It is occasionally asked if bonsai horticultural requires an understanding of how trees, are or whether it is a creative science experiment to create a living art piece of an ever-changing medium.
Bonsai is a combination of both horticulture and science: it satisfies the basic human need to create, train, care, and enjoy.
Of all the tree types, it lives the longest due to the permanent structure of the trunk and branches.
Starting from the outer ring and working towards the center, the bark, cambium, sapwood, hardwood, and pith.
The cambium layer produces the wood fibers and the vessels which carry sap towards to the aerial parts of the tree. As the tree grows, these vessels become more numerous, with the new layers superimposed on the old, which gradually cease to play an active part in the life of the tree.
In this way, the annual life cycle of the tree produces successive layers of compressed xylem (Xylem are the rings which do transport water and food, as the xylem ages, it becomes less efficient until it ceases to have any function at all and becomes heartwood), clearly visible in the section of the trunk. Each of these growth rings corresponds to one year in the life of the tree. To determine the age of the tree all you need to do is count the rings.
In a well-kept Bonsai Tree, the production of cambium is continuous and regular, as long as the water supply is constant. A tree in the wild and natural surroundings may have to cope with dramatic variations in climate which can considerably influence its growth.
On the outside of the trunk are two darker, softer layers. The innermost layer is the phloem between the outer bark and the cambium (pronounced ‘flo-em’), which distributes sugars from the leaves to the other parts of the tree, giving them the energy to grow. If you ring bark a tree, cut a strip of bark away around the trunk and the tree will die, not because the crown of the tree is starved of water, but because the roots don’t get the sugars they need to survive from the leaves.
When you damage the phloem by deep-pruning a branch or allowing training wire to crush the bark, it interrupts the flow of essential sugars. This may lead to the death of the roots below the damaged area. Each year a new layer of phloem is produced, but this doesn’t normally lead to the formation of such clearly visible rings as the xylem.
Enveloping the phloem is bark, which varies in thickness and texture according to species. Bark is made up of an accumulation of old, spent phloem, and has a variety of practical purposes. The bark is waterproof, so it prevents moisture from leaking out of the phloem. It is also home to small structures, called lenticels, which permit the trunk and the branches that allow gas exchange between the atmosphere and the internal tissues, (to let the tree ‘breathe’). Another function that bark performs is to protect the phloem from impact, abrasions, and attack by a variety of insects or fungal infections.
Between the xylem (sapwood) and the phloem is what may justly be called the most crucial part of the tree, the cambium. This layer is just one cell thick and shows as a bright green film when the outer ‘skin’ (the phloem) of a twig is scratched away. In spite of its thinness, the cambium is highly active.
Throughout the growing season, the cells are constantly dividing, producing new xylem cells on the inside and new phloem cells on the outside. When winter comes it slows down, almost to a standstill, while a new ring forms.
The cambium is able to adjust its work rate to the growing conditions of the tree. In situations in which a tree can’t get sufficient water or nutrients or when the tree is confined to a pot, it slows down the rate of cell division
When a tree is adequately fed and watered, the cambium speeds up, producing thicker annual rings. In bonsai. We are aware that life in a pot is bound to affect the vigor of a tree, so we must attempt to counterbalance this restriction by creating a very efficient root system and feeding it well.
If the cambium is kept as active as possible, the trunk thickens more rapidly, which helps the bark to mature, increasing the tree’s value. The cambium is extremely versatile, so much so that it is even able to alter the nature of new cells to perform any number of essential tasks.
When grafting, it’s vital to get the two foreign cambium layers to meet exactly, in order to ‘fuse’ together. Once fusion has successfully taken place, the new xylem and phloem cells that it produces within the union are able to function as continuous pathways.
If you cut through a branch in summer, you will eventually find a ring of fresh buds crowding around the cut between the wood and bark.
These have been developed by the cambium layer, which has modified its function in response to losing the supply of hormones produced by actively growing shoots and buds. Adventitious buds (ones which are produced at random) growing from the older branches and trunks of trees are also generated by the cambium in response to stress higher up the tree. When cuttings are taken, the cambium generates the new roots for the new tree. It also gives rise to new roots during the process known as air-layering.
CROSS-SECTION THROUGH A LEAF
- Cuticle: A waxy layer that prevent water loss by evaporation. The cuticle is transparent and very thin to allow maximum light penetration.
- Upper Epidermis: A protective layer of cells that produces the cuticle. The epidermis is also transparent and very thin to allow maximum light penetration.
- Palisade Mesophyll: Rod-shaped cells that contain large numbers of chloroplasts for photosynthesis. These cells are located close to the leaf surface to maximize light absorption. They are upright, elongated, and tightly packed together in order to increase the surface area for light absorption. Chloroplasts are found near the palisade cell surface to maximize light absorption and to reduce the distance that carbon dioxide and oxygen have to diffuse (to/from the chloroplast stoma)
- Spongy Mesophyll: These cells are smaller than those of the palisade mesophyll and are found in the lower part of the leaf. They also contain chloroplasts, but not quite as many. These cells have large air spaces between them that allow carbon dioxide and oxygen to diffuse between them. The air spaces also give these cells a large surface area to maximize the diffusion of carbon dioxide into the cell and oxygen out of the cell.
- Vein: Tree veins consist of xylem (vessels that carry water) and phloem (vessels that carry dissolved nutrients such as sugar). These vessels play an essential role in transporting water to the chloroplasts in the mesophyll tissues for photosynthesis. They also transport the sugar produced by photosynthesis away from these cells to the rest of the tree tissues to be used as an energy source or stored.
- Lower Epidermis: A protective layer of cells. The lower epidermis produces a waxy cuticle too in some tree species. The lower epidermis contains pores called stomata that allow carbon dioxide and oxygen to move in and out of the tree respectively.
- Stomata: Tiny pores (small holes) surrounded by a pair of sausage-shaped guard cells. These cells can change shape in order to close the pore. In very hot conditions water inside the leaf evaporates and the water vapor can escape through the stomata. Closing them prevent reduces water loss, but also limits the diffusion of carbon dioxide and oxygen in and out of the leaf. (Credit: Ben Himme)
The leaves are the food factories of the tree. The leaves transport water supplied by the roots and carbon dioxide from the atmosphere, and convert them to complex sugars, making use of sunlight to energize the reaction, in a process known as photosynthesis. The sunlight is captured by the green chlorophyll, which acts as a catalyst for the chemical reaction that takes place. Even red-leaved species feature green chlorophyll, but this may be masked by the red pigmentation which is present in greater abundance.
During the day, the leaves take in oxygen and carbon dioxide through pores, called stomata, which are usually found on the underside of each leaf. At night they expel carbon dioxide by respiration. The stomata are able to open and close in response to the ambient temperature and humidity, thus controlling the rate at which water evaporates from the leaf. Some water has to, of necessity, evaporate all the time in order that a fresh supply of moisture is kept flowing up towards the leaves from the roots.
Leaves vary tremendously between species. Large, flat leaves capture the maximum amount of light. Narrow pine needles are adapted to be drought resistant. They don’t get weighed down by heavy layers of snow, but have a large surface area in comparison with their volume.
This way they are still able to capture plenty of light. Some species have waxy leaves in an attempt to prevent excessive evaporation, while others have leaves covered in dense hairs, which achieve the same result. All leaves have one thing in common: they emerge from buds that form at the tips of shoots (apical buds), often in clusters.
On conifers, buds also form at random points along each shoot, of which the precise number and density depend upon the length of the shoot and the vigor of the tree.
On broadleaved trees, a bud appears at the base of each leaf stalk (petiole). These buds are called axillary buds.
When pruning or pinching growing shoots on a bonsai, it’s important to consider the location of such buds because these are the points from which new shoots will emerge. If the bud faces left, then the shoot will grow to the left; if it faces right, the shoot will bear right. If the bud faces up or down the shoot will grow up or down.
Buds are miraculous things. Their outer scales are modified leaves whose function is to protect the delicate contents. Inside each mature bud is a tightly packed shoot, complete with its first few leaves, the apical bud, and even embryonic axillary buds. Buds vary in character as much as leaves do. In the winter months, buds provide a reliable means of identifying deciduous trees even without any leaves to go by.
New shoots also vary from tree to tree. During the summer months, leaves make sugars, and when eventually they become redundant, they turn brown and fall. In the case of deciduous trees, by their first autumn, the leaves are worn out, whereas the needles (or leaves) of conifers often last for two years, or even longer.
The joint between the petiole (leaf stalk) and the shoot seals off in deciduous trees, and the chlorophyll breaks down, allowing the residual pigments to show.
This is what causes that brilliant blaze of autumn color that people who live in temperate zones are privileged to enjoy. Spectacular autumn colors cannot always be relied upon. It will depend upon the health and vigor of the individual trees, and the feeding program that was followed in the preceding summer.
Weather also plays a part; in wet years the autumn colors are poor. However, when the autumn is clear with sunny days and frosty nights, the colors produced will be intense.
While nothing can be done about the weather, it is possible to enhance the chance of your achieving a show of the best possible color by maintaining control over the feeding and watering of your trees.