Meteorology
Pu blishing Company, Amsterdam -Printed in The Netherlands
TER TRANSPIRED BY TREES IS INDICATED BY HEAT PULSE
Mountain Forest and Range Experiment Station, Forest Service, U.S. Department of
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Fort Collins, Colo. (U.S.A.)
;�p',celvc�a May 18, 1971)
ABSTRACT
Swanson , R. H., 1972. Water transpired by trees is indicated by heat pulse velocity. Agric. Meteorol.,
10: 277-281.
Daily total transpiration was linearly related to heat pulse velocities measured once each day at
mid-day or averaged for 4- to 14-hour periods centred on mid-day . The correlation coefficients were
liigher than 0.98 in every case. Because the sky was clear throughout m ost of this study, heat pulse
Vel ocities were quite constant throughout the daylight hours. If variable cloudiness had occurred,
HPV's for the longer time periods would probably have shown significantly higher correlations than
the one reading at mid-day.
Heat pulse velocity was shown to be a function of transpiration and water movement into storage.
If HPV's were negligible during darkness, then an HPV taken during daylight was fairly indicative of
the same hour's transpiration. Significant HPV's during darkness indicated m ovement into stem
'ltOrage rather than vapor loss alone, so that any hour's transpiration value was essentially independent
of the comparable hour's velocity reading.
INTRODUCTION
How do we determine the amount of water used by a tree? The heat pulse method
l;(Huber and Schmidt,1937; Marshall,1958) offers promise. In this method, the velocity
;of a heat pulse moving upward in the xylem is measured as an indication of sap water
movement. Heat pulse velocity is usually measured at some convenient height on a tree,
which results in a considerable potential for storage above the measurement plane.
Functionally:
HPV= f(T, S)
(1)
where HPV= heat pulse velocity; T= transpiration (vapor loss from leaves); and S = water
stored in tree parts.
Transpiration is directly indicated by HPVonly if !lS = 0 and the functional relationship
,(l)etween HPVand Tis known.
; ,.
Present address: Forest Hydrology Research, Canadian Forestry Service, Edmonton, Alta., Canada.
278
R. H.
If transpiration rates are to be estimated from heat pulse velocity measurements, under
what circumstances does AS 0,and what is the relationship between HPV and Tin trees1
These two questions were examined for an artificial regime within one tree.
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METHOD
Heat pulse velocity and weight loss from a potted tree were measured simultaneously
for two soil-drying cycles at the Southwest Soil and Water Conservation Laboratory,
Tempe, Arizona. The study tree was an Aleppo pine (Pinus halepensis, mill.) 6 cm in
diameter at the butt,S m tall,and 6 years old. Its total basal area was 24.6 cm2,of whic)!
23.1 cm2 was wet xylem. The container volume was approximately 1 m3 and was enc1o�
in a plastic sheet, partially open at the top but water-tight at the sides and bottom. The
tree was placed on a weighing lysimeter to obtain a sensitive platform for measuring weight
changes. Weather during the approximately 2-week study period varied from hot and <iJ¥
(93°F,1.057 in Hg VPD) to hot and humid (91°F,0.885 in Hg VPD). Data obtained
during days in which rain fell were excluded from the study.
Heat pulse velocity was measured once during each hour with probes and recording
apparatus (Swanson,1967a). Two thermistor probes,2.28 mm in diameter,and a heat
source,1.02 mm in diameter,were located in the tree butt at midpoint of the wet xylei
The entire heat pulse-sensor group was covered with an insulating layer of fiberglass
an aluminium foil shield to reduce insolation.
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Weight changes of 10 g or more were sensed by the lysimeter. Losses during rain-freej�
periods were considered transpiration; evaporation from the plastic-enclosed pot was
assumed to be negligible. Three readings of lysimeter weight were obtained every 10 Ohtf
15 minutes and recorded on punched paper tape. Each set of three readings was avera
'
to obtain weight for that time interval.
Plant internal water stress was used to indicate the degree of soil water availability.
Stress was measured by the pressure bomb technique (Scholander et aI.,1965). Read�
were made to the nearest psig at OOhOO,06hOO, 12hOO, and 18hOO daily. No attempt was
made to obtain soil moisture directly - it was reasoned that stress measuremen ts were �
better indicator of soil moisture tension and water available to the tree than that indie.
by the soil moisture measurements.
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RESULTS
Daily transpiration amounts (7) are closely correlated with 1-,4-,6-,or 14-h dY" .....·'�,f
of heat pulse velocity (HPV) as shown in Fig.I. All of the correlation coefficients are
higher than 0.98. Heat pulse values from each time period centred around mid-day
linearly and equally well correlated with total daily weight loss of the tree. The "".,v_'·>",
linearity was unexpected. It was anticipated that drying of the soil and the
stresses (>30 atm) would affect the conducting xylem area,moisture content,and
in such a way as to make the relationship curvilinear at best (Swanson,1967b).
279
OF WATER TRANSPIRED BY TREES
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20
15
10
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HPV (cm/hr)
Total daily transpiration versus I-h, 4-h, and 6-h HPV centered on mid-day. HPV for 14 daylight
was nearly identical for the 6-h HPV regression line.
Heat pulse velocity directly indicates the hourly transpiration rate only at the lower
The phase and magnitude of the correspondence for mean internal water stress
16 atm on July 19, 1967 are shown in Fig.2. The scatter of data was excessive and
hourly readings should not be used to predict transpiration. It is not clear whether
scatter was due to errors in weight measurement (caused by wind influencing the
readout) or to erratic velocity measurements. It is reasonably evident there was
significant DS term at that time.
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Hour of the day
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Llig.2. Diurnal phase diagram of transpiration and heat pulse velocity at daily average stress of 16 atm.
280
R. H. SWANSON:
Heat pulse velocity was poorly related with hourly transpiration rates at higher stresse$;
Fig.3 shows the phase and magnitude at 27 atm of internal water stress on July 23rd,
2 days before rewatering. The effect of stem storage replenishment and discharge is
evident. Were it not for the four readings from 21hOO to 24hOO,there would be no
apparent correlation between hourly HPV and hourly T. One possible explanation for
these four low readings is that transpiration and change in stem storage were both near
.'
zero. The stress measured at 18hOO was 30.0 atm, and at midnight,28.9 atm. This small
relief in internal stress could indicate little transfer of water from soil to plant,and hence;
limited sap movement. The tree probably suffered permanent damage during this period;'
it failed to regain its former appearance after irrigation, even though the stress
measurements later returned to 11 atm.
0.8
r------,
0.7
' - ' Transpiration
. -. -Heot pulse velocity
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20 ;:,
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Hour of the doy
22
24
Fig.3. Diurnal phase diagram of transpiration and heat pulse velocity at daily average stress of 27
Fig.2 and 3 tend to support the model of eq.I. They also suggest that hourly heat
pulse velocity data must be used with caution to indicate hourly transpiration. (Similar
diurnal heat pulse velocity curves have been obtained in a forest situation. Data from
lodgepole pine and Engelmann spruce growing intermixed at the Fraser Experimental
Forest,Colorado,indicated movement at night during July and September but not in .
mid-June. The night-time velocities were interpreted as indicating low moisture
(Swanson,1 967a). Fig.2 and 3 above tend to confirm this interpretation.)
DISCUSSION
Heat pulse velocity values have been used by others to estimate relative tr�.n.'nir,�tl(
rates (Ladefoged,1963; Doley and Grieve, 1966). My results tend to confirm the
of such use within a daily time context. Single-hour,mid-day heat pulse velocity
are not necessarily representative for an entire day. In this study,I-h HPV values were
OF WATER TRANSPIRED BY TREES
281
because daylight velocities were reasonably uniform, days were mostly clear with
insolation, and the tree was under high internal water stress. For partly cloudy
more usual case, I would expect 4-, 6-,Or 14-h HPV values to show better
with daily transpiration amounts.
change and transpiration both appear in HPV measurements made in the main
refore, transpiration and HPV should be more closely related when velocity
ments are made in twigs. However, it would be difficult to index transpiration
for whole trees through twig measurements There is also a physical limitation
size of material into which probes can be inserted. More than 2 cm of xylem are
d; less tissue than this dried due to air entry when probes are emplaced.
do heat pulse velocity measurements indicate transpiration alone, and not
into storage as well? The most likely way to find out is to plot diurnal HPV's
hourly. If sap moves much after sundown, there is little chance of good correlation
any velocity and corresponding hourly transpiration values.
B. and Schmidt, E., 1937. Eine Kompensationsmethode zur thermoelektrischen Messung
lanjgsalner Saftstrome. (A compensation method for the thermoelectric measurement of slow sap
Ber. Dtsch. Botan. Ges., 55,514-529.
K.,1963. Transpiration of forest trees in closed stands. Physiol. Plant., 16: 378-414.
C., 1958. Measurement of sap flow in conifers by heat transport. Plant Physiol.,
-396.
P. F., Hammel, H. T.,Bradstreet,E. D. and Hemingsen,E. A.,1965. Sap pressure in
plants. SCience, 148,339-346.
R. H., 1967a. Seasonal course of transpiration of lodgepole pine and Engelmann spruce. In:
W. E. Sopper and H. W. L ull (Editors),International Symposium on Forest Hydrology. Pergamon,
London,pp. 419-434. (Appendices containing instrumentation descriptions available upon request
from Rocky Mountain Forest and Range Experiment Station,Fort Collins, Colorado 80521.)
R. H.,1967b. Improving tree transpiration estimates based on heat pulse velocity measure­
ments. IUFRO Kongr., 14, Munich, Sect. 01, 252: 252-263.