- introduction: how trees grow
How do trees grow?
By using the energy of sunlight to make organic matter out of simpler substances.
Does the sunlight just warm them up?
No, although it does do that. Like all green plants, trees contain a pigment called chlorophyll
which absorbs some of the sunlight, allowing part of its energy to be turned into a chemical
During the second stage of this process of photosynthesis, the chemical energy is used to build in carbon dioxide absorbed from the air to form sugars (D 10 in Manual 4).
What happens to these sugars?
Of the large amount produced in a tree, some sugars are:
Does the tree keep on storing more and more?
Yes it may do so, if there is enough light. However, quite a lot of its sugars, starch and structural parts are:
Tropical trees are often the main producers in the food chains, on which all the consumers (most other living organisms) depend (D 10).
But what has all this got to do with tree nurseries?
Growing good trees successfully is determined by the general principles which control tree growth. Practical experience is important too, but it is most effective when combined with a grasp of the basic elements of tree biology.
How does a tree actually get bigger?
It does so because some of its cells:
What are cells?
They are the living units of which the tree is made, which often:
Note: All living organisms release carbon dioxide to the air as they break down organic matter to release energy.
And what are they like?
For example, the cells in the root possess all the instructions for making fruits, but these remain unused.
Why is that important?
Because it means that:
In theory, any living cell could be turned into a whole tree.
How many cells are there in a big tree?
A single leaf can contain several hundred, and a large tree many millions.
Well, why not turn a good tree into millions of young ones?
Unfortunately, there are a lot of barriers to such micropropagation (A 5 in Manual 1). For example:
Cross-section through young elongated stem.
Does that mean that micropropagation can't help?
No it doesn't, because:
However, for most tree species the best prospects are to root leafy, juvenile cuttings in a shaded poly-propagator (Manual 1), and grow them on in a tree nursery, because this approach:
What are cells like inside?
The living part usually includes:
Cells may also contain:
Outside is the cell wall, which limits the tendency of the living part to go on getting bigger, and if strengthened may give support to the tree even after the cell has died.
How do cells divide?
The instructions in the nucleus are copied exactly, the two versions separate, and new cell membranes and walls are formed between them. Most cell divisions result in two identical daughter cells, one of which often continues dividing while the other expands and becomes specialised.
Can any cell divide?
Any living cell can theoretically do so, but in practice most cell division happens:
Cell division also occurs in other young, rapidly growing parts of the tree, such as elongating fine roots, extending stem internodes and expanding young fruits. It also takes place when new growing points are formed, and in response to damage.
How do cells enlarge?
Water tends to enter the cell more strongly than to leave it. The pressure this exerts stretches the new thin cell wall, making the cell bigger until the strengthening wall and neighbouring cells restrain the enlargement.
|Where most cell division happens. Stylised young tree, showing many root tips, three shoot tips, and the layer between wood and inner bark.|
What kinds of specialised cells are formed?
Some common examples are:
Don't things go wrong sometimes?
Yes, occasionally a cell is formed that has incorrect instructions. However, in most cases there are specific structures in the cytoplasm which destroy such faulty cells. The contents are broken down and other cells take over.
What affects growth and division of cells?
Some of these effects will be mentioned in sheets C 11–15 and C 34.
O2 = oxygen
CO2 = carbon dioxide
H2O = water
- expanding root systems
What kinds of roots do trees have?
In a young nursery tree, the root system generally consists of:
In some tropical trees part of the root systems are above-ground, for example as stilt roots or as ‘breathing’ roots, found especially when the soil is waterlogged.
How does a root grow?
By some of its cells dividing, becoming larger and more specialised (C 10). Within one actively growing root, there are usually zones where cells:
|Section through a young root.|
1 — root cap. 2 — cells dividing.
3 — cells dividing, elongating and becoming specialised.
4 — zone no longer elongating.
5 — surface cells. 6 — boundary cells.
7 — storage cells. 8 — conducting cells.
9 — branch root.
Do all roots start off the same?
In some tree species, young root tips are of two distinct types:
In seedlings (Manual 2), the first root is generally thick, making branch roots that may be
thick or thin;
In cutting that have been treated with auxin (A 40 in Manual 1), both types may be formed, but without added auxin only thin roots may sometimes be produced.
In palms, the roots are usually very thick when formed, and remain the same diameter, without branching.
How do the roots of other trees get thicker?
By producing a cylindrical layer of cells that then divide continually, forming water-conducting and strengthening cells to the inside and sugar-conducting cells to the outside (C 10). Because this layer of dividing cells lies deep inside the tissues, the root can actually get thinner as the outer parts are lost, before thickening later on. Like stems (C 12), thickened roots also produce bark to the outside.
And how do branch roots form?
A small group of cells divide and form into a root tip, which then grows in the normal way, bursting out into the soil through the outer tissues of the original root. Older, thickened roots can also produce branch roots, especially after disturbance or injury.
When cuttings form roots (Manual 1), a similar process happens near the base of the stem, which is stimulated by adding small amounts of auxins.
Can I increase root growth by adding auxins?
No, applying these hormones will make root elongation slow down or stop altogether.
Do roots normally grow all the time?
Growth in length of roots is often continuous in the tropics, though with periods of faster and slower growth. Similarly, there are often peaks in the formation of new thin roots. However, roots might stop growing completely if subjected to:
Is growth in thickness of roots continuous?
It generally is, unless the trees become leafless (C 12).
What influences how fast the roots grow?
In the nursery, root growth is likely to be most rapid when:
What other things slow down root growth?
Root growth will tend to decrease or even stop if, for example:
Why is too much water bad for root growth?
Because the water drives out the air from the spaces between the soil particles, so that the root cells run short of oxygen. Such conditions can also favour micro-organisms that cause diseases (C 45). Waterlogging can happen if:
Tree species from mangrove woodland, freshwater swamps, and seasonally flooded forest and savanna generally thrive in soils liable to flooding. Some of them have:
But isn't the purpose of roots to supply water?
Well, it is one of their four vital functions, which are:
How is the water actually taken up by the tree?
When a fine, absorbing root is in close contact with small, moist soil particles, it is easy for water to pass through the thin walls of its surface cells. Because of the dissolved substances they contain, water is then taken up strongly through their cell membranes (C 10). Most of it moves from cell to cell into the water-conducting ‘pipes’ of the root, and then into the shoot system.
Water may also pass into the roots from the extensive network of fungal threads of mycorrhizas (C 30–31).
Are nutrients taken up the same way?
If the nutrient is dissolved in water, it can enter the cell walls easily. However, the next stages are different (C 14), because:
Once absorbed, the nutrients may be:
What sort of substances are manufactured in the roots?
Using sugars arriving from the leaves, the root system usually:
How about anchorage?
Starting in the nursery, the root system also serves to fix the tree into the ground, keeping it upright and stopping it from being blown over (and in flooded sites from being washed away). When the tree is older, a set of large near-horizontal roots with sinkers may anchor the tree firmly, while in other tree species stilt-roots may assist in stability.
Aren't nursery root systems too weak to give much stability?
Young trees do need shelter (C 25, C 46), and could still be damaged by occasional fierce storms (C 3). However, their roots generally soon provide anchorage because:
Poor, diseased or spindly trees are more likely to prove unstable than good planting stock (C 4).
Do roots ever turn into shoots?
Root tips do not turn into shoot tips.
Certain cells on older roots occasionally form buds (suckers), for example in Milicia (Chlorophora) and Cordia alliodora (A 3 in Manual 1).
Remove section of root
Dig up sucker + root
Can root systems exist by themselves?
Only in the laboratory, and when provided with a sugar supply. Roots usually contain few or no sugar-making cells (C 10), and so are just as dependent upon photosynthesis in green shoots as are humans, other animals and decomposers (D 10 in Manual 4).
Does that mean that the shoots are more important?
- growing stems, leaves and branches
How do shoots differ from roots?
In the way they are organised, how they grow and what they do.
Do they have the same kinds of cells?
Sugar-making cells are found in large numbers in the shoot, and only rarely in roots. Many of the other kinds of cells are found in both.
What differences in organisation are there?
The chief points are that:
In addition, shoots have a much thicker waxy covering on surface cells (C 10), which reduces water loss.
|Section through a young shoot.|
A - shoot tip. B - first pair of leaf projections.
D - third pair of young leaves (the second [C] cannot be seen). E - third pair of young buds.
F - conducting cells. G - fifth pair of leaves expanding. H - fifth pair of buds.
I - third and fourth internodes extending.
How do stems and leaves grow?
They are both formed at shoot tips, which have zones of:
What makes stems and leaves different?
Although both of them arise from the same growing point, they differ because:
Section through a leaf.
1 - thick waxy layer. 2 - upper surface cell. 3 - upper sugar-making cell. 4 - lower sugar-making cell.
5 - lower surface cell. 6 - thin waxy layer. 7 - guard cells and holes. 8 - conducting cells.
Don't they also differ in what they do?
Yes, that is so. Generally:
However, most young stems also contain some chloroplasts and guard cells, while in a few tree species, such as Acacia mangium, short flattened stems act in place of leaves.
What happens to the leaf projections at the shoot tips of trees?
What about the shape of leaves?
This varies greatly between different species, to some extent from one tree to another of the same species, and even within a single tree. For example, leaves may have:
Many features of leaf shape are useful in identifying one tree species from another.
How big can leaves get?
Their final length can vary between a few millimetres (for instance in Casuarina and Cupressus)
and several metres (for example in palms). Widths range from about 1 mm to 1 m.
In mountains, tree leaves are usually smaller than at lower elevations, and the same is often true in drier compared with wetter environments.
Which features of leaves are the most important?
The number of leaves and the leaf area are usually key points, whether for a single leaf, a tree crown or an entire stand.
What determines leaf area?
Besides the genetic potential of the individual tree (C 5), many other factors influence the number of leaves formed and how big they grow. These often include:
Do I want the leaves to be as big as possible?
Yes, in a well-established tree, growing in the ground; but
No, in a young tree in the nursery (C 34).
How long do leaves live?
Usually between 3 and 15 months, although in a few tree species they may last for several years. In a rapidly growing nursery tree, some of the first-formed leaves may live for only a few weeks.
What happens to their contents when they die?
In a few species, such as some palms, it is normal for the bottoms of dead fronds to remain attached to the trunk.
Why do some trees naturally lose all their leaves at once?
This is a common feature of:
Deciduous trees shed all their leaves well before they produce any new ones, so that the tree
stands leafless for weeks or months;
Leaf-exchanging trees shed their leaves at about the same time that they produce new ones, so the tree may be leafless for a short time only;
Evergreen trees never stand leafless, because they either:
Is it drought that makes leaves fall off?
Although many leaves may fall during the early part of the dry season, there are probably several other factors at work, such as:
Poor watering, rough handling or sudden changes in the amount of light reaching nursery trees can sometimes be followed by a lot of leaf shedding (C 40–43, C 47).
So shoots don't necessarily grow all the time?
Shoot growth is continuous in young trees of a considerable number of species; but
Some seedlings and most saplings and older trees show periodic flushing, followed by one to eleven months in which no new leaves are expanding or stems extending.
Is it rainfall that starts shoots growing again?
Perhaps yes in a few species, especially in dry areas; but
Probably no in many others, because flushing commonly occurs before the first rains.
Do all the buds grow into branches?
No. Some buds may:
Common types of branching.
Which buds become branches?
Trees generally have regular patterns of branching that produce a growth habit and crown shape that are characteristic of the species, and sometimes of the individual clone (A 11–13 in Manual 1).
Does this matter much?
Yes it does, because different branching habits and crown shapes are best for trees needed for shade, shelter, wood production or fruits.
For instance, straight stems are valued for timber, pulpwood and poles (C 36–38 in Manual 4), but undomesticated tree species (C 5) often contain stems that are crooked, forked or even grow as multiple stems. Some may have large branches that leave big knots in the wood, reducing its strength and value.
How do stems get thicker?
By division of a cylindrical layer of cells lying nearer the outer surface than in roots (C 11). The cells it produces on the inside become specialised as wood cells (water-conducting, strengthening or storage), while those formed to the outside are sugar-conducting or strengthening cells (C 10).
Will all these cells stay alive?
Dividing layers and storage cells may live for many years, and sugar-conducting cells for a shorter time, but many of the other kinds die quickly. The outer part of the wood (sapwood) contains some living cells, but in the inner heartwood they are all dead, though they generally remain strong.
What happens to the surface of the tree when it grows?
When a leaf falls off, its scar has usually been already protected.
As a stem grows in thickness, a second layer of dividing cells is formed nearer its surface. This produces the bark, and the original surface cells die. The bark may become smooth, ridged, scaly or peeling, but looser areas remain that allow some continued gas exchange.
Cutting a test plant shows that water did not reach all the soil.
- maintaining the water balance
What is meant by the water balance?
Trees contain large quantities of water, but if they lose more than they gain, there may soon be problems.
How much water is there in a tree?
A considerable proportion of it consists of water. For example:
Is it important that there should be so much?
What happens if a tree runs short of water?
It is under water stress (C 41). Moderate, temporary water stress is normal, but a lack of substantial amounts of water for longer periods can lead to premature leaf-shedding, shoot die-back or death of the whole tree, unless it:
And if not, it wilts?
If nursery trees wilt, it means that the water shortage is decreasing the pressure in the cells.
This should be avoided, as they are already under severe stress, leading to a check in growth,
damage or even death.
Note: young trees with thick, leathery leaves and tough stems may not wilt although suffering from pronounced water stress.
Why do trees in leaf lose such a lot of water?
Because it evaporates so easily from leaves, especially from their moist internal surfaces.
Perhaps only around 10% is lost from the outer surfaces of leaves, stems, aerial roots, flowers and fruits, because:
Less than 1% is actually used up in photosynthesis and other chemical reactions.
How does such a lot come from inside the leaves?
Because there are:
The bigger the arrow, the more water is evaporating.
Can they be closed?
Yes, each small hole can be opened or closed by two special guard cells. Generally the holes are open during daylight hours, but closed:
It is also common for many of the holes to close around midday during sunny weather.
But isn't that when sugar production could be fastest?
Yes it is. Light levels are often greatest then, but with higher air temperatures evaporation will also be very rapid. A balance is struck by the plant, capturing carbon dioxide but stopping excessive water loss.
In practice, unshaded or lightly shaded leaves in the nursery can receive plenty of light for photosynthesis from about 0800–1200 and 1500–1700.
What influences how much water is lost by a tree?
How quickly can water be lost from leaves?
If you break off a leaf and keep it in the sun, it may be only a minute or two before it wilts. Water has been lost so rapidly that most cell activities will already have been affected. Unless the water balance is quickly restored, the leaf will soon die.
How much water does that mean in a day?
Each attached leaf could lose many times its own weight in water in a single hour. So a small tree may lose litres a day, and a big emergent tree in the canopy of a tropical forest might be losing hundreds of litres an hour.
Where does all this water come from?
It has to come from the soil, by way of the root system, trunk, branches and twigs.
However does it get up a tall tree?
Most of the water travels in the dead water-conducting cells (C 10), which are like long, miniature pipes.
Aren't they too small to carry all that water?
No, because there are a lot of them, all the way from the absorbing zones in young roots (C 11)
to the small veins in the leaves.
Their diameter is usually much less than 0.5 mm, but this and their wall structure help these tiny water columns to travel up and not break.
How do the roots pick up so much water from the soil?
A well-developed root system has a very large surface area of fine, growing roots in contact with moist soil particles. Water enters the root easily because:
The surface area of the root system may be enlarged still further by:
What affects how fast it comes in?
The rate at which water is taken up by the root system depends primarily on how much water is being lost from the shoots of the tree, which draws it up from the roots. However:
But is there enough water in the soil?
For an established tree there may be, but its root system must be able to:
For a young tree in a container, a good root system (C 4) and adequate watering (C 43) are clearly vital. If the potting soil becomes dry, water may still be lost rapidly unless most of the small holes in the leaves have closed.
Is there any way of saving young trees that are wilting severely?
Note: do not water if the soil is moist, because it will not help, and it could cause harm by waterlogging the soil (C 11), and encouraging root diseases (C 45).
Can shoots take up any water directly?
Generally no, because rainfall does not normally enter the small holes in the leaves.
Occasionally they might do so, for example when the soil is very dry, and:
Although leaves and young stems depend on rain taken up by the roots, if water fills their internal air spaces they cannot function properly.
What about trees which have shed their leaves naturally?
These lose much less water than leafy plants, and may even contain sap under pressure. They should be watered sparingly (C 43), but not allowed to dry up.
Time after root disturbance
Are there some other practical guide-lines about water balance?
Other suggestions for maintaining the water balance are found elsewhere in the Manuals.
|%RH = relative humidity (per cent). Immediately the poly-propagator is opened, the air becomes much drier, even when a hand-sprayer is used - - - -.,|
- absorbing, making and moving substances
What kind of substances does the tree absorb?
Some pollutants can also be absorbed by the shoots of trees (D 16, D 26 in Manual 4).
What are nutrients?
Simple chemical substances that are important to or essential for the growth of all living organisms (D 13 in Manual 4).
Which are the main nutrients?
Relatively large amounts of these three nutrients are required for good growth of both young nursery plants (C 33–34) and older trees (D 13 in Manual 4).
Are other nutrients needed as well?
Where do all these chemicals come from?
Which is the most important source?
Under trees, the litter is usually by far the most important. Most nutrients are very efficiently recycled in natural woodland ecosystems, so that they are used over and over again, with only very small losses in run-off and leaching (D 13).
What happens when the trees are cut down?
Nutrient supply generally becomes a serious problem, because:
So recycling of nutrients is one of several important reasons for having plenty of different kinds of trees (C 1; and D 30 and D 53 in Manual 4) in managed ecosystems.
Recycling of nutrients.
But does this apply to young trees in the nursery?
Young nursery trees start off with a supply of nutrients stored in the seed, or present in the cutting when it is taken. But sooner or later they depend for nutrients on what is available in the potting mixture or nursery bed.
What about adding fertilisers?
If a balanced fertiliser (C 33–34) is added to a poor soil, the young trees may be able to take up more nutrients and grow better, unless:
It is generally better to concentrate on choosing good nursery soils (C 23) and preparing
favourable potting mixes (C 6).
Use cheap and readily available materials like composts and wastes (C 33), and avoid expensive fertilisers except when they are specially needed.
How do nutrients actually get into the tree?
Small amounts of nutrients can also be absorbed by leaves, if for example a foliar feed is sprayed on to them.
What about mycorrhizas?
Yes, these are very important in the uptake of nutrients (C 30–31; and D 32 in Manual 4), because the fungal threads can:
How about nitrogen-fixing trees?
These form nodules with closely associated micro-organisms that have the unique capacity to fix N from the air, where it is very abundant, into nutrient form (C 32 and D 32).
Are sugars also nutrients?
No, they are more complex organic substances that green plants manufacture from simple chemicals containing carbon (C), using the energy of sunlight (C 10).
Does that include carbon dioxide?
Yes, the energy that is captured during photosynthesis is used to make sugars by adding more carbon to organic acids, which also contain hydrogen (H) and oxygen (O). Some of the latter is released into the atmosphere.
What other kinds of substances do trees manufacture?
Amongst the most important are those used in:
Do trees make anything else?
Yes, a very large number of different chemicals. Some have been widely studied, while many others are yet to be identified. For example, some may:
How do these substances move around in the tree?
Those that do so will move:
Some substances are formed, broken down and reformed many times over.
However, others are made within a cell and never move, becoming part of the permanent tissues of the tree.
Where do sugars move?
From the sugar-making cells in the leaf and stem to neighbouring cells, and then through the sugar-conducting cells to all other cells in the shoot and the root system.
What are they used for by the tree?
Besides storage, sugars are broken down (respired) (C 10) in each living cell, releasing energy for:
How do nutrients move?
Simple nutrients, once through the inner boundary layer of an absorbing root, travel mainly
through the water-conducting cells to other parts of the tree.
Organic nutrients, already partly built up, may pass either by the same route or through the sugar-conducting cells.
How is all this controlled?
In at least 3 ways:
For example, when a leafy stem cutting is detached from a stockplant (Manual 1), the natural auxins (produced by the shoot tip and moving towards the base) can help to regenerate a complete plant by stimulating cells there to form new root tips. Synthetic auxins applied to the base can often increase this process (A 40).
What else do plant hormones do?
Various hormones appear to help keep a balance between the two parts. For example:
Are hormones well understood?
Although control over the growth and development of large trees is a central part of biology, there is still much to be learnt. Much more research is needed (C 7, C 15).
- experiments with young trees in pots
What experiments can be done in pots?
Many different kinds. It is often best to concentrate on those which are:
Are there some examples?
What is the main aim of basic studies of tree growth?
To add a bit to our understanding of some of the complex interactions (C 69-K) between:
How would I discover better growing conditions?
By using the most successful local techniques, and then:
What about problem-solving?
Some nursery problems can be tackled by:
Other problems may need formal or informal experiments to find out what might be wrong.
Is it possible to test out a new tree species in pots?
Yes, this is often a useful first step, before trying them out in field trials (D 29 in Manual 4). One might discover, for instance:
Before writing off the species as a failure, remember that it could be the provenance, seed-lot or clones used (C 5) that were unsatisfactory.
How would I set about doing an experiment?
Supposing I want to compare pots of different sizes?
They should preferably be:
How many treatments would be needed?
Between two and four is often a good idea, rather than making the experiment too large.
And how big should the different pots be?
The most important aspect of pot size is the volume of soil within, which will provide both rooting space and a nutrient supply for the young tree. So you could choose convenient pot sizes that:
What would that mean in practice?
If, for instance, you chose tapered pots where the diameters at the level of the soil at the top were 9.5, 12.25 and 15.5 cm, this would give soil volumes for the root systems of about 450, 900 and 1850 cubic centimetres. The medium-sized pots would hold about twice the soil of the small ones; and similarly the large containers would have about double the contents of the medium-sized pots.
How can I check the volume of my containers?
See sheet C 63-D for this.
Should all the containers have the same number of holes?
No, the medium-sized pot would need to have more holes than the small one, so that excess water drained off in a similar way. The large containers would need even more holes to maintain comparable conditions of soil moisture.
How would I set about grading the young trees?
Supposing you have decided to have 25 replicates for each of the 3 treatments:
What about applying the treatments?
Some important points to plan for are:
How about labelling and records?
The best way is to:
Are individual labels really necessary?
Although in this experiment you can easily see which treatment is which, remember that:
Note: experiments using clonal plants (Manual 1) can give greater precision than those with seedlings, provided that the trees of each clone are allocated equally to the 3 treatments. In such cases you might perhaps write the treatment details on the label bearing the clone number.
Where should the pots be kept?
It may be useful to put an extra line of similar trees in pots around the experiment, so that none of the treated trees are ‘edge plants’ subject to different conditions.
How should the experiment be laid out?
In such a way that no treatment (or genetic origin) is specially favoured. To avoid bias it is often recommended that a completely random arrangement of the pots should be used. However, you also need to take into account the following points:
For problems (1) and (2), a compromise might be to randomise short lines of trees of the same treatment and pot size. Layouts such as Randomised Blocks (D 55 in Manual 4) can reduce problem (3), and Latin Squares can avoid it altogether.
What would the Blocks consist of?
In the pot size experiment, five triplets could be randomly assigned to each of 5 Blocks, making five sets of 15 plants. Each Block would be laid out together in a convenient and compact pattern within the whole experiment.
But supposing all the pots won't fit into one place?
If necessary, some of the Blocks could be put in one area, and the rest in a second place, according to the available space. As long as the whole Block is kept together, it would even be possible to put each Block in a separate place, as long as they were all reasonably similar.
Do experimental trees need special care?
Yes they do (C 48), in order to:
How about assessing and analysing the results?
In order to be able to judge properly (C 55, C 67–69), it is important:
Some research workers decide beforehand what should be assessed, but others prefer to choose at the time what to measure and when to do it. If combined with regular observations, the second approach may allow one: