484
Ch. Komer
unexplained "time-dependent" reductions in g relative to morning values
frequently occur at equal PFD and vpd (phase 3, see previous paragraph).
The resultant diurnal curve for g most frequently found in the field approximates a triangular shape with a steep short ascent and a variable descent,
which, in the case of humid weather (with vpd never falling below a critical
level) may even form a trapezoid. Symmetrical bell-shaped diurnal courses
(midday peaks) which were reported in the earlier literature are actually
very rare in the field. The same is true for the so-called midday depression
of g. In some regions, diurnal variations of gael in response to environmental
influences are quite regular, so that even a simple statistical function with
daytime as the only independent variable can be utilized. This was shown to
work in the Amazonian forest by Dolman et al. (1991).
22.9 Conclusions and Recommendations for Further Research
The purpose of this reconsideration of the vanatIOn of leaf conductance
measured for the major types of global vegetation in the field was to provide
a simplified and reliable summary of the appropriate data to be used in
large-scale models. It became apparent that to achieve this goal, it was not
necessary to differentiate among types of woody vegetation in terms of
maximum leaf diffusive conductance. The major gaps of knowledge with
respect to leaf conductance are in the tropics and the subtropics, especially
in tropical grasslands and tropical seasonal forests. Humid tropical forests
are also under-represented in the literature, considering their great significance for global carbon storage, energy balance, and their great susceptibility to changing moisture supply.
Since gmax rarely occurs in the field, the seasonal variation of gpot and the
diurnal reduction of g induced by vpd and/or transpirational flux per se
become major determinants of the water loss from vegetation. However,
further refinements of our knowledge of leaf conductance will be of limited
value unless we achieve a better data base of leaf area index and its seasonal
variation. Future campaigns for improving the current database of leaf
conductance should thus include protocols for seasonal leaf area dynamics.
Studies also need to account for the relative abundance of species within a
certain vegetation type. Priority should be given to screening programs
which include large numbers of species and individuals rather than very
detailed studies on only a few species and sites (Korner 1991).
Acknowledgments. I am grateful to Susanna Pelaez-Riedl for her assistance in literature
search, to Bernd Schappi for compiling new conductance data, to J. Arnone and the
editors for their helpful comments, and to L. Strasser for her unwavering effort in typing
succeeding versions of this manuscript. Figure 22.4 is reproduced with permission from
Elsevier Science publishers B.Y., Amsterdam.
Ch. Komer
unexplained "time-dependent" reductions in g relative to morning values
frequently occur at equal PFD and vpd (phase 3, see previous paragraph).
The resultant diurnal curve for g most frequently found in the field approximates a triangular shape with a steep short ascent and a variable descent,
which, in the case of humid weather (with vpd never falling below a critical
level) may even form a trapezoid. Symmetrical bell-shaped diurnal courses
(midday peaks) which were reported in the earlier literature are actually
very rare in the field. The same is true for the so-called midday depression
of g. In some regions, diurnal variations of gael in response to environmental
influences are quite regular, so that even a simple statistical function with
daytime as the only independent variable can be utilized. This was shown to
work in the Amazonian forest by Dolman et al. (1991).
22.9 Conclusions and Recommendations for Further Research
The purpose of this reconsideration of the vanatIOn of leaf conductance
measured for the major types of global vegetation in the field was to provide
a simplified and reliable summary of the appropriate data to be used in
large-scale models. It became apparent that to achieve this goal, it was not
necessary to differentiate among types of woody vegetation in terms of
maximum leaf diffusive conductance. The major gaps of knowledge with
respect to leaf conductance are in the tropics and the subtropics, especially
in tropical grasslands and tropical seasonal forests. Humid tropical forests
are also under-represented in the literature, considering their great significance for global carbon storage, energy balance, and their great susceptibility to changing moisture supply.
Since gmax rarely occurs in the field, the seasonal variation of gpot and the
diurnal reduction of g induced by vpd and/or transpirational flux per se
become major determinants of the water loss from vegetation. However,
further refinements of our knowledge of leaf conductance will be of limited
value unless we achieve a better data base of leaf area index and its seasonal
variation. Future campaigns for improving the current database of leaf
conductance should thus include protocols for seasonal leaf area dynamics.
Studies also need to account for the relative abundance of species within a
certain vegetation type. Priority should be given to screening programs
which include large numbers of species and individuals rather than very
detailed studies on only a few species and sites (Korner 1991).
Acknowledgments. I am grateful to Susanna Pelaez-Riedl for her assistance in literature
search, to Bernd Schappi for compiling new conductance data, to J. Arnone and the
editors for their helpful comments, and to L. Strasser for her unwavering effort in typing
succeeding versions of this manuscript. Figure 22.4 is reproduced with permission from
Elsevier Science publishers B.Y., Amsterdam.