenty of sunlight, deep soil and weed-free ground.
When might I particularly want to induce trees to flower?
Can any tree be made to flower?
Is much known about how to stimulate flowering?
Reproduction in most trees is very difficult to study, because of their long life-cycles, large size when flowering starts, and great variability. Relatively little is therefore clearly established about the many factors that could be involved in the onset of maturity, and in the initiation of flowers. However, there are now opportunities for ‘miniaturising’ experimental research which are likely to increase our ability to induce flower formation.
What sorts of factors might play a part?
There appear to be two groups - those which provide a:
There is also evidence that damage to the main stem bark or the root system may sometimes induce flower formation.
|Ringed main stem.|
Can any of these be of practical help?
Yes they can. For example:
Which method is most likely to work?
Bark ringing (or girdling) is a centuries-old way of stimulating flowering in trees. It involves removing a narrow, complete ring of bark from a woody shoot, and has been shown to be effective in many tree species of the north temperate zone, and in Terminalia ivorensis, mango and rubber.
But doesn't that cut off the shoot from the root system?
Yes it does - partial ringing (two overlapping part-rings) is often less effective, though it is sometimes used because the wounds heal better. However even in complete ringing:
|Positions for bark ringing of a woody shoot to induce flowering.|
How should I set about ringing some shoots?
The best approach might be to plan a simple experiment on some suitable trees that could be mature (B 10) or nearly so, and are growing in reasonably bright conditions. First decide whether:
A first experiment might use 12–20 pairs of branches or whole trees (see advice on experiments in sheet D 6 in Manual 4).
When should I do the ringing?
Unless the shoots are growing continuously, try and choose a season when new leaves are just beginning to emerge. This could be the most likely time:
How do I make a complete ‘ring’?
5 mm for shoots of a diameter of about 8–20 mm;
10 mm if they are 20–40 mm thick; and
15 mm for those over 40 mm in diameter;
What else should I do?
|Ringed branch with support for weakness caused by ring.|
Won't the ring be a point of weakness?
Yes it can be, especially with slender shoots, unless it heals over rapidly and completely. You could support:
Support is particularly needed if treatment is effective and there is a heavy fruit crop.
Can ringing work with trees that are still juvenile?
It can do so in certain north temperate tree species, so it might be worth a try.
A more likely technique could be accelerated growth. Growing a young tree as fast as possible sometimes encourages it to start flowering sooner. So you could try:
But I've heard that stress helps trees to flower!
Yes, this is often claimed, although it is unlikely to apply to all kinds of stress (C 41 in Manual 3). An interesting example is the formation of numerous flowers on young rooted cuttings of Triplochiton scleroxylon growing slowly in relatively small pots in glasshouses in Scotland under long daylengths (see Leakey et al., 1981 in B 51).
Could two methods be combined?
Yes. Accelerated growth is a treatment designed to shorten the juvenile period, after which it might be that different conditions (such as ringing or nutrient stress) might perhaps induce the formation of flowers.
Are there any kinds of trees that are easy to stimulate?
Yes, cypress trees (members of the family Cupressaceae, such as Cupressus and Callitris, can be readily and reliably induced to form large numbers of male and female cones by treating them with small amounts of the plant hormone gibberellic acid (GA3).
This treatment is very effective on small or large shoots, and even in young trees.
|Injected tree with plant hormone to stimulate flowering.|
How is it done?
Calculations for treating 18–20 shoots with GA3.
|Length of branch||Height of tree||Recommended dose of GA3 per shoot||Weigh out GA3||Dissolve in alcohol and make up to||Treat shoot with||Minimum size of the 2 holes;|
* 4 holes needed
Will GA3 work in other trees?
No, it only induces coning in conifers of the Cupressaceae and Taxodiaceae.
A mixture of GA4 and GA7 sometimes stimulates cone formation in pines, but the effects are not as pronounced and reliable as for the cypresses.
In broadleaved trees, gibberellins can even prevent flower initiation.
What about other chemicals?
Paclobutrazol (an inhibitor which blocks the natural formation of gibberellins) has been
reported to increase flowering in avocado, eucalypts and mango.
Ethephon (which releases ethylene gas in plant tissues) tended to encourage female flowers on male trees of Carica papaya.
How low on trees can fruits form?
Depending on the species, the genetic origin and the site, this can vary from one to around forty metres. In one study of four tree species in Gabon (see Voysey in B 51), the lowest fruiting branches averaged between 6 and 15 metres from the ground, with individual trees ranging between 2 and 22 metres.
Can I expect the fruiting height to be reduced by these treatments?
Ringing main stems tends to keep the fruiting height lower, because the rate of height growth is usually reduced. Moreover, branches arising below the ring may grow into replacement leading shoots, which can then in turn be ringed.
Gibberellic acid treatment means that you can choose the height at which cones are formed. If applied to the upper part of the tree, the whole crown could be removed when the many female cones are ripe but not yet open, encouraging lower branches to form replacement leaders for later injection.
Is there any other way of avoiding having to climb high up?
Lower fruiting heights have been achieved in Nigeria (see Okafor in B 51) by combining;
Aren't some fruit trees grafted on to special rootstocks?
For apple trees, a series of clonal rootstocks were developed several decades ago, and specific pairings of these with particular clonal scions are now recommended. Besides still greater uniformity, these also allow choices as to vigorous or reduced vegetative growth, and of other characteristics. Clonal rootstocks have only been produced for a few commercially grown tropical fruit trees, but seedling rootstocks are likely to be quite adequate for most other species.
If flower buds are stimulated, shall I get seeds?
It is a big step towards that aim, although ‘wastage’ can still occur, as with natural flowering (B 12).