Decay in a tree is not always visible from an external inspection, as it progresses to the outer limbs from pith decay in the main trunk. Decay can progress slowly, if the tree is left in-situ in it's natural environment. Physical stresses to the tree (eg. Relocation/root damage) can accelerate the decay process.
Invasive tree roots can undermine foundations works and cause damage to structures including piped services.
Developmental sites often required land clearing for relevant construction. Disturbance to a root zone on a tree can lead to canopy die-back and tree decline. A risk of a tree falling can be caused by excavating/undermining the root system structure to a point which cannot compensate for the size of the tree and the wind sail of the canopy. Equipment strike upon bark, can lead to a penetration wound, which will invite tree decay and subsequent insect infestation.
Lightning and wind can cause damage to limbs and bark, which can subsequently lead to ingress of decay, and tree decline. When a tree's bark is penetrated or removed, this area can become an ingress point for insect or decay, particularly in soft fleshy trees such as Brachychiton rupestris.
By using a VTA (visual tree assessment) on mature trees, external damage and decay can be detected. The risk of falling limbs is increased when the decay in the outer limbs gets to a size that effects the structural integrity of the affected limb. The future effect of decay on limbs considered at risk, need to be monitored by a qualified Arborist within a predefined schedule, and preventative hazard reduction steps are to be taken as appropriate.