Principles of terrestrial ecosystem ecology.pdf

Depth (m)
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Chaparral
1.6
ψplant Quercus -0.2
1.8
Adenostoma -1.4
Heteromeles -2.1
2.0 Rhamnus -2.8
Grassland
ψplant Sitanion < -4.0
-5.0 -4.0 -3.0 -2..0 -1.0 0
Soil water potential , ψ soil (MPa)
Figure 4.8. Soil water profiles in adjacent shrub and
grassland communities at the end of the summer
drought period. Predawn water potentials are a good
index of the soil moisture and the degree of drought
stress experienced by the plant. (Redrawn with permission
from Oecologia; Davis and Mooney 1986.)
Water Movement Through Plants
The vapor-pressure gradient from the leaf
surface to the atmosphere is the driving force
for water movement through plants. Water
transport from the soil through the plant to
the atmosphere takes place in a soil-plantatmosphere
continuum that is interconnected
by a continuous film of liquid water. Water
moves from the soil through the plant to the
atmosphere along a gradient in water potential.
The low partial pressure of water vapor in air
relative to that inside the leaves is the major
Water Movements Within Ecosystems 83
driving force for water loss from leaves, which
in turn drives water transport along a pressure
gradient from the roots to the leaves, which in
turn drives water movement from the soil into
the plant. The rate of water movement through
the plant (J p) (Eq. 4.7) is determined by the
water-potential gradient (the driving force;
DYt) and the resistance to water movement, just
as described for water movement through soils
(Eq. 4.6). As in soils, the resistance to water
movement through the plant depends on
hydraulic conductivity (Lp) and path length (l).
(4.7)
The movement of water into and through the
plant is driven entirely by the physical process
of evaporation from the leaf surface and involves
no expenditure of metabolic energy by
the plant. This contrasts with the acquisition of
carbon and nutrients for which the plant must
expend considerable metabolic energy (see
Chapters 5 and 8).
Roots
DYt
Jp = Lp l
Water moves through roots along a waterpotential
gradient from moist soils to the
atmosphere during the day and sometimes to
dry surface soils at night. In moist soils, the cell
membranes, which are composed of hydrophobic
lipids, provide the greatest resistance to
water movement through roots (see Fig. 8.3).
This membrane resistance to water flow is
greatest under conditions of low root temperature
or flooding, so plants that are not adapted
to these conditions experience substantial
water stress in cold or saturated soils. In dry
soils the contact between the root and the
soil accounts for the greatest resistance to
water flux through the root. Plants overcome
this resistance primarily by increasing allocation
to the production of new roots (see
Chapter 6).
In dry environments, there is a strong vertical
gradient in soil water potential due to the
low soil water potential in dry surface soils.
However, water moves slowly through the soil
because of the low hydraulic conductivity of dry
soils. During the day, when plants lose water