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R. Losch and E.-D. Schulze
morphogenetic, structural coordination may be the case of hazel (Corylus
avellana) shrubs growing in a hedgerow in North Germany: from porometer
measurements of diurnal courses throughout the season by Linnenbrink
(unpublished) south-facing leaves had 1.4 (±0.18 SE) greater transpirational
water loss per unit leaf area than north-facing ones. Transpiration of leaves
in the center of the shrub was still less. But the greater water loss of southfacing leaves under more severe evaporation conditions is nearly equal to
the sum of transpirational water loss of all the leaves calculated for each
canopy compartment. Based on differences in the specific leaf area, an outer
or more south-facing, an inner or more north-facing crown, and an inner,
shaded portion of the canopy can be distinguished. Since only one-fifth of
the foliage belongs to the sun-exposed part of the canopy and more than
half of the leaf area is in the shadow of the shrub interior, on average
the sun foliage has only a 12% higher gross transpiration than the latter
(Lilienfein et al. 1991). Similarly, differences exist between transpiration
rates of leaves in different canopy positions and canopy-related water loss
from the periphery and the interior of co-occurring Alnus and Sambucus
shrubs (Weisheit, Linnenbrink, unpubl. data). Xylem sap flow rates differ
considerably between north- and south-facing branches of Carya illinoensis,
resulting in a 40% smaller amount of water transported to the more shaded
parts of the crown (Steinberg et al. 1990). Normalized in relation to the
supplied partial leaf area, however, flow rates in all parts of the tree are in
the same order of magnitude. This points to a versatile allocation of trunk
water supply to the different parts of a canopy, according to the respective
actual demands (Hinckley and Ritchie 1970). Similarly, Cregg et al. (1990)
determined biomass development, leaf conductance, and predawn xylem
potentials of a thinned Pinus taeda stand. Also in this case, leaf area,
transpiration, and water potential interact in such a way that homeostasis of
water relations results.
A relationship exists between leaf area and maximal stomatal conductance of Saccharum so that transpiration from a plant does not exceed a
certain maximum value even if total leaf area increases (Meinzer and Grantz
1990). This results from a decrease in area-related maximal conductance
once leaf area has exceeded a value of 0.2m2 plane 1 . Upon partial removal
of the transpiring leaves, a rapid stomatal adjustment occurred, reestablishing the relationships between maximum conductance and remaining leaf
area to the previous level. Leaf water balance at maximal transpiration was
independent of leaf area, e.g., leaf water potentials remained constant while
transpiration per plant increased. The ratio of transpiration to the soil-xylem
water potential difference, taken as the hydraulic conductivity, decreased
with increasing total leaf area if unit-area related, and came to a constant
maximal value at leaf areas greater than OAm2 plant -1 if related to the
total leaf area of a plant. This is the same pattern as that exhibited by
stomatal conductance. Extending the earlier "pipe" model of Waring et
al. (1982), which proposed a relation between leaf area index and xylem
R. Losch and E.-D. Schulze
morphogenetic, structural coordination may be the case of hazel (Corylus
avellana) shrubs growing in a hedgerow in North Germany: from porometer
measurements of diurnal courses throughout the season by Linnenbrink
(unpublished) south-facing leaves had 1.4 (±0.18 SE) greater transpirational
water loss per unit leaf area than north-facing ones. Transpiration of leaves
in the center of the shrub was still less. But the greater water loss of southfacing leaves under more severe evaporation conditions is nearly equal to
the sum of transpirational water loss of all the leaves calculated for each
canopy compartment. Based on differences in the specific leaf area, an outer
or more south-facing, an inner or more north-facing crown, and an inner,
shaded portion of the canopy can be distinguished. Since only one-fifth of
the foliage belongs to the sun-exposed part of the canopy and more than
half of the leaf area is in the shadow of the shrub interior, on average
the sun foliage has only a 12% higher gross transpiration than the latter
(Lilienfein et al. 1991). Similarly, differences exist between transpiration
rates of leaves in different canopy positions and canopy-related water loss
from the periphery and the interior of co-occurring Alnus and Sambucus
shrubs (Weisheit, Linnenbrink, unpubl. data). Xylem sap flow rates differ
considerably between north- and south-facing branches of Carya illinoensis,
resulting in a 40% smaller amount of water transported to the more shaded
parts of the crown (Steinberg et al. 1990). Normalized in relation to the
supplied partial leaf area, however, flow rates in all parts of the tree are in
the same order of magnitude. This points to a versatile allocation of trunk
water supply to the different parts of a canopy, according to the respective
actual demands (Hinckley and Ritchie 1970). Similarly, Cregg et al. (1990)
determined biomass development, leaf conductance, and predawn xylem
potentials of a thinned Pinus taeda stand. Also in this case, leaf area,
transpiration, and water potential interact in such a way that homeostasis of
water relations results.
A relationship exists between leaf area and maximal stomatal conductance of Saccharum so that transpiration from a plant does not exceed a
certain maximum value even if total leaf area increases (Meinzer and Grantz
1990). This results from a decrease in area-related maximal conductance
once leaf area has exceeded a value of 0.2m2 plane 1 . Upon partial removal
of the transpiring leaves, a rapid stomatal adjustment occurred, reestablishing the relationships between maximum conductance and remaining leaf
area to the previous level. Leaf water balance at maximal transpiration was
independent of leaf area, e.g., leaf water potentials remained constant while
transpiration per plant increased. The ratio of transpiration to the soil-xylem
water potential difference, taken as the hydraulic conductivity, decreased
with increasing total leaf area if unit-area related, and came to a constant
maximal value at leaf areas greater than OAm2 plant -1 if related to the
total leaf area of a plant. This is the same pattern as that exhibited by
stomatal conductance. Extending the earlier "pipe" model of Waring et
al. (1982), which proposed a relation between leaf area index and xylem