Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 211

Over time, water uptake will decrease as the water poten-
tial difference,

constant, known heat source is applied to a segment of stem
or trunk. This locally heats the transpiration stream present
in the xylem by about 1-6 C. Under steady-state conditions,
heat added must equal heat lost, and this heat loss is
quantified in four directions: conduction up the stem; con-
duction down the stem; conduction out through the heating
element; and convection in the transpiring water. The flux of
heat is balanced using Fourier's Law (Vieweg and Ziegler
1960; Sakuratani 1981; Baker and Van Bavel 1987).
A commonly utilized heat-balance sap-flow meter is the
Flow32-1K

Dc , decreases, until no water is diffused into

adjacent cells.
As water in the soil pores enters the root hair cells by
diffusion, the water can reach the xylem only after first
passing through multiple cell walls through the symplastic
pathway or through cell walls and spaces through the
apoplastic pathway. As water moves by diffusion, it
encounters cell membranes along its path, which provide a
large resistance to flow. Once in the xylem, however, these
resistances to water flow by diffusion are no longer present,
because dead xylem cells no longer contain cytoplasm or
cell membranes. In roots, these resistances are quantified as
root hydraulic conductivity, L p , and can be estimated by
rearranging Eq. 9.1 as

(Dynamax, Houston, TX), a portable, nonin-
vasive sap-flow sensor that requires no calibration. The flow
meter consists of a flexible heater enclosed in insulation that
is wrapped around the trunk, stem, or branch to be examined
and is covered by foil to reflect incident radiation (Steinberg
et al. 1990a) (Fig. 9.1 ). This is limited to smaller trees or
branches of larger trees.
Constant heat also can be applied and heat loss recorded
on larger trees using thermocouple dissipation probes
(TDPs) (Probe12; Dynamax, Houston, TX; Fig. 9.2 ). The
heated thermocouple can be readily observed using infrared
photography (Fig. 9.3 ). Note that the heated probe is cooled
and the heat plume travels upward in the direction of sap
flow.
Using the thermocouples estimates sap velocity , and to
calculate sap flow the velocity is multiplied by the stemwood
area. Typical sap-flow results, for the large poplar tree in
Fig. 9.1 , are shown in Figs. 9.4 and 9.5 .
In the heat pulse-method, sap-flow velocity is defined as
the time required for a heat input of known quantity to travel
from the heat source, such as a heater, to a thermocouple
located a known distance up the stem (Huber 1932). These
results of sap velocity also need to be multiplied by the
stemwood area to yield sap flow.
Regardless of which sap-flow approach is used, certain
considerations are common to both. For example, transpira-
tion can exceed sap flow before the local solar noon, and sap
flow can exceed transpiration in the afternoon and evening,
(Steinberg et al. 1989). This indicates that a lag time exists
between a plant's water demand and the water supply, prob-
ably on account of water storage or capacitance (Dugas et al.
1992). In smaller trees such as whips or 1-year-old trees the
sap flow tends to be equivalent to transpiration, as there is no
lag time on account of a lack of stem-water storage
(Wullschleger et al. 1998). Moreover, sap flow often ceases
in mid-afternoon, in relation to stomatal closure in response
to leaf temperatures (Dugas et al. 1992)—this can be seen in
Fig 9.4 .
In addition to the lag time between transpiration and
measured sap flow, solar orientation is an important factor
to consider when making sap-flow measurements. Some
researchers suggest installing the heater device in the after-
noon, when trunk diameter

™

L p ¼ W v =Dc (9.2)

Root hydraulic conductance can be measured using the
pressure chamber approach, in which a piece of root material
is placed in the chamber and pressurized to reverse water
held in tension to determine the value of Dc for use in

Eq. 9.2 .
The few taproots that characterize most plants, relative to
more abundant roots in the shallow parts of the soil horizon,
have much higher root hydraulic conductivities than shallow
roots. It has been suggested that this is a result of a difference
in physiology of these deeper roots, which tend to have long
and continuous xylem (Le Maitre et al. 1999). The higher
root hydraulic conductivities also reflect the fact that the
potential for groundwater to be encountered increases with
increasing root depth penetration, and water potentials
become less negative with depth nearer the capillary fringe.
As taproots age, however, they tend to become more suber-
ized and have lower hydraulic conductivities than young
taproots.

9.1.3 Sap Flow

For all the methods available to examine the water status of
trees at a phytoremediation site, the measurement of sap flow
is the only method available to directly determine the flow of
water. The sap flow method is based on either a heat-balance
or heat-pulse equipment. Measurement of sap flow does not,
however, yield the fraction derived from groundwater—a
commonly held assumption, perhaps only defensible under
drought conditions. Techniques to determine the fraction of
sap flow derived from groundwater are presented later in this
chapter.
Measurement of sap flow uses heat as a tracer and a mass
balance of added heat to determine sap velocity. In general, a is the smallest due to