Environmental Engineering Reference
In-Depth Information
Fig. 9.4
Sap flow, in grams per hour (g/h) measured on a branch of the
dormant hybrid poplar tree shown in Fig.
9.1
in Charleston, SC, for
6 days, where the depth to groundwater is between 2 and 4 ft
(0.6-1.2 m) below land surface. Sap flow was low during the hot
afternoons. This created a daily sap-flow curve of two peaks in high
flow. This flow rate represents only part of the total flow of the tree
(Landmeyer unpublished data, March 15, 2005).
Fig. 9.6
Sap-flow volume, in liters (L), as determined by the heat-
balance and load-cell methods (Modified from Ferro et al. 2001).
flow, individual trees grown in separate containers are
placed on a scale, called a load cell. Over time, the loss of
water from each tree is computed from the loss in weight of
each tree. A control tree from which all branches have been
removed also is placed on a scale to determine the water loss
from the root zone. In Ferro et al. (2001), each tree also was
measured for water flow using the TDP sap-flow method. In
this manner, sap flow was calibrated to the water-loss
method. Ferro et al. (2001) reported that the total water use
of the plants by sap-flow monitoring was 45% of the total
water use as determined by the load cell method (Fig.
9.6
). It
was unclear if this rather large difference could be explained
by soil moisture losses from the soil not related to transpira-
tion, however, which would not have been measured by the
sap-flow method, or by the large standard deviations evident
for ever increasing water losses.
Sap flow has been measured for phytoremediation
systems at certain sites in the United States and can provide
some information regarding the range of sap-flow rates to be
expected at sites in similar areas. Sites where sap flow has
been measured occurred when at least one of the researchers
had a background in plant physiology or forestry and had
some experience in making sap-flow measurements at
uncontaminated sites. For example, at the site near Fort
Worth, TX (Air Force Plant 4 described in Chap. 8), sap
flow was measured in whips and 1-year-old plantings
installed at the same time. These are the size of tree most
commonly planted at phytoremediation sites. Sap flow for
both whips and 1-year-old trees was higher during the sum-
mer, but the 1-year-old trees had higher sap-flow rates than
the whips as measured on a per-tree basis, by almost a factor
of 2, or 0.61 compared to 0.34 kg/h/tree (Vose et al. 2000)
(Fig.
9.7
). This difference can be explained by the fact that
the 1-year-old plants had a larger average diameter, 2.9 in.
(7.6 cm), compared to the 1.8 in. (4.7 cm) diameter of the
Fig. 9.5
Sap flow, in grams per hour (g/h) measured on a branch of the
dormant hybrid poplar tree shown in Fig.
9.2
using the TDP method,
Elizabeth City, NC. The lower sap flow from July 13 to July 14 was the
result of relative humidity near 100% each day, whereas the other days
did not exceed 75%.
Individual tree-based sap flow could be the mean daily sap-
flow measurement. Sap-flow measurements could be taken
on trees with different diameters or cross-sectional areas,
and a regression line fit to explain how these two variables
move with respect to each other. This equation could then be
used to estimate the sap-flow rate at a particular site if the
cross-sectional area is known (Rural Industries Research and
Development Corporation 2000). Another method to apply
sap-flow measurements made on individual trees to an entire
stand is to relate the sap flow for a particular tree to its
measured leaf area, as
LAI
, and is discussed later in this
chapter.
Sap-flow measurements also can be calibrated using load
cells (Ferro et al. 2001). In this method of computing sap
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