Many urban trees are continually wounded – usually to more minor extents (mower damage, minor acts of vandalism), though vandalism may also cause major wounding, as may pruning work (for both necessary and unnecessary reasons) – particularly where the work is over-zealous and not in line with BS 3998:2010. If a tree has not recovered from the previous pruning event, further pruning will only serve to worsen the state of the tree over-and-above its previous ‘worst’ state – pushing the tree towards strain (Shigo, 1986).
Such continued attrition to a tree means that compartmentalisation of the wounds must occur. The process of compartmentalisation requires the tree to wall-off the infected area, in turn reducing its energy-storing capacity as otherwise-functional parenchyma cells within sapwood and rays are locked away. When trees receive a large array of minor wounds or a few major ones (or a mixture of both) continually over time, the progressive locking away of potential storage capacity means the tree cannot keep energy reserves needed to even sustain its current mass – and as defence reactions take priority over other tree life processes (Shigo, 1986; Shigo, 1991), continued wounding will also mean the tree has less energy for growth and maintenance of mass. Once such a threshold is reached and crossed therefore, mass reduction is an inevitability. The tree is strained.
Sources:
Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.
Shigo, A. (1991) Modern Arboriculture. USA: Shigo and Trees Associates.
Arboriculture 