466
Ch. Korner
T = To and P = Po, To and T are the absolute temperature at O°C and the
real temperature in K, P and Po are the local pressure and the standard
pressure of 1000 both in hPa (=mbar).
22.3.1 Definition of Maximum Leaf Conductance for Water Vapor
Since diffusion porometers became easily available by the end of the 1970s,
thousands of plant studies on leaf conductance were undertaken all over the
world. In fact, about 90% of the database currently available dates from
after 1979, when our first review was completed (Korner et al. 1979).
However, a closer look at this novel flood of porometer studies showed that
only a small fraction suits the current modeling needs. In order to explain
this dilemma, I need to define the criteria under which data were accepted
for the present purpose.
Maximum diffusive conductance is defined here as the seasonal maximum
conductance found in fully developed but not senescent leaves of adult
plants, growing in their natural environment. Unlike the first review (Korner et
al. 1979), all data were omitted from plants grown in nurseries, plantations
(with few exceptions), greenhouses, growth chambers, all data from seedlings, potted plants, fertilized or watered plots, all data taken from shaded
plant parts (e.g., understory) or exposed to any sort of manipulation. This
strict treatment of the literature narrows the database to less than onequarter of the published work. In some cases, as for instance in prairie or
steppe grasses, only three papers out of several dozens heading under these
key words passed the "sieve". However, it is imperative to adopt such a
discriminating approach, since it is well documented that leaves of young
plants or plants grown under protected conditions, as well as leaves in the
lower part of plants, exhibit stomatal (and photosynthetic) responses rather
different from those that control the major part of vapor flux in the upper
part of natural vegetation (e.g., Franich et al. 1977; Hinckley et al. 1978;
Korner et al. 1979; Leverenz et al. 1983; Beadle et al. 1985; Lichtenthaler
1985). With the above criteria, we concluded with 380 gmax values, some
species with multiple references, and with the abundance of data biased
toward a few biomes.
A closer inspection of this data set showed that numerous maximum
leaf conductances were totally unrealistic. For instance, the relatively
large data set for evergreen conifers includes gmax values between 12 and
584mmolm- 2 s- 1 , while the majority of values range between 100 and
300. If, for instance, one would believe all published numbers for Pinus
banksiana, gmax of this species varied between 29 and 404 during peak
season (with no drought stress), leaving the reviewer with the choice of
ranging P. banksiana among the conifers with the smallest or highest con-
Ch. Korner
T = To and P = Po, To and T are the absolute temperature at O°C and the
real temperature in K, P and Po are the local pressure and the standard
pressure of 1000 both in hPa (=mbar).
22.3.1 Definition of Maximum Leaf Conductance for Water Vapor
Since diffusion porometers became easily available by the end of the 1970s,
thousands of plant studies on leaf conductance were undertaken all over the
world. In fact, about 90% of the database currently available dates from
after 1979, when our first review was completed (Korner et al. 1979).
However, a closer look at this novel flood of porometer studies showed that
only a small fraction suits the current modeling needs. In order to explain
this dilemma, I need to define the criteria under which data were accepted
for the present purpose.
Maximum diffusive conductance is defined here as the seasonal maximum
conductance found in fully developed but not senescent leaves of adult
plants, growing in their natural environment. Unlike the first review (Korner et
al. 1979), all data were omitted from plants grown in nurseries, plantations
(with few exceptions), greenhouses, growth chambers, all data from seedlings, potted plants, fertilized or watered plots, all data taken from shaded
plant parts (e.g., understory) or exposed to any sort of manipulation. This
strict treatment of the literature narrows the database to less than onequarter of the published work. In some cases, as for instance in prairie or
steppe grasses, only three papers out of several dozens heading under these
key words passed the "sieve". However, it is imperative to adopt such a
discriminating approach, since it is well documented that leaves of young
plants or plants grown under protected conditions, as well as leaves in the
lower part of plants, exhibit stomatal (and photosynthetic) responses rather
different from those that control the major part of vapor flux in the upper
part of natural vegetation (e.g., Franich et al. 1977; Hinckley et al. 1978;
Korner et al. 1979; Leverenz et al. 1983; Beadle et al. 1985; Lichtenthaler
1985). With the above criteria, we concluded with 380 gmax values, some
species with multiple references, and with the abundance of data biased
toward a few biomes.
A closer inspection of this data set showed that numerous maximum
leaf conductances were totally unrealistic. For instance, the relatively
large data set for evergreen conifers includes gmax values between 12 and
584mmolm- 2 s- 1 , while the majority of values range between 100 and
300. If, for instance, one would believe all published numbers for Pinus
banksiana, gmax of this species varied between 29 and 404 during peak
season (with no drought stress), leaving the reviewer with the choice of
ranging P. banksiana among the conifers with the smallest or highest con-