Biomechanics, as defined by Webster’s Dictionary, is “the study of the mechanical laws relating to the movement or structure of living organisms.” In tree biomechanics this includes consideration of internal (tree’s own size and weight) and external (wind, anchorage) forces put on the structure which contribute to compression and tension in the plant tissue.
A tree is a “self-optimising mechanical structure,” meaning the growth of new wood aims to eliminate stress concentrations, maintaining a uniform stress distribution throughout the tree, this is called the axiom of uniform stress. It is for this reason that trees are usually wide at the base and tapered at the top. It is this innate process that must be considered when pruning is undertaken.
After losing its leading shoot the lateral succeeding branch of the tree will reposition itself over the trunk’s centre of mass, this is called the minimum lever arm and refers to the distribution of weight along a branch that affects the load at the base. It is a self-correcting counter-gravity growth phenomenon.
A curved stem with load bending against the direction of curvature, is called a hazard beam: they tend to split through the centre, to the overall effect of straightening the curve. Narrow forked co-dominant stems, which are equal in diameter, with included bark that continue radial growth results in pressure pushing the forked limbs apart, these are called tensile forks.
Disfigurations that require pruning include: bulges, swelling in response to an irritant (bacteria, cavities, pruning cuts, etc); buckling, the irreversible deformation of a plant structure due to load; cracks/splits, material fracture in a tree usually along the grain; and ribs, long protrusions along the grain in response to structural weakness.
So, how do these stresses affect the approach to pruning?
Pruning without knowledge or consideration of how the tree will respond to the cut can be more detrimental than beneficial in the long run as the effect may be counter to what was intended. Never forget you are working with a living organism that will respond accordingly to the stresses put upon it and do it’s best to ensure it’s own ongoing health.
When haphazard pruning is completed (ie pruning back to a boundary line in a neighbourly dispute without attention to the whole tree), larger problems often occur – in addition to the tree looking an eyesore. Poor pruning can promote growth of adventitious branches, which grow with a minimal wood content, making them spindly and weak. Presence of these branches indicate the tree has undergone additional stress. A tree will also respond to pruning in the wrong timescale: a fruit tree may minimise fruit production or choose to stop fruiting all together when pruned off season. As always, if in doubt about the best care for your trees, consult a Qualified Arborist.
Next month… Correct pruning techniques.
Andrew Noakes, of NZ Arb Consultancy, is a local Qualified Arborist and TRAQ Certified Assessor with over 30 years experience caring for trees in a variety of situations. firstname.lastname@example.org