"Andrew Ostrander" wrote:

Your point is overstated. If the leader of a blue spruce is broken or cut
off, one nearby young branch will take over as the leader, and the tree will
continue to grow like a tree, not a bush. Last year I had 3 twigs vying to
be the leader and growing vertically. Now the dominant one has been
determined, I wonder how

The highest bud becomes dominant and releases auxins that prevent the
lower ones from becoming dominant or growing vertically. It is an
especially strong response in Colorado Blue Spruce. An older spruce
can take several years to recover and develop a new leader. There are
documented cases where removing the leader without creating a dominant
bud has resulted in such stiff competition among buds on the highest
whirl that a new leader never formed and the Blue Spruce were
essentially turned into bushes.

Sometimes 2 leader candidates both continue to grow vertically and you get 2
trunks. Apical dominance seems to be effective only near the very top, and
once they grow long enough with side branches, neither will turn horizontal.

Apical dominance is caused by the upper most bud releasing auxins that
keep lower buds from becoming dominant or growing vertically. When
pruning Blue Spruce Christmas trees, care is taken to cut at a 45 degree
angle just above a bud so only one bud gains dominance and only one
leader forms. Any time that dominance is transfered, vertical growth is
stunted. Since Blue Spruce are slow growing trees, this stunting can be
a problem.

In Christmas tree production, pruning is done both to control size and
shape but to also insure a dominant bud.

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