Sexual reproduction in trees

What are the various stages in sexual reproduction by trees?

1. Maturation: the parent trees develop to the stage in life when they can reproduce;
2. Flower initiation: some of their growing points form flowers (B 11) or cones;
3. Flowering: these expand and open;
4. Pollination: pollen from the male parts is transferred to the female parts;
5. Fertilisation: a pollen grain germinates and forms a pollen tube that penetrates the ovule, where a male gamete fuses with a female gamete, forming a single new cell;
6. Fruit and seed development: this cell develops into an embryo, inside a seed, inside a fruit (B 12);
7. Dispersal: the fruits ripen and the seeds become detached from the seed parent (B 13) and;
8. Germination: seeds grow into seedling trees (B 40).

That's quite a long and complicated process, isn't it?

Yes it is. Yet it regularly results in:

1. natural regeneration of nearly all the different species within an intact stand (D 2 in Manual 4);
2. spreading of colonising trees into farmland and other clearings; and
3. the opportunity to collect seeds of many species for artificial regeneration.

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:

1. the progeny are roughly, but not exactly, like the parents; and may sometimes differ considerably from them; and so
2. there is quite a lot of variation amongst which to select (B 21–24); and
3. the species can become adapted to altered conditions over very long periods of time, and might even evolve into something substantially different.

Why aren't the seedlings just like the parents?

Because:

1. the genetic information is re-sorted before the gametes are formed, with some new combinations being formed;
2. only half the genetic potential from each parent goes to form a seed;
3. abrupt changes in inherited characters happen occasionally, called mutations; and
4. the seedling grows in a different environment from its parents.

How do those longer-term changes happen?

Adaptation and evolution occur because:

1. some progeny are by chance better fitted to the prevailing conditions than others: and
2. these are more likely to survive and produce pollen and seeds than other progeny.

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:

1. quite small, or shedding their petals after only a short time;
2. produced at irregular times, and not by all trees of the species simultaneously;
3. not formed by young trees, and usually occurring high up in older ones.

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:

1. they are of a species and genetic origin which only flowers occasionally;
2. it is the wrong time of year;
3. the season when flowers can be formed may have been unfavourable (B 11, B 13); or
4. other trees above may be casting too heavy a shade for them to initiate flowers.

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:

1. the most favourable parent trees may already have been felled;
2. nearly all the trees of the desired species may have already been removed, so that any available seeds might be inbred (B 22) or empty (B 13); or
3. the characters of the individuals that flower first, and most heavily, may not be appropriate for what you want to grow your trees for.

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:

1. felling some superior trees, and rooting leafy cuttings taken from the juvenile coppice shoots in a poly-propagator (A 21, A 31 in Manual 1);
2. multiplying seedlings from small seed-lots into clones, or testing larger seed-lots before doing so (A 10–13); or
3. grafting mature, potentially flowering shoots on to seedling rootstocks (A 3; and B 24).

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:

1. continuously in a few species, including Guiera senegalensis, Rhizophora mangle and Trema orientalis;
2. repeatedly, two to four times a year, in a few others, such as Ficus sumatrana;
3. regularly at about the same time each year, for example in Cedrela, Gmelina and teak;
4. at irregular intervals, which is quite common; and sometimes
5. synchronously with others of the same species every 2–10 years, as in dipterocarps and Triplochiton scleroxylon.

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:

1. genetic differences between individual trees in flowering tendency or sex;
2. varied local environments in which they are growing; and
3. seasons that are more favourable to flower initiation, or less so.

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 simultaneously; but
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:

1. regular checking is needed if flowering is not to be missed; and
2. any cross-pollination (B 23) has to be done quickly, while the female parts are still receptive.

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:

1. it has opened;
2. pollen is being shed, or can easily be removed; andlor
3. the stigma or other female parts are displayed.

How does the pollen reach the female parts?

Generally by:

1. wind carrying it from one flower to another, as for example in conifers and in some of the tallest trees in the tropical forest;
2. insects such as bees, butterflies, moths and flies transferring it (the commonest way);
3. birds such as humming-birds or bulbuls visiting the flowers; or
4. bats doing the same, for instance in Ceiba pentandra and Freycinetia.

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:

1. Non-specific, such as Xerospermum noronhianum, with a wide range of pollinators;
2. Usually specific, like Ceiba and several species of Shorea;
3. Completely dependent on one species of pollinator, as for example some figs, which are only pollinated by one type of wasp.

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:

1. using a step ladder to collect pollen and apply it gently to receptive female parts with a very small brush (B 23);
2. taking small, free-flowering shrubs growing in pots into a greenhouse or other sheltered place to do specific pollinations; or
3. in certain species, trying to stimulate a greater pollen supply (B 14).

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:

1. self-pollination, where the pollen comes from the same tree, or from vegetatively propagated trees of the same clone (A 11 in Manual 1);
2. without pollination, in a few species where fertile seed can be formed by the female flower alone.

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:

1. Male parts shedding pollen a day or two before, or after, the period when the female parts of the same flower are receptive;
2. Male flowers or cones being produced in another part of the tree from the female parts;
3. Male flowers or cones being formed on one tree and female parts on another; and
4. Inability of pollen from the same tree (or clone) to produce viable seeds (self-incompatibility.

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:

1. plan when and where to collect the seed needed for the nursery (B 3, B 24);
2. be able to take seed from selected trees or stands (B 21–22); or
3. get ahead with a tree breeding programme (B 23).

There are a lot of different kinds of fruits, aren't there?

Yes; the main distinction is into:

1. fleshy fruits, where the fruit tissues remain alive and juicy when they ripen; and
2. dry fruits, in which they dry right out and the cells die.

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 (seed leaves).
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:

1. fleshy fruits generally become softer, change colour and may become palatable;
2. dry fruits and female cones lose water and may split open.

The fruits and seeds are now ready for dispersal.

How do they get distributed, under natural conditions?

1. By simply falling straight down to the ground;
2. By being carried somewhat farther because the fruits or seeds have ‘wings’ that make them spiral obliquely;
3. By being eaten by bats, birds, monkeys or other kinds of animal, taken from the tree or the ground; and the seeds being scattered in their droppings;
4. Through being taken and buried by certain animals; or occasionally
5. Through being carried away in rivers or the sea to somewhere the seeds can germinate.

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:

1. the moist seeds from fleshy fruits usually die if they are dried, and so need to be sown without delay;
2. some kinds of seeds may be already germinating within the fruits, as for example cocoa, many dipterocarps and Lovoa trichilioides. These need to be handled very carefully, and transferred to a good germination medium at once, or collected as wildings (B 44);
3. in a few species, such as Rhizophora, the seeds even develop into seedlings while still on the mother tree, and can be planted directly where they are to grow; but

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:

1. put in hot water, exposed to the full sun or left in a closed vehicle;
2. taken from a warm place and suddenly put in a refrigerator or deep freeze; or
3. dried too rapidly.

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:

1. choosing genetic origins that flower early and regularly;
2. planting at a spacing that is wide enough for the crowns to be well-lit, but close enough for pollination to be easy;
3. assisting natural pollination by collecting and applying pollen (B 23);
4. scaring off birds and monkeys; and
5. watching out for pests and diseases.

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:

1. select genetic origins where flowering is delayed and sparse, but vegetative growth or stem straightness is above average, and propagate them vegetatively;
2. plant at a spacing and in a planting pattern (D 54 in Manual 4) that will maximise leaf and stem production; and if possible
3. use a flower induction technique (B 14) on the best parent trees (B 22) when seeds are needed, and do some assisted pollination if the flowers are accessible.

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:

1. empty, never having contained any embryo (B 10);
2. no longer viable, since the embryo has died;
3. slow germinators, taking months to come up (B 40);
4. quiescent, because the conditions are unsuitable for germination; or
5. dormant, with specific barriers preventing germination.

Why would seeds be empty?

1. Because there was little or no pollen produced while the ovules were receptive (B 11);
2. As there were few pollinators around at the time;
3. Since unfavourable weather conditions hampered pollination and/or fertilisation (B 12);
4. Because there were genetic barriers to fertilisation and/or embryo development; or
5. Since seed insects had eaten the embryos (B 30).

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:

1. have a very short natural life-span unless they germinate, as is the case for many non-storing, moist seeds (B 12);
2. were storable, dry seeds, but were kept in unfavourable conditions (B 33);
3. have been stored for so long that they have reached the end of their capacity to survive; or
4. were damaged by seed pests or diseases (B 30).

What might make seeds I have sown quiescent?

1. Dry conditions - if they had not been soaked or taken up enough moisture (B 46) from the germination medium (B 42) to become fully hydrated, or if they had dried out again;
2. Temperature range - below about 15°C and above around 35°C, tropical seeds may not germinate.

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:

1. be spread out over a longer period; or
2. occur in response to a change to more favourable conditions.

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:

1. shaking smaller seeds with sharp grit and a little water in a closed container, to scratch the coats;
2. rubbing larger ones on a file or sandpaper, or making a very small nick with nailclippers or scissors, and then soaking for a few hours in water (B 46), as in Albizia zygia and Leucaena; (NOTE: do this far away from the small scar where the seed was attached, or the root tip may be damaged);
3. slightly cracking the fruit wall, as reported for Calamus deeratus and for teak, using a vice.
4. placing the seeds in a small amount of almost boiling water and leaving until cool (e.g. Acacia, Albizia, Cassia, Ochroma, Prosopis); or putting in hot water (80 °C) for 2 minutes (e.g. Leucaena); (NOTE: these treatments soften the coat, but the seeds will die if they stay very hot for more than a short time); or even
5. putting in moderately strong hydrochloric or sulphuric acid for a few minutes, and then washing the seeds repeatedly. NOTE: Wear eye protection and gloves, and avoid splashing acids - they are dangerous!

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:

1. separating the seeds from most or all the fruit pulp and washing them thoroughly (B 32); and
2. leaving them in cold water in a bucket for a day or two, changing the water twice a day to stop it going foul; or if necessary
3. treating the seeds with hydrochloric acid for 1–2 hours. This replaces passage through the gut of an animal, which quite often breaks dormancy in nature. See warning in (e) above.

How about seeds that need light?

Breaking the dormancy of light-requiring seeds is very simple:

1. cover them with just a thin layer of a light-coloured grit or coarse sand (B 43), so that the seeds are not in the dark; and
2. shade the seed trays or nursery beds with pale or light brown material (B 41), and not green foliage, because the light quality can make a difference (D 11 in Manual 4).

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:

1. if possible, sowing at a time of year when temperatures are less uniform;
2. covering the seed bed or seed trays with polythene sheeting to raise the day temperatures of the germination medium to about 35°C. (Try this out first without seeds, and use an office punch to make small holes in the polythene until the desired temperature is obtained on a hot, sunny day.)

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;

3. taking seed trays from the nursery to an air-conditioned room from about 1630 to 0730; or
4. if available. germinating at about 30°C day and 20°C night temperature in controlled environment chambers.

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.)

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:

1. the trees are not yet mature (B 10);
2. the species has years with little or no flower formation (B 11);
3. the individual trees selected as parents (B 22) remain vegetative;
4. pollination and/or fruit set have been insufficient (B 11–12);
5. the fruits are inaccessible or regularly eaten by animals, or the seed is quickly shed; or
6. seed is available, but it is empty (B 13), or comes from only a few parent trees and so may perform poorly (B 22).

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?

1. To start or increase fruit or seed production in:
   1. a species where flowering is sparse;
   2. young trees that may be old enough to flower (B 10);
   3. a fruit orchard or selected wild fruit trees;
   4. a seed stand (B 22) or seed orchard (B 23); or
2. To make cross-pollinations possible or easier between:
   1. individual trees selected for favourable characters (B 22);
   2. useful but differing provenances of the same species (B 21);
   3. closely related species, in order to try and make hybrids between them (B 23).

Can any tree be made to flower?

No, since:

1. some trees may be too small and still juvenile (B 10);
2. a tree's reserves may be temporarily be too low, if for instance herbivores had recently eaten all the leaves; or
3. the present environment may be completely unsuitable for flower initiation (B 11).

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:

1. background potential for flowering, probably including:
   1. the tree having grown big enough for it to have changed from the juvenile to the mature stage (B 10);
   2. its crown receiving sufficient light for there to be adequate stores of starch; and a
2. flower-inducing trigger, possibly involving:
   1. sunnier and drier conditions than usual;
   2. particular temperature regimes, such as increased diurnal fluctuations (D 11 in Manual 4); or
   3. relatively small changes in day-length.

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:

1. where possible the potential parent trees can be chosen or planted at a sunny site;
2. either a spacing can be chosen that will allow enough light to reach the crowns when they are big enough to flower and fruit, or the same can be achieved by adequate early thinning; and
3. a specific flower induction technique can be tried out on sample trees.

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:

1. water and some dissolved nutrients can still travel up in the wood (C 14 in Manual 3);
2. sugars can still be manufactured in the ringed shoots; and
3. the dislocation in the movement of hormones and other substances seems to favour reproductive activity beyond the ring, and vegetative growth beneath it.

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:

1. you do not want to damage the main stem, in which case choose pairs of similar, vigorous, relatively unshaded branches. Randomly select one of each pair to be ringed, right at its base, and keep the other as an untreated control; or
2. you would like the treated trees to remain short and branched, in which case pair up whole trees that are similar in size and light conditions, randomly select one of each pair as a control and ring the main stem of the other just above some vigorous lower branches.

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:

1. to have an effect before the time of year at which flowers are initiated (B 12); and
2. for the bark to separate easily from the new wood (C 12 in Manual 3).

How do I make a complete ‘ring’?

1. Select an appropriate width of ring, such as for example:

   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;

2. Using a sharp knife, make two parallel cuts down as far as the new wood, the chosen distance apart, all the way round the shoot;
3. Join the two cuts and take out the ring of bark; and
4. If possible, coat the cut surfaces at once with a wound paint, or other non-toxic paint.

What else should I do?

1. Label the treated and control shoots or whole trees, and record details of your experiment (C 54 in Manual 3);
2. Protect the experimental plot carefully; and
3. Watch out for and record any differences between ringed and unringed parts, such as changes in colour, or in the rate or duration of shoot growth (C 12 in Manual 3); and
4. If flower buds are produced, assess the numbers on each control and ringed shoot, by counting or scoring (C 68).

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:

1. ringed branches with a loop of strong string from a branch above; and
2. ringed trees by ropes to pegs or to the base of other trees.

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:

1. growing seedlings in very good potting soil in large containers (C 6 in Manual 3);
2. digging a larger planting hole, and adding compost and good topsoil (Manual 5);
3. watering regularly and using plenty of mulch (C 33); or
4. regularly giving small amounts of a fertiliser that contains some extra potassium.

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?

1. From the table below, estimate the dose of GA₃ appropriate for the size of branch or tree to be treated;
2. Weigh out enough GA₃ for all the shoots to be treated, and dissolve it in alcohol to make up the stated small volume;
3. Drill 2 or more downwardly directed holes into a branch, or the main stem, big enough to receive the stated volume of GA₃ solution;
4. Take this up into a 1 ml graduated pipette with a fine needle attached, and slowly inject the liquid into the holes in turn, avoiding spilling and keeping the tube containing the GA₃ solution closed to reduce evaporation;
5. After a few minutes, close the holes with paint; and
6. Label the treated shoots with the dose and date, and write a simple record.

Calculations for treating 18–20 shoots with GA₃.

* 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;

1. selecting individual fruit trees with earlier and copious flowering and fruiting;
2. grafting mature scions or buds from these clones on to seedling rootstocks (A 3 in Manual 1); and
3. planting the grafts as improved fruit trees.

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).