464
Ch. Komer
is only given in connection with' a number of other biotic and climatic
variables. Canopy structure and canopy climate may, in some instances,
become stronger biotic determinants of vapor flux than leaf conductance
itself (e.g., McNaughton and Jarvis 1991). For example, Roberts et al.
(1990) provide field data about the relative significance of stomata and
aerodynamic boundary layer conductance within a tropical rainforest canopy. On an average, boundary layer conductance of leaves in the understory
was similar to gmax, but was six times larger than gmax (i.e., of little significance) at the top of the canopy. Kelliher et al. (1993) illustrated that the
aerodynamic conductance for water vapor of grass swards is about eight
times smaller than that of forest canopies. As a consequence, they found the
proportional sensitivity of canopy transpiration to changes in g to be >0.7 in
conifers and as low as 0.3 in grassland. This indicates that the significance of
g for predicting fluxes will vary with canopy structure, in particular with leaf
area density. The often unknown relative abundance of species within a
certain type of vegetation also provides a limitation to the utilization of
species based data for the estimation of vegetation means.
Furthermore, plant canopies are not only composed of various species,
but bear leaves of various age and leaves which are exposed to contrasting
"microclimates" that influence their morphology and physiology. Thus, the
main problem associated with this attempt at characterizing leaf conductance is the overwhelming diversity of plant species and functional leaf types
(Korner 1993) that cover the continents, contrasted by an otherwise small
number of adequate field studies. Therefore, besides providing a rather
provisional catalog of conductance parameters for certain vegetation types,
the second aim of this chapter is to highlight the major gaps on the global
vegetation map that need to receive top priority in future field measurement
campaigns.
22.3 How Was the Data Set Compiled?
In order to characterize stomatal control of vapor diffusion on a leaf basis,
three types of information are required:
- the maximum diffusive conductance of leaves,
- the minimum diffusive conductance of leaves, and
- the functions that control conductance between these extremes.
Coarse ranges of maximum and minimum conductances for certain
groups of species can be distilled from the literature on relatively safe
grounds (Korner et al. 1979; Korner 1993). However, on the scales required
in biosphere modeling, the third requirement, the functions for stomatal
dynamics, is difficult to fulfill. Despite the fact that some very detailed
Ch. Komer
is only given in connection with' a number of other biotic and climatic
variables. Canopy structure and canopy climate may, in some instances,
become stronger biotic determinants of vapor flux than leaf conductance
itself (e.g., McNaughton and Jarvis 1991). For example, Roberts et al.
(1990) provide field data about the relative significance of stomata and
aerodynamic boundary layer conductance within a tropical rainforest canopy. On an average, boundary layer conductance of leaves in the understory
was similar to gmax, but was six times larger than gmax (i.e., of little significance) at the top of the canopy. Kelliher et al. (1993) illustrated that the
aerodynamic conductance for water vapor of grass swards is about eight
times smaller than that of forest canopies. As a consequence, they found the
proportional sensitivity of canopy transpiration to changes in g to be >0.7 in
conifers and as low as 0.3 in grassland. This indicates that the significance of
g for predicting fluxes will vary with canopy structure, in particular with leaf
area density. The often unknown relative abundance of species within a
certain type of vegetation also provides a limitation to the utilization of
species based data for the estimation of vegetation means.
Furthermore, plant canopies are not only composed of various species,
but bear leaves of various age and leaves which are exposed to contrasting
"microclimates" that influence their morphology and physiology. Thus, the
main problem associated with this attempt at characterizing leaf conductance is the overwhelming diversity of plant species and functional leaf types
(Korner 1993) that cover the continents, contrasted by an otherwise small
number of adequate field studies. Therefore, besides providing a rather
provisional catalog of conductance parameters for certain vegetation types,
the second aim of this chapter is to highlight the major gaps on the global
vegetation map that need to receive top priority in future field measurement
campaigns.
22.3 How Was the Data Set Compiled?
In order to characterize stomatal control of vapor diffusion on a leaf basis,
three types of information are required:
- the maximum diffusive conductance of leaves,
- the minimum diffusive conductance of leaves, and
- the functions that control conductance between these extremes.
Coarse ranges of maximum and minimum conductances for certain
groups of species can be distilled from the literature on relatively safe
grounds (Korner et al. 1979; Korner 1993). However, on the scales required
in biosphere modeling, the third requirement, the functions for stomatal
dynamics, is difficult to fulfill. Despite the fact that some very detailed