The request that starts it
Someone rings and asks for a tree to be "lopped" — usually meaning cut back hard, often by a third or more of its height. The tree is a eucalypt, it is close to the house, and a storm last month made everyone nervous.
The instinct is reasonable. The treatment is not.
What a cut through a leader actually does
A eucalypt has dormant buds under the bark along the trunk and major limbs. Cut the leaders out and those buds fire all at once, producing a dense flush of epicormic shoots below the cut.
Those shoots have two problems. They grow fast — a metre or more a year is normal on an angophora or a Sydney blue gum in a decent season — and they attach only to the outer wood at the point they emerged. They are not anchored the way an original branch is, which develops a branch collar and years of overlapping wood at the union.
Meanwhile the cut itself is a large wound in the middle of a limb, which a eucalypt cannot compartmentalise well. Decay works down from the stub into the limb the new shoots are growing out of.
Five years later
The tree is back to something near its original height, carrying more foliage mass than before because there are now dozens of shoots where there were a handful of branches. That mass sits on decaying stubs. The centre of gravity is higher and the attachment strength is lower than it was before anyone touched it.
That is the failure you were trying to prevent, made more likely and put on a timer.
It is also expensive. A topped tree needs repeat work every few years to manage the regrowth, and the eventual removal is harder because climbing a tree full of weak unions is more dangerous than climbing a sound one.
What to ask for instead
Crown reduction to Australian Standard AS 4373, where the outer limbs are shortened back to a suitable lateral branch that can take over as the new leader. Crown thinning, where interior branches are removed so wind passes through the canopy instead of loading it. And deadwooding, which removes the material that actually falls first.
All three are described on the pruning page, and together they typically remove far less live crown than a topping job while doing considerably more for stability.
When removal is honestly the answer
Sometimes the tree is in the wrong place and no pruning regime fixes it — a large gum three metres from the house on shallow sandstone, or a tree with basal decay. In that case, remove it properly and plant something appropriate. Topping is not a cheaper version of removal. It is a more expensive version of doing nothing, with a worse outcome attached.
For a second opinion on a tree you have been quoted to lop, call (02) 9538 7974, or read how failure risk is assessed first.