" I recently reduced a white pine that had been topped 25 yrs ago according to the owner. The cavity was about 35% with a lead of about 30' above it. This is a removal by installment job now."
I hear this said quite often, but do not understand why there is such little belief in the tree's ability to codit and retrench and grow on. A 35% hollow may not be significant.
Alex Shigo’s A New Tree Biology (1986) introduced many new
concepts about tree pruning that we are still struggling to understand and apply. For every general rule we make there are exceptions,many confirmed by new research. Pruning branches back to the branch protection zone (BPZ) at
their origins is generally good,unless too much heartwood is
exposed to decay.
Removing codominant branches also invites decay, because they have no BPZ. What are our options? Reduction
of stems and scaffolds is sometimes considered, like topping, inadvisable because BPZs are not present.
However, the same kind of chemicals and anatomical structures exist at other nodes—“enlarged portions of stems
where leaves and buds arise”—especially those where the terminal bud was set.
“Topping is done internodal…” according to Shigo, and the ANSI (2008) standardsagree. As Shigo goes on to say, “…proper crown reduction is done at nodes, or at crotches.”