Leaf Diffusive Conductances in the Major Vegetation Types of the Globe
467
ductances ever recorded. Similar curiosities have been published for all
vegetation types and reflect either a still widespread ignorance of porometer
calibration or calculation errors. Since there is an obvious bias of such
erroneous values towards rather small conductances, their inclusion would
have resulted in unrealistically low average maximum leaf conductances,
even when using the median instead of the arithmetic mean.
Although I have tried to omit only the most extreme values, the data set
may still include a few unsecure extreme values. Therefore arithmetic
means have been supplemented with median values where sample size was
sufficient. Where multiple reports for a single species were available, the
mean from all these reports was used to calculate the means for a vegetation
type. A full documentation of all individual original data is not possible
here; however, I have selected a few references as examples for each biome.
Nevertheless, the means I present include more information than is presented in these references.
A key problem in defining g in a comparative survey is the choice
between projected leaf area or total leaf surface area as the reference for
leaf transpiration. Papers with unclear information in this respect had to be
disregarded. All values presented here are based on a projected leaf area
basis. Although this may affect the comparative value in an ecological
context (Korner et al. 1979) this reference was chosen, because the leaf area
index (LAI) is defined as projected leaf area per unit of land area, and
LAI is the second important vegetation variable that controls total canopy
conductance (Schulze 1982). In plant species with thick leaves such as
needles, ericoid or festucoid leaves, g may become three to five times as
large as if based on a total surface area basis. The dilemma is particularly
serious in conifers, for which most researchers have adopted total surface
area, while most people working with flat leaves use projected leaf areas.
22.3.2 Definition of Minimum Leaf Conductance for Water Vapor
Minimum conductance (complete stomatal closure) is difficult to measure.
For most species, porometers or gas exchange systems are not accurate
enough to determine the low vapor fluxes that occur after stomatal closure.
Furthermore, neither darkness (night values) nor high CO 2 levels or low
ambient humidity can mimic the effect of desiccation stress in nature, where
minimum conductance becomes ecologically meaningful. Therefore repeated
weighing of detached, and thus desiccating, leaves (and thereby forcing
stomata to complete closure) has often been adopted as a useful approximation. On the other hand, such severe and fast desiccation again is rare in
the real world, and thus is of more theoretical value. In the course of severe
drought, most species reduce their total leaf area and control vapor loss by
467
ductances ever recorded. Similar curiosities have been published for all
vegetation types and reflect either a still widespread ignorance of porometer
calibration or calculation errors. Since there is an obvious bias of such
erroneous values towards rather small conductances, their inclusion would
have resulted in unrealistically low average maximum leaf conductances,
even when using the median instead of the arithmetic mean.
Although I have tried to omit only the most extreme values, the data set
may still include a few unsecure extreme values. Therefore arithmetic
means have been supplemented with median values where sample size was
sufficient. Where multiple reports for a single species were available, the
mean from all these reports was used to calculate the means for a vegetation
type. A full documentation of all individual original data is not possible
here; however, I have selected a few references as examples for each biome.
Nevertheless, the means I present include more information than is presented in these references.
A key problem in defining g in a comparative survey is the choice
between projected leaf area or total leaf surface area as the reference for
leaf transpiration. Papers with unclear information in this respect had to be
disregarded. All values presented here are based on a projected leaf area
basis. Although this may affect the comparative value in an ecological
context (Korner et al. 1979) this reference was chosen, because the leaf area
index (LAI) is defined as projected leaf area per unit of land area, and
LAI is the second important vegetation variable that controls total canopy
conductance (Schulze 1982). In plant species with thick leaves such as
needles, ericoid or festucoid leaves, g may become three to five times as
large as if based on a total surface area basis. The dilemma is particularly
serious in conifers, for which most researchers have adopted total surface
area, while most people working with flat leaves use projected leaf areas.
22.3.2 Definition of Minimum Leaf Conductance for Water Vapor
Minimum conductance (complete stomatal closure) is difficult to measure.
For most species, porometers or gas exchange systems are not accurate
enough to determine the low vapor fluxes that occur after stomatal closure.
Furthermore, neither darkness (night values) nor high CO 2 levels or low
ambient humidity can mimic the effect of desiccation stress in nature, where
minimum conductance becomes ecologically meaningful. Therefore repeated
weighing of detached, and thus desiccating, leaves (and thereby forcing
stomata to complete closure) has often been adopted as a useful approximation. On the other hand, such severe and fast desiccation again is rare in
the real world, and thus is of more theoretical value. In the course of severe
drought, most species reduce their total leaf area and control vapor loss by