198
R. Losch and E.-D. Schulze
ing to bulk water shortage in the aerial parts of a plant might not be
sensitive enough to fulfill such a requirement. Again, direct signals from the
water uptake interface, the roots, to the water loss interface, the stomata,
could be a suitable solution for such control.
In order to be efficient, such a regulation needs information about the
water transport capacity of the conducting pathway. Knowledge is scarce
about the means of such a communication. From parallel increases of water
uptake and transpiration not accompanied by greater water potential gradients, a variable hydraulic conductivity has been inferred in several studies
(e.g., Aston and Lawlor 1979; Meinzer et al. 1988). Such variable wholeplant conductivities are usually attributed to altered root resistances for
water uptake (Steudle 1993). They are short-term effects, and the calculated
changes of conductivity mostly result in an improvement of the acropetal
water transport capacity. In the case ofxylem embolism, this will deteriorate,
and stomata have to respond by closure. Sperry (1986) has demonstrated
such a sequence of events as occurring in the palm, Rhapis excelsa. Jones
and Sutherland (1991) modeled the stomatal responses to xylem embolism
and emphasized that optimal stomatal control of plant water relations
required information about leaf and soil water potentials.
The species-specific balance between average leaf conductances and shoot
conductivities will be developed during ontogeny. A positive correlation
between conductance and conductivity has been shown to exist in several
species. For example, Kiippers (1984) reported that Central European
hedgerow shrubs differed in the efficiency of their conducting sapwood area.
He suggested that a relationship exists between the average hydraulic conductivity calculated as the seasonal mean of transpiration change per change
of leaf water potential and the ratio of sapwood area/supplied leaf area. A
low water supply capacity due to a peculiar xylem anatomy might also
influence the occurrence of a species in the gradient between humid and
arid habitats. This can be deduced from comparative studies on the water
relations of Canarian laurel forest trees (Losch 1993a). The species which
compose this vegetation differ among each other with respect to plasmatic
drought tolerance, stem hydraulic conductivity, and water loss avoidance.
The latter is quantified as percentage water loss per time from desiccating
saturated leaves under controlled conditions. All laurel forest species turned
out to be much less efficient with respect to all these three parameters when
compared with Mediterranean sclerophyllous species. It can be calculated
from these data and the maximum leaf conductances of the laurophyllous
species (unpubl.) that in nearly half of the species leaf water contents will
fall below critical values if the saturation deficit of the ambient air decreases
below lOgm- 3 for prolonged periods.
Coordinated growth of trees was demonstrated for Nothofagus, in which
the maximum transpiration rate per tree was linearily related to the sapwood area and the circumference of the tree (Fig. 9.5). The slope of this
relation may be taken as an indication for the requirement for stem growth
R. Losch and E.-D. Schulze
ing to bulk water shortage in the aerial parts of a plant might not be
sensitive enough to fulfill such a requirement. Again, direct signals from the
water uptake interface, the roots, to the water loss interface, the stomata,
could be a suitable solution for such control.
In order to be efficient, such a regulation needs information about the
water transport capacity of the conducting pathway. Knowledge is scarce
about the means of such a communication. From parallel increases of water
uptake and transpiration not accompanied by greater water potential gradients, a variable hydraulic conductivity has been inferred in several studies
(e.g., Aston and Lawlor 1979; Meinzer et al. 1988). Such variable wholeplant conductivities are usually attributed to altered root resistances for
water uptake (Steudle 1993). They are short-term effects, and the calculated
changes of conductivity mostly result in an improvement of the acropetal
water transport capacity. In the case ofxylem embolism, this will deteriorate,
and stomata have to respond by closure. Sperry (1986) has demonstrated
such a sequence of events as occurring in the palm, Rhapis excelsa. Jones
and Sutherland (1991) modeled the stomatal responses to xylem embolism
and emphasized that optimal stomatal control of plant water relations
required information about leaf and soil water potentials.
The species-specific balance between average leaf conductances and shoot
conductivities will be developed during ontogeny. A positive correlation
between conductance and conductivity has been shown to exist in several
species. For example, Kiippers (1984) reported that Central European
hedgerow shrubs differed in the efficiency of their conducting sapwood area.
He suggested that a relationship exists between the average hydraulic conductivity calculated as the seasonal mean of transpiration change per change
of leaf water potential and the ratio of sapwood area/supplied leaf area. A
low water supply capacity due to a peculiar xylem anatomy might also
influence the occurrence of a species in the gradient between humid and
arid habitats. This can be deduced from comparative studies on the water
relations of Canarian laurel forest trees (Losch 1993a). The species which
compose this vegetation differ among each other with respect to plasmatic
drought tolerance, stem hydraulic conductivity, and water loss avoidance.
The latter is quantified as percentage water loss per time from desiccating
saturated leaves under controlled conditions. All laurel forest species turned
out to be much less efficient with respect to all these three parameters when
compared with Mediterranean sclerophyllous species. It can be calculated
from these data and the maximum leaf conductances of the laurophyllous
species (unpubl.) that in nearly half of the species leaf water contents will
fall below critical values if the saturation deficit of the ambient air decreases
below lOgm- 3 for prolonged periods.
Coordinated growth of trees was demonstrated for Nothofagus, in which
the maximum transpiration rate per tree was linearily related to the sapwood area and the circumference of the tree (Fig. 9.5). The slope of this
relation may be taken as an indication for the requirement for stem growth