∆
V
i
= {
π[(
R
orig_i
+
∆R)
2
– R
orig_i
2
]}L
i
f
(8)
Then
∆V
i
values for a particular stem segment were compared
with the
∆Q
i
for that segment.
Water potential measurements
After foliage expansion was complete, water potential and
transpiration values were obtained during summer 1996 from
two branchlets of each of the study trees (heights ranged from
56 to 65 m) with a pressure chamber (Soil Moisture, Santa
Barbara, CA) and an LI-1600 porometer (Li-Cor, Lincoln,
NE). Measurements were taken at predawn and solar noon in
both aluminum-foil-covered and uncovered branchlets and
then soil-to-leaf hydraulic resistance was calculated by the
Ohm’s Law analog. The study trees had statistically identical
values at predawn and solar noon once height was accounted
for. Details are provided in Bauerle et al. (1999). Three spe-
cific hydraulic resistances were calculated and then compared.
First, water potential values taken at solar noon in uncovered
branches were plotted against the corresponding transpiration
rate—the slope of this line is the hydraulic resistance to water
flow and provides an estimate of resistance between roots and
foliage (i.e., a long-distance resistance, see Elfving et al. 1972,
Camancho et al. 1974). Second, water potential values taken at
solar noon in covered branches were plotted against the tran-
spiration rate measured in the paired uncovered branch (i.e., a
short-distance resistance, see Brooks et al. 2003). Third, leaf
specific conductivity (LSC or 1/resistance) was calculated for
August 29, with a sap flux density of 0.08 kg m
– 2
h
– 1
(at
56.7 m) as the estimate of transpiration rate for the upper
crown and predawn and solar noon water potentials were cor-
rected for the gravitation potential (to provide the water poten-
tial gradient). These values were compared against each other
and against previously reported values.
Data collection and logging
The study began on July 24 and ended on October 15, 1996
(75 days in total). Data were measured every minute and
stored as 15-min means over 2-week periods or every minute
for 3-day periods. Data stored at minute intervals included the
following measurements on Psme 1373: the N and S sap flow
sensors at 4 m, all sap flow sensors at 51 m, and all branch sap
flow sensors and both dendrometers (at 4 and 46 m). Data
stored as 15-minute means were obtained from the stem sap
flow sensors at 4 m on the E and W sides of Psme 1373.
Results
Stem tissue water content of sample trees
Evaluation of cores at the stem base (4 m, the tree had 250 mm
thick bark at that height) showed that phloem and xylem dry
matter (i.e., mainly cell walls) represented about 27%
vol
of to-
tal tissue volume of the Douglas-fir Psme 1373. The fraction
of water was about 5%
vol
for the bark, about 10%
vol
for the
heartwood, about 32%
vol
for the phloem and around 44%
vol
for
the sapwood. Similar values were found in the other two sam-
pled trees. Values at the mid-crown (46 m) were similar to
those at the stem base except that heartwood water content was
only about 6%
vol
. The sapwood of Psme 1373 was about 5 to
8 cm deep (i.e., 13 to 20% of the xylem radius) at 4 m, and
about 4 to 5 cm deep (18 to 27% of the xylem radius) at 46 m.
When considering the whole tree (see Figure 1), the sapwood
represented about one third of the total xylem volume (5363
versus 16,993 dm
3
, i.e., about 56 mm when expressed on a
crown projected area basis) and of that about one quarter was
free water (or about 1217 dm
3
of water, i.e., about 13 mm).
This volume of water represented the majority of total free wa-
ter (85%) in the tree. The total amount of free water in the stem
phloem of Psme was over 161 dm
3
(i.e., 11% of the total free
water) and in the branch sapwood and phloem it exceeded
71 dm
3
(i.e., 5% of total tree water). The needle free water
fraction was 47 dm
3
(or 3.3% of total tree water). When one
considers the vertical distribution of free water in the upper
crown (above 51 m), there was about 4 dm
3
in the stem
phloem, 21 dm
3
in the stem sapwood, 8 dm
3
in the branch sap-
wood and phloem, and 7 dm
3
in the needles. Thus, free water
in the treetop totaled 41 dm
3
and represented about 3% of the
total free water in the tree. In contrast, 97% of the total free wa-
ter in the tree was found below 51 m, with 6, 39 and 52% in the
middle crown, lower crown and bare stem, respectively (Ta-
ble 1).
Water storage and daily transpiration
Diurnal courses of sap flow over 10 days from late July to Oc-
tober (Figure 2 shows four days selected at roughly monthly
intervals), as estimated by the heat balance method, illustrate
the magnitude of the temporal variation in sap flow in the stem
of Psme 1373 measured at 4 and 51 m. The measurements at
4 m height capture sap flow at the base of the tree and for the
entire crown of the tree, whereas the measurements at a height
of 51 m (i.e., upper crown) capture only the upper 6 m of the
crown (representing 20% of this tree’s crown length and carry-
ing about 33% of the total foliage). Whole-tree transpiration
during clear days ranged between 150 and 300 dm
3
day
– 1
over
the study period (about 1.6 to 3.2 mm day
– 1
when expressed
on a crown projected area basis). Phillips et al. (2003) found
that maximum daily water use in a nearby, but larger and taller,
Douglas-fir (~65 m) did not exceed 370 dm
3
day
– 1
on clear
days.
For Psme 1373 for August 1 (Figure 3), Q
t
(equal to Q
crown
)
was 196 dm
3
(i.e., the integrated value under “crown total”; or
under “stem total”). Upper crown and stem sap flow on the
same day above 51 m was 128 dm
3
, i.e., 65% of the total (= the
highest seasonal value). When considering 10 clear days, the
upper crown transpired on average 50 to 65% of the total tree
water loss. Based on calculations of Q
lower crown
(= Q
crown total
–
Q
upper crown
), water loss from the lower crown averaged 89 dm
3
day
– 1
(62 to 114) or 45% (36 to 55) of the total tree water loss.
Expressed as flow density per leaf area, the values were 0.34
and 0.85 dm
3
m
– 2
day
– 1
for the whole tree and upper crown,
respectively. Thus for clear days, water loss from the upper
crown (or stem) was almost equal to water loss from the rest of
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DYNAMICS OF TREE WATER STORAGE AND STEM DIAMETER CHANGE
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