Principles of terrestrial ecosystem ecology.pdf

92 4. Terrestrial Water and Energy Balance
however, is often relatively small. Maximum
stomatal conductance of individual leaves is
relatively similar among natural ecosystems
(Körner 1994, Kelliher et al. 1995). Woody
and herbaceous ecosystems, for example, have
similar stomatal conductance of individual
leaves (Körner 1994) and similar surface conductance
of entire ecosystems (Kelliher et al.
1995). Crops, however, which have about 50%
higher stomatal conductance than does natural
vegetation, also have about 50% higher surface
conductance (Schulze et al. 1994, Kelliher et al.
1995). There are currently insufficient data to
know whether ecological variation in stomatal
conductance associated with gradients in soil
fertility causes similar variation in surface
conductance and therefore evapotranspiration
from ecosystems.
In summary, aerodynamic roughness, which
depends on plant height and the number of
roughness elements, is the main way in which
vegetation influences evapotranspiration from
dry canopies under conditions of adequate
water supply. Stomatal conductance exerts
additional control in some ecosystems. It also
has an increasingly important control over
evapotranspiration as soil moisture declines.
In other words, stomatal conductance accounts
for temporal variation in evapotranspiration in
response to variation in soil moisture, but
surface roughness is the major factor explaining
differences in evapotranspiration among
moist ecosystems.
Vegetation structure also influences the relative
importance of different climatic variables
in regulating evapotranspiration. In aerodynamically
rough, well-mixed canopies such as
open-canopied forests, the moisture content of
the air within the canopy is similar to that of the
bulk air above the canopy. Under these wellcoupled
conditions, evapotranspiration is determined
more by the moisture content of the
air (and the accompanying stomatal response)
than by net radiation (Waring and Running
1998). In short canopies, by contrast, the air
adjacent to the leaves is mixed less readily with
the bulk air above the canopy, allowing evapotranspiration
to increase the moisture content
within the canopy environment. In other words,
the canopy air becomes decoupled from condi-
Table 4.5. Decoupling coefficient of vegetation
canopies in the field under conditions of adequate
moisture supply.
Vegetation Decoupling coefficient a
Alfalfa 0.9
Strawberry patch 0.85
Permanent pasture 0.8
Grassland 0.8
Tomato field 0.7
Wheat field 0.6
Prairie 0.5
Cotton 0.4
Heathland 0.3
Citrus orchard 0.3
Forest 0.2
Pine woods 0.1
a A completely smooth surface has a decoupling coefficient
of 1.0, and a canopy in which the air is identical to that in
the atmosphere has a decoupling coefficient of zero.
Data from Jarvis and McNaughton (1986) and Jones (1992).
tions in the bulk atmosphere. In these smooth
canopies, evapotranspiration is determined
more by net radiation than by the moisture
content of the bulk air, just as when canopies
are wet.The decoupling coefficient is a measure
of the degree to which a canopy is decoupled
from the atmosphere (Table 4.5) (Jarvis and
McNaughton 1986). It is determined primarily
by canopy height. In summary, net radiation is
the dominant environmental control over evapotranspiration
in short canopies, whereas the
vapor pressure deficit is the dominant control
in tall canopies, when water is freely available
(Waring and Running 1998).
Changes in Storage
Water inputs that exceed outputs replenish
water that is stored in soil and groundwater.
Water that enters the soil is retained until the
soil reaches field capacity. Additional water
moves downward to groundwater. In cold climates
in winter, most of the precipitation input
is stored above ground in the snowpack. The
snowpack substantially increases the quantity
of water that an ecosystem can store and
the residence time of water in the ecosystem.
Stored water supports evapotranspiration
at times when evapotranspiration exceeds precipitation;
the declines in soil moisture during