210
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
have important consequences for plant gas exchange, which may be extended to understanding
changes in ecosystem water fluxes.
Whole plant hydraulic conductance can be calculated using the pressure chamber and stem flow
methods already described (Cohen et al. 1983; Tyree et al. 1994). The pressure difference across the
plant can be taken as the difference between soil 'I'
estimated by predawn leaf water potential ('1'1) and
midday '1'1. The pressure difference can be further
divided into above- and belowground components
by adding measurements of midday stem xylem
pressure near the root crown. Pressure chamber
measurements of a leaf or small branch attached to
the main stem at ground level, and wrapped in advance with aluminum foil to eliminate flow, are
used to measure stem xylem pressure. The hydraulic conductance of the whole plant, stem, or root
system can be calculated by dividing the transpiration rate measured with sap flow methods by the
appropriate pressure difference. These calculations
assume the contribution of capacitance to the transpiration stream is negligible and that predawn '1'1
accurately reflects the 'l's where water uptake occurs
(Tyree et al. 1994).
Direct measurements of hydraulic conductance
are also made by applying a pressure difference
across the roots or shoots and measuring the flow
through the system (Sperry and Pockman 1993;
Kolb et al. 1996). Roots of potted plants can be
measured by enclosing the soil and root system in
a gas-tight container and measuring flow when the
chamber pressure is increased (Saliendra and Meinzer 1992) or decreased by applying partial vacuum
(Kolb et al. 1996). Until recently, field measurements have been more difficult because of the difficulty of generating a pressure difference without
disturbing the root-soil interaction. The high pressure flowmeter (HPFM) (Tyree et al. 1993) permits
the simultaneous application of pressure and measurement of flow into the cut stem or detopped root
system. As originally developed, the HPFM
method required the application of constant pressure over a period of hours until steady state flow
was observed. During this extended measurement
of roots, the accumulation of solutes caused by reverse flow through the roots changed the measured
hydraulic conductance (Tyree et al. 1994). To minimize the effects of solute accumulation, a modified
method measures flow as the applied pressure is
increased at a constant rate (3 to 7 kPa sec - I, Tyree
et al. 1995). Using this approach, the hydraulic conductance of the measured tissue is calculated as the
slope of the relationship between flow rate and applied pressure, and evidence suggests that this technique provides improved measurement of hydraulic
conductance (Tsuda and Tyree 1997).
The development of the HPFM provides one
method for measuring stem and root hydraulic conductances. Besides application to whole-plant
physiology (Tyree et al. 1995), the availability of a
technique for measurement of water transport
through entire, intact root systems may prove useful
in future ecosystem studies. In particular, measurements of whole-root systems and shallow and deep
roots of the same plants may improve our understanding of how the characteristics of plant water
transport influence ecosystem water fluxes.
Summary
Ecosystem studies are becoming increasingly important for monitoring and solving today's problems, such as those associated with global environmental change. This chapter is meant to be
integrated with Chapters 11 and 12 for designing a
well-conceived study of ecosystem water fluxes.
Our chapter describes the necessary background
and accompanying field measurements that may be
taken for interpreting canopy fluxes and eddy covariance data. Promising methods include time domain reflectometry, remotely sensed data, and stem
flow measurements. By combining such measurements with those discussed in the other chapters of
this section, net ecosystem water fluxes across the
landscape can be determined meaningfully and interpreted mechanistically.
Acknowledgments This research was supported by
grants from the National Science Foundation (DEB
97-333333), NIGEC DOE, and the Andrew W.
Mellon Foundation, and a USDA postdoctoral fellowship (#98-35100-6079) to W.T.P. M. Shawn
Brumbaugh, J.R. Ehleringer. w.A. Hoffmann, and
E.G. Jobbagy provided helpful suggestions on the
manuscript.
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
have important consequences for plant gas exchange, which may be extended to understanding
changes in ecosystem water fluxes.
Whole plant hydraulic conductance can be calculated using the pressure chamber and stem flow
methods already described (Cohen et al. 1983; Tyree et al. 1994). The pressure difference across the
plant can be taken as the difference between soil 'I'
estimated by predawn leaf water potential ('1'1) and
midday '1'1. The pressure difference can be further
divided into above- and belowground components
by adding measurements of midday stem xylem
pressure near the root crown. Pressure chamber
measurements of a leaf or small branch attached to
the main stem at ground level, and wrapped in advance with aluminum foil to eliminate flow, are
used to measure stem xylem pressure. The hydraulic conductance of the whole plant, stem, or root
system can be calculated by dividing the transpiration rate measured with sap flow methods by the
appropriate pressure difference. These calculations
assume the contribution of capacitance to the transpiration stream is negligible and that predawn '1'1
accurately reflects the 'l's where water uptake occurs
(Tyree et al. 1994).
Direct measurements of hydraulic conductance
are also made by applying a pressure difference
across the roots or shoots and measuring the flow
through the system (Sperry and Pockman 1993;
Kolb et al. 1996). Roots of potted plants can be
measured by enclosing the soil and root system in
a gas-tight container and measuring flow when the
chamber pressure is increased (Saliendra and Meinzer 1992) or decreased by applying partial vacuum
(Kolb et al. 1996). Until recently, field measurements have been more difficult because of the difficulty of generating a pressure difference without
disturbing the root-soil interaction. The high pressure flowmeter (HPFM) (Tyree et al. 1993) permits
the simultaneous application of pressure and measurement of flow into the cut stem or detopped root
system. As originally developed, the HPFM
method required the application of constant pressure over a period of hours until steady state flow
was observed. During this extended measurement
of roots, the accumulation of solutes caused by reverse flow through the roots changed the measured
hydraulic conductance (Tyree et al. 1994). To minimize the effects of solute accumulation, a modified
method measures flow as the applied pressure is
increased at a constant rate (3 to 7 kPa sec - I, Tyree
et al. 1995). Using this approach, the hydraulic conductance of the measured tissue is calculated as the
slope of the relationship between flow rate and applied pressure, and evidence suggests that this technique provides improved measurement of hydraulic
conductance (Tsuda and Tyree 1997).
The development of the HPFM provides one
method for measuring stem and root hydraulic conductances. Besides application to whole-plant
physiology (Tyree et al. 1995), the availability of a
technique for measurement of water transport
through entire, intact root systems may prove useful
in future ecosystem studies. In particular, measurements of whole-root systems and shallow and deep
roots of the same plants may improve our understanding of how the characteristics of plant water
transport influence ecosystem water fluxes.
Summary
Ecosystem studies are becoming increasingly important for monitoring and solving today's problems, such as those associated with global environmental change. This chapter is meant to be
integrated with Chapters 11 and 12 for designing a
well-conceived study of ecosystem water fluxes.
Our chapter describes the necessary background
and accompanying field measurements that may be
taken for interpreting canopy fluxes and eddy covariance data. Promising methods include time domain reflectometry, remotely sensed data, and stem
flow measurements. By combining such measurements with those discussed in the other chapters of
this section, net ecosystem water fluxes across the
landscape can be determined meaningfully and interpreted mechanistically.
Acknowledgments This research was supported by
grants from the National Science Foundation (DEB
97-333333), NIGEC DOE, and the Andrew W.
Mellon Foundation, and a USDA postdoctoral fellowship (#98-35100-6079) to W.T.P. M. Shawn
Brumbaugh, J.R. Ehleringer. w.A. Hoffmann, and
E.G. Jobbagy provided helpful suggestions on the
manuscript.