Leaf Diffusive Conductances in the Major Vegetation Types of the Globe
479
5 and 10% found most frequently. Generally, stomata are not completely
closed in the dark, and some species - not only CAM species - keep them
almost fully open (e.g., Stalfelt 1963; Gross and Ph am-Nguyen 1978). In
CAM species, parasitic species, and subarctic species, "night" time stomatal
opening is the rule (e.g., Nobel and Hartsock 1984; Press et al. 1988;
Gauslaa and Odasz 1990). PFD required for attaining gpot under otherwise
unrestricted environmental conditions is difficult to define, since PFD
response curves are usually published only for photosynthesis. g tends to
peak sharply at somewhat lower PFDs than photosynthesis, which increases
more assymptotically. As a rule, most late successional woody species reach
50% of gpot around 3-5% of maximum daily PFD (ca. 50-100 Jlmol photons
m- 2 S-I) and reach >90% of gpot between 10 and 20% of maximum PFD
(e.g., Benecke et al. 1981; Whitehead et al. 1981; Goldstein et al. 1985;
Ullmann et al. 1985). In graminoid leaves growing at steep angles, light
saturation of g occurs at somewhat higher levels (ca. 40% of maximum
PFD, e.g., Monson et al. 1986). It should be noted that these light responses
are dependent on leaf angle and competitive shading within shoots or tussocks. A clear distinction needs to be made between data for (1) leaves
or shoots "bent" into a horizontal position during porometry or cuvette
enclosure, (2) leaves or shoots illuminated in their natural position when
measured, and (3) whole canopy light responses (Kimes et al. 1980; Carter
and Smith 1985; Korner and Cochrane 1985; Oker-Blom and Smolander
1988). The above estimates of PFD responses of g are valid for unshaded
leaves measured in an approximately natural position. In dense conifer
shoots this will allways include some self-shading between adjacent needles.
In addition, stomata were found to respond very sensitively to dawn light
and may not perfectly track PFD during the day. Instead, they may remain
in an open "waiting" position dispite periodic shading, which frequently
occurs in dense plant canopies (e.g., Young and Smith 1979; Weber et al.
1985; Knapp and Smith 1987; Hollinger 1987; Pearcy and Seemann 1990).
22.8.2.2 Moisture Responses
As mentioned in Section 22.8.1, the most important long-term moisturerelated control of vegetation water loss operates via LAI and/or species
replacement. Only within a certai,n range of moderate water stress and over
short periods of time, stomatal control, plays an important role. Although a
number of further details of moisture control of stomata opening still need
to be clarified (Schulze et al. 1987; Passioura 1988; Bennett 1990), at least
three mechanisms seem to be employed (with decreasing frequency of
employment): (1) direct responses to the speed of vapor loss and/or ambient
vapor pressure deficit (sensu Lange et al. 1971; Hall and Kaufmann 1975;
Losch and Tenhunen 1981; Turner et al. 1984; a possible mechanism is
479
5 and 10% found most frequently. Generally, stomata are not completely
closed in the dark, and some species - not only CAM species - keep them
almost fully open (e.g., Stalfelt 1963; Gross and Ph am-Nguyen 1978). In
CAM species, parasitic species, and subarctic species, "night" time stomatal
opening is the rule (e.g., Nobel and Hartsock 1984; Press et al. 1988;
Gauslaa and Odasz 1990). PFD required for attaining gpot under otherwise
unrestricted environmental conditions is difficult to define, since PFD
response curves are usually published only for photosynthesis. g tends to
peak sharply at somewhat lower PFDs than photosynthesis, which increases
more assymptotically. As a rule, most late successional woody species reach
50% of gpot around 3-5% of maximum daily PFD (ca. 50-100 Jlmol photons
m- 2 S-I) and reach >90% of gpot between 10 and 20% of maximum PFD
(e.g., Benecke et al. 1981; Whitehead et al. 1981; Goldstein et al. 1985;
Ullmann et al. 1985). In graminoid leaves growing at steep angles, light
saturation of g occurs at somewhat higher levels (ca. 40% of maximum
PFD, e.g., Monson et al. 1986). It should be noted that these light responses
are dependent on leaf angle and competitive shading within shoots or tussocks. A clear distinction needs to be made between data for (1) leaves
or shoots "bent" into a horizontal position during porometry or cuvette
enclosure, (2) leaves or shoots illuminated in their natural position when
measured, and (3) whole canopy light responses (Kimes et al. 1980; Carter
and Smith 1985; Korner and Cochrane 1985; Oker-Blom and Smolander
1988). The above estimates of PFD responses of g are valid for unshaded
leaves measured in an approximately natural position. In dense conifer
shoots this will allways include some self-shading between adjacent needles.
In addition, stomata were found to respond very sensitively to dawn light
and may not perfectly track PFD during the day. Instead, they may remain
in an open "waiting" position dispite periodic shading, which frequently
occurs in dense plant canopies (e.g., Young and Smith 1979; Weber et al.
1985; Knapp and Smith 1987; Hollinger 1987; Pearcy and Seemann 1990).
22.8.2.2 Moisture Responses
As mentioned in Section 22.8.1, the most important long-term moisturerelated control of vegetation water loss operates via LAI and/or species
replacement. Only within a certai,n range of moderate water stress and over
short periods of time, stomatal control, plays an important role. Although a
number of further details of moisture control of stomata opening still need
to be clarified (Schulze et al. 1987; Passioura 1988; Bennett 1990), at least
three mechanisms seem to be employed (with decreasing frequency of
employment): (1) direct responses to the speed of vapor loss and/or ambient
vapor pressure deficit (sensu Lange et al. 1971; Hall and Kaufmann 1975;
Losch and Tenhunen 1981; Turner et al. 1984; a possible mechanism is