Environmental Engineering Reference
In-Depth Information
Fig. 3.12
The entry of water and
solutes into the vascular system of
a plant can occur by two
pathways. Water and solutes
follow a path from the sediment
or pore space to a root hair by
diffusion, through the
intracellular space to the cell
walls of the cortex to the
endodermis through the
Casparian strip and finally the
xylem (Modified from Curtis
1983).
Evidence exists in Hultline (2003) and Leenhouts et al.
(2006) that water in roots in the shallow soils also
can move in the opposite direction of hydraulic lift; that
is, downward from wetter soils to roots that exist in
deeper, drier sediments. This process, called hydraulic
push, may explain how the roots of phreatophytes follow
the fluctuation of the water table and how such plants can
become established after germination, grow deeply into
relatively dry sediments, and reach declining water tables
over time.
Brooks et al. (2009) present evidence that the subsurface
water used by some plants is not necessarily from recent
precipitation and infiltration but rather from residual water
that has been entrained and, therefore, represents water from
multiple past precipitation events. In essence, their data of
water stable isotopes indicates that all of recent infiltration
does not necessarily move rapidly from the land surface to
the water table, also called translator flow. Plants that grow
in the dry Mediterranean climate of the study area have
adapted to these dry conditions by essentially mining this
trapped water and, therefore, would not be classified as
phreatophytes that rely on groundwater.
plants from soil to air is called transpiration, from the Latin
trans
for across and
spiro
for to breathe. Moreover, from the
entry of water into root hairs to the release of water vapor
from the leaves, no to very little energy is expended by the
plant—plants can be considered,
therefore, as efficient
parasites of the sun, air, and water.
But how can water be transported from the roots to the
leaves at the top of the tallest trees? Some ancient coast
redwoods in California's Redwood National Park are over
370 ft (112 m) tall, weigh about 1.6 million pounds, and are
about 600 years old; in fact, the oldest was 2,200 years old
but was logged in 1933. The coast redwood is
Sequoia
sempervirens
, whereas other sequoias can be found in the
Sierra Nevada (
Sequoiadendron giganteum
) and China
(
Metasequoia glyptostroboides
). These are not only the
tallest but the fastest growing conifers in North America,
which is significant because conifers primarily are slow
growing as they have adapted to low-moisture climates.
Fossils of similar trees have been found in sediments that
date back to the Jurassic Era some 160 MYa in areas much
more widespread around the world than today.
The current, more isolated, distribution of redwood trees
can be explained primarily as the result of abundant soil
moisture (Rundel 1972; Preston 2008). Moreover, these
trees were classified by Robinson (1958) as phreatophytes.
Rundel (1972) reported that during seasonal dry periods
when precipitation was low, relatively high soil moistures
were measured beneath redwood tree roots, supplied by
groundwater. This groundwater had to have been recharged
elsewhere during times of higher precipitation. In fact,
3.5
Vascular Tissues, Leaves,
and Transpiration
At its simplest, a plant takes advantage of the radiant energy
of the sun to make food, and can control to some extent the
rate of water evaporation. This movement of water through
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