All living organisms have an interplay with electricity. They use, transmit, absorb or generate electrical energy. All living cells have electrical potential that can be measured in one way or another. In the case of trees, there are two useful ways we can use this electrical potential; the determination of the decay state of the wood and the extent and activity of the cambium.
DETERMINING THE CONDITION OF WOOD: In the case of wood there are two stages of wood; living wood and inert wood. Living wood or symplast is the layer of wood, often termed sapwood, that is actively involved in the transmission of water, oxygen and nutrients. Symplast has a normal life expectancy of between 6 – 15 years, after that it becomes inert – true wood.
The conversion of Symplast to true wood sees the lignification of the cells. At this time all soluble nutrients are extracted and recycled into the symplast. This leaves the True Wood virtually deionised. Deionisation is a protective state as it is effectively devoid of free minerals that would be required by microflora, decay organisms, to colonise and digest the True Wood.
Through wounding, by fire, mechanical damage such as storm or at the hand of man, this true wood responds to this stimulus and the damaged area is compartmentalised. See Section 3. The wound within the compartmentalised areas becomes colonised by a long succession of micro, meso and macro flora and fauna. The type and extent of the degradation of the wounded section of the tree is determined by the species of tree, the tree’s vigour and vitality, and the subspecies compartmentalisation strength.
Large hollows will form in trees that are low in all of these factors. This is the result of all tissue being (digested) decayed away to the outer extent of the wood tissue present at the time of wounding, the Barrier Zone. Tissue formed subsequent to the wounding event will remain unaffected. In trees that are strong, in all these areas, the wounded wood may simply become stained. It is important to understand that Decay organisms do not move at will throughout a tree they only decay wound altered wood.
The electrical conductivity changes through the decay process: Healthy True Wood, no wounds, few free ions, very low conductivity thus very high resistance. Conversely, the decay process frees ions, particularly potassium ions, and the conductivity resistance progressively drops as the decay process progresses, the end point will be a hollow.
In order to pass the special 2.7mm diameter, 300mm long probe into the tree, a 3mm diameter hole is drilled into the tree. This is a tool that not only drills the hole but it also provides a torque moment to the wrist of the drill operator. The torque generated by the resistance between the wood and the motion of the drill increases and decreases as the drill bit passes through the True Wood. In sound wood, high torque is referred to the drillers hand, in decayed wood lower resistance is felt, and in a hollow no resistance is felt.
With years of experience drilling and dissecting trees a third factor comes into play, which is Reaction Tissue. Trees are living organisms that respond to many stimuli, for example; light, gravity, wind and load changes through growth.
Thus if a wound decays away trunk tissue, a zone of different strength will result. This weakened zone will, under load conditions, flex and move differently to normal tissue that is above or below this localised weak point. This movement will stimulate abnormal growth in a localised area. This is referred to as Reaction Tissue as it is generated in reaction to the localised weakness.
Reaction tissue will be formed wherever a weakness is identified by the tree; this induces an electrical field that stimulates the cells into abnormal quantities and types of cells. These cells are much denser and structurally stronger and drill differently, with higher torque resistance, than normal wood.
VITALITY OF THE TREE: The more interesting use of the Shigometer and the Shigometry system is the use of a different special probe. This probe consists of a number of different styles of pins that are attached to the front of the probe. These pins are inserted into the bark through the cambial layer that exists just under the bark.
The cambium is the meristematic tissue that generates the tree. Phloem – Bark on the outside and Xylem – Wood on the inside. The initially living conductive tissue called Symplast. Symplast is the networks of the living in the bark and the wood. The section on the inside of the cambial zone is the Sapwood. Over time of apwood changes to become true wood, at this stage the cells lignification and deionised. Deionisation is a protective state.
There are two issues that are of particular interested in the management of trees, genetics and site conditions, Shigo refers to these as Vigour and Vitality.
VIGOUR IS GENETIC ABILITY, WHILE VITALITY IS WHAT YOU DO WITH THE ENVIRONMENTAL OPPORTUNITIES THAT EXIST.
Where there is no limitation factors the genetic potential of the tree will be expressed as its Vigour. The higher the Vigour the more cells there are within the symplast the thicker the layer the lower the resistance to an electrical current. However, as limiting factors come into play, the thickness of the Symplast will diminish and the resistance will escalate.
The example of using the Genetic Vigour of a line of plants is: Take a line of plants put them in controlled conditions, say in nursery pots, that all have the same potting mix with the same nutrients and moisture. The probe can then be used to identify the genetic Vigour of each of the plants. This will allow the plants to be graded according to the Vigour categories.
This selection process is used for planting into sites with different growth potential. The high Vigour plants, are planted in the poorer sites, the middle group, into the average sites and the low Vigour plants, into the best sites. In this way, an even stand of trees can be established.
Alternately, you can take a stand of trees that are established in similar conditions, soil moisture and nutrients, and obviously healthy and growing well. These trees can be probed; the reading recorded, totalled and divided by the sample number to establish a mean reading for such trees.
Then if you find trees of the same species that are growing in substantially the same conditions as the first group, but with poor crown health, you can compare the Cambial Resistance Readings for these trees which will be higher.
In both cases the reading is not an absolute, rather it is used for comparison between trees of different healthy states. The reading if a log scale so 1 = 10, thus the resistance difference, between average symplast health 2, of a tree with a reading of 1 shows it to be 10 times less resistant than the norm and a symplast reading of 3 makes it 10 times higher resistance to the mean.
Thus healthy vigorous symplast provides very low resistance reading due to their thickness and the high level of mineral ions. Where as depleted sick trees with thin symplast have progressively higher resistance readings.
Why does this work? The first group of healthy vigorous trees have low resistance due to the thick cambial layer, growing fast and the Symplast living for a longer period. The poor trees will have a thinner Symplast thus higher resistance and are growing slower. See Section 5 about energy, but put simply all organisms are in a constant battle between Living and Dying. Living takes energy and resisting death requires even more energy. Trees with high resistance cambial readings are loosing the battle for life. They are producing less energy than they are using in life and death resisting processes. They are running down their energy reserves see Section 6 Managing The Supply And Demand Of Energy.
If you produce less energy than you require you are committed to death. Eventually you get to a tipping point where no matter how much conditions improve, the organism is so depleted it cannot restore the organism to full health. It may slow the inevitable death but it cannot regain its youth.
The vast majority of trees established in the bush and then transposed into the urban landscape are committed to death by this transition. However, at the time of change they were healthy sound trees with full energy reserves. They were like the cut flower in the vase – alive but committed to death. Bush trees generally die within a period of 1 – 5 tree years.
The best conservation is to remove the old trees and replant new stock into the new site conditions. This is after all what nature does. It makes a clearing in the forest and a progression of renewal follows until the climax woodland species are once again dominant. The matrix of renewal occurs across the natural landscape so there are many trees of different ages and conditions across the forest. This process ensures healthy sound safe trees that grow to their natural climax. The alternative is mature trees that decline and die becoming hazards and are very expensive to remove but of more importance is that they are often not replaced.