Leaf Diffusive Conductances in the Major Vegetation Types of the Globe
481
described by Nonami et al. 1990), (2) responses to rhizosphere moisture
depletion mediated by a root signal (Gollan et al. 1985, 1986), and (3) leaf
water potential. The mechanism mentioned last seems to be rarely employed and, within a given day, water potential tends to reach its lowest
value when g reaches its maximum (e.g., Schulze et al. 1972, 1975; Schulze
and Hall 1982; Nilsen et al. 1983). There seems to be a surprisingly wide
r.ange of water potentials over which no feedback control of g occurs (e.g.,
for Pinus sylvestris and Picea abies, values down to -1.8 and -2.0 MPa,
respectively, Korner and Perterer, unpubl.). Tenhunen et al. (1987b) conclude "under natural conditions, diurnal changes in 'P seem not to exert a
major influence on stomatal aperture". Since rhizosphere moisture is difficult to define in a diurnal cycle (the questions of soil layer and microcompartments of soil around roots), its usefulness for modeling g lies more
in the long-term determination of gpO! (Sect. 22.8.1). Therefore, ambient
humidity, expressed as vapour pressure deficit (vpd) of the air (or more
exactly between air and the substomatal airspace in leaves) becomes the·
most significant moisture-related predictor of g during a day. Figure 22.3
shows some examples. Most studies showed nearly linear negative responses
once vpd exceeded a threshold of 5 to 12hPa (mostly around 10, Korner
1985; Korner and Cochrane 1985; Roberts et al. 1990).
22.8.2.3 Time-Dependent Processes
In addition to light and moisture effects, almost all plants investigated show
hysteresis phenomena in their diurnal course of leaf conductance, generally
not taken into account in modeling. The same set of climatic conditions
could "produce" a higher leaf conductance during the morning than in the
afternoon, (e.g., Jarvis 1976; Lange et al. 1982; Korner and Cochrane 1985;
Kiippers et al. 1986; Pereira et al. 1987). Such unexplained time-dependent
responses are often not found under laboratory conditions. Evidently it will
not be possible to recommend a unique set of response characteristics for
each type of vegetation. This afternoon reduction of g compared to morning
values under equal climatic conditions is in the order of 20% (see above
references) .
Fig. 22.3a-c. Examples of humidity (vpd) responses of g in woody plant species in the
field. a Range of responses for six Acacia species in semi-arid shrub land (Ullmann et al.
1985). b Response of old and new foliage in Pinus sylvestris at low altitude in the Alps
(derived by boundary analysis from several hundred data points collected over two years,
1. Perterer, unpuhl.). c Response of Eucalyptus pauciflora from four different altitudes in
the Australian Snowy Mountains. Data from phase II (Fig. 22.6) only. (Korner and
Cochrane 1985)
481
described by Nonami et al. 1990), (2) responses to rhizosphere moisture
depletion mediated by a root signal (Gollan et al. 1985, 1986), and (3) leaf
water potential. The mechanism mentioned last seems to be rarely employed and, within a given day, water potential tends to reach its lowest
value when g reaches its maximum (e.g., Schulze et al. 1972, 1975; Schulze
and Hall 1982; Nilsen et al. 1983). There seems to be a surprisingly wide
r.ange of water potentials over which no feedback control of g occurs (e.g.,
for Pinus sylvestris and Picea abies, values down to -1.8 and -2.0 MPa,
respectively, Korner and Perterer, unpubl.). Tenhunen et al. (1987b) conclude "under natural conditions, diurnal changes in 'P seem not to exert a
major influence on stomatal aperture". Since rhizosphere moisture is difficult to define in a diurnal cycle (the questions of soil layer and microcompartments of soil around roots), its usefulness for modeling g lies more
in the long-term determination of gpO! (Sect. 22.8.1). Therefore, ambient
humidity, expressed as vapour pressure deficit (vpd) of the air (or more
exactly between air and the substomatal airspace in leaves) becomes the·
most significant moisture-related predictor of g during a day. Figure 22.3
shows some examples. Most studies showed nearly linear negative responses
once vpd exceeded a threshold of 5 to 12hPa (mostly around 10, Korner
1985; Korner and Cochrane 1985; Roberts et al. 1990).
22.8.2.3 Time-Dependent Processes
In addition to light and moisture effects, almost all plants investigated show
hysteresis phenomena in their diurnal course of leaf conductance, generally
not taken into account in modeling. The same set of climatic conditions
could "produce" a higher leaf conductance during the morning than in the
afternoon, (e.g., Jarvis 1976; Lange et al. 1982; Korner and Cochrane 1985;
Kiippers et al. 1986; Pereira et al. 1987). Such unexplained time-dependent
responses are often not found under laboratory conditions. Evidently it will
not be possible to recommend a unique set of response characteristics for
each type of vegetation. This afternoon reduction of g compared to morning
values under equal climatic conditions is in the order of 20% (see above
references) .
Fig. 22.3a-c. Examples of humidity (vpd) responses of g in woody plant species in the
field. a Range of responses for six Acacia species in semi-arid shrub land (Ullmann et al.
1985). b Response of old and new foliage in Pinus sylvestris at low altitude in the Alps
(derived by boundary analysis from several hundred data points collected over two years,
1. Perterer, unpuhl.). c Response of Eucalyptus pauciflora from four different altitudes in
the Australian Snowy Mountains. Data from phase II (Fig. 22.6) only. (Korner and
Cochrane 1985)