From experience in assessing trees for figure, such as flame, we can suggest that it is not directly related to size, age, or environmental conditions such as soil type, rainfall, temperature, elevation and aspect. Although these variables may have some effect on flame formation they are not directly related.
A theory that best explains our experience with Blackwood (Acacia melanoxylon) trees that exhibit flame is that the phenotype of an individual must contain a gene or group of genes responsible for reduced cell wall strength, resulting in compression of the wood (xylem tissue). This may be evident in trees as young as 30-40 years of age and can extend from the roots, up the trunk and out into the branches. This helps to explain why, for example, 50 trees within the same area may display no flame and one individual will have extreme flame.
In combination with a weak cell structure, exposure to an increase in compressive stress due to nearby clearing or disturbance, internal rotting or rotting to one side may also explain, and is common amongst, trees that exhibit flame. The increase in compressive stress as a result of nearby clearing may come about through an increase in above ground biomass as the tree invests more energy into leaves and branches due to increased space and light avaliability. The larger above-ground biomass puts more compressive stress on the trunk of the tree causing it to compress laterally. An increase in stress due to clearing may also be due to the increased effects of wind on stability. Internal rotting or core rotting puts increased stress on the remaining wood, causing it to compress laterally, as would rott occuring in any region that maintains the structural stability of the tree.
Flame formation may therefore develope slowly over the life of the tree (phenotype) or appear relatively suddenly (combination of phenotype and disturbance or rotting). In both formation types, the cambium region is also laterally compressed forming zig-zag patterns when viewed from the outside of the tree (shown below). Further lateral growth of the xylem tissue will therefore result in flame formation.