468
Ch. Korner
reduced leaf area index. Hence, there will be a difference between the
lowest value of. leaf conductance observed in the field, glow (with a large
measurement error), and the much lower "lowest" leaf conductance gmin
that can be determined (technically quite accurately) in detached leaves in
the laboratory. For modeling purposes glow is more useful, while text books
usually refer to gmin. Wherever possible, I will include both values in order
to avoid confusion between these two "minimum" conductances. The data
base for gmin is much smaller than for gmax and is only approximate.
22.3.3 Definition of Stomatal Response Functions
The prediction of leaf conductances smaller than gmax in the field requires a
hierarchy of response functions. First is the seasonal variation of the diurnal
maximum value of g, gpot, which is a function of the overall predisposition
of a plant - a combined effect of phenological control (including photoperiodism effects) and environmental prehistory. In deciduous vegetation
such as drought or temperate deciduous forests and woodlands, as well as
grasslands, the annual course of the leaf area index is by far the most
important variable, periodically setting gmax to zero, simply because leaves
are absent. When leaves are present, soil moisture is the prime determinant
of gpot in these areas. In zonal grasslands phenological changes in plants
usually parallel the course of seasonal soil moisture, but, in the absence of
drought, reproductive development may set limits to leaf function, simply by
leaf die back in the course of diaspore ripening.
In evergreen vegetation the major determinants of seasonal variations in
gpot are again soil moisture and, in the temperate, boreal, and subarctic
zone, low temperatures. The prediction of soil drought stress for plants
requires a large set of subroutines, including those for plant water consumption. There is great uncertainty about how the moisture-related signal
ultimately controls gpot and how this may vary among species (see Sect.
22.8).
Predictions of gpot on the basis of minimum temperatures are easier, since
they do not depend on a capacitor such as the soil water reserve, and
response thresholds are narrower, simply because soil water freezes near
o dc. Some examples for low temperature control of gpot will be provided in
Section 22.8.
Once the highest value of g that may be reached on a given day (= gpot)
is set, the diurnal climatic and soil moisture conditions become the main
determinants of the actual g, gaet. The major problem then is that all these
environmental variables are effective only over certain ranges and interactive influences may differ from single-factor influences. Besides assigning
response functions for the "responsive range", thresholds and saturation
levels need to be defined. Again, a few examples of response characteristics
for species from important types of vegetation will be presented.
Ch. Korner
reduced leaf area index. Hence, there will be a difference between the
lowest value of. leaf conductance observed in the field, glow (with a large
measurement error), and the much lower "lowest" leaf conductance gmin
that can be determined (technically quite accurately) in detached leaves in
the laboratory. For modeling purposes glow is more useful, while text books
usually refer to gmin. Wherever possible, I will include both values in order
to avoid confusion between these two "minimum" conductances. The data
base for gmin is much smaller than for gmax and is only approximate.
22.3.3 Definition of Stomatal Response Functions
The prediction of leaf conductances smaller than gmax in the field requires a
hierarchy of response functions. First is the seasonal variation of the diurnal
maximum value of g, gpot, which is a function of the overall predisposition
of a plant - a combined effect of phenological control (including photoperiodism effects) and environmental prehistory. In deciduous vegetation
such as drought or temperate deciduous forests and woodlands, as well as
grasslands, the annual course of the leaf area index is by far the most
important variable, periodically setting gmax to zero, simply because leaves
are absent. When leaves are present, soil moisture is the prime determinant
of gpot in these areas. In zonal grasslands phenological changes in plants
usually parallel the course of seasonal soil moisture, but, in the absence of
drought, reproductive development may set limits to leaf function, simply by
leaf die back in the course of diaspore ripening.
In evergreen vegetation the major determinants of seasonal variations in
gpot are again soil moisture and, in the temperate, boreal, and subarctic
zone, low temperatures. The prediction of soil drought stress for plants
requires a large set of subroutines, including those for plant water consumption. There is great uncertainty about how the moisture-related signal
ultimately controls gpot and how this may vary among species (see Sect.
22.8).
Predictions of gpot on the basis of minimum temperatures are easier, since
they do not depend on a capacitor such as the soil water reserve, and
response thresholds are narrower, simply because soil water freezes near
o dc. Some examples for low temperature control of gpot will be provided in
Section 22.8.
Once the highest value of g that may be reached on a given day (= gpot)
is set, the diurnal climatic and soil moisture conditions become the main
determinants of the actual g, gaet. The major problem then is that all these
environmental variables are effective only over certain ranges and interactive influences may differ from single-factor influences. Besides assigning
response functions for the "responsive range", thresholds and saturation
levels need to be defined. Again, a few examples of response characteristics
for species from important types of vegetation will be presented.