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Tree water storage and its diurnal dynamics related to sap flow and changes in stem volume in old-growth Douglas-fir trees

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Diurnal and seasonal tree water storage was studied in three large Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) trees at the Wind River Canopy Crane Research site. Changes in water storage were based on measurements of sap flow and changes in stem volume and tissue water content at different heights in the stem and branches. We measured sap flow by two variants of the heat balance method (with internal heating in stems and external heating in branches), stem volume with electronic dendrometers, and tissue water content gravimetrically. Water storage was calculated from the differences in diurnal courses of sap flow at different heights and their integration. Old-growth Douglas-fir trees contained large amounts of free water: stem sapwood was the most important storage site, followed by stem phloem, branch sapwood, branch phloem and needles. There were significant time shifts (minutes to hours) between sap flow measured at different positions within the transport system (i.e., stem base to shoot tip), suggesting a highly elastic transport system. On selected fine days between late July and early October, when daily transpiration ranged from 150 to 300 liters, the quantity of stored water used daily ranged from 25 to 55 liters, i.e., about 20% of daily total sap flow. The greatest amount of this stored water came from the lower stem; however, proportionally more water was removed from the upper parts of the tree relative to their water storage capacity. In addition to lags in sap flow from one point in the hydrolic pathway to another, the withdrawal and replacement of stored water was reflected in changes in stem volume. When point-to-point lags in sap flow (minutes to hours near the top and stem base, respectively) were considered, there was a strong linear relationship between stem volume changes and transpiration. Volume changes of the whole tree were small (equivalent to 14% of the total daily use of stored water) indicating that most stored water came from the stem and from its inelastic (sapwood) tissues. Whole tree transpiration can be maintained with stored water for about a week, but it can be maintained with stored water from the upper crown alone for no more than a few hours.
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Summary Diurnal and seasonal tree water storage was stud
-
ied in three large Douglas-fir (Pseudotsuga menziesii [Mirb.]
Franco) trees at the Wind River Canopy Crane Research site.
Changes in water storage were based on measurements of sap
flow and changes in stem volume and tissue water content at
different heights in the stem and branches. We measured sap
flow by two variants of the heat balance method (with internal
heating in stems and external heating in branches), stem vol-
ume with electronic dendrometers, and tissue water content
gravimetrically. Water storage was calculated from the differ-
ences in diurnal courses of sap flow at different heights and
their integration. Old-growth Douglas-fir trees contained large
amounts of free water: stem sapwood was the most important
storage site, followed by stem phloem, branch sapwood,
branch phloem and needles. There were significant time shifts
(minutes to hours) between sap flow measured at different po
-
sitions within the transport system (i.e., stem base to shoot tip),
suggesting a highly elastic transport system. On selected fine
days betweenlate July and early October, when daily transpira
-
tion ranged from 150 to 300 liters, the quantity of stored water
used daily ranged from 25 to 55 liters, i.e., about 20% of daily
total sap flow. The greatest amount of this stored water came
from the lower stem; however, proportionally more water was
removed from the upper parts of the tree relative to their water
storage capacity. In addition to lags in sap flow from one point
in the hydrolic pathway to another, the withdrawal and replace
-
ment of stored water was reflected in changes in stem volume.
When point-to-point lags in sap flow(minutes to hours near the
top and stem base, respectively) were considered, there was a
strong linear relationship between stem volume changes and
transpiration. Volume changes of the whole tree were small
(equivalent to 14% of the total daily use of stored water) indi
-
cating that most stored water came from the stem and from its
inelastic (sapwood) tissues. Whole tree transpiration can be
maintained with stored water for about a week, but it can be
maintained with stored water from the upper crown alone for
no more than a few hours.
Keywords: dendrometer, flow rate differences, heat balance
method, time shift, tissue free water content, vertical profile.
Introduction
Most analyses of plant water relations regard the soil as the
sole source of transpired water. Roberts (1976) reported that
the amount of free water from storage in Pinus sylvestris L.
trees and stands is insignificant relative to daily or seasonal
transpiration. Similarly, Tyree and Yang (1990) concluded that
stored water is not a significant source of water for transpira
-
tion in most woody plants. Holbrook (1995) in her review of
stem water storage stated: “Its [Stem water storage] role in
maintaining high levels of photosynthetic carbon gain during
periods of drought, however, is limited to plants with inher
-
ently low transpiration rates (i.e., CAM succulents and per
-
haps large conifers). However, Ladefoged (1963), Hinckley
and Bruckerhoff (1975), Waring and Running (1978), Waring
et al. (1979) and Èermák et al. (1976, 1982) have suggested
that internal water storage in both elastic and inelastic tissues
may be important in supporting diurnal and seasonal
transpiration of woody plants. In special situations, it has been
observed that internal storage can provide a significant propor
-
tion of the total diurnal and even seasonal water use by a plant
(e.g., Èermák et al. 1982, 1984, Goldstein et al. 1984, 1998,
Borchert 1994). If storage is minimal in large trees, then water
loss would either result in severe water deficits or prolonged
stomatal closure. Either of these outcomes would have conse
-
quences for growth and survival. Therefore, we contend, as
suggested by older work with trees, that stored water plays a
biologically significant role.
Water transport in large old-growth trees occurs over long
distances via conducting elements that may have low hydrau
-
lic conductivities (Gartner 1995, Sperry 1995, Ryan and Yoder
1996). Even in short-stemmed woody plants, there may be a
Tree Physiology 27, 181–198
© 2007 Heron Publishing—Victoria, Canada
Tree water storage and its diurnal dynamics related to sap flow and
changes in stem volume in old-growth Douglas-fir trees
JAN ÈERMÁK,
1,2,3
JIØÍKUÈERA,
4
WILLIAM L. BAUERLE,
5
NATHAN PHILLIPS
6
and
THOMAS M. HINCKLEY
3
1
Institute of Forest Ecology, Mendel University of Agriculture and Forestry, 61300 Brno, Czech Republic
2
Corresponding author (cermak@mendelu.cz)
3
College of Forest Resources, University of Washington, Seattle, WA 98195, USA
4
Environmental Measuring Systems Inc., 61300 Brno, Czech Republic
5
Department of Horticulture, Clemson University, Clemson, SC 29634, USA
6
Department of Geography, Boston University, Boston, MA 02215, USA
Received September 5, 2005; accepted April 1, 2006; published online November 1, 2006
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
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 sapwoodheartwood 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
qq
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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in the stem cylinder between 4 and 51 m (Q
t_med
;kg
stem
h
–1
)
was derived as the difference between the inflow (Q
t_4m
) into,
and outflow (Q
t_51m
) from, this section:
Q
t_med
= Q
t_4m
Q
t_51m
(2)
where Q
t_4m
is assumed daily total tree water use. Similar cal
-
culations were made for branches:
Q
crown_med
= Q
crown_tot
Q
crown_top
(3)
where Q
crown_med
is sap flow for the mid-crown, Q
crown_tot
is sap
flow for the whole crown (equal to that for the whole tree) and
Q
crown_top
is sap flow measured just below the top.
Estimation of water storage and tissue volume
We estimated wood and foliage water storage gravimetrically
(biometric samples), hydrometrically (based on sap flow) and
volumetrically (with dendrometers). For some of the water
content estimates, it was necessary to determine bole, branch
and foliage volumes. Total volumes of aboveground tree tis
-
sues including the stem and foliage were estimated biometri
-
cally. Volumes of stem xylem, phloem and bark, and branch
xylem, phloem and bark, were estimated from diameters and
cores taken at stem heights of 4, 46 and 51 m.
Foliage volume was calculated from (1) measurements of
height above ground, diameter, length and foliage volume of
all live branches (Ishii et al. 2002) and (2) estimated foliage
quantity based on sapwood basal area and branch size and po
-
sition. Sapwood cross-sectional area at any height on the bole
of a Douglas-fir is related linearly to the amount of foliage
above that point (Long et al. 1981). In addition, our two esti
-
mates of foliage quantity were compared with a third estimate
derived from sapwood cross-sectional area at 4 m (McDowell
et al. 2002).
The longitudinal or vertical section of the stem in Figure 1
was reconstructed from the sapwood cross-sectional area us
-
ing a tree stem form factor (Korf et al. 1972, Philip 1994) with
total stem volume converted to sapwood and phloem volumes.
Free water volume in the sapwood (V
w_free
/V, expressed as a
percentage of total sapwood fresh volume, V) was calculated
by subtracting the volume of water in the heartwood (V
w_htrw
,
taken as mostly physically bound) from that in the sapwood
(V
w_sapw
) (Zimmermann and Brown 1971, Kravka and Èermák
1995):
V
V
V
V
V
V
w_free
w_sapw
w_htrw
=
(4)
Changes in stem radius of Psme 1373 at heights of 4 and
46 m were measured with a temperature compensated elec
-
tronic radial dendrometer (DR-01, EMS Brno, Czech Repub
-
TREE PHYSIOLOGY ONLINE at http://heronpublishing.com
DYNAMICS OF TREE WATER STORAGE AND STEM DIAMETER CHANGE 183
Figure 1. Sample tree (right)
showing the positions of sap
flow sensors at heights of 4, 46
(B), 51 (M) and 56 m (T).
Vertical pattern of stem form
(A; delimiting sapwood and
heartwood) and free water
content in the sapwood,
phloem and needles (B) in the
old-growth Douglas-fir sample
tree (Psme 1373). Horizontal
bars represent 1-m thick layers
above ground, including tree
stem, branches and needles.
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lic). A steel radial rod was inserted through a 7-mm diameter
hole (which extended 80 mm through the sapwood and was a
little wider than the rod so that the rod was not touching the
sapwood) and screwed tightly into the heartwood. A magnetic
sensor (Diana Inc., U.K.), whose sensitive point was in direct
contact with a smooth bark surface (located 5 cm below the
rod), was fastened to the rod; its temperature was measured
with an attached platinum thermometer. The bark was re
-
moved and smoothed to a distance about 1 mm from the bark
cambium and phloem. The dendrometers were insulated and
shielded as described for the sap flow sensors.
Gravimetric water storage estimates
Stem tissue water content was estimated by classical methods.
Bark, phloem, and xylem radial water contents (% of volume)
were measured gravimetrically on 5.2-mm diameter cores
taken with an increment corer (Suunto, Finland). Immediately
after sampling, each core was protected by tightly wrapping it
in aluminum foil and stored in a shielded polyethylene bag.
Within 24 h, each core was cut into short pieces of known
length; these were individually marked, weighed, oven-dried
at 90 °C for 24 h and re-weighed. The specific mass of xylem
dry matter was assumed to be 1.54 g cm
–1
and volume was at-
tributed to three fractions: dry matter, water and air (Kravka
and Èermák 1995). The phloem was assumed to contain the
same fraction of free water as the xylem sapwood. The amount
of free stored water was calculated by multiplying the volume
of a particular tissue by the volumetric percentage of free wa-
ter. We defined free water as the amount of water measured in
the sapwood after subtracting the amount of water measured in
the heartwood.
Needle water content was measured at 55.9, 51.1, 44.2, 39.1
and 26.4 m in current-year, 1- and 2-year old foliage sampled
from the south side of the crown. Needle free water was esti
-
mated based on percent water content values obtained from
small samples multiplied by the quantity of foliage in 1-m
zones. Samples were taken in late October when tissues were
well hydrated. Needles were oven-dried at 65 °C for 72 h. The
foliage area, mass and volume in these zones were estimated
knowing total foliage parameters (from sapwood area) and its
distribution along the stem (derived from the distribution of
branch foliage volumes).
Hydrodynamic water storage estimates (sap flow)
For Psme 1373, we had estimates of the total foliage and its
vertical distribution and the amount of water lost from six
branches (q
br
). When q
br_mean
(kg
m
leaf
–2
h
–1
) for each crown sec
-
tion was multiplied by the leaf area for that section and then
summed to give Q
crown
(kg h
–1
), Q
crown
overestimated Q
t
,be
-
cause branch sap flows were measured in branches at the outer,
more exposed edges of the crown, which overestimated water
loss for that section. To correct this error, q
br_mean
was con
-
verted to total sap flow for each section of the crown by using
apparent leaf area A
app
(m
2
part
–1
) and the formula:
Q
crown
= q
br_mean
A
app
(5)
where A
app
was determined in an iterative process so that Q
crown
equaled Q
t
for each day (assuming that there were no water
losses from a stem without foliage). Daily total Q
crown
was first
calculated by multiplying q
br_mean
by leaf area (A
actual
) and this
value was compared with the daily summed Q
t
derived from
sap flow data. Actual leaf area was reduced and Q
crown
was re
-
calculated. The process was continued until Q
crown
matched Q
t
for that day. At that point, A
app
for the tree crown was known.
The change in stored water (Q;dm
3
) at any time in the
whole tree (or in a specified part of the tree) became discern
-
ible when the difference in sap flow between the stem and
small branches was calculated:
Cum (
branch stem
t
t
∆∆QQ Qt=
+
–)
1
(6)
where t is the time step and can range from the length of time
between data logging to the entire day. Negative values of Q
occur between sunrise and early to mid-afternoon and repre
-
sent times when water stores are being depleted. Positive val
-
ues of Q indicate refilling of depleted water storage tissue,
which occurs from mid-afternoon to well into the evening or
until the next dawn.
Because the daily totals of flow, Q
crown
and Q
t
, were equal
(only small differences can be expected between consecutive
days), data collected over short periods within a day can be
compared to estimate the amount of water extracted from tree
storage during a particular day (W
stor
):
W
stor
= (±) Q = Q
t
Q
crown
(7)
where, for each recorded time step (1 or 15 min) during a diur
-
nal course, a series of differences, +Q and Q resulted.
Their summation for the morning hours provided an estimate
of the use of stored water (–Q), whereas their summation in
the late afternoon and evening hours (+Q) gave an estimate
of the refilling of stored reserves.
Volumetric water storage estimates (dendrometers)
The diurnal curve of cumulated Q should reflect changes in
tissue water content and thus should be comparable with diur
-
nal changes in the volume of extensible tissue (e.g., sapwood,
phloem and a negligible part of cork bark (see Molz and
Klepper 1973, Hinckley and Bruckerhoff 1975)), measured as
R with dendrometers in addition to water changes in inelastic
tissue (e.g., cavitation). This comparison can be made only
when Q and R are expressed in comparable units. First, re
-
corded data changes in stem radius (R based on an initial ra
-
dius, R
orig
) were converted to changes in stem cross-sectional
area (A). These changes were expressed on a volume basis,
V, by multiplying the length (L) of the corresponding stem
segment, L
i
(or lengths of upper, middle and lower part of the
stem) by a stem form parameter (f ):
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TREE PHYSIOLOGY VOLUME 27, 2007
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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 185
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the tree below and seemed more stable than water loss from the
lower crown. In addition, the uppermost foliage always tran-
spired disproportionately more water (> 2.5× in relation to
needle area) than that of the middle and lower crown. Similar
results were noted by Èermák and Kuèera (1990) in large Nor-
way spruce trees.
Diurnal course and time lag
The diurnal course of sap flow closely followed foliar transpi
-
ration and thus started first in branch tips, then in branches,
somewhat later in the stem near branches and with the most
pronounced delay in the stem furthest from the foliage. The
top 6 m of the crown and the whole crown appear to begin tran-
spiring almost at the same time; integrated sap flow in
branches of the upper crown showed no significant time shift
compared with the crown total (see Figure 3; maximum lag
was less than 15 min), or with stem sap flow at 51 m. In con-
trast, a pronounced time shift was noted between stem sap
flow at 51 m and at 4 m. Sap flow at the stem base lagged that
of the whole crown by 1 to 2 h. The time lags were even more
pronounced after sunset. Transpiration from the upper crown
or total crown ceased about 2030 h, almost 4 h before sap flow
approached zero at 51 m and 4.5 h before it approached zero at
4 m. During the morning hours, the water balance of the tree
stem between 4 and 51 m was negative. For about 2 h, outflow
to the upper stem was greater than inflow from the lowermost
stem (below 4m). The balance became positive again at about
1000 h when input into the stem at 4 m was greater than output
at 51 m. Depleted water storage was recharged in the afternoon
and at night until the early morning hours of the next day when
transpiration resumed.
Similar to the timing of sap flow lags, temporal variation in
sap flow decreased from the individual branch to the whole
crown (data not shown) and from the upper stem to the lower
stem. Sap flow variation was higher in small, foliated branches
and decreased in the main stem with increasing distance from
the foliage. As the distance from the transpiring surface in
-
creased, more transpiring surfaces were integrated and more
tissue buffering capacity was involved, thus the diurnal curves
in the lower stem appeared quite smooth (see Figure 3).
Diurnal courses of sap flow and changes of stored water
Diurnal changes in water storage were varied over the growing
season, but had the same general pattern. Stored water was de
-
186 ÈERMÁK, KUÈERA, BAUERLE, PHILLIPS AND HINCKLEY
TREE PHYSIOLOGY VOLUME 27, 2007
Figure 2. Seasonal course of daily sap flow in the old-growth Doug
-
las-fir sample tree (Psme 1373) and daily global radiation in the Wind
River experimental plot. Short lines indicate 10 individual days with
fine weather selected for detailed analysis of water storage, arrows in
-
dicate days shown in following figures.
Table 1. Volumes and amounts of free water in parts of the Douglas-fir sample tree, Psme 1373.
Volume (dm
3
) Free water (dm
3
)
Upper Mid- Lower Bare Tree Upper Mid- Lower Bare Tree
crown crown crown stem total crown crown crown stem total
> 51m 4651m 2346m < 23m > 51m 4651m 2346m < 23m
Stem sapwood 92 186 1931 2912 5122 = 81.0% 21 42 438 661 1163 = 81.6%
Stem phloem 13 21 204 316 554 = 8.8% 4 7 58 76 145 = 10.2%
Stem sapw + phl 105 207 2135 3229 5676 = 89.7% 25 49 497 737 1308 = 91.8%
Branch sapwood 29 77 135 0 242 = 3.8% 7 17 31 0 55 = 3.8%
Branch phloem 6 16 29 0 51 = 0.8% 2 5 9 0 16 = 1.1%
Branch sapw + phl 35 93 164 0 293 = 4.6% 8 23 40 0 71 = 5.0%
Sapwood total 121 263 2067 2912 5363 = 84.8% 28 60 469 661 1217 = 85.4%
Phloem total 19 38 233 316 606 = 9.6% 6 12 67 76 161 = 11.3%
Sapwood + phl total 140 301 2299 3229 5969 = 94.4% 34 71 536 737 1379 = 96.7%
Needles total 54 105 198 **** 357 = 5.6% 7 14 26 **** 47 = 3.3%
Wet tissues total 194 405 2497 3229 6326 = 100% 41 86 562 737 1426 = 100%
3.1% 6.4% 39.5% 51.0% **** 2.9% 6.0% 39.4% 51.7% ****
Stem total 237 608 5948 10200 16993 = 100% **** **** ***** ***** ****
1.4% 3.6% 35.0% 60.0% **** **** **** ***** ***** ****
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
pleted mostly during morning hours and replenished during
the afternoon, particular changes were dependent on weather
conditions.
On a daily basis, water withdrawn from storage equaled wa
-
ter returned to storage (consequently Q
t
= Q
crown
, which fits for
day-to-day changes, but not for a growing season). The results
presented in Figure 3 illustrate the within-day behavior of
stored water, whereas data in Figure 4 represent the total water
used from storage. The mean daily total quantity of water
withdrawn from storage for the whole tree Psme 1373 was
about 45 dm
3
(varying between 34 and 53 dm
3
) and for its up
-
per crown it was about 13 dm
3
(9–27 dm
3
). The relative quan
-
tity of stored water used from the whole tree was appreciable
and represented about 23% (2031%) of daily total sap flow,
compared with only about 7% (5–16%) of daily sap flow used
from the upper crown. When expressed as a percentage of total
free water, total stored water used on clear days was 3.0%
(2.33.6%) and that from the upper crown was 0.9%
(0.6–1.8%): therefore, it was a relatively small fraction of total
free water, likely reflecting our definition of free water (see
Figure 1). However, as a percentage of total water lost by tran
-
spiration, the daily use of stored water (~23%) represented a
biologically significant quantity.
Water withdrawn from storage came from both the upper
stem (> 51m) and the lower stem (< 51m); however, the quan
-
tity coming from the lower stem was 3 to 5 times that from the
upper stem. Most of the stored water (75%) came from the
zone between 4 and 51 m (see Figure 3 and 4). The volume of
free water in the tree was an order of magnitude larger below
46 m than above 46 m (Table 1). For the upper part of the tree
(i.e., above 51 m), water used from storage was about 10 dm
3
.
Both elastic (phloem, needle, etc.) and inelastic tissue volumes
were small in the top of the tree (see Figure 1) and between 38
and 65% of these volumes could be withdrawn. There was
considerable temporal variability in water removed from or re-
turned to storage in the upper part of Psme 1373 (Figure 4).
Although there was a net depletion of water from storage in
the morning and early afternoon, there were short periods of
recharge. The pattern for the tree below 51 m was easily di-
vided into distinct phases of depletion and recovery (Figure 4).
As shown in Table 1, the amount of free water in the upper
stem was considerably less than in the lower stem (140 versus
5969 dm
3
, or 2.3% of the total). Thus, the percentages of free
water observed in the two regions differed (~24 versus ~4%
for the upper stem and whole tree, respectively) even though
the absolute amounts were in the opposite direction (~10 ver
-
sus ~50 dm
3
, respectively; see Figure 4). From the standpoint
of proximity (when compared with soil water) and volume, the
sapwood of the lower stem was most important. This was evi
-
dent in the distribution of free water in different tissues (stem
sapwood and phloem, branch sapwood and phloem and need
-
les) and at different heights (upper middle and lower crown
and bare stem below crown; see Figure 1).
Diurnal changes in stem volume and stored water
The radius of the stem (R) measured at 4 and 46 m changed ap
-
preciably during a 24-h period (Figures 5 and 6). Values of R
reported in this study (with observed daily amplitude of about
0.1 mm) largely reflect volumetric changes in elastic tissues
and associated changes in their water content as first suggested
by MacDougal (1925), Arcikhovskiy (1931), and Molz and
Klepper (1973). The maximum radius was noted between
0730 and 0800 h at both 4 and 46 m. At 46 m, a minimum oc
-
curred around 1400 h; there were no further changes in stem
radius until 1900 h, when radius increased rapidly. In contrast,
stem radius was minimal at 1730 h at 4 m. Stem radius contin
-
ued to increase through the night, but at a lower rate. The
steeper slope during recovery at night likely illustrates rehy
-
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DYNAMICS OF TREE WATER STORAGE AND STEM DIAMETER CHANGE 187
Figure 3. Diurnal measures of sap flow in the old-growth Douglas-fir
sample tree (Psme 1373) at the base of the stem and for the upper
crown on four selected days with fine weather (August 1 and 24, Sep
-
tember 10, October 8). Crown totals represent sap flow measured in
branches distributed at six locations in the crown. Stem totals repre
-
sent flow measured at the stem base (at a height of 4 m). Upper crown
represent flows measured in branches close to the tree top (above
51 m), upper stem represents flow measured in the stem at the height
of 51 m.
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
dration, whereas the gentler slope late at night and early in the
morning suggests growth. Diurnal changes in stem volume
paralleled the cumulated difference in sap flows during day
-
time and volume increases during night, when no storage wa
-
ter is extracted from tissues. The time shift, however, was
larger for the whole tree than for the upper crown.
The slightly greater dimensional changes in the upper stem
radius than in the lower stem radius confirmed earlier findings
of Dobbs and Scott (1971). For Psme 1373, the diurnal pattern
of stem shrinkage (–R and Q), refilling (+∆R and +Q) and
growth (+R) was similar at the base of the stem to that in the
upper crown. However, there were large differences in the tim
-
ing of changes in stem volume and water storage in the upper
stem versus the lower stem, 15 min versus 3 h (Figure 6). The
delay in the upper stem increased substantially during the
growing season, whereas the delay in the lower stem remained
constant. If these time shifts are taken into account and the late
night and early morning increases due to growth are excluded,
then the relationship between a volume change and a change in
water storage was linear during most of the daytime—from
about 0800 to 2100 h (Figure 7).
Diurnal changes in stem volume (V calculated from mea
-
sured R with Equation 7, Figure 6) were strongly related to
changes in the quantity of water removed from storage Q)
(cf. Figures 6 and 7). Stem volume decreased with increasing
transpiration (and water depletion from storage, Q) early in
the day and increased with decreasing transpiration (and grad
-
ual refilling of storage, +Q) later in the day. Despite these
changes, growth or a net day-to-day increase in volume oc
-
curred only during the night when transpiration approached
zero and internal storage comparments had been largely re
-
filled.
When measured volume changes were expressed as frac
-
tions of free water for different tissues, they appeared highest
for needles of the whole crown, followed by phloem and other
wet tissues. The situation was similar when daily storage was
evaluated the same way, but the significantly higher percent
-
age volume change occurring in the upper crown indicated
188 ÈERMÁK, KUÈERA, BAUERLE, PHILLIPS AND HINCKLEY
TREE PHYSIOLOGY VOLUME 27, 2007
Figure 4. Diurnal courses of differ-
ences between transpiration of crown
foliage (sap flow in branches) and sap
flow in the stem for the whole tree
(whole crown and stem base; left) and
upper crown (above height of 51m;
right) in the old-growth Douglas-fir
sample tree (Psme 1373) for four se
-
lected days with fine weather (Aug
-
ust 1 and 24, September 10 and Octo
-
ber 8).
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
higher tensions there (Figure 8). Changes in stem volume are
caused by transpirational extraction of water from tissues and
when taking into account the time shift (larger but almost con
-
stant for the whole tree, smaller but gradually increasing in the
upper crown) both processes are linearly related. Soft tissues
account for only a small proportion of stored water, most of
which is in the sapwood.
Seasonal changes in daily water storage and stem volume
The amount of water withdrawn from storage on clear days
was either relatively stable (upper crown and stem; Figure 8A:
open rectangles) or increased as the season progressed (whole
tree; Figure 8A: solid rectangles). For the whole tree, the
amount of water withdrawn from storage each day increased
from about 40 dm
3
in early August to 50 dm
3
in late September
and ranged from 20 to 30% of daily sap flow. For the upper
stem, water withdrawn from storage (~10 dm
3
day
–1
) averaged
about 10% of the water lost from the upper stem (96 to
128 dm
3
; Figure 8B) and was relatively stable. The amount of
water withdrawn daily from the whole stem represented 2 to
5% of the free stored water (again increasing from August to
October). For the same period, but considering only the upper
part of the stem, 20 to 25% of free water was used during the
day (Figure 8C). There was a disproportionate use of stored
water from the top of the tree.
Earlier in the season when about 40 dm
3
of stored water was
used by the entire tree (Figure 8A), about 6 dm
3
came from
changes in elastic tissues or about 15% of the total. This per
-
centage decreased over the season. Daily volume changes in
elastic tissues (averaged 6 and 0.4 dm
3
day
–1
for the whole tree
and upper crown, respectively; Table 2) were small compared
with the total quantity of free water used from storage. If ex
-
pressed as a percentage of free water, diurnal water used from
elastic tissues of the entire tree never exceeded 0.5%. For the
upper stem, it increased from 0.7 to 1.0% over the study pe
-
riod. Water from elastic tissue was never a substantial percent
-
age of the total, about 14% of daily water used for transpira-
tion derived from storage or about 1% of the free water. On
average about 45 and 10 liters (about 23 and 10% of transpired
water) were taken from storage when considering the whole
tree and its upper part, respectively. This percentage did not
change much for the whole tree during the study (it increased
from about 20% in the fall to 31% in midsummer), but the per-
centage increased substantially in the upper part of the tree
(the upper stem supplied twice as much in October, up to 27%,
compared with earlier in the summer), even though tree water
loss was about 33% lower at this time of the year.
Water turnover rate
When considering just free stored water from the upper crown
and from the whole tree and their corresponding sap flows,
stored water could meet transpirational needs for a little more
than a third of a day (0.32 to 0.42) and for about a week (6.3 to
8.4), respectively (Table 3), with no clear seasonal variation.
Transpiration and water potential relationship
Figure 9 illustrates a constant decline in water potential as
transpiration increased at the tops of the three study trees. The
two slopes, short- and long-distance hydraulic conductivity,
were similar and linear. The water potential difference be
-
tween the two lines at a given transpiration rate corresponds to
the frictional potential. Frictional potential is another negative
constraint, in addition to the gravitational potential, that can
decrease leaf water potential in these tall trees.
Discussion
Tree water storage in stems and branches
Sapwood and heartwood water contents at the stem base of the
studied trees were similar to those observed in other conifer
-
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DYNAMICS OF TREE WATER STORAGE AND STEM DIAMETER CHANGE 189
Figure 5. Diurnal measures of stem radius (R) at the height of 4 m
(left) and 46 m (right) in the old-growth Douglas-fir sample tree
(Psme 1373) during selected days with fine weather (August 1 and 24,
September 10 and October 8). The radius displayed is real, but is rela
-
tive to the measuring device—the actual radius would include the dis
-
tance from the center of the stem to the measuring point (from the
position at 4 m, this was about 500 mm).
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
ous trees (Waring and Running 1978, Sellin 1991b, Èermák
and Nadezhdina 1998, Kravka et al. 1999), which also fit for
needle water content (Èermák et al. 1983). The extremely low
heartwood water content in the upper stem (46 m) of Psme
1373 was probably indicative of long-term desiccation. Unfor
-
tunately, additional and more extensive sampling was not per
-
mitted at the protected site.
Based on tissue volumes, water contents and assumptions
about free water as a proportion of total water, we estimated
that Psme 1373 contained 1426 dm
3
of free water (Table 1).
Most of this water was in the stem sapwood below a height of
46 m and represented the greatest source of stored water used
daily in transpiration. Water stored in tissues above 46 m and
in elastic tissues was also used, but these sources were less im
-
portant. Similar results were noted by Phillips (unpublished
data) for a nearby 65-m-tall Douglas-fir tree. In contrast, water
storage in the upper half of the stem of Pinus pinaster Ait. was
reported to be more significant than in the lower portion of the
stem (Loustau et al. 1996); however, the pine trees studied by
Loustau et al. (1996) were considerably smaller than Psme
1373.
Water can be stored extracellularly or intracellularly in plant
tissues (Arcikhovskiy 1931, Holbrook 1995). In contrast to
extracellular stem water, intracellular water in trees is mostly
confined to living tissues between the bark and the newly de
-
rived xylem. These tissues are highly elastic and may undergo
considerable dimensional changes during the day (Dobbs and
Scott 1971, Klepper et al. 1971, Goldstein et al. 1984, Milne
1989, Franco-Vizcaino et al. 1990, Holbrook and Sinclair
1992a, 1992b). For this large tree (Psme 1373), these intra
-
cellular stores represent less than 1% of daily water use.
Extracellular water storage can be substantial in trees (Waring
and Running 1978, Kravka et al. 1999) and involves water held
by capillary forces in the sapwood as well as water released as
a result of cavitation (Zimmermann 1983, Tyree and Yang
1990). Easily available (“free”) extracellular or capillary water
could represent a substantial fraction of water (Holbrook
1995). Under severe drought, water released by cavitation may
support survival by preventing desiccation of fast-growing tis
-
sues (Dixon et al. 1984). Tyree and Yang (1990) concluded
that water stored by small trees is mainly capillary water or
water released by cavitation and may comprise a large volume;
however, they also stated that the stored water in small trees is
typically released at either very high or very low water poten
-
tials and thus has no significant role under most conditions, al
-
though Zweifel et al (2001) reported that sap flow is buffered
by storage in small trees.
Our results for the large Psme 1373 tree suggest otherwise.
As water is transpired from the foliage, water is withdrawn
from needle tissue reserves (small), tensions develop in the
190 ÈERMÁK, KUÈERA, BAUERLE, PHILLIPS AND HINCKLEY
TREE PHYSIOLOGY VOLUME 27, 2007
Figure 6. The change in stored water at
any time (Cum Q) and stem volume
(V) calculated for the whole tree (left)
and for the upper crown (above 51 m;
right) in the old-growth Douglas-fir
sample tree (Psme 1373) during se-
lected days with fine weather (August
1 and 24, September 10 and October
8). Scales differ because of the large
range of values.
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
vascular tissue (e.g., Bauerle et al. 1999 for Psme 1373) and
water may then be withdrawn from the xylem and phloem. The
data presented in Table 1 and Figure 4 demonstrate that there
was a large quantity of free stored water and most of this water
was in the xylem sapwood below the most active portion of the
crown. If these stores were biologically unimportant, there
should be only small lags in sap flow between the branches and
the upper set of stem sap flow measurement points (51 m) and
from the upper stem to the lower stem (4 m). Our data, which
are within the range described by other authors (Table 4),
demonstrated large lags, especially during refilling. Recently,
there have been several publications documenting the role of
the stem as a usable reservoir of water (e.g., Perämäki et al.
2001, Sevanto et al. 2002, Meinzer et al. 2006). Considered
alone, however, the treetop behaved like a small tree, showing
minimal time lags and much less reliance on storage.
Tissue and whole-plant water relations
Three factors appear to influence the time lag between sap
flow and transpiration. First, distance is important. Short dis
-
tances between measurement points (e.g., the foliage at the tip
of small twigs and the base of the twig) should result in rela
-
tively small lags. Second, the resistance to flow within the con
-
ducting elements is important: the smaller the diameter of the
conducting elements, the greater the resistance. A conducting
system containing cavitated elements would have an even
greater resistance. According to electric circuit analogies of
water flow through plant tissues, this resistance will have an
effect on the characteristic response time and corresponding
time lags of the tissue (Schulte 1993). Third, buffering capac
-
ity, or the quantity and availability of stored water, should ex
-
ert an influence. For most plants, a combination of these
factors affects the time lags observed. In 3-m-tall reeds (Phra
-
gmites spp.), Rychnovská et al. (1980) observed a lag between
sap flow at the stem base and foliage transpiration in the order
of minutes. In contrast, several other authors have noted much
greater lags (tens of minutes to hours) for a variety of woody
species (Morikawa 1974, Hinckley and Bruckerhoff 1975,
TREE PHYSIOLOGY ONLINE at http://heronpublishing.com
DYNAMICS OF TREE WATER STORAGE AND STEM DIAMETER CHANGE 191
Figure 7. Relationships between the
change in stored water at any time
(Cum Q) and stem volume (indicative
of a volume of water), calculated for
the whole tree (left) and upper crown
(right) in the old-growth Douglas-fir
sample tree (Psme 1373) during se-
lected days with fine weather (Aug-
ust 1 and 24, September 10 and
October 8, see Figures 5 and 6). The
originally curvilinear relationships be-
came almost linear when considering
time shifts amounting to about 3 hours
at the stem base, constant over the
growing season, but increasing from 15
minutes (midsummer) to 1.5 hours
(fall) at the upper crown.
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
Èermák et al. 1982, 1984, Schulze et al. 1985, Loustau et al.
1996, Phillips et al. 1996, Goldstein et al. 1998, Zweifel and
Häsler 2001).
Diurnal courses of sap flow and changes of stored water
As first suggested by Ladefoged (1963), Morikawa (1974),
Èermak et al. (1976) and Waring and Running (1978), large
old-growth trees appear to have a large reservoir of water
available on a daily or seasonal basis. Waring and Running
(1978) estimated that the total storage capacity of any
old-growth Douglas-fir forest is 267 m
3
of water per hectare
(or 26.7 mm) and 75% of this water is stored in the stem sap
-
wood. However, the data of Waring and Running have been
criticized because they used a narrow-bore increment corer to
collect samples (although the maximum error would likely be
less than 10%; Morales et al. 2001). The absolute values of di
-
urnal water depletion observed during the summer from our
study of an old-growth Douglas-fir tree, 40 to 50 dm
3
,ap
-
peared to match the estimates of Waring and Running and the
measurements of Phillips et al. (2003) with an even larger,
nearby Douglas-fir tree (~ 65 m). However, our values gener-
ally exceed other published values (Èermák et al. 1976, 1982,
1984, Schulze et al. 1985, Goldstein et al. 1998, Kravka et al.
1999).
Our results suggest that the upper part of the stem is subject
to much greater desiccation than the lower part. The upper
canopy loses water more rapidly than the lower canopy be-
cause of the structure of this old-growth forest; the upper can-
opy is exposed to direct sunlight and is exposed to warmer,
windier and drier conditions than the mid- and lower canopy.
The upper canopy approaches the situation of a solitary tree
(Èermák et al. 1984, Èermák and Kuèera 1990, Parker 1997,
Ishii et al. 2000, Ishii et al. 2002, Parker et al. 2002). Not only
is the top of the tree exposed to drier conditions, but to a gravi
-
tational tension resulting in more negative water potentials,
higher δC
13
values, and lower stomatal conductances (Ryan
and Yoder 1996, Bauerle et al. 1999, Woodruff et al. 2004).
Greater use of free water and lower heartwood water contents
all indicate greater desiccation, likely accounting for top
dieback of many of the large Douglas-fir trees in the crane
circle at the Wind River site.
Diurnal changes in stem volume and stored water
Diurnal changes in stem dimensions were thought to occur in
tissues external to the rigid xylem (MacDougal 1925, Arcik
-
hovskiy 1931, Dobbs and Scott 1971, Molz and Klepper 1973,
Molz et al. 1973, Lassoie 1973, 1979, Hellkvist et al. 1974,
Braekke and Kozlowski 1975, Hinckley and Bruckerhoff
1975, Vogel 1994). These changes can be either positive (in
-
creases due to growth or rehydration) or negative (decreases
due to dehydration). Irvine and Grace (1997) demonstrated
that dimensional changes are not restricted to tissues external
to the sapwood; however, as pointed out by Zweifel et al.
(2000), these sapwood dimensional changes are small when
192 ÈERMÁK, KUÈERA, BAUERLE, PHILLIPS AND HINCKLEY
TREE PHYSIOLOGY VOLUME 27, 2007
Figure 8. Daily water storage in the old-growth Douglas-fir tree
(Psme 1373) during the growing season (A; values represent means
for individual days), total daily water loss and fraction of that water
taken from storage from the whole tree (B), and from the upper part of
the tree (C).
Table 2. Mean daily water use and change in stem volume in a
Douglas-fir tree, Psme 1373, in the whole tree and upper crown
(51–57m).
Upper crown Tree total
Daily stored water use (dm
3
) 10.9 50.4
Daily tissue volume change: (% vol)
Sapwood 9.4 0.90
Phloem 191.2 31.3
Total 8.2 0.8
Daily stored water use relative to free water fraction: (% vol)
Sapwood 41.6 4.1
Phloem 4.1 3.7
Total 23.1 3.4
Daily elastic tissue volume change (dm
3
) 0.38 6.11
Daily elastic tissue volume change relative to: (% vol)
Daily stored water use 3.5 0.81
Free water 13.0 0.41
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
compared with those of tissues external to the sapwood.
Changes in stem dimensions are useful for modeling and re
-
cording stem water potentials, particularly if lags are incorpo
-
rated (e.g., Zaerr 1971, Molz and Klepper 1973, So 1979,
Herzog et al. 1995, Irvine and Grace 1997, Zweifel et al. 2000.
Intrigliolo and Castel 2006).
Changes in response to hydration are attributed to the lateral
transfer of water between these tissues and the conducting xy
-
lem (Molz and Klepper 1973). Hinckley and Bruckerhoff
(1975) in white oak, Lassoie (1979) in Douglas-fir and Anto
-
nova et al. (1995) in Scots pine noted three general patterns of
dimensional changes in trees during the growing season. First,
during periods of high soil water content and low evaporative
demand, stem diameter increased from one morning to the
next and often during the day there was either no decrease or
just a reduction in the rate of increase in diameter. This pattern
was largely characterized by growth. Second, during periods
of high soil water content and high evaporation demand, there
was an increase in stem diameter from one morning to the
next, but during the day there could be an appreciable de-
crease. A mixture of growth and tissue rehydration character-
ized this pattern. Third, during periods of low soil water
content and high evaporation demand, there was a daytime re
-
duction in stem diameter with only partial recovery overnight.
Only changes in hydration characterized the last pattern. We
observed only Pattern 2 in Psme 1373. Recently, these three el
-
ements have been integrated in a model (Steppe and Lemeur
2004).
Seasonal changes in daily water storage and stem volume
A critical assumption in our calculations of the total amount of
stored water was that there was no net change from day-to-day
(i.e., complete refilling occurred). Three lines of evidence sug-
gested that this assumption was likely justified: first, predawn
water potentials were consistently high in Psme 1373 (as well
as other Douglas-fir trees measured within the crane circle),
second, plotting the radial changes from day-to-day did not
demonstrate a progressive decline in radius, and third, on Au-
gust 8, we added over 800 liters (8.5 mm) of water to the soil
surrounding the study tree in an effort to reduce any water defi-
cits. Loustau et al. (1996) made a similar assumption in their
calculations for the water relations of Pinus maritima Poir.
During an extraordinarily dry summer, Hinckley and
Bruckerhoff (1975) noted a continuous loss of stem diameter
in a white oak tree. They assumed that day-to-day volume
changes in extensible tissues reflected a net loss in stem water
content—these day-to-day decreases were linearly related to
decreases in predawn water potential. Similarly, Waring and
Running (1978) observed a progressive decrease in sapwood
water content over the growing season in old-growth
Douglas-fir trees; maximum water contents of 100% satura
-
tion were observed in late February and March and a minimum
of 50% was reached in mid-August. Similar conclusions were
drawn by Èermák and Nadezhdina (1998) for adult Norway
spruce trees, where sapwood was maximally hydrated in early
spring and dehydrated substantially, especially in the inner
sapwood, during a summer drought. Similar results were
found in broadleaf species (Tatarinov and Èermák 1999).
These and other studies indicated that our assumption of no net
change in tissue water content might have resulted in a slight
overestimation of the daily water use from storage: however,
predawn water potential did not change appreciably during the
summer in our study trees.
Although the total change in water volume (i.e., the amount
used) in elastic tissues of the stem was small and mostly con
-
stant over the growing season (about 6 dm
3
for the whole trees
and 0.3 dm
3
for the treetop; see Figures 6 and 8), daily total
TREE PHYSIOLOGY ONLINE at http://heronpublishing.com
DYNAMICS OF TREE WATER STORAGE AND STEM DIAMETER CHANGE 193
Figure 9. Relationship between transpiration (mmol m
–2
s
–1
) and wa
-
ter potential (Ψ, MPa) in the exposed upper crown of old-growth
Douglas-fir trees. Each value is the mean of two readings on each of
three trees and three measurement days. Short distance hydraulic re
-
sistance () is the slope of the regression for Ψ of uncovered leaves at
solar noon minus Ψ of aluminum-foil-covered leaves Ψ versus the un
-
covered foliage’s transpiration rate. Long distance hydraulic resis
-
tance () is similarly determined, except the values shown are the Ψ
of uncovered leaves at solar noon minus the Ψ of covered leaves at
predawn versus the corresponding solar noon values of transpiration
in the uncovered foliage (it was assumed that transpiration in the cov
-
ered foliage was zero).
Table 3. Water turnover rate, i.e., theoretical mean time during which transpiration can be supported by free water storage in different tree parts and
tissues of Douglas-fir 1373 (values of total water storage for corresponding tree parts were taken from Table 1).
Tree part Time interval Stem (xyl + phl) Branch (xyl + phl) Sapwood (stem + bran) Phloem (stem + bran) Needles Tree total
Upper crown Days 0.228 0.073 0.255 0.055 0.064 0.374
Hours 5.5 1.8 6.1 1.3 1.5 9.0
Entire tree Days 6.57 0.36 6.12 0.81 0.236 7.17
Hours 158 9 147 19 6 172
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
water used from storage was much greater and increased as the
season proceeded (from about 35 to 55 dm
3
). Most of the
stored water in the study tree was located in the relatively rigid
stem sapwood below 46 m. Water released by cavitation of
vascular elements may prevent desiccation of leaves and other
living tissues (Dixon et al. 1984, Tyree and Yang 1990) but the
loss of hydraulic conductivity due to cavitation may result in
further decreases in water potential leading to runaway xylem
cavitation (Sperry 1995). However, a balance seems to exist
between use of water from cavitated elements and loss of hy
-
draulic conductivity due to cavitation (Sellin 1991a, Èermák
and Nadezhdina 1998, Sperry et al. 1998, Domec and Gartner
2001).
There is increasing evidence that in large trees sapwood hy
-
draulic capacity maybe vastly more than sufficient to meet
transpiration needs under favorable conditions. In Quercus
robur L. and Laurus azorica (Seub.) Franco, it was found that
only about 2% of all stem xylem conducting elements were
theoretically needed to supply water for rapid transpiration
(Krejzar and Kravka 1998, Èermák et al. 2001 and Morales et
al. 2001), whereas almost 100% of all conducting xylem ele
-
ments must function in petioles. Theoretically, the majority of
conducting elements in stems could be embolized without sig
-
nificantly affecting stem hydraulic conductivity. For Laurus
azorica, stem vessels represented the largest store of free wa
-
ter. Because the sapwood does not conduct water uniformly
with depth (Swanson 1971, Èermák et al. 1984, 1992, 2004,
Phillips et al. 1996, Èermák and Nadezhdina 1998, Jimenez et
al. 2000, Nadezhdina et al. 2001), and does not dehydrate uni
-
formly with depth (Èermák and Nadezhdina 1998) it should
become a more important source of water as sapwood depth
and volume increase. Under such circumstances, the less-con-
ducting part of the sapwood can serve as a source of stored wa-
ter, while having a minimal impact on hydraulic conductivity.
If such stored water is used more than once, cavitated elements
must be refilled. Domec and Gartner (2002) suggested that the
latewood, because of the smaller diameter of its conductive el
-
ements and is greater resistances to flow, may cavitate first and
provide water to the transpiration stream. Loss of latewood
would not severely impact whole sapwood water conduction.
Waring and Running (1978) hypothesized that refilling of
cavitated xylem conduits occurs over winter. However, more
recent evidence suggests that refilling of cavitated elements
may occur diurnally (Zwieniecki and Holbrook 1998, Hol
-
brook et al. 2002). Irvine and Grace (1997) noted a linear rela
-
tionship between xylem dimension and xylem water potential,
suggesting water loss of individual xylem elements and di
-
mensional changes as a result of the loss. They hypothesized
that the conducting xylem could lose volume without cavi
-
tating. Second, their study suggests that water can be released
in this way within the normal range in plant water potentials.
Recently, using snap-freezing of roots and stems and cryo
-
scanning electromicroscopy, Shane and McCully (1999) and
McCully (1998, 1999) observed that as many as 60% of the
stem or root conducting vessels may be cavitated. They also
observed rapid refilling of cavitated vessels, thereby offering a
much more dynamic view of the role and behavior of water in
conducting elements. Both the refilling of cavitated elements
and the release of water by conducting elements without cavi
-
tation needs further study.
194 ÈERMÁK, KUÈERA, BAUERLE, PHILLIPS AND HINCKLEY
TREE PHYSIOLOGY VOLUME 27, 2007
Table 4. Comparison of estimates of water used from storage in various tree species.
Species Height (m) Age (year) Quantity (dm
3
day
–1
) (%) Reference
Acer saccharum Marsh. > 9 30–100 Not given (17%) Tyree et al. 1991
Anacardium excelsum (Bertero and Balb. ex. Kunth) Skeels. 35 Mature 54 (14%) Goldstein et al. 1998
Carya illinoensis Wangenh. 4 5 4 (3%) Steinberg et al. 1990
Cecropia longipes Pittier. 18 Mature 4.0 (9%) Goldstein et al. 1998
Ficus insipida Willd. 30 Mature 25 (15%) Goldstein et al. 1998
Larix decidua Mill. 18 70 18 (4%) Schulze et a l. 1985
Luehea seemannii Triana. 29 Mature 16 (12%) Goldstein et al. 1998
Malus pumila Mill. 2.5 9 0.7 (20%)
1
Landsberg et al. 1976
Nothofagus fusca (Hook F.) Oerst. 34 300400 5–10 (48%) Köstner et al. 1992
Picea abies (L.) Karst. 30 80 9 (14%) Schulze et al. 1985
Picea abies (L.) Karst. 0.6–1.2 46 (2–15%) Zweifel et al. 2000
Pinus maritima Mill. 24 64 10–13 Loustau et al. 199
Pinus sylvestris L. 15 41 2030 (3050%) Waring et al. 1979
Prunus avium L. 6 15 1 (min 4%) Èermák et al. 1976
Pseudotsuga m.1373 57 450+ 4070 (2030%) This study
Pseudotsuga m. 091 65 450+ 2274 (29–17%) Phillips et al. 2003
Pseudotsuga menziesii (Mirb.) Franco. 19,15, 6 40 1.8, 1.0, 0.1 (5%) Lassoie 1979
Quercus robur L. 32 100 1031 (1522%) Èermák et al. 1982
Salix fragilis L. 10 30 3% (1% stem vol) Èermák et al. 1984
Schefflera morototoni (Aubl.) Maguire, Steyerm. and Frodin. 20 Mature 0.9 (2.5%) Tyree et al. 1991
Spondias mombin L. 23 Mature 8.7 (11%) Goldstein et al. 1998
Thuja occidentalis L. 10 Not given 2.5 (22%) Tyree 1988
1
Estimated as 2 h of transpiration divided by a nominal 10 h.
by guest on May 15, 2011treephys.oxfordjournals.orgDownloaded from
In summary, diurnal and seasonal changes in stored water
are reflected in decreases in sapwood water content (e.g., War
-
ing and Running 1978, Èermák and Nadezhdina 1998), volu
-
metric changes in elastic tissues (MacDougal 1925, Stewart
1967, Molz and Klepper 1973, Morikawa 1974, Hinckley and
Bruckerhoff 1975) and volumetric changes in mature xylem
elements (Zimmermann 1983, Irvine and Grace 1997). In gen
-
eral, the water used from storage on any given day is rather
small compared with the volume of free water in the tree.
Water turnover rate
On a whole-tree basis, the stem was about 18 times more im
-
portant in supplying water for transpiration than the branches,
but only three times greater if only the upper crown was con
-
sidered (Table 4). Its sapwood was about 7.5 times more im
-
portant than phloem, although this difference was reduced to
about 4.6 times when only the upper crown was considered.
The needle compartment shared a simlar relative importance
to that of the branches or phloem. The length of time for which
free water can supply transpiration from storage was much
longer than that estimated for herbaceous plants (e.g.,
Rychnovska et al. 1980), but similar to values mentioned for
other woody species.
Hydraulic resistance
Our results confirm the need to account for hydraulic architec-
ture, including the distribution of storage elements, in
whole-tree process modeling (Tyree 1988). Friction during
high rates of transpiration adds a significant constraint that is
manifested in the form of greater tensions or more negative
water potentials at the tops of tall trees (Bauerle et al. 1999,
Koch et al. 2004, Woodruff et al. 2004). The increase in water
potential gradients with transpirational water movement, how
-
ever, followed a constant resistance for both short and long
distances (cf. Camacho et al. 1974, Wenkert 1983, Ryan et al.
2000). Although our data followed a simple constant resis
-
tance analog (see Figure 9), the combination of long distance
sap transport components caused branchlet water potential to
fall below –1.5 MPa at the treetops. Recently, Woodruff et al.
(2004) concluded that the gravitational component of water
potential is a significant contributor to the decline in leaf
turgor with increasing height. Although the gravitational com
-
ponent of water potential contributes 0.01 MPa m
–1
to the xy
-
lem tension gradient, our data indicate that the frictional
potential added yet another negative constraint that can de
-
crease leaf water potential and affect leaf turgor, further sup
-
porting the hydraulic limitation hypothesis of Ryan and Yoder
(1996). For Psme 1373, the observed leaf specific conductivity
was 1.132 mmol m
–2
s
–1
MPa
–1
and was slightly greater (i.e.,
less resistance) than the value of ~0.8 mmol m
–2
s
–1
MPa
–1
re
-
ported by Irvine et al. (2004) in old-growth ponderosa pine.
Irvine et al. (2004) also observed that LSC was six times
greater in young, smaller trees than in tall, old trees.
In conclusion, tissues of old-growth Douglas-fir trees con
-
tain large amounts of free water. Stem sapwood appears to be
the most important, followed by stem phloem, branch sap
-
wood, branch phloem and needles. There are significant time
lags (minutes to hours) between sap flows measured at differ
-
ent positions within the transport system (i.e., stem base to
shoot tip). These shifts suggest that the transport system is
highly elastic. Moreover, on clear days, the daily quantity of
water used from storage ranges from 25 to almost 75 liters
(i.e., about 20 to 30% of daily sap flow). Our results suggest
that the source of this water varies spatially and the greatest
amount of water comes from the lower stem; however, more
water is transpired from the tree top. In addition to positional
lags in stem flow, the withdrawal and refilling of water storage
components is reflected in changes in stem volume. There is a
strong linear relationship between volume changes and tran
-
spiration when time shifts (minutes to hours near the top and
the base of the stem, respectively) are considered. The volume
changes are small (only about 14%) compared with the
amount of stored water used daily, indicating that most of the
stored water comes from inelastic tissues (i.e., sapwood),
which can supply water for transpiration of the whole tree for
about a week, but only for several hours from tissues in the up
-
per crown taken separately. The disproportionate use of stored
water from the top of the tree, the drier microclimate of the up
-
per canopy, and the more negative water potentials found in
the tops of tall trees appear to cause greater desiccation and
resultant top-dieback.
Acknowledgments
The authors thank Dr. David Shaw, Mr. Buz Baker and Mr. Mark
Creighton for their assistance at the Wind River Canopy Crane
Research site. Ms. Sarah McCarthy provided needle water content
data and Drs. Horaki Ishii and Nate McDowell provided leaf area
data. This research was supported by the Biological and Environmen
-
tal Research Program (BER), U.S. Department of Energy, through the
Western Regional Center of the National Institute for Global Environ
-
mental Change (NIGEC) under Cooperative Agreement No.
DE-FC03-90ER61010. The authors also thank the U.S. Forest Ser
-
vice for providing the Rose Canopy Platform.
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TREE PHYSIOLOGY VOLUME 27, 2007
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... Multiple studies have reported the relevance of the internal storage of SW as a water reserve for transpiration in several species, depending on wood density, SW area and also on wood anatomy (Köcher et al., 2013;Phillips et al., 2003). The stem water reservoir does not only buffer transpiration demand during soil water shortage (Köcher et al., 2013) but also contributes to daily water use, accounting for 5%-22% of the daily water use in broadleaved trees (Köcher et al., 2013) and for 20%-25% in conifers (Cermak et al., 2007;Phillips et al., 2003). By measuring water potential variations in the trunk, Betsch et al. (2011) proved that water is exchanged between the elastic tissues and the vertical transpiration flow within the tree. ...
... The importance of SW as source water for leaf transpiration increases with depth (Cermak et al., 2007), and most of the water flow is largely confined in the outer SW (Steward, 1967). In ring-porous species such as oak trees, the axial flow is restricted to either the last or last few annual rings (Cermak et al., 1992;Granier et al., 1994;Kozlowski & Winget, 2015;Matheny et al., 2017) where the width of the latewood vessel creates sufficient transport capacity to supply the entire crown (Gartner, 1995). ...
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... Despite the increasing number of studies reporting the contribution of capacitance to daily transpiration in trees (Phillips et al., 2003;Borchert and Pockman, 2005;Cermak et al., 2007;Scholz et al., 2011;Köcher et al., 2013), we did not observe a significant buffering of Ψ stem from the effects of Ec by the plant capacitance in the two herbaceous studied here. In woody species, the extent of the lag between stem and branch flow (used as a surrogate for transpiration) measured with sap flow has been often correlated with the magnitude of tree capacitance, with longer time lags though indicate a greater plant capacitance (Goldstein et al., 1998;Phillips et al., 2003;Scholz et al., 2011;Köcher et al., 2013). ...
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Monitoring plant transpiration (Ec) is important for quantifying irrigation amounts and maintaining optimum soil moisture conditions for maximum growth potential. However, real-time quantification of Ec dynamics under field conditions is still challenging due to technical limitations. Here, we present a new approach for monitoring plant water use using continuous measurement of stem water potential dynamics (Ψ stem) under non-limiting water conditions. According to Darcy's law, if daytime root to stem hydraulic conductance (Kr-s) remains constant and the contribution of plant capacitance to Ec is minimal or accounted for, then daytime Ec could potentially be continuously inferred from Ψ stem dynamics. We investigated the viability of this approach by first quantifying the dynamics of daytime Kr-s and the transient buffering effects of plant capacitance in irrigated plants of two crop species with contrasting physiology and life history: the perennial herbaceous Tanacetum cinerariifolium and the annual monocot Triticum aestivum. Daytime Kr-s dynamics and the effects of plant capacitance were determined by continuously and simultaneously measuring Ψ stem with optical dendrometers and Ec with gravimetric method under naturally variable conditions of light, temperature and humidity. Kr-s remained relatively stable throughout the day and the effects of plant capacitance were negligible in both species under field conditions implying that the dynamics of Ec can be inferred from Ψ stem. This was clearly supported by the close agreement between measured Ec and that predicted from the optically derived Ψ stem. We conclude that optical dendrometry has the potential to continuously monitor both Ec and plant hydration status at high accuracy and temporal resolution under naturally variable atmospheric conditions and optimum water supply. Combining these two aspects of the optically derived Ψ stem should create new opportunities to monitor crop water use and maintain plant hydration at optimum levels for maximum productivity in various crops under field conditions.
... Therefore, the increase of udaily of about 25 % at breast height is probably water that remains in the stem and is not directly transported to the crown. This suggests a refilling of internal water storages in spruce stems upon drought release, although the complete refilling might take several weeks or months (Čermák et al. 2007, Hao et al. 2013, Köcher et al. 2013). ...
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As climate change progresses, the frequency and duration of drought stress events are increasing. While the mechanisms of drought acclimation of trees received considerable attention in recent years, the recovery processes remain critically understudied. We used a unique throughfall exclusion experiment in a mature temperate mixed forest consisting of the more isohydric Norway spruce and more anisohydric European beech, to study the recovery and resilience after drought release. We hypothesized that pre-dawn water potential (ΨPD) of both species will increase within one day after watering, while the recovery of stomatal conductance (gs) and the reversal of osmoregulation will be significantly delayed in the more isohydric spruce. Further, we hypothesized that the xylem sap flow density (udaily) will not fully recover within the growing season due to the strong drought impact. After five years of summer drought, trees showed significantly reduced ΨPD, udaily and increased osmoregulation in leaves, but only isohydric spruce displayed increased leaf ABA concentrations. In line with our hypothesis, ΨPD and gs recovered within one day in beech. Conversely, isohydric spruce showed delayed increases in ΨPD and gs. The delay in recovery of spruce was partially related to the replenishment of the stem water reservoir, as indicated by the missing response of udaily at the crown base compared with DBH level upon watering. However, udaily fully recovered only in the next growing season for beech and was still reduced in spruce. Nevertheless, in both species, osmotic acclimations of leaves were reversed within several weeks. While both species displayed full resilience to drought stress in water-related physiology, the recovery time was in several cases, e.g., udaily, ΨPD and gs, shorter for beech than for spruce. With future increases in the frequency of drought events under ongoing climate change, tree species that recover more quickly will be favored.
... As transpiration (Ep) occurs in the plant leaves, a tension arises in the evaporative surface and extends to all water-storing organs. This rapid response to atmospheric changes causes systematically diurnal diameter changes in all plant parts, including the stem, branches, roots, leaves, and fruits [90,91,[96][97][98]. As a result, as Ep increases, water loss increases, leading to a decrease in trunk diameter. ...
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Transpiration of 22 individual trees in a mature plantation of spruce in Drahanska Vrchovina uplands, Moravia (Querci-fageta abietis forest-type group) was studied through the measurement of sap flow by the stem section heat balance method over six years. Relative transpiration of trees was calculated as the ratio of daily totals of their actual transpiration and potential evapotranspiration (standard crop evapotranspiration, E1). This was expressed in % of the canopy projected area unit and in m2 per tree as the "effective crown area" when a ratio of whole tree transpiration to E1 was considered. Significant relationships were found between the sap flux density (expressed per stem section 1 cm wide) measured at breast height and tree size. Relative transpiration of canopy area unit (Trcl) reached in average about 1/3 of potential cvapotranspiration, it was very variable between individual trees as well as between different periods of time during growing seasons. Highest values occurred in medium size trees, while in small and large trees they were mostly smaller and less variable. Higher Trcl occurred in medium and big trees with relatively narrow crowns. Trcl had two limits, gradually appearing outside of a rather wide range of soil water storage values between 100 to 135 mm per 40 cm layer. Canopy water storage capacity reached in average 0.6 mm - within the wide range of 0.1 to 1.7 mm, which was dependent on social positions of trees.
Article
The three-dimensional light environment within the canopy of a tall coniferous forest was sampled to quantify its variation and localize the sites of radiation absorption. Broadband visible (PAR) and ultraviolet radiation (UVB) were measured around midday in midsummer in an old-growth Douglas-fir/Western Hemlock forest in the Cascade Range in southern Washington using sensors suspended from the gondola of a large tower crane. Patterns of vertical transmittance varied greatly between locations and showed abrupt transitions from bright to dark conditions at varying heights. The average light field in this canopy with trees to 60 m was resolved into three functional zones. Above 40 m from the ground is a 'bright zone' where light was reliably intense and predominantly in the direct beam component, and below 12 m, a 'dim zone,' where light was reliably low and mostly diffuse. Between these levels is a 'transition zone,' with a steep vertical gradient in light transmittance, high horizontal variation, and a mixture of beam and diffuse components. The pattern of UVB light was very similar to that of PAR. From the general transmittance profile the vertical structure of the canopy was estimated to have a peak density of foliage at 12 m (less than one quarter of the stand height) with declining densities above and below. The 'bottom-heavy' canopy structure found in this study differs markedly from the 'top-heavy' profiles reported from managed or young stands.
Chapter
This chapter considers interactions between stem water transport, xylem structure, vegetative phenology, and stomatal regulation of gas exchange. The significance of stem water transport, detailed in the first part, is apparent in its influence on leaf water status and, ultimately, in how the leaf water status is linked to the regulation of gas exchange and other leaf-level processes affecting whole plant carbon gain. The importance of shoot k1 on leaf water status increases dramatically in response to drought and freezing stress because of physical limitations on xylem transport. The cavitation response of a plant unambiguously limits the xylem pressure range over which water transport is possible. The remainder of this chapter explains the mechanisms of cavitation in stems caused by freezing and water stress, and the implications of cavitation for adaptation to environmental stress. The role of cavitation in controlling water use may explain why many plants experience limited safety margins from failure of water transport. In particular, the occurrence of extensive xylem cavitation in stems and roots forces a new perspective on the stomatal regulation of water loss. The importance of cavitation for changing whole-plant hydraulic conductance, and the dependence of stomatal conductance on hydraulic conductance, reveals a link that can potentially explain variation in water use efficiency, drought survival, and signaling processes linking water stress to the stomatal reaction.
Chapter
This chapter discusses the patterns of variation in xylem structure found within a woody plant, and emphasizes what is known and what is not known about the functional consequences of this variation for shoot water movement and mechanics. The first section reviews the typical structure of xylem within a tree. There is more information on softwood (gymnosperm) than hardwood (woody angiosperm) anatomy, and therefore many of the paradigms of wood anatomy are based on softwoods. The anatomical variation described here results in systematic variation in efficiency of water transport through the stem. The hydraulic properties discussed here are hydraulic conductivity (kh) and specific conductivity (ks) in the axial direction. Stems experience short- and long-term stress (force per unit area) from a variety of causes, such as gravity, wind, weight of snow or a maturing fruit, removal of a branch, partial failure of the anchorage system, or growth and development. This chapter has emphasized optima for mechanics and hydraulics separately, but the trade-offs between the two must be considered more fully. In the ranges of wood densities and water demands that plants have, we do not even know whether there are trade-offs between mechanics and hydraulics, partly because one must define the hydraulic and mechanical criteria in order to try such analysis.
Article
Water, solute and pressure potentials have been investigated in shoots in the canopy of a Sitka spruce stand using the pressure chamber. Water potentials of - 15 bar or lower frequently occurred in the upper parts of the canopy on warm, sunny days of low vapour pressure deficit although there was never a shortage of water in the soil. Root water potentials ranged between -0.5 and -4.5 bar. Gradients of water potential of up to 2 bar m-1 occurred in the trunk and larger gradients occurred in the primary and secondary branches. On overcast or wet days the water potentials in the canopy were higher and the gradients smaller. It was concluded that the resistance to flow in the trunk was largely responsible for the large drop in potential between roots and leaves. The resistance per unit length was estimated as 107 bar s m-4 in the trunk and was considerably higher than in the angiosperms or pines for which comparable data are available. The solute potential of shoots in the canopy varied between -13 and -24 bar depending on the time of year. There was some tendency for solute potentials to be lower higher up in the canopy. From typical Hofler diagrams constructed from the water and solute potentials, the change in pressure potential (turgor pressure) with water content was determined, and a pressure dependent bulk modulus of elasticity derived. A water potential of -15 bar was equivalent to a fall in pressure potential to about 40% of that at full turgor. Empirical equations relating solute, pressure and water potential to water content were determined.
Article
Many trees of tropical dry forests flower or form new shoots soon after leaf shedding during the dry season, i.e., during a period when trees are likely to be severely water stressed. To resolve this apparent paradox, phenology and seasonal changes in tree water status was monitored during two consecutive dry seasons in >150 trees of 37 species growing at different sites in the tropical dry lowland forest of Guanascaste, Costa Rica. Tree development during the dry season varied considerably between species and between sites of different moisture availability. Leaf shedding, flowering, and shoot growth (flushing) were strongly correlated with seasonal changes in tree water status, measured by conventional and newly developed techniques. Tree water status varied with the availability of subsoil water and a variety of biotic factors such as structure and life-span of leaves, time of leaf shedding, wood density and capacity for stem water storage, and depth and density of root systems. Observed tree species differed widely in wood density (from 0.19 to 1.1. g/cm^3) and stem water storage capacity (400-20% of dry mass), which was highly correlated with the degree of desiccation during drought. Only hardwood trees at dry upland sites, lacking stem water storage and access to subsoil water, desiccated strongly (stem water potential