I hope to show that such criticism is unwarranted; that stress is always present in nature; and that if we take the time to understand how trees cope with stress in a natural setting, we are more able to reproduce the appearance of stress in our bonsai, and more able to assist them to cope, develop and flourish.
The art of Bonsai is concerned with trees under stress. Not for we avid bonsai-ists the pampered tranquillity of the potted plant, or the tap-root luxury of the trees in the richly cultivated garden.
Ours is concern for the tree determined to survive under all manner of sparse and unsym- pathetic environments -- the windswept head- land, the rugged cliff face, the barren soil, the areas of fire and disease.
In trying to duplicate the appearance of stress in nature, the bonsai grower seeks to under- stand the physical and biological effects of such pressures on trees; how various types of trees re-act to various pressures; and how trees utilize particular qualities which enable them to survive in even the most hostile situation.
I intend to look at two forms of potentially stressful environments which occur on Australia's eastern coastline: the salt saturated environment, and the exposed windswept headland.
Forests of Mangrove of various types flourish
on coastal mud flats and salt marshes and are
often partially covered by sea and coastal rivers.
Mangroves are found along the whole of the
Australian coast, from the tropical far north
extending down to the cool climate of southern
Victoria. The Mangrove is an example of the
plant group called "halophytes". Halophytes
are plants which grow and complete their life
cycle in saline soils; the Salt content of the soil
ranging from about one-tenth to full strength
sea water.
How does the Mangrove cope with this potentially stressful situation? According to Osmond, 1979, halophytes can survive because they have the capacity to transport salt rapidly from root to shoot and to accumulate the salt in high concentration in the leaf cells. The saline tolerant plant uses this salt as a means of regulating pressure within its leaf cells. On the other hand, salt sensitive species cannot control large amounts of salt. The salt spills over to the leaf tissue, which is unable to cope with it. It has been shown that all plants have the same amount of salt in their tissues, but that halophytes have this unique ability to control the salt.
One way in which the mangrove manages to survive in salt water is through its development of a system of vertical root branches called pneumatophores or "cobblers' pegs". These rise closely packed from the mud to act as breathing tubes, enabling the plant roots to get oxygen through the thick mud.
Another curious adaptation, which enables the Mangrove to survive, is through its seeds. The embryo seeds, which form on the lower branches, germinate before dropping. While still on the parent tree, they develop two very long shoots which are thickest at the bottom and point downwards. When the seedling drops, one end sticks in the mud and anchors it, preventing the seed being washed away by waves or tide. The root develops at an amazing rate and takes hold in the mud, allowing the plant to develop and begin its life in water. Coastal vegetation above high tide level - contains many species which are not regularly exposed to high levels of salt in the soil. These plants, however, are exposed to heavy salt loads m the form of salt spray.
It is thought that this ability to cope with salt spray is determined by the low wetability and absorption of leaf surfaces, so that an excessive amount of salt spray does not penetrate in quantity into the leaf through open stomata (leaf pores).
In halophytes, the wax-like fibrils or fibres are angled so as to prevent the salt entering through the pores. In cases such as the Norfolk Island Pines at Manly ocean beach, Sydney, the heavy use of surfactants (ungraded detergents) caused these wax fibres to weld together exposing the stomata. This allowed excessive amounts of salt to poison the trees.
Wind is another environmental factor which places great stress on plants near the coastline. Wind affects water loss from both plant and soil. Its most important influence is its drying action which increases evaporation, thus emphasizing the effects of any water shortage. Wind tends to increase transpiration by thinning the 2 mm. boundary layer of air above the leaf of the plant. It also bends the leaf causing expansion and contraction of the intercellular space (space between cells).
Wind modifies the form of plants during their growth. In general, strong winds cause plants to be squatter with shorter internodes and larger root systems. Trees vary greatly in response to the force of strong winds. Pines, picea and oaks often tend to be flattened against the ground, while other trees in the same habitat remain erect.
When developing shoots are subjected to strong wind-pressure from a constant direction, their form and position may become permanently altered or deformed. Deformation is not necessarily accompanied by dwarfing. Moist winds can mould the form of the trunk without appreciably reducing its size.
Trees with inclined trunks are commonly observed on ridges along the coast, where they are exposed to unidirectional and severe winds.
Trees in this type of environment make several adaptations. The most efficient tree form on windy habitats has low streamlined contours and numerous crowded branches. Wind injury to terminals continually prune the tree back to this compact shape.
Adaptation of the tree itself may include flattening of the trunk, root and branches in a plane parallel to the wind direction. Growth rings are chiefly active on the leeward side of the trunk, with most branches emerging from the leeward side or being bent permanently in the leeward direction.
To minimize damage, plants tend to grow together in communities to form a continuous carpet over the earth. In this extensive cover, it is usual to find particular species of plant dominating all other species. Like plants tend to grow together; plants suitable to that particular habitat.
When growing bonsai, we can learn from this
stress and adaptation in nature in two main
ways :
Firstly, we need to study the conditions
under which particular trees grow so that we are
able to create a friendly environment for our
bonsai even while attempting to reproduce the
appearance of stress.
What I have discussed appears to suggest that nature provides plants with the ability to either exclude stress (e.g. waxy fibres of the pineapple plant); or to cope with stress (e.g. cobblers' pegs and seed shoots of the Mangrove). We have seen that nature is selective in the type of trees it will allow to grow in an environment. If, as bonsai-ists, we hope to reproduce nature, we must be equally selective in the plants we choose. For example, it would not be legitimate to use plants in a sea-scope setting that could not cope with the extremes of this type of environment in a natural setting.
Secondly, I suggested in my introduction
that Bonsai, by its very nature, places plants
under stress - stress caused by wiring, potting,
and pruning. Stress by wiring, for example,
causes death or displacement of a certain
amount of cells within a branch.
The same effect is caused by wind (over a
much longer period) in a natural setting.
Recognition that we are placing trees under
stress emphasizes the importance of after-care.