How does the Bonsai tree works ?

If we understand how trees function will greatly increase our enjoyment of bonsai and gives us much-needed confidence in caring for the bonsai trees. And it will be easy to carry out the watering, pruning, and shaping of bonsai trees.

1. Root: 
Usually, when your bonsai begins to look sick, it is an indication of root disorders. Roots have three functions, First of all, they provide anchorage, holding the tree firm in the ground. Secondly, they absorb moisture and soluble nutrients from the soil. Thirdly, they store sugars during dormancy to provide energy for the first flush of growth in spring.

Japanese Maple (Acer) Bonsai ( Bonsai Empire)

Anchorage:  
To hold a tree firm in its pot, the roots must be distributed all around the trunk and must grow sideways rather than downwards. One-sided root systems are unstable and most likely also result in a one-sided structure. During repotting the bonsai trees, initially, the wire used to hold a tree firm, but it's only temporary and is unattractive and also used to tie the branches for different styles. If the roots lack vigor or decaying, they will not fill the pot and this causes further damage to the roots.

Absorbing Water and Nutrients: 
Healthy growing roots show plump and white at the growing tips and it is the most active part. The very tip is protected as it's a way through soil by a hard cap, which is constantly being worn away and replaced. Behind this, the white part of the root is clothed in minute root hairs which are composed of single-cell and can be almost invisible to the naked eye. Water and nutrients are readily absorbed by these root hairs due to the enormous combined surface area. Root hair production is stimulated by moisture and oxygen present in the soil. Absorption of water takes place through a process called Osmosis.

Root Burn: 
 If the concentration of salts outside the roots is greater than that of the inside, the osmosis is reversed and the water passes out of the roots back into the soil to equalize the solutions. The tree will wilt and begin to shed young shoots. This is why you should never use fertilizers over the recommended rates or when the roots are inactive. Similarly feeding directly after repotting, when there are few root hairs can have a similar effect. Regular prolonged watering to flush the soil clean of residual salts is a wise precaution. 
       
Nutrient Storage:
 Older, thicker roots develop bundles of sap conducting cells called the phloem, which is also present in trunk and branches. These cells conduct the sugars from the leaves and distribute them to all parts of the plant, whenever they need for growth, including the roots. In late summer and autumn, when growth slows down and eventually ceases, the phloem becomes plump with excess sugars, which they are stored there until they needed to support new growth in spring.

Transplanting trees and pruning roots in autumn causes considerable loss of stored sugars which will retard spring growth. This is why you should wait until the buds begin to swell, which indicates at least some of the stored sugars have been returned to the growing points, before pruning the roots. The expectations are some flowering plants that seem to produce even more flowers and fruit if given a hard time and repotted in autumn.

2. Trunks and Branches:
        The main purpose of trunks and branches is to support as much foliage as possible where it will receive the most light and air. They also, naturally have to conduct water, nutrients, and sugars from the roots to the leaves and back. In a mature tree, most of the tissue forming the trunk and branches-the the heartwood-is dead. It has become lignified and hardened and is responsible for the tree's strength.

The Cambium:
                 If you gently scratch a twig with your fingernail, you will notice a bright green layer just below the surface-cambium. It is a single layer of cells surrounding the trunk, branches, and shoots. It can initiate new buds, new roots, as well as fuse with the cambium of another plant, such as in the process of grafting.
                When a thick branch is cut through during the growing season, it produces new shoots that emerge as a crown from between the bark and the sapwood in an attempt to replace the lost foliage. Most of the shoots will die off through overcrowding and lack of light, but the strongest will continue to grow vigorously.
                The cambium is also responsible for producing the healing tissue that 'rolls' over wounds. If you look closely at a recent pruning cut, you can see how this tissue emerges from between the bark and the wood.

Cross Section of the tree trunk

The Xylem:
                 On the inside of the cambium, the new cells it produces form the Xylem, which conducts the water upwards. It is the formation of new xylem each year that creates the familiar annual rings. The xylem remains active for a year or more, depending on the species, and while active forms are called sapwood. This is the lighter colored group of rings surrounding the heartwood.
                  The production of new xylem is also what makes branches set in position when trained with wire. Once the tensile strength of the new xylem is sufficient to counter that of the old wood, the wire may be removed and the branch will stay in place. The wood in young shoots and branches is composed entirely of xylem, which is malleable and will readily adopt new shapes. Older branches that contain heartwood take longer to set.

The Phloem:
                On the outside of the cambium, the new cells form the phloem, which distributes the sugars manufactured by the leaves to all other parts of the plant. As old phloem cells are replaced by new ones each year, they, in turn, harden and become the bark. As the year's passes, the bark thickens and in most cases becomes Corky and flakes, peels. The precise formation of phloem cells and the length of their useful lives differ between one species and another. This explains why different trees display their own unique characteristic bark patterns as they mature.

3. Leaves:

Each leaf converts water from the soil and carbon dioxide from the air into essential sugars in a process called photosynthesis. Some shade is preferable for almost all the plants grown in containers, but deep shade will cause problems. Too much sun although providing the necessary light will cause the leaves to overheat. They will rapidly close their breathing pores (stomata) to reduce the water evaporation This causes the leaf to 'shut down' until the sun becomes less intense. During this time, the leaf is not manufacturing sugars because the process relies on a constant passage of water through the leaf and a supply of carbon dioxide, which is absorbed through the same stomata.

Leaf types:

  

  Plants that normally live in semi-shade, such as azaleas and Japanese maples, have delicate, thin-skinned leaves that easily become brown and withered if grown in full sun or exposed to drying winds. Delicate leaves such as those on Japanese Maples, are easily scorched if exposed to drying winds.  Plants growing in semi-shade will bear large, deep green leaves. The same variety grown in a small bonsai pot in full sun will have much smaller leaves, which will not display the same richness of color. Plants that are exposed to the hot sun have thick, leathery leaves, often with a waxy coating that helps prevent water evaporation. A waxy coating is also used by species that prefer cold conditions. Most conifers have waxy leaves to protect from forming on the needles.

Buds:

On most species, there is a tiny, embryonic bud at the base of each leaf stalk (petiole). As the bud opens in spring, the central core elongates to form the shoot, with leaves distributed at intervals (internodes) along its length. As it extends, the bud at the growing tip is constantly undergoing a cycle of opening, extension, and regeneration. If this is removed, the energy will be diverted into the next one or two buds back down the shoot.

Each bud is surrounded by scales, which can be anything from green, through browns to bright red, depending on the species. The scales are designed to protect the delicate, partly formed leaves in the bud from the sun, rain, frost and insect attack. They are, in fact, modified leaves and also have embryonic buds at the base of each scale. This explains why a mass of new shoots emerges from the short stub left when hard pruning current year's growth.

Buds can also be formed on old wood in reaction to more severe pruning or drought or an attack of fungal disease. The cambium layer works to regenerate lost foliage by rapidly developing new buds that force their way through the bark. These are called adventitious buds and can appear on branches, trunks, and even on old roots near the surface of the soil.

In bonsai cultivation, the production of adventitious buds is of key importance because they are selectively used to grow new shoots to replace outgrown or congested areas of foliage. As the tree begins to stir from its dormancy in spring, the buds start to swell. Tiny, pale-colored lines appear at the edge of each scale as they begin to separate. This indicates an increase in root activity and signals that time for repotting before growth is too far advanced.

Autumn Colour:
Different color pigments varying from bright yellow through reds to purple. Many of these pigments are in the leaves when the shoots emerge from the buds but are masked by the presence of chlorophyll. Other pigments are the result of chemical changes that take place in early autumn. As autumn approaches, the leaves cease producing sugars and the chlorophyll breaks down and is reabsorbed.

Autumn color can be enhanced by keeping your trees in a warm, sunny spot during the day in late summer and early autumn but as cold as possible at night. This maximizes the color-forming breakdown of substances during the day, but their redistribution to other parts of the plant is hindered by the night-time cold.

Don't worry if your bonsai lose their leaves before full-sized trees lose theirs. This is quite common and, although disappointing, does no harm.

Reference:
1. Bonsai Basics by Lewis Colin.