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water may then be withdrawn from the xylem and phloem.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.
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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
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