considerable delay between water loss from the foliage and
water uptake by the roots. For example, Hellkvist et al. (1974)
noted a 6-hour difference between the drop in foliage water
potential and a drop in root water potential in relatively young
Picea sitchensis (Bong.) Carr. trees.
A study of the water relations of large old-growth trees at the
Wind River Canopy Crane Research facility has enabled us to
re-examine the topic of stored water. The presence of a tall
Liebherr high-rise construction crane (top of mast 87 m) pro-
vided access to about 2.3 ha under the 75 m jib. During the
summer of 1996, we measured foliar water potential, stem wa-
ter content, stem and branch sap flux and stem dimensional
changes in a 57-m tall Douglas-fir tree, one of the tallest single
trees ever equipped with instruments to monitor in vivo water
flux dynamics. (Much taller trees have since been studied;
e.g., Koch et al. 2004.) Measurements were taken at multiple
heights and positions. Additional, but spatially limited, mea-
surements of stem sap flux and stem water content were made
on two other Douglas-fir trees.
Materials and methods
Study site and study trees
The Wind River Canopy Crane Research site is located near
the Columbia River in Washington. Details about its location,
soil and climate have been described elsewhere (Shaw et al.
2004).
Three
dominant
Douglas-fir
(Pseudotsuga
menziesii
(Mirb.) Franco; hereafter Psme) trees were sampled for stem
tissue water content and one Douglas-fir, Psme 1373, was se-
lected for intensive short-term study of tree water storage
based on branch and stem sap fluxes, twig water potential and
stem dendrometer measurements. Detailed biometric and
physiological measurements were made on Psme 1373, and
supplemental measurements were made on the other study
trees to validate absolute values and patterns. The sample trees
were between 450 and 480-years-old, their heights were in the
upper 20% of trees in the crane circle. Psme 1373 was 1.29 m
in diameter, 57 m tall and had a live crown length of 31 m and a
projected crown area of 95 m
2
. Sampling heights were par-
tially dictated by the presence of the Rose Canopy Platform at
46 m and a 4.5 m mountaineering ladder above the platform.
Additional information about Psme 1373 has been presented
by Bauerle et al. (1999).
Sap flow measurement and calculation
Six stem sap-flow, six branch sap-flow and two dendrometer
sensors were installed on Psme 1373. The sensors were posi-
tioned to capture the vertical and circumferential variation in
sap flow and the vertical variation in dimensional changes in
elastic stem tissues. Two sensors on opposite crown sides
(South, North) were placed on branches at heights of 46, 51
and 56 m, four sap flow sensors were installed near the stem
base at a height of 4 m (from cardinal points) and two in the up-
per stem at 51 m (again on opposite stem sides; see Figure 1).
Sap flow in the main stem was measured by a stem heat bal-
ance (THB) method applied to a stem section with internal (di-
rect electric) heating of tissues (Èermák et al. 1973, 1982,
2004, Kuèera et al. 1977, Tatarinov et al. 2005). The method
used five stainless steel 25 × 1 mm rectangular electrodes that
were inserted in parallel at 20 mm distances into the sapwood
to the depth of the sapwood–heartwood boundary. A compen-
sating system of eight thermocouples (Cu-Cst) was used
(Èermák and Kuèera 1981) with two EMS P-2 sap flow meters
producing constant power (1 W) and data loggers (Environ-
mental Measuring Systems, Brno, Czech Republic). Sensors
were insulated with 2-cm-thick open-porous polyurethane
foam, shielded from radiation by aluminum foil, and protected
from rain by a polyethylene sheet fastened to stem surface with
sealing wax. Because the upper stems of the old-growth
Douglas-fir trees in this forest are exposed to high radiation
loads, the stem immediately above and below the two sets of
sensors in the upper part of Psme 1373 was shielded with a
2-m-long section of aluminum foil. Sap flow in six branches
was measured by a method similar to that for the main stem,
but applying EMS Baby-1 sensors with flexible external heat-
ing and sensing (based on Èermák et al. 1984, 2004 and
Lindroth et al. 1995). Study branches were at tips of healthy,
full-sized branches. Branch tips averaged 13.4 mm in diameter
and carried ~100 g
DW
of needles (~0.37 m
2
of foliage). Branch
sensors were insulated with foam and shielded with a sil-
ver-coated mylar sheath. The ends of the mylar sheaths were
fastened to the smooth bark surface with polyethylene tape.
Both variants of the THB method measure total sap flow
within selected stem sections delimited by electrodes (inte-
grating the radial pattern of flow by combination of two
thermocouples placed at different depths and bulk heating of
tissues) or in branches, where circumferential heating was ap-
plied (Èermák et al. 2004, Tatarinov et al. 2005 and literature
cited therein).
The diurnal course of sap flow within the stem (Q
t
) was
compared with that of branches (
q
br
) located above that point.
Branch sap flow (g m
sw
– 2
h
– 1
on a sapwood area basis) in each
crown section was assumed to be the mean of sap flow in the
branches (q
sh_mean
; g m
sw
– 2
h
– 1
) at the top and bottom of that
crown section and converted to a leaf area basis q
sh_mean
(g m
leaf
– 2
h
– 1
):
q
q
q
br_mean
br_ bottom
br_ top
=
+
2
(1)
where q
br_bottom
is the mean sap flow of the two branch sensors
at the bottom of the crown section under consideration and
q
br_top
is the mean of the two branch sensors at the top.
Because branch sap flow was not measured below 46 m, we
assumed that branch sap flow measured at 46 m would decay
in a linear fashion and approach zero at the base of the live
crown (26 m). Previous studies comparing sap fluxes from the
largest to the smallest tree in a stand (e.g., Èermák and Kuèera
1990, Martin et al. 1997, 2001, Tatarinov et al. 2000) and those
studies comparing branches within the crown of a single tree
(e.g., Hinckley et al. 1994) support this assumption. Sap flow
182
ÈERMÁK, KUÈERA, BAUERLE,
PHILLIPS AND HINCKLEY
TREE PHYSIOLOGY VOLUME 27, 2007
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