Environmental Engineering Reference
In-Depth Information
Another characteristic of plants which has implications
for phytoremediation is the rate of growth of certain plants.
Kudzu (
Pueraria lobata
) can grow up to 1 ft (0.3 m) per day,
as can some other vines. In the southeastern United States,
kudzu is considered today to be an invasive plant, but was
initially introduced by the federal government to reduce
erosion in soils that were heavily planted during the cotton
industry boom in the early 1900s, and to be used as a
landscape plant, called porch vine. Kudzu can thrive in
nutrient-poor soils because as a member of the legume
family, kudzu creates nitrate in the root zone by the fixation
of atmospheric nitrogen, as is discussed in Chap. 11. Some
trees, such as poplars often used for phytoremediation, can
grow 10 ft (3 m) or more in one growing season, if nothing
needed to support this growth rate is limited. By comparison,
some plants grow only a few inches per year.
What causes this difference in growth rate in plants? Is it
difference in water use? Although water can increase cell
turgor and result in cellular elongation, it does not necessar-
ily affect the
rate
of cell division. The primary reason that
some plants grow faster than others is because they possess a
longer section of active meristem cells, up to 2-ft (0.6 m)
long in some plants such as poplars, relative to those that
have smaller sections of meristems and exhibit slower
growth. As such, the individual cell-growth rates are the
same for each plant, but plants that grow faster have more
cells dividing across a greater length than in slower growing
plants. Because massive amounts of energy are needed to
support the enlarged area of growth, fast-growing plants
typically are characterized by having larger leaves, and
more of them, to make food or extensive and deep root
systems for food storage and water and nutrient uptake.
Both of these characteristics are why phreatophytes, such
as willows and poplars, are successful for phytoremediation
if groundwater is within reach of roots. In essence, the use of
poplars to remediate contaminated groundwater is a result of
poplars having large sections of stem cells to support, which
drives the need to interact with groundwater.
Fast growth rates are more common for obligate
phreatophytes along riparian habitats where water is not
limited. Riparian plants also have a high rate of seed dis-
persal and rapid germination rates to take advantage of
infrequent floods. In fact, the growth and seed-dispersal
traits that ensure the survival of many generations of riparian
trees are similar to those of plants considered to be weeds.
Three examples of fast-growing riparian plants will be
discussed here; tamarisk, eucalyptus, and melaleuca. The
first two can be used in phytoremediation projects.
Tamarisk (
Tamarix
spp.
)
, or saltcedar, was introduced in
the United States around 1860 for shade, wood, and flood
control. It probably came from Europe, or the area around
the Mediterranean Sea, because tamarisk also is the name of
a river in the Pyrenees (Van Hylckama 1974). It also is
mentioned in the
Bible
, and wood from these trees is found
at many sites of antiquity in the Middle East. Alternatively,
it is possible that Spanish conquistadors may have brought
the plant with them when they invaded Mexico in the six-
teenth century. Although originally intended to be a
cultivated plant, tamarisk's ability to tolerate high-salinity
conditions and its fast growth have allowed its range to be
uninhibited since about the 1930s following a tree-planting
campaign, called the shelterbelt project, to slow soil erosion
after the Dust Bowl (Robinson 1965). Saltcedar trees pro-
duce large quantities of seeds; one tree can produce 500,000
seeds each year, with growth rates up to 10 ft (3 m) per year
after germination.
The most aggressive of the
Tamarix
species are
T. pendantra
and
T. gallica
. In particular, these species
have out-competed other trees to dominate the riparian
corridors in at least 15 of the 17 western states, including
Arizona, New Mexico, Texas, Oklahoma, Kansas, Colorado,
Utah, California, Nevada, Oregon, Nebraska, Idaho,
Montana, Wyoming, and South Dakota (Robinson 1965).
In some cases, its establishment occurred after floods
denuded large areas of native riparian species. Also, the
damming of many rivers for water supply and flood control
in this part of the United States has favored the tamarisk over
native species, such as willow and cottonwood, which rely
on the natural ebb and flow of flooding for seed dispersal.
Because saltcedar thrives along river banks in low topo-
graphic areas, the roots generally need to be no more than
25 ft (7.6 m) deep, because the depth to the water table is
shallow in groundwater discharge areas. Because the
saltcedar is fast growing, large volumes of groundwater are
needed to support its growth. And because no economic
benefit is derived from the wood or fruit, groundwater used
by the trees and not returned to the basin is considered to be
consumptive. Robinson (1965) reported that groundwater
used by saltcedar can approach 9 acre-feet (11,097 m
3
) per
acre (4,047 m
2
) in the southwestern United States. These are
the trees used by Gatewood et al. (1950) in the tank studies
of the effect of phreatophytes on groundwater resources
discussed in Chap. 1.
The eucalyptus trees (
Eucalyptus globules
) found in the
western United States came fromAustralia and may have been
introduced by railway owners who planted these trees along
the rights-of-way to provide a source of lumber for the ties as
well as shade from the sun. It was soon found, however, that
the wood was prone to splitting after the rails were attached.
These trees grow fast, at about 10 ft (3 m) per year. Eucalyptus
thrives not because it can grow along rivers and use shallow
groundwater like the tamarisk but that it can utilize deep
groundwater that is unavailable to most plants. The roots of
eucalyptus are called lignotubers and can store starch and
water for increased drought survival—the native people of
Australia use the roots as a source of water.
Search WWH ::
Custom Search