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.
References
Antonova, G.F., V.P. Cherkashin, V.V. Stasova and T.N. Varaksina.
1995. Daily dynamics in xylem cell radial growth of Scots pine
(Pinus sylvestris L.). Trees 10:24–30.
Arcikhovskiy, V.M. 1931. Suction of water by woody species under
its artificial injection through holes in conducting tissues. Trudy po
lesnomu opytnomu delu, CLOS, XI: 69–141. In Russian.
Bauerle, W.L., T.M. Hinckley, J. Èermák, J. Kuèera and K. Bible.
1999. The canopy water relations of old-growth Douglas-fir trees.
Trees 13:211–217.
Borchert, R. 1994. Soil and stem water storage determine phenology
and distribution of tropical dry forest trees. Ecology 75:
1437–1449.
Braekke, F.H. and T.T. Kozlowski. 1975. Shrinkage and swelling of
system of Pinus resinosa and Betula papyrifera in northern Wis-
consin. Plant Soil 43:387–410.
Brooks, J.R., P.J. Schulte, B.J. Bond, R. Coulombe, J.C. Domec,
T.M. Hinckley, N. McDowell and N. Phillips. 2003. Does foliage
on the same branch compete for the same water? Experiments on
Douglas-fir trees. Trees 17:101–108.
TREE PHYSIOLOGY ONLINE at http://heronpublishing.com
DYNAMICS OF TREE WATER STORAGE AND STEM DIAMETER CHANGE
195
Downloaded from https://academic.oup.com/treephys/article-abstract/27/2/181/1664618
by guest
on 25 July 2018