Leaf Diffusive Conductances in the Major Vegetation Types of the Globe
465
studies on stomatal responses to climatic variations were conducted in lea"es
of crop species and some temperate zone woody species, the information
available for the majority of other vegetation types is, at best, anecdotal.
Short-term stomatal responses to light and moisture supply are the simpler
parts of relationships. More complicated are predictions of seasonal changes
and phenological changes. On a vegetation basis, further complications arise
from mutual leaf shading and aerodynamic uncoupling from the climate
measured outside the plant canopy by meteorological stations. The present
survey does not account for these additional determinants of evapotranspiration. Attempts at modeling these interactions at the canopy level were
published, for instance, by Federer (1979), Hall (1982), Kaufmann (1983),
Jarvis et al. (1985), Caldwell et al. (1986), Landsberg (1986), Baldocchi et
al. (1987) and Baldocchi (1989).
The present chapter is divided in two major parts. The first part provides
a reassessment of the knowledge on maximum leaf conductance incorporating the information that accumulated in addition to that compiled in the
1979 review (Korner et al. 1979). This will be supplemented with a brief
account on representative values for minimum conductance. The second
part contains a collection of stomatal response characteristics observed in
plants from major vegetation types. This latter part may assist in designing
approximative stochastic functions of stomatal responses.
Five types of leaf conductances (g) are distinguished throughout this
chapter:
gmax The absolute maximum g achieved in the field following the criteria
listed in the next paragraph.
gpot The potential daily maximum of g reached under a given situation. gpot
varies throughout the year depending on phenology, soil moisture, and
low temperature stress. Only under optimum, peak-season conditions
will gpot equal gmax'
gmin The lowest conductance a leaf can reach when stomata are completely
closed as a result of desiccation stress. Other environmental influences
(e.g., high CO2 , low light) do not normally lead to the same low gmin'
glow The lowest conductance normally found in the field. In almost all cases
glow will be higher than gmin'
gaet The actual leaf conductance occurring under a given climatic constellation during a certain day. The maximum value of gaet during a specific
day equals gpot.
All values for conductance are presented as molar flux values (mmol
m- 2 s- 1 ). For the sake of comparison, the interconversion of conductances
from velocity values (cm s -1) was set to 20°C and a barometric pressure of
1000 hPa. Under these conditions the numerical value of g in molar units is
410 times greater than in cm s -1. For other conditions the equation g (mmol
m- 2 s- 1 ) = g (cms- I ). k· (TofT)· (PfPo) needs to be applied. k is a constant derived from the mole volume of air and has the value of 1122.73 for
465
studies on stomatal responses to climatic variations were conducted in lea"es
of crop species and some temperate zone woody species, the information
available for the majority of other vegetation types is, at best, anecdotal.
Short-term stomatal responses to light and moisture supply are the simpler
parts of relationships. More complicated are predictions of seasonal changes
and phenological changes. On a vegetation basis, further complications arise
from mutual leaf shading and aerodynamic uncoupling from the climate
measured outside the plant canopy by meteorological stations. The present
survey does not account for these additional determinants of evapotranspiration. Attempts at modeling these interactions at the canopy level were
published, for instance, by Federer (1979), Hall (1982), Kaufmann (1983),
Jarvis et al. (1985), Caldwell et al. (1986), Landsberg (1986), Baldocchi et
al. (1987) and Baldocchi (1989).
The present chapter is divided in two major parts. The first part provides
a reassessment of the knowledge on maximum leaf conductance incorporating the information that accumulated in addition to that compiled in the
1979 review (Korner et al. 1979). This will be supplemented with a brief
account on representative values for minimum conductance. The second
part contains a collection of stomatal response characteristics observed in
plants from major vegetation types. This latter part may assist in designing
approximative stochastic functions of stomatal responses.
Five types of leaf conductances (g) are distinguished throughout this
chapter:
gmax The absolute maximum g achieved in the field following the criteria
listed in the next paragraph.
gpot The potential daily maximum of g reached under a given situation. gpot
varies throughout the year depending on phenology, soil moisture, and
low temperature stress. Only under optimum, peak-season conditions
will gpot equal gmax'
gmin The lowest conductance a leaf can reach when stomata are completely
closed as a result of desiccation stress. Other environmental influences
(e.g., high CO2 , low light) do not normally lead to the same low gmin'
glow The lowest conductance normally found in the field. In almost all cases
glow will be higher than gmin'
gaet The actual leaf conductance occurring under a given climatic constellation during a certain day. The maximum value of gaet during a specific
day equals gpot.
All values for conductance are presented as molar flux values (mmol
m- 2 s- 1 ). For the sake of comparison, the interconversion of conductances
from velocity values (cm s -1) was set to 20°C and a barometric pressure of
1000 hPa. Under these conditions the numerical value of g in molar units is
410 times greater than in cm s -1. For other conditions the equation g (mmol
m- 2 s- 1 ) = g (cms- I ). k· (TofT)· (PfPo) needs to be applied. k is a constant derived from the mole volume of air and has the value of 1122.73 for