208
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
11 and 12. In all but the simplest systems, these
methods can obscure the contributions of different
vegetative constituents to total water fluxes. To understand the contributions of individual species and
to predict changes in ecosystem water fluxes associated with vegetation change, ecosystem-level
measurements must be combined with measurements of individuals from the dominant species or
functional types.
Numerous studies have addressed leaf-level gas
exchange and plant water use, but scaling such
measurements to provide reliable estimates of stand
or ecosystem fluxes is difficult (Jarvis 1993; Ansley
et al. 1994; Verhoef 1997). In this section, we address two approaches with considerable promise for
ecosystem studies: sap flow methods and measurements of whole plant hydraulic conductance. Sap
flow methods are used for measuring water fluxes
through stems of a variety of species. When combined with canopy or eddy correlation measurements, these methods help partition the contribution of species and plant functional groups to
ecosystem water fluxes. They can also be coupled
with stable isotope measurements (see Chapter 12)
to estimate the volume of water transpired from different soil sources. The second useful approach is
measuring the hydraulic conductance of entire root
and shoot systems. As context for ecosystem studies, hydraulic measurements provide a mechanistic
perspective on the contribution of different species
and functional types.
Sap Flow Measurements
Sap flow techniques estimate the axial flow rate of
water through root or stem xylem by applying heat
and measuring the temperature of surrounding tissues. The methods used can be divided into three
general groups: heat pulse, heat balance, and thermal dissipation techniques. Taken together, these
methods permit measurement of sap flow through
the roots and stems of herbs, grasses, crops, shrubs,
and trees spanning a wide range of basal diameters
(Lott et al. 1996; Baker and van Bave11987; Allen
and Grime 1995; Kostner et al. 1992; Barrett et al.
1995). Several recent reviews cover various aspects
of these methods (Pearcy et al. 1989; Swanson
1994; Edwards et al. 1996; Smith and Allen 1996).
Heat Pulse Method
The heat pulse method measures sap velocity
through the xylem of woody stems. Two temperature probes are inserted in a stem above and below
a heating element that is also inserted in the xylem
of a woody stem. The lower temperature probe is
positioned closer to the heater than the upper probe.
Following the application of a 1- to 2-sec heat
pulse, the temperature of both probes is measured
continuously to determine the time (te) required for
the two probes to reach the same temperature. The
heat pulse velocity (Vh) is calculated as:
(13.11)
where Xl and Xu are the distances between the
heater and the lower and upper temperature sensors,
respectively. Measured values of Vh are converted
to sap velocity by accounting for sensor materials,
wound effects, installation geometry, and the loss
of heat to the stem during the measurement (e.g.,
Marshall 1958; Swanson and Whitfield 1981).
Measuring the sap flow of entire stems often requires probes inserted to different depths and at different points around the stem to account for stem
variation in sap flow. Integration of the resulting
sap flow profile yields whole-plant water use
(Green and Clothier 1988).
Heat Balance Methods
As the name suggests, heat balance methods apply
heat (P) to the plant and measure the contribution
of sap flow to the distribution of that heat. The
amount applied is calculated from the heater resistance and the voltage. For the stem heat balance
method, generally used with stems <120 mm in
diameter, a heater is wrapped around the circumference of the stem and surrounded by insulation.
Thermocouples are positioned to measure the radial
and vertical temperature differences used to solve
the heat balance (Sakuratani 1981; Baker and van
Bavel 1987). For larger-diameter stems, a trunk
sector heat balance method applies heat internally
to a stem sector using embedded electrodes and
thermocouples to measure vertical and lateral temperature differences (Cermak et al. 1984). The heat
balance for both approaches is given by:
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
11 and 12. In all but the simplest systems, these
methods can obscure the contributions of different
vegetative constituents to total water fluxes. To understand the contributions of individual species and
to predict changes in ecosystem water fluxes associated with vegetation change, ecosystem-level
measurements must be combined with measurements of individuals from the dominant species or
functional types.
Numerous studies have addressed leaf-level gas
exchange and plant water use, but scaling such
measurements to provide reliable estimates of stand
or ecosystem fluxes is difficult (Jarvis 1993; Ansley
et al. 1994; Verhoef 1997). In this section, we address two approaches with considerable promise for
ecosystem studies: sap flow methods and measurements of whole plant hydraulic conductance. Sap
flow methods are used for measuring water fluxes
through stems of a variety of species. When combined with canopy or eddy correlation measurements, these methods help partition the contribution of species and plant functional groups to
ecosystem water fluxes. They can also be coupled
with stable isotope measurements (see Chapter 12)
to estimate the volume of water transpired from different soil sources. The second useful approach is
measuring the hydraulic conductance of entire root
and shoot systems. As context for ecosystem studies, hydraulic measurements provide a mechanistic
perspective on the contribution of different species
and functional types.
Sap Flow Measurements
Sap flow techniques estimate the axial flow rate of
water through root or stem xylem by applying heat
and measuring the temperature of surrounding tissues. The methods used can be divided into three
general groups: heat pulse, heat balance, and thermal dissipation techniques. Taken together, these
methods permit measurement of sap flow through
the roots and stems of herbs, grasses, crops, shrubs,
and trees spanning a wide range of basal diameters
(Lott et al. 1996; Baker and van Bave11987; Allen
and Grime 1995; Kostner et al. 1992; Barrett et al.
1995). Several recent reviews cover various aspects
of these methods (Pearcy et al. 1989; Swanson
1994; Edwards et al. 1996; Smith and Allen 1996).
Heat Pulse Method
The heat pulse method measures sap velocity
through the xylem of woody stems. Two temperature probes are inserted in a stem above and below
a heating element that is also inserted in the xylem
of a woody stem. The lower temperature probe is
positioned closer to the heater than the upper probe.
Following the application of a 1- to 2-sec heat
pulse, the temperature of both probes is measured
continuously to determine the time (te) required for
the two probes to reach the same temperature. The
heat pulse velocity (Vh) is calculated as:
(13.11)
where Xl and Xu are the distances between the
heater and the lower and upper temperature sensors,
respectively. Measured values of Vh are converted
to sap velocity by accounting for sensor materials,
wound effects, installation geometry, and the loss
of heat to the stem during the measurement (e.g.,
Marshall 1958; Swanson and Whitfield 1981).
Measuring the sap flow of entire stems often requires probes inserted to different depths and at different points around the stem to account for stem
variation in sap flow. Integration of the resulting
sap flow profile yields whole-plant water use
(Green and Clothier 1988).
Heat Balance Methods
As the name suggests, heat balance methods apply
heat (P) to the plant and measure the contribution
of sap flow to the distribution of that heat. The
amount applied is calculated from the heater resistance and the voltage. For the stem heat balance
method, generally used with stems <120 mm in
diameter, a heater is wrapped around the circumference of the stem and surrounded by insulation.
Thermocouples are positioned to measure the radial
and vertical temperature differences used to solve
the heat balance (Sakuratani 1981; Baker and van
Bavel 1987). For larger-diameter stems, a trunk
sector heat balance method applies heat internally
to a stem sector using embedded electrodes and
thermocouples to measure vertical and lateral temperature differences (Cermak et al. 1984). The heat
balance for both approaches is given by: