Alteration of Root Architecture in Field-grown Liners
Alteration of the architecture of structural roots by early stages of nursery
production is only beginning to be recognized as a possible contributor
to deep structural roots. To understand how this affects root system
architecture, an understanding of development of root architecture
under natural conditions is needed.
In Nature
The primary root emerges from the seed and grows straight down in response to gravity. When more compacted, less aerated conditions in deeper soils are encountered, growth of the primary root will slow. This may occur quite close to the surface in dense or poorly drained soils, and in species with weak primary roots, or somewhat deeper in strong taprooted species, and on well drained sites. When growth of the primary root slows, growth of the small lateral roots near the soil surface increases. As growth shifts, these lateral roots start to develop into the large, shallow, more-or-less horizontal roots that form the root flare.
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In Nursery
The first step in production of tree lining
out stock (liners) begins with producing seedlings in dense
beds. Seeds are planted very shallow, so the roots develop
very close to the soil surface. Most species are grown for
one season in these seedling beds.
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At the end of the first season, the seedlings are mechanically
harvested and the primary root is pruned to approximately
10 cm. The next spring, seedlings are replanted in rows,
maintaining the same depth as in the seedling bed. Late in
the summer of this second year, cultivar buds may be grafted
on to these rootstock plants, 1-2 inches above ground. (photo
by: J. Frank Schmidt & Son,
Co.) |
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Before growth begins in the third year, the stem of the rootstock
plant is cut back to just above the bud graft. Trees sold
as seedling stock (not grafted), are also cut back to a low
lateral bud. Evidence of cutting the stem back will be visible
for several years. |
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Since the root system has already been established for
a year before cutting back, the new growth is rapid and the
stem can be trained into a single straight trunk in one year.
If harvested at this stage, these liners are referred to
as whips. They may also be grown 1 or 2 years longer to be
sold as branched liners. |
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The transplanting and pruning operations at the end of the first
year in the nursery are never experienced by trees in nature.
How does the root system react to this process? Roots are
regenerated from the cut end of the primary root. Growing
conditions in the soil around this cut end are ideal for
root growth and the regenerated roots grow rapidly. Most
of the shallow laterals do not persist. |
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Adventitious Root Flare
The roots regenerated from the cut primary
root are adventitious roots, induced by pruning. As the tree
increases in size, they form what can be called an adventitious
root flare several inches below the natural location for
the root flare. Sometimes a few laterals remain on the “root
shank” (the remaining portion of the primary root above
the adventitious root flare). The pattern seems to be set
at time of seedling transplanting, but much more information
is needed. We are still learning about species variation. |
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The distance between the ground line and adventitious root
flare must be as small as possible. If the distance is too
large, the adventitious root flare will develop too deep. |
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