474
Ch. Komer
seven woody vegetation types considered, mean values for Amax vary insignificantly (p = 0.127) and average at 9.1 ± 1.6!lmolm-2s-1. Although
this subs ample contains only field data for 55 woody species, it appears that
these types of vegetation do not differ significantly in leaf-based rate of
maximum photosynthesis, similar to what applies to gmax' Only by including
species or morphotypes with extreme high or extreme low gmax and Amax
such as succulents or the various types of herbaceous plants (as was done in
our review, Korner et al. 1979), a significant variation in gmax and Amax
among plant groups is achieved. It seems that the inclusions of those other
life forms has overshadowed the rather small differences that exist among
the major types of woody vegetation. In their review on canopy photosynthesis Ceulemans and Saugier (1991) present data about meteorologically
measured net CO2 fluxes above various forest types which - although at
a rather different scale - lead to a similar conclusion: various types of
deciduous forest, spruce and pine forest, and the tropical rain forest average
at 20 ± 3 !lmol m - 2 S -1 land area (extremes for a ponderosa pine forest of 12
and a young rubber plantation of 40 not included). Taken as a maximum
flux rate (not specified by Ceulemans and Saugier), these canopy CO2 fluxes
are about twice as high as the mean Amax obtained here.
The surprisingly high value for photosynthesis in conifers again reflects
the fact that the projected leaf area is about 2.6 times larger than the overall
needle surface area on which photosynthesis data are traditionally based. If
one assumes a mean CO2 mixing ratio in the atmosphere of 340 !lmol per
mol during the period 1975-1985 in which most of these data were collected, the "all species regression" of Amax versus gmax indicates a mean c/ca
ratio of about 0.77 (Cj = 262!lmol mol-I). In conclusion, it can be assumed
in accordance with earlier attempts that conductance and assimilation data
can be approximately interconverted using the functions listed in Table 22.4.
22.7 Minimum Leaf Diffusive Condnctances
The minimum leaf conductance of plants varies between 1120 in herbaceous
plants and almost 11100 of maximum conductance in evergreen conifers
(Korner 1993; Table 22.5). Most broadleaved shrubs and trees can reduce
their leaf conductance to 1140 to 1160 of the maximum value. Succulents
form an exception with their ability to reduce the conductance of their
surfaces by a factor of 300 and more.
However, except for CAM plants, such reductions rarely operate in the
field. Minimum daytime leaf conductances in the field (glow) in the most
important woody vegetation types of more humid regions rarely drop below
5% of the maximum and often stay above 25% of the maximum (Table
22.5). Diurnal courses of leaf conductances in plants from arid regions
Ch. Komer
seven woody vegetation types considered, mean values for Amax vary insignificantly (p = 0.127) and average at 9.1 ± 1.6!lmolm-2s-1. Although
this subs ample contains only field data for 55 woody species, it appears that
these types of vegetation do not differ significantly in leaf-based rate of
maximum photosynthesis, similar to what applies to gmax' Only by including
species or morphotypes with extreme high or extreme low gmax and Amax
such as succulents or the various types of herbaceous plants (as was done in
our review, Korner et al. 1979), a significant variation in gmax and Amax
among plant groups is achieved. It seems that the inclusions of those other
life forms has overshadowed the rather small differences that exist among
the major types of woody vegetation. In their review on canopy photosynthesis Ceulemans and Saugier (1991) present data about meteorologically
measured net CO2 fluxes above various forest types which - although at
a rather different scale - lead to a similar conclusion: various types of
deciduous forest, spruce and pine forest, and the tropical rain forest average
at 20 ± 3 !lmol m - 2 S -1 land area (extremes for a ponderosa pine forest of 12
and a young rubber plantation of 40 not included). Taken as a maximum
flux rate (not specified by Ceulemans and Saugier), these canopy CO2 fluxes
are about twice as high as the mean Amax obtained here.
The surprisingly high value for photosynthesis in conifers again reflects
the fact that the projected leaf area is about 2.6 times larger than the overall
needle surface area on which photosynthesis data are traditionally based. If
one assumes a mean CO2 mixing ratio in the atmosphere of 340 !lmol per
mol during the period 1975-1985 in which most of these data were collected, the "all species regression" of Amax versus gmax indicates a mean c/ca
ratio of about 0.77 (Cj = 262!lmol mol-I). In conclusion, it can be assumed
in accordance with earlier attempts that conductance and assimilation data
can be approximately interconverted using the functions listed in Table 22.4.
22.7 Minimum Leaf Diffusive Condnctances
The minimum leaf conductance of plants varies between 1120 in herbaceous
plants and almost 11100 of maximum conductance in evergreen conifers
(Korner 1993; Table 22.5). Most broadleaved shrubs and trees can reduce
their leaf conductance to 1140 to 1160 of the maximum value. Succulents
form an exception with their ability to reduce the conductance of their
surfaces by a factor of 300 and more.
However, except for CAM plants, such reductions rarely operate in the
field. Minimum daytime leaf conductances in the field (glow) in the most
important woody vegetation types of more humid regions rarely drop below
5% of the maximum and often stay above 25% of the maximum (Table
22.5). Diurnal courses of leaf conductances in plants from arid regions