Dieback
Short-term stress management is manifested by the dieback of the less productive and vital crown sections or iterations. This may see the inner branches of the crown or larger crown iterations die, while the remaining sections of the tree apparently maintain normal leaf density, colour and vitality.
This is often due to a sudden or short term event, either natural or man made stresses such as, drought, soil disturbance and the like. Once these stresses are alleviated growth will return to normal and new crown iterations will occur.
Decline
Decline is manifested as an overall crown thinning, reduction in leaf colour and reduced extension growth or reduced vitality. This is normally the end result of a chronic malaise that depleted the energy reserves of the tree. Typically, decline is followed by flushes of sprout shoots along the branches and trunks. A particularly precocious flowering event may also occur followed by sudden vascular collapse.
Eucalypts are particularly good at generating sprout shoots along their branches and trunks. Sprout shooting from advantageous buds form part of their fire resistance ability that some Eucalypts have developed over millennia. Unfortunately, in many cases such sprouts on trunks and branches are in response to the excessive heating of that section from the sun, due to the lack of shade from a healthy crown. These sprouts often wither and die as the available resources are extinguished. The root system of many declining trees had long ago been killed or starved to death. The tree is now simply wicking water up through its vascular system or absorbing it from dew or rain. This occurs in the same way as a cut flow in a vase of water. Such trees may live for decades until the crown “Suddenly” collapses. However, there is no such thing as “Sudden” in the life of a tree. Failure in any form, or death, is the end result of a very extended and complicated process.