Environmental Engineering Reference
In-Depth Information
Manual pruning can be done at phytoremediation sites
and is done to achieve the same effect, which is to protect the
overall health of the plant. To understand what happens
when a plant is pruned, it is necessary to understand how a
plant grows at both the macroscopic tissue and microscopic
cellular levels. Then and only then can the decision when
and where to prune be made. Some of this information was
covered in Chap. 3. Essentially, pruning is the selective
removal of plant tissue to achieve a goal of a certain plant
form or to remove older or diseased wood, such as stems and
branches. The mere act of pruning, however, ironically
enhances the formation of new growth, by causing dormant
buds beneath the cut to become active and start to grow. One
stem terminal bud, for example, if removed by pruning, will
cause two new stems to take its place—this will increase the
fullness of the plant.
The act of making a pruning cut is important not only for
the shape of the plant but also its health. Only sharp blades
should be used, and the cut made as close as possible to the
main stem. Any amount left behind that projects from the
main stem will die, and start to rot. This rot can then have an
opportunity to invade the main plant. Also, the pruning cut
should be made from the bottom up if using a scissor-type
blade, but from the top down if a saw is needed to remove a
large branch. In this manner, the weight of the branch helps
keep the cut open and the saw from binding. However, an
initial small cut from the bottom up close to the main stem
will prevent
Similar to the question posed previously regarding the
best time to plant, there are many answers to the question of
the best time to prune. In general, pruning can be done
anytime it is convenient for the person doing the pruning.
There are positive and negative consequences, however, that
should be considered before a cut is made based on conve-
nience. Pruning can be accomplished during the winter when
plants are mostly dormant, especially when removing dead
branches. Winter pruning results in the production of wood
at the expense of leaves or flowers or fruit. Pruning of live
branches removes wood that contains many dormant buds.
As a result, less leaves will be produced the following year
because energy will be spent on new wood production,
formed from adventitious buds, to replace the pruned
wood. Fall pruning also can be done to remove old dead
growth but will cause the remaining buds to be more numer-
ous and grow faster in the spring. Late spring pruning should
be avoided because the commencement of sap flow and
higher
ET
p
lead to wounds that leak sap, which can become
infected.
From this information, a general rule to be followed in
most areas regarding pruning is that it should be done when
the plant is dormant, typically between the fall and spring.
This is because more food stored in the roots will be avail-
able for fewer parts of the shoots in the spring when top-
growth resumes. This is counterintuitive, because pruning
often is done in an effort to decrease the size and shape of a
plant. For poplars, this is after leaf drop in more temperate
areas of the Southeastern United States or in the spring in the
cooler northern areas. During dormancy, most deciduous
trees contain water stored in the xylem sapwood and even
transferred to the heartwood prior to when the leaves
dropped. Also, after the leaves drop, no new sugar
compounds are formed by photosynthesis, so no additional
sap is being formed. This is more of a problem for slow-
growing trees that typically are not used in phytoremediation
studies. All in all, pruning during dormancy produces cuts
with very little fluid leakage. Very little pruning should be
done the first year after planting a phytoremediation system,
especially if whips or cuttings are used, other than removal
of dead or damaged areas or removal of the terminal bud. For
plants such as birches or maples, summer pruning is advised,
because fall pruning causes too much leakage of downward
moving sap. Pruning should be done when temperatures are
above freezing, as the wood is too brittle when temperatures
are at or below freezing.
With respect to pruning commonly used phyto-
remediation plants, eucalyptus trees require special care.
This is because they tend to drop a lot of plant parts, called
slash, that can include very large branches. These can be
removed to limit the potentially negative consequences of
such dropping, but keep in mind that reduction of a tree to its
the bark from tearing as the down-cut
is
finished.
Although pruning typically is associated with selective
removal of plant biomass above ground, pruning also can be
performed on root material. Root pruning can be accom-
plished to decrease root competition for resources at
phytoremediation sites where the trees typically are planted
within a few feet of each other. Some planting approaches
also can decrease future root competition when cuttings or
trees are installed in holes lined with impermeable materials,
typically designed to limit lateral root growth and encourage
deep root growth. The effect of root pruning on plant and
water relations tends to be site and tree specific, however.
Woodall and Ward (2002) reported that following root prun-
ing of tree crops in Australia, sap flow in some trees was
unchanged but ceased in others. They also reported that soil
moisture increased in sediments after the roots of plants
were removed, as the sink for the water had been eliminated.
The perceived benefit of increased plant biomass, health, and
water relations associated with root pruning may become
more apparent at sites with relatively close tree spacing.
Moreover, there may be genetic controls on root pruning
that are plant specific and, therefore, the success of this
approach may be hard to predict, or may result in varying
degrees of success.
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