Environmental Engineering Reference
In-Depth Information
terminal growth will limit the
LAI
and keep transpiration
below optimum levels.
The care for evergreens, especially the thin leafed or
needle-type conifers, is slightly different than for deciduous
trees. Evergreens drop their leaves like deciduous trees, but
because of their efficient use of water and ability to grow in
areas where water availability is limited, they drop their
leaves every 2 to 3 years. Some evergreens, such as the
baldcypress, that grow where water is not limited, drop
their leaves annually, hence the adjective bald. One of the
problems with pruning thin-leafed evergreens relative to
deciduous trees is that rapid growth from the lateral buds
does not occur. Hence, if too much pruning is done, there
will not be enough leaf-surface area to make food for the
plant, and it literally will starve to death. Therefore, it is not
recommended to do heavy pruning of such trees; rather, light
pruning should be accomplished throughout the year on a
more frequent basis.
There always is some level of concern that the benefits of
pruning come at the cost of essentially wounding the plant in
the process. However, plants naturally lose limbs all the
time, as can be seen the day following a major wind storm.
In most cases, these limbs are older and partially dead
anyway; or in the case of the lower shaded limbs of large
conifers, the loss is a benefit to the plant because the limbs
were a net drag on photosynthesis. Remember that the outer
layer of all plant parts consists of a tough layer of epidermal
cells, the first line of defense between the plant and the
hostile external world. These cells can produce suberin in
the cork cells beneath the bark. In most plant parts, naturally
occurring tannins are produced and act as natural antimicro-
bial and antifungal compounds. After a wound or pruning
cut is made, the exposed cork cells die, and the cutin and
suberin are released and flow over the cut. Then parenchy-
mal cells rapidly grow and cover the exposed area with a
callus. To encourage a high rate of such coverage is the
reason why pruning cuts are made as close as possible to
the main trunk. Moreover, pruning is indeed not as harmful
as sometimes envisioned and is best proven by the fact that
the parts removed from the plant can often survive on their
own, as occurs in vegetatively propagated plants.
To prevent organisms, such as fungal spores, from
penetrating the entire plant by way of the phloem that may
be exposed following damage by a storm, these cells when
broken release callose, that essentially plugs the sieve tubes
so that the exposed phloem cannot flow. In conifers, broken
cells release resin produced by the duct cells of the afore-
mentioned rays, which harden when exposed to air. Other
examples of such wound healing and sealing compounds
include gums, which are used to thicken some foods, and
latex. Other compounds include the tannins, which are
discussed in Chap. 11.
Poplar trees can be cut flush to the ground in the event of
problems with top growth or damage by storms. This is
purposefully done during coppice silviculture to produce
and harvest trees for paper products or as an alternative
energy source. That certain trees regrow after such cutting
has been known for some time:
For there is hope for a tree,
if it be cut down, that it will sprout again,
and that its shoots will not cease.
Though its root grow old in the earth,
and its stump die in the soil,
yet at the scent of water it will bud,
and put out branches like a young plant.
Job 14:7-9 (RSV)
Why is all of this information about pruning important to
those concerned with phytoremediation of contaminated
groundwater? First, pruning will have to be done. Pruning
makes more sense when the action is underscored by an
understanding of plant physiology and the purpose behind
the physiology, which is for the cells of a multicellular plant
to essentially all work together to make and redistribute
food. This is analogous to vineyard owners that purposely
girdle grape plants below the branches that contain the
grapes by removing the phloem after the fruit has formed.
This ensures that all the food created by the leaves is routed
to fruit production, size, and sugar content rather than to the
roots.
As with any investment, there are concerns for the health
of a phytoremediation crop after it has been installed. A
farmer will worry about changes in the weather and
the appearance of disease or infestation. It is no different
for a grove of plants used for the phytoremediation of
contaminated groundwater. The question is, should prophy-
lactic control measures be used? Some plants can immunize
themselves after mild exposure to certain pathogens. This is
described more fully in Chap. 11. Chemical as well as
biological agents can be used, but need to be accounted for
in any assessment monitoring.
7.3.12 Growing Season Length and Effect
on Acceptance of Phytoremediation
A widely held assumption regarding the relation between
plants, water use, and groundwater is that deciduous trees,
such as hybrid poplars, are relatively passive and inert with
respect to water use during the dormant season (Interstate
Technology Regulatory Council 2009). This assumption is
valid and understandable for the most part, because the lack
of leaves during the dormant season indicates that water loss
from subsurface sources to the atmosphere does not occur
through transpiration.
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