13. Measuring Water Availability and Uptake in Ecosystem Studies
209
(13.12)
where qv, q" ql, and qf are the vertical, radial, and
lateral heat loss by conduction and the heat transferred by convection in the xylem sap, respectively.
The lateral heat loss term ql is only applicable in
the trunk sector heat balance method since stem
heat balance method applies heat evenly around the
stem circumference. In both methods, qf is calculated by subtracting qv, qr' and ql from P. Sap flow
is then calculated using qf and the specific heat of
water.
To determine qr accurately, both heat balance approaches require measurements during periods of
no flow for estimating the thermal conductance of
insulation around the heater (Baker and van Bavel
1987).
Thermal Dissipation
The thermal dissipation method uses the installation of two probes in a woody stem, one probe directly above the other (Granier 1985, 1987). The
upper probe contains a thermocouple junction and
a heater powered by constant current, while the
lower probe contains only a thermocouple. The
temperature difference between probes (AT) is a
function of the sap flow rate past the installation;
the maximum difference occurs under conditions of
no flow (ATo) and the difference decreases as sap
flow carries more heat away from the upper
(heated) probe. Addition of a thermocouple array
helps remove the confounding effects of naturally
occurring vertical and lateral temperature gradients
in the stem (Goulden and Field 1994). According
to an empirical calibration for three tree species
(Granier 1985), volumetric sap flux density (uv) is
given by:
(
ATo - AT\ 1.231
Uv = 0.000119
AT -;
(13.13)
Although this calibration may apply to other woody
species (Granier et al. 1990), independent calibration may be required (Smith and Allen 1996).
Advantages and Disadvantages
The primary advantage of these methods is that
they provide continuous measurements of sap flow
in individuals over a wide range of plant sizes and
functional types. It is thus possible to partition total
plant water fluxes into the contributions of different
species, at least in relatively simple systems. Simultaneous measurements at ecosystem and wholeplant scales can be combined in this way to address
questions about the role of different species, life
forms, or functional groups for ecosystem functioning (e.g., Ham et al. 1990; Kostner et al. 1992).
Sap flow methods are not without difficulties.
Calibration is one important problem. The stem
heat balance, trunk sector heat balance, and thermal
dissipation methods depend on reliable measurements under no-flow conditions. If these conditions
cannot be met, the accuracy of the methods is questionable (Smith and Allen 1996). A recently developed heat balance gauge may avoid this requirement through the incorporation of two heating
elements (Peressotti and Ham 1996). In some installations, field values obtained using these methods can be checked by comparison with measurements of hydraulic conductance through the portion
of the measured tissue containing the sap flow installation (e.g., Goulden and Field 1994).
Several other potential problems should also be
considered. Heterogeneity in flow may confound
whole-plant scaling when measuring flow in only
a portion of the stem (Edwards et al. 1996). The
accuracy of sap flow measurements may be compromised by tissue damage caused by prolonged
heating effects (Smith and Allen 1996), toxic effects of silicone compounds used to improve thermal contact between heater and stem (Wiltshire et
al. 1995), or wound responses and physical damage
where portions of the apparatus are installed (Barrett et al. 1995). Finally, an error due to heat storage
in the stem has been detected in some applications
(Grime et al. 1995).
Whole Root/Shoot
Hydraulic Conductance
Measurement of whole-plant hydraulic conductance is useful for mechanistic studies of ecosystem
water fluxes and in determining the relative importance of individual plant species to total plant transpiration. Hydraulic conductance is defined as the
mass flow rate for a given pressure difference (e.g.,
kg sec - 1 MPa -1) and may increase with the production of new xylem or decrease with cavitation
caused by freezing or water stress. Seasonal and
interspecific changes in hydraulic conductance can
209
(13.12)
where qv, q" ql, and qf are the vertical, radial, and
lateral heat loss by conduction and the heat transferred by convection in the xylem sap, respectively.
The lateral heat loss term ql is only applicable in
the trunk sector heat balance method since stem
heat balance method applies heat evenly around the
stem circumference. In both methods, qf is calculated by subtracting qv, qr' and ql from P. Sap flow
is then calculated using qf and the specific heat of
water.
To determine qr accurately, both heat balance approaches require measurements during periods of
no flow for estimating the thermal conductance of
insulation around the heater (Baker and van Bavel
1987).
Thermal Dissipation
The thermal dissipation method uses the installation of two probes in a woody stem, one probe directly above the other (Granier 1985, 1987). The
upper probe contains a thermocouple junction and
a heater powered by constant current, while the
lower probe contains only a thermocouple. The
temperature difference between probes (AT) is a
function of the sap flow rate past the installation;
the maximum difference occurs under conditions of
no flow (ATo) and the difference decreases as sap
flow carries more heat away from the upper
(heated) probe. Addition of a thermocouple array
helps remove the confounding effects of naturally
occurring vertical and lateral temperature gradients
in the stem (Goulden and Field 1994). According
to an empirical calibration for three tree species
(Granier 1985), volumetric sap flux density (uv) is
given by:
(
ATo - AT\ 1.231
Uv = 0.000119
AT -;
(13.13)
Although this calibration may apply to other woody
species (Granier et al. 1990), independent calibration may be required (Smith and Allen 1996).
Advantages and Disadvantages
The primary advantage of these methods is that
they provide continuous measurements of sap flow
in individuals over a wide range of plant sizes and
functional types. It is thus possible to partition total
plant water fluxes into the contributions of different
species, at least in relatively simple systems. Simultaneous measurements at ecosystem and wholeplant scales can be combined in this way to address
questions about the role of different species, life
forms, or functional groups for ecosystem functioning (e.g., Ham et al. 1990; Kostner et al. 1992).
Sap flow methods are not without difficulties.
Calibration is one important problem. The stem
heat balance, trunk sector heat balance, and thermal
dissipation methods depend on reliable measurements under no-flow conditions. If these conditions
cannot be met, the accuracy of the methods is questionable (Smith and Allen 1996). A recently developed heat balance gauge may avoid this requirement through the incorporation of two heating
elements (Peressotti and Ham 1996). In some installations, field values obtained using these methods can be checked by comparison with measurements of hydraulic conductance through the portion
of the measured tissue containing the sap flow installation (e.g., Goulden and Field 1994).
Several other potential problems should also be
considered. Heterogeneity in flow may confound
whole-plant scaling when measuring flow in only
a portion of the stem (Edwards et al. 1996). The
accuracy of sap flow measurements may be compromised by tissue damage caused by prolonged
heating effects (Smith and Allen 1996), toxic effects of silicone compounds used to improve thermal contact between heater and stem (Wiltshire et
al. 1995), or wound responses and physical damage
where portions of the apparatus are installed (Barrett et al. 1995). Finally, an error due to heat storage
in the stem has been detected in some applications
(Grime et al. 1995).
Whole Root/Shoot
Hydraulic Conductance
Measurement of whole-plant hydraulic conductance is useful for mechanistic studies of ecosystem
water fluxes and in determining the relative importance of individual plant species to total plant transpiration. Hydraulic conductance is defined as the
mass flow rate for a given pressure difference (e.g.,
kg sec - 1 MPa -1) and may increase with the production of new xylem or decrease with cavitation
caused by freezing or water stress. Seasonal and
interspecific changes in hydraulic conductance can