Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 268

Fig. 11.11 The effect of the
exclusion of salts on the salinity
of pore water in the sediments in
the root zone as a mangrove tree
uses water. Modified from data
presented in Greenwood et al.
(2006). One meter is equivalent to
3.2 ft.

was studied by Shannon et al. (1999). Cuttings of eight
poplar species, including DN-34 and OP-367, were exposed
to salt between 3.3 and 7.6 dS/m were observed to have
reduced growth rates and shed leaves prematurely. The
authors concluded that the poplars studied were not very
tolerant to salt, especially in relation to salt-tolerant euca-
lyptus trees ( Eucalyptus spp. ). The study by Shannon et al.
(1999) supports the observation of higher salt tolerance in
DN-34, with no defoliation at 5.53 dS/m—this salinity is
equivalent to about 3,000 mg/L of total dissolved salt. Even
for the more salt-tolerant poplar species, however, the nega-
tive effects of salinity on growth are enhanced when transpi-
ration is higher and evaporation rates are higher.
Such tolerance to salt by phreatophytes also may be an
adaptation to acquiring water from deeper within the soil
zone than can be reached by shallower plant roots. Nilsen
et al. (1983) reported that Honey Mesquite ( Prosopis
g landulosa ) growing in the Sonoran Desert of California
used groundwater from between 4 and 6 m below land
surface. Near-surface, pore-water soil moisture was
characterized by increased salinity relative to deeper depths
because of soil-water evaporation but also hydrologic lift.
Consequently, soil-water potentials in the surface soils were
lower, between

data suggest a mechanism by which water stress could be
avoided.

11.3.2 Extreme Temperatures

Prevailing temperatures affect the types of plants that can
grow in a particular area. Because plants cannot respond to
changes in temperature that occur in a location on a seasonal
basis like migratory animals, they have had to adapt to these
changes. In fact, most plants in temperate areas actually need
for temperatures to change to promote sexual reproduction.
For example, certain flowers that grow from bulbs, such as
daffodils, have to undergo a period of cooler weather before
they will grow, produce flowers, and can be pollinated.
Deciduous trees also have a similar response to changes in
temperature. Close inspection of some flowering trees dur-
ing early dormancy reveals buds that do not open unless they
are exposed to a period of cooler temperatures.
Temperatures affect plants not only changing over time
but also over space. Temperature changes with a change in
elevation, and this produces a gradient of plants. An example
is the lack of plant growth beyond a certain elevation and
temperature decrease in mountains, which is called the tim-
berline. In such colder conditions or even climates, the few
plants that do grow consist of mosses and grasses.

4.0 and 5.0 MPa relative to deeper soils

that had 0.2 MPa. This gradient caused deeper groundwa-

ter to flow vertically upward. The authors report that these