Environmental Engineering Reference
In-Depth Information
simple and inexpensive tree mats can be used around indi-
vidual installed plants to suppress weed growth. This
approach, however, may be problematic at sites where
thousands of trees need to be planted.
A few liabilities are inherent in the installation of a
phytoremediation system for the hydrologic control of
contaminated groundwater. The biggest one is the amount
of work that must be done below grade where many utilities
are known, or thought, to be located. A utility survey is
essential before any work begins that involves movement
of the subsurface soil. A utility survey usually is a free
service. Local ordinances regarding the maximum height
of plants located near roads, intersections, or flight paths
also must be followed. Many of these liabilities can be
addressed, however, even before the start of planting as
part of site assessment and characterization.
genes have been added to proffer protection against disease
or infestation.
The appropriate perspective must be kept in mind, how-
ever, during planting. Phytoremediation plants are not a
food crop, but rather a tool to remove groundwater or
contaminants at sites. Some losses to infestation are to be
expected but can be easily remedied. The SRWC industry
has demonstrated that even cutting trees to the ground can
result in extensive new growth, and removal of damaged
parts of trees stimulates new growth.
The footprint of the site boundaries controls the maxi-
mum size of the planting. As could be expected, if a constant
transpiration rate can be assumed for a given genus, the more
plants that are installed per site the more water that will be
transpired from the site. George et al. (1999) provide data for
conditions in Australia that indicate that for every 10%
increase in planted area, the water table decreased 1.3 ft
(0.4 m). These results indicate that plants that are installed
as closely as possible to each other at a site maximizes
biomass and, therefore, potential measurable changes in
the water budget.
However, the goal of a phytoremediation system is not
necessarily to establish biomass but that the biomass
interacts with groundwater. As such, Meinzer et al. (1996;
not O.E. Meinzer of the USGS!) reported that the transpira-
tion from a stand of koa plants (
Acacia koa
) was higher at a
larger spacing of 8 ft by 8 ft (2.5 m by 2.5 m) relative to a
closer spacing of 3 ft by 3 ft (1 m by 1 m). This was in
contrast to closer spaced plants that had higher biomass. In
general, transpiration and stomatal conductance increased to
a greater extent with a wider-spaced planting in response to
increased
VPD
, solar radiation, and wind speed.
7.2.2 Site Plants and Planting
Because the installation of plants at a phytoremediation site
leads to a grove, orchard, or plantation, it is worthwhile to
consider how the planting relates to future plant-care duties.
On the one hand, plants can be viewed as tools to solve an
environmental problem and can be considered strictly from a
mechanistic standpoint. Alternatively, each plant can be
viewed as a living entity that has to survive on its own ration
of air, water, and soil nutrients. In this view, plants are
installed with a full awareness of the soil properties (as
previously discussed) prior to the installation of the first
plant. In the first viewpoint, the planting design can be
constrained purely by limitations not necessarily related to
the health of an individual tree or its interaction with ground-
water. For example, the decision to plant along a particular
spacing interval between trees may be based on the width of
the tractor axle needed to maintain the area between rows. In
most cases, these two extremes of planting mindset should
be avoided and a compromise selected.
Conventional wisdom is that in order to establish a more
ecologically efficient system of plants, a monoculture should
be avoided. This wisdom is based on experience that a mixed
planting that consists of different species and even genera is
more resistant to infestation threats. This is partly true,
because if one plant in a monoculture is attacked by a pest,
few limits prevent all from being attacked. However, even if
multiple genera and species are planted, most often the
plants used are vegetative cuttings of a common parent
and, therefore, are derived from vegetative reproduction
and tend to lose resistance to infestation over time relative
to plants derived from sexual reproduction. This liability can
be avoided by using plants that have resulted from sexual
reproduction or by using transgenic plants to which other
7.2.2.1 Seedlings
In many cases, selective pressure during evolution has
resulted in plants that can reproduce themselves by using
more than one method. This includes sexual reproduction
and asexual reproduction.
Plant sexual reproduction occurs when the pollen pro-
duced from the stamen of a male plant enters the pistil of the
female plant of the same species (or different in the case of a
hybrid), fuses with the female egg, and forms a zygote. In the
ovary, this ultimately becomes the embryo of a new plant, as
a seed, encased in fruit in angiosperms but exposed in
gymnosperms. Under natural conditions, the seed is dis-
persed, germinates, and grows into another plant. Under
nursery conditions, these plants, or seedlings, are cared for
in individual pots until they are at least 1-year old. Seeds
from hybridized plants often result in less desirable
characteristics than the parent, which is why most are sterile
and are propagated by using vegetative methods as discussed
below.
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