Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 184

7.3.10 Climate and Vapor Pressure Deficit

per a given area and, therefore, can potentially transpire
more water and groundwater relative to areas of bare soil
that are not planted.
Leaf size also is important in considering the closest tree
spacing necessary to support hydrologic goals and protect
plant health. Most trees have differences in leaves even on
the same tree, with leaves grown in the shade near the
ground being larger than leaves grown in direct sun near
the top of trees. This is the result of smaller sized leaves at
the top that can dissipate heat and larger leaves at the bottom
that dissipate less heat but need to be large to capture fleeting
levels of light.
The widely used Populus genera supply various
organisms with a source of food to encourage their presence,
which could jeopardize a phytoremediation planting. Pests
are mostly the larval stages of various species of beetles
( Coleoptera ) and butterflies ( Lepidoptera ). For beetles,
such as the cottonwood leaf beetle, the larval stage migrates
from eggs that winter over in the leaf litter to the underside
of leaves where they amass in groups that consume the leaf
material. Rather than using the presence of these and other
organisms as the criterion to enact control measures, it
should be the extent of the infestation that is used to decide
if foliar application of insecticide should be used. In all
healthy ecosystems, there always will be some level of
host and predation activity—this is to be expected.
Another interesting factor to consider when deciding
which plants to use to interact with contaminated groundwa-
ter is the rate of growth. The adage of faster is better often
can become the primary criterion for plant selection at a site.
Choosing a plant that can grow faster relative to alternatives
has the potential economic advantage of installing smaller,
less expensive trees, and yet being able to demonstrate
closed canopy conditions within a similar timeframe as if
older and larger plants were installed. However, a fast
growth rate does not necessarily indicate that more water
will be moved through the plant and, hence, translate into
successful phytoremediation for hydrologic control. For
example, many pine trees can achieve fast growth rates
similar to hardwoods, such as poplars and willows, but use
less water per tree. Lower transpiration rates for pines that
have similar growth rates to poplars can be explained by
pines' lower LAI —plants can be fast growers but use less
water because the total LAI is lower. Fast-growing plants
that have lower water use due to lower LAI include Leyland
Cypress (the naturally occurring hybrid Cupressocyparis x
Leylandii ); dawn redwood ( metasequoia glyptostroboides );
loblolly pine ( Pinus taeda )—one of the few pines that actu-
ally prefer wetter soils such that its name is derived from
moist depressions called loblollies; slash pine ( Pinus
elliottii ); and pond pine ( Pinus serotina ).

The water content of the air is an important control on the
success of phytoremediation of contaminated groundwater.
The transpiration gradient is set in motion at the leaf surface
in response to the humidity difference in vapor pressure
between the water vapor in the stomata relative to the air.
Given all factors being equal, more water will be transpired
by a plant during conditions of drier than more humid air.
This is one reason why caution needs to be exercised when
trying to apply results from one phytoremediation site to
another, because the air humidity characteristics may be
radically different between sites, even though the same
plant may have been used and the depth to water table are
equal.
Ambient air contains some level of moisture. This mois-
ture, as water vapor, exerts a partial pressure on its surround-
ings. As the amount of water vapor increases, the partial
pressure also increases. As this occurs, the partial pressures
of the other atmospheric gases, such as CO 2 , oxygen, and
nitrogen, must decrease to conserve the total pressure,
760 mm Hg at sea level (defined hereafter as the North
American Datum of 1983, NAD 83). For example, if the
water vapor content of the air is 5%, the partial pressure of
water vapor is 38 mmHg; the balance of which (722 mm Hg)
will be left to the other gases. Warm air requires more water
vapor to reach saturation than cool air. The absolute humidity
of the air is the partial pressure of water vapor, and saturated
humidity is the total amount of water vapor that the air can
hold at a specific temperature; therefore, relative humidity is
the current percentage of total saturation.
Specifically, vapor pressure, VP , is the pressure that
water vapor in air exerts on its surroundings. The maximum
VP ,or VP m , is the vapor pressure of a molecule when it has
come into equilibrium with its confining space. For exam-
ple when water is heated, the VP m occurs when water boils
at the same rate that water condenses. This VP m can be
decreased, however, by solute addition. The ambient VP ,
VP e , is the measured VP at conditions less than saturation.
The vapor pressure deficit, VPD , between VP m and VP e
represents the magnitude of the deficit between what VP
is and what it can be at a given temperature. If VP e is less
than VP m , a gradient is established so that water will move
to balance the vapor pressures. Relative humidity is the
more commonly used and recognized term for the ratio of
the VP e / VP m (100).
The VPD of an area will affect plant transpiration and,
therefore, phytoremediation potential. As a plant grows and
has more and larger leaves and increasing LAI , the transpi-
ration rate should also increase, especially if the VPD is
high. This relation between LAI and VPD is depicted in
Fig. 7.10 . For two Eucalyptus species, when the LAI was
highest during the summer (December in the southern