Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol16.1996:
Tree Physiology 16, 247--255
© 1996 Heron Publishing----Victoria, Canada
Environmental regulation of xylem sap flow and total conductance of
Larix gmelinii trees in eastern Siberia
A. ARNETH,1,2 F. M. KELLIHER,3 G. BAUER,1 D. Y. HOLLINGER,3,4 J. N. BYERS,3
J. E. HUNT,3 T. M. MCSEVENY,3 W. ZIEGLER,5 N. N. VYGODSKAYA,6 I. MILUKOVA,6
A. SOGACHOV,6 A. VARLAGIN6 and E.-D. SCHULZE1
1
Universität Bayreuth, Lehrstuhl für Pflanzenökologie, Box 101251, 95440 Bayreuth, Germany
2
Present address: Lincoln University, Department of Soil Science, P.O. Box 84, Lincoln, New Zealand
3
Manaaki Whenua----Landcare Research, P.O. Box 69, Lincoln, New Zealand
4
Present address: U.S. Forest Service, P.O. Box 640, Durham, NH 03284, USA
5
Comenius University, Department of Biophysical and Chemical Physics, Mlynska Dolina F1, CS-842 15, Bratislava, Slovak Republic
6
V.N. Sukachov Laboratory of Biogeocenology, Russian Academy of Sciences, Leninsky Prospect 33, 117071 Moscow, Russia
Received March 28, 1995
Summary Xylem sap flow and environmental variables were
measured on seven consecutive midsummer days in a 130-yearold Larix gmelinii (Rupr.) Rupr. forest located 160 km south of
Yakutsk in eastern Siberia, Russia (61° N, 128° E, 300 m asl).
The site received 20 mm of rainfall during the 4 days before
measurements, and soil samples indicated that the trees were
well watered. The tree canopy was sparse with a one-sided leaf
area index of 1.5 and a tree density of 1760 ha −1. On a clear
day when air temperature ranged from 9 to 29 °C, and maximum air saturation deficit was 3.4 kPa, daily xylem sap flux
(F) among 13 trees varied by an order of magnitude from 7 l
day −1 for subcanopy trees (representing 55% of trees in the
forest) to 67 l day −1 for emergent trees (representing 18% of
trees in the forest). However, when based on xylem sap flux
density (F′), calculated by dividing F by projected tree crown
area (a surrogate for the occupied ground area), there was only
a fourfold range in variability among the 13 trees, from 1.0 to
4.4 mm day −1. The calculation of F′ also eliminated systematic
and large differences in F among emergent, canopy and subcanopy trees. Stand-level F′, estimated by combining halfhourly linear relationships between F and stem cross-sectional
area with tree size distribution data, ranged between 1.8 ± 0.4
(standard deviation) and 2.3 ± 0.6 mm day −1. These stand-level
F′ values are about 0.6--0.7 mm day −1 (30%) larger than daily
tree canopy transpiration rates calculated from forest energy
balance and understory evaporation measurements.
Maximum total tree conductance for water vapor transfer
(Gtmax, including canopy and aerodynamic conductances), calculated from the ratio of F′ and the above-canopy air saturation
deficit (D) for the eight trees with continuous data sets, was 9.9
± 2.8 mm s −1. This is equivalent to a leaf-scale maximum
stomatal conductance (gsmax ) of 6.1 mm s −1, when expressed
on a one-sided leaf area basis, which is comparable to the
published porometer data for Larix. Diurnal variation in total
tree conductance (Gt ) was related to changes in the above-can-
opy visible irradiance (Q) and D. A saturating upper-boundary
function for the relationship between Gt and Q was defined as
Gt = Gtmax (Q/[Q + Q50 ]), where Q50 = 164 ± 85 µmol m −2 s −1
when Gt = Gtmax /2. Accounting for Q by excluding data for Q
< Q85 when Gt was at least 85% of Gtmax , the upper limit for the
relationship between Gt and D was determined based on the
function Gt = (a + blnD)2, where a and b are regression
coefficients. The relationship between Gt and D was curvilinear, indicating that there was a proportional decrease in Gt with
increasing D such that F was relatively constant throughout
much of the day, even when D ranged between about 2 and 4
kPa, which may be interpreted as an adaption of the species to
its continental climate. However, at given values of Q and D,
Gt was generally higher in the morning than in the afternoon.
The additional environmental constraints on Gt imposed by
leaf nitrogen nutrition and afternoon water stress are discussed.
Keywords: boreal forest, larch, stomatal conductance, transpiration.
Introduction
The eastern Siberian taiga is dominated by Larix gmelinii
(Rupr.) Rupr. with a forested area of 1 × 106 km2 (Shridenko
and Nilssen 1994). The species forms pure stands regenerated
by recurrent fire (Schulze et al. 1995). A common habitat
feature of Larix is an extremely cold winter when physiological activity ceases (Gower and Richards 1990). The trees thus
complete needle development, growth, flowering and seed
development during relatively short summers. During the approximately 90-day snow-free growing season, average rainfall is only 110 mm. This is about half the annual precipitation
of 210 mm, whereas the corresponding potential evaporation
rate is 370 mm (Müller 1982), indicating that there must be
severe environmental regulation of the forest evaporation rate.
248
ARNETH ET AL.
However, despite the possible influence of this ecosystem’s
energy balance on the regional and global climate (Shukla and
Mintz 1982), we are not aware of any published research about
Larix forest evaporation.
The objective of this paper was to examine the effects of
environmental variables on xylem sap flow of L. gmelinii
during a 7-day period in midsummer. We calculated total tree
conductances for water vapor transfer (Gt ) from xylem sap flux
density and above-canopy air saturation deficit measurements,
and developed a multiplicative environmental constraint Gt
model for diagnosis, rather than strictly for prognosis (Granier
and Loustau 1994). Adaptation of L. gmelinii to its continental
climate is discussed.
Materials and methods
Site description
Measurements were made in a 130-year-old L. gmelinii forest
located 160 km south of Yakutsk in the extreme continental
part of eastern Siberia, Russia (61° N, 128° E, 300 m asl).
Characteristics of the vegetation were determined by measurement and destructive sampling in a representative 0.5-ha plot
(Schulze et al. 1995). Average tree height was 12 m, although
some canopy emergents were nearly 20 m tall, average stem
diameter was 110 mm, stand density was 1760 ha −1, and
one-sided leaf area index was 1.5 (Schulze et al. 1995). The
evergreen, broad-leaved understory vegetation was about 0.05
m tall with a one-sided leaf area index of 1.0. Understory plant
species included Arctostaphlos uva ursi, Vaccinium vitis idaea
and V. uliginosum.
According to the USDA taxonomic classification, the soil
was an inceptisol known as Pergelic Cryochrept. There was a
30-mm deep leaf litter horizon, and the top 50-mm deep
mineral soil horizon was enriched with humus (carbon content
= 10%). The texture of the underlying soil horizons was silt
loam. Because of underlying permafrost, the soil temperature
at a depth of 700 mm was only 1--2 °C. Soil water release
measurements were made in the laboratory on undisturbed
cores. Roots were generally confined to the upper 300 mm that
had an available soil water storage capacity of about 45 mm.
Xylem sap flow measurements
Xylem sap flow was measured at a height of 1.5 m in 13 trees
on a second 204-m2 plot (Table 1). The plot was a polygon
determined by connecting bisects of outer plot trees and their
nearest neighboring trees outside the plot (Figure 1). Twenty
small L. gmelinii and two Betula platyphyllos Sukachev trees
were present in the polygon plot (Table 2). There was about
twice as many large (> 160 mm diameter) trees on the polygon
plot (23% of total) than on the representative 0.5-ha plot.
Mass flow rate of water through the xylem of trees (xylem
sap flux, kg s −1, Fi) was estimated from half-hourly steadystate energy balances of undisturbed sapwood (to a depth of
50 mm) heated by electrodes to a constant 2 K (Th) above the
temperature of unheated sapwood (Tu) (UP Umweltanalytische Produkte, Munich, Germany), located at the same height
but 0.2 m away, as:
Fi =
k iP i
,
cw(Th − Tu)
(1)
where Pi is the power input to tree i (W, for an average time of
30 min), cw is the specific heat of water, and ki = Ci /[d(e − 1)]
with Ci, d and e being stem circumference of tree i, distance
between electrodes and number of electrodes, respectively
(Èermák et al. 1973, Pearcy et al. 1989). Power input is the
difference between power supplied to the stem and losses
Table 1. Some dimensional characteristics of 13 Larix gmelinii trees, and their associated xylem sap fluxes and maximum total tree conductances
(Gtmax ) on July 26, 1993, a clear summer day in eastern Siberia. Xylem sap flux densities may be calculated by dividing xylem sap fluxes by
projected crown area or sapwood area. Tree social position is indicated by the canopy emergence class as emergent, canopy and subcanopy.
Tree
Height
(m)
Stem diameter
(mm)
Projected crown area
(m2)
Sapwood area
(m2 × 10 −3)
Daily sap flux
(kg day −1)
Maximum sap flux
(kg h −1)
Gtmax
(mm s −1)
Emergent
19
29
34
32
5
2
19.5
18.5
16.0
19.5
19.0
19.5
253
244
178
255
204
221
28.4
15.4
15.8
6.7
10.3
10.1
7.64
5.51
6.34
8.58
5.51
6.60
66.9
41.9
37.1
23.0
44.4
43.2
6.3
3.7
3.1
2.2
3.9
1.1
3.1
3.5
3.5
5.5
7.2
7.3
Canopy
9
27
1
13
25
26
17.5
15.5
18.5
13.5
16.0
13.0
140
178
170
129
146
138
3.5
12.2
13.2
5.2
3.1
4.7
4.81
5.03
4.82
3.63
4.22
3.75
14.3
12.1
20.1
14.2
10.3
13.7
1.3
1.1
2.0
1.4
1.6
1.6
5.2
1.7
1.7
3.3
9.0
4.1
Subcanopy
16
11.0
116
4.3
2.72
6.8
0.8
3.0
XYLEM SAP FLOW AND TOTAL CONDUCTANCE OF LARCH IN SIBERIA
249
Figure 1. Map of projected crown areas of
the 35 trees in the representative 204-m2
(polygon) plot. The tree numbers appear at
the stem locations and the additional symbol B indicates a birch tree. The crowns of
the measured trees are shaded. Also shown
are a scale, north arrow, and locations of
the 15 neighboring trees whose crowns
reached the plot boundary.
Table 2. Some average (± standard deviation) dimensional characteristics of the 33 Larix gmelinii trees in a representative 204-m2 plot in eastern
Siberia. Tree social position refers to the canopy emergence class as emergent, canopy and subcanopy.
Social position
Number of trees
Height
(m)
Stem area
(m2 × 10 − 2)
Projected crown area
(m2)
Crown depth
(m)
Emergent
Canopy
Subcanopy
6
9
18
18.2 ± 1.4
14.6 ± 3.2
7.3 ± 2.4
4.03 ± 1.03
1.59 ± 0.51
0.28 ± 0.20
14.5 ± 9.7
6.9 ± 6.2
2.9 ± 2.8
7.7 ± 2.9
5.0 ± 2.2
2.3 ± 2.2
caused by convection by xylem sap flow and also by conduction and radial heat loss to the surrounding air.
Conduction and radial heat losses were constant because of
the constant heated--unheated temperature difference, and
were equal to the minimum power input obtained during darkness. Forest evaporation, determined from micrometeorological measurements, was zero during the night (Kelliher et al.
1994). Heat losses, which were determined each night for
every tree, were generally constant for each tree throughout the
2--5-day measurement period (see below). For a 1-m vertical
band centered at the height of instrumentation, the stems of
measured trees were surrounded by 200-mm thick fiberglass
insulation and covered with aluminized mylar sheeting. The
electrodes were inserted on branchless stem sections at about
1.5 m, facing west, north or east. This arrangement avoided
possible sap flow irregularities caused by branches and direct
sun exposure during most of the day.
Within 2 to 5 days after installation of a sap flow measurement system, resin production increased the resistance be-
tween electrodes such that we could not generate enough
power to maintain (Th − Tu) = 2 K. Consequently, measurement
systems were regularly shifted throughout the study. The number of trees measured varied between 7 and 13.
Comparison of sap flow for trees differing in size requires
normalization on an area basis (sap flux density, F′, kg m −2
s −1). We used a vertical projection of the crown perimeter for
each tree, determined from anascope measurements (Figure 1),
as an estimate of the tree’s occupied ground area. For comparison with tree canopy evaporation, we estimated F′ for the
polygon plot based on the tree size distribution data and the
half-hourly relations between Fi and stem cross-sectional area.
The relations were linear and passed through the origin with
coefficients of determination (r 2) ranging from 0.40 in the
early morning before sap flow had started in all trees to 0.75
during the rest of the day.
Plot level F′ estimates were scrutinized by comparison with
tree canopy transpiration rates (Et) determined from a combination of energy balance and understory evaporation rate (Eu)
250
ARNETH ET AL.
measurements described by Kelliher et al. (1994). Briefly, Et
was estimated as (Ra − H − Eu), where Ra is the available energy,
and H is the sensible heat flux density. We determined Ra from
measurements of the above-canopy net radiation and ground
heat flux after Kelliher et al. (1992). We measured H with a
three-dimensional sonic anemometer. Five lysimeters, thinwalled plastic sleeves (370 × 370 × 150 mm deep) encasing
undisturbed understory vegetation--forest-floor--soil cores,
were weighed at 2-h intervals to determine Eu.
Estimating total tree conductance for water vapor transfer
Total tree conductance for water vapor transfer (Gt ) was estimated from the ratio of F′i and air saturation deficit (D),
measured above the forest at a height of 22.5 m with an
ultraviolet absorption hygrometer (Model KH2O, Campbell
Scientific, Logan, UT). When F′i and D are expressed in units
of mm h −1 and kPa, respectively, Gt = k(F′i/D), where k is a
weakly temperature-dependent conversion factor (= 38 at
20 °C; Köstner et al. 1992). The conductance Gt includes
stomatal, leaf boundary layer (gb) and eddy diffusive aerodynamic (gaM ) conductance components because we are accounting for conductances from the average height of stomata in the
tree canopy to the height of a D measurement made above the
forest (Thom 1972). The sonic anemometer measurements
indicated gaM = 0.1u, where u is the wind speed above the
canopy (Jarvis et al. 1976). Daytime wind speeds during the
measurement period were generally low, between 1 and 2 m
s −1, so gaM = 0.1--0.2 m s −1. We set gb = 0.17 m s −1, an average
for several coniferous species given by Jarvis et al. (1976). For
the tree canopy, we multiplied by the leaf area index to estimate an average gb = 0.26 m s −1. The series sum of gaM and gb
is therefore at least 10 to 20 times larger than Gt. Consequently,
Gt closely resembles the tree’s canopy conductance (Köstner
et al. 1992).
We examined the optimal response of Gt to above-canopy
visible irradiance (Q, measured with a calibrated galiumarsenide photodiode) and D by a combination of boundary-line
analysis (e.g., Livingston and Black 1987) and a multiplicative
constraint-function model (e.g., Granier and Loustau 1994).
We first fitted a hyperbolic saturating function to the upper
boundary of a plot of Gt and Q as Gt = Gtmax (Q/[Q + Q50]),
where Q50 is the value of Q when Gt = Gtmax /2. We then
examined the upper limit of the decrease in Gt with increasing
D under nonirradiance-limiting conditions (Q > 85% of the
saturating value) as Gt = (a + blnD)2, where a and b are
regression coefficients.
We did not include the effects of soil water deficit in our Gt
model. However, the site had 20 mm of rain in the 4 days
preceding the study, and gravimetric forest floor and soil
samples, which were collected daily to a depth of 700 mm,
indicated that the trees were well watered throughout the study.
Results and discussion
Xylem sap flow rate
On a clear day (July 26, 1993) when air temperature ranged
from 9 to 29 °C, and maximum air saturation deficit was
3.4 kPa, daily xylem sap flux (F) among 13 measured trees
varied by an order of magnitude from 7 to 67 l day −1 (Table 1).
The lowest value of F was for an 11-m tall subcanopy tree that
was representative of 55% of the trees in the forest in terms of
size, based on an inventory of 875 L. gmelinii trees in the
0.5-ha plot (data not shown). The highest daily xylem sap flux
was for a 20-m tall emergent tree representing 18% of the plot
trees. For emergent, canopy and subcanopy trees, respectively,
average values of F could be normalized with respect to emergent tree values as 1.00, 0.33 and 0.16.
There was, however, substantial variation in the daily xylem
sap flux and dimensional characteristics of trees within each of
the three canopy emergence classes (Tables 1 and 2). Adopting
a pipe model of xylem sap flow (Shinozaki et al. 1964) and
assuming a constant depth of physiologically active sapwood,
because of the generally small ring of sapwood (Table 1) and
the adequate root water supply during measurements, we
found a statistically significant linear relation between stem
area and sapwood area (r 2 = 0.4, Figure 2A), and between stem
area and daily F (r 2 = 0.6, Figure 2B). The slope was interpreted to indicate the stem growth requirement for water transport (Köstner et al. 1992). Stem area increased by about 0.001
m2 for each additional liter of water transpired per day during
the summer growing season.
When F was converted to xylem sap flux density (F′),
Figure 2. The relationship between stem cross-sectional area (m2 ×
10 −2) and sapwood area (m2 × 10 − 3, panel A) and daily xylem sap flux
(l day −1, panel B) in eastern Siberia on July 26, 1993, a clear summer
day. The lines are regressions through the origin with slopes of 0.18
(r 2 = 0.4, panel A) and 9.6 (r 2 = 0.6, panel B).
XYLEM SAP FLOW AND TOTAL CONDUCTANCE OF LARCH IN SIBERIA
obtained by dividing F by projected tree crown area, there was
only a fourfold range in variability among the 13 trees, from
1.0 to 4.4 mm day −1 (Figure 3, Table 1). Variation in F′
appeared to be normally distributed with a median (2.7 mm
day −1) nearly equal to the mean (2.9 mm day −1) and a standard
deviation (1.1 mm day −1) equivalent to about one third of the
range. Conversion to F′ also significantly decreased the systematic and large differences in F among emergent, canopy
and subcanopy trees. Average values of F′ for the three canopy
emergence classes were normalized as 1.00, 0.80 and 0.52,
respectively.
Comparable F and F′ measurements were made in eight
Picea abies (L.) Karst. trees in a Danish plantation (K.
Schelde, 1995, personal communication). For these approximately 20-m tall trees, F ranged from 11 to 37 l day −1 on a clear
summer day. Calculation of F′, based on an estimation of
projected crown area, reduced the variability to 1--1.8 mm
day −1.
Measurements of F and calculations of F′ were also made
for 14 representative trees in a vertically structured 36-m tall,
natural broad-leaved forest with a leaf area index of 7 located
near Maruia, New Zealand (Kelliher et al. 1992, Köstner et al.
251
1992). On a clear late-summer day, F′ ranged from 0.1 to 1.4
mm day −1. Approximately 50% of daily stand transpiration
emanated from emergent trees representing only 3--4% of the
tree population. Variation in F′ was not normally distributed in
horizontal space because F′ depended on the location and size
of neighboring trees. The low variability of F′ in our study
probably reflects the relatively simple tree canopy architecture
and low leaf area index in eastern Siberia.
Xylem sap flow began at about 0600 h local time or approximately 1 h after sunrise with little difference in the timing
among trees of differing size or canopy emergence class (Figure 3). On clear days, F′ varied sinusoidally reaching a broad
4-h long maximal plateau between the times of maximum
irradiance and air saturation deficit. Although the magnitude
of this plateau varied among trees of different sizes and sometimes among canopy emergence classes (Table 1), a remarkably similar diurnal variation in F′ occurred for all trees.
However, F′ for a given irradiance or air saturation deficit, or
combination of the two, was different in the morning and
afternoon (Figure 4). These hystereses and the accompanying
implications for modeling F′ are discussed in the next section.
Nevertheless, there was a close correspondence between
Figure 3. Half-hourly averages of above-forest visible irradiance (Q) and air saturation deficit (D), xylem sap flux density (F′), and the associated
total tree conductances for water vapor transfer (Gt ) during a 4-day measurement period: emergent tree (e), canopy tree (c) and subcanopy tree
(sc). The numbers in brackets refer to the tree’s position in the sap flow plot as indicated in Figure 1.
252
ARNETH ET AL.
Daily flux densities on the 204-m2 polygon plot
Figure 4. Daytime measurements of half-hourly xylem sap flux density and above-canopy visible irradiance (Q, panel A) and air saturation deficit (D, panel B) for emergent Tree 19 on July 24, 1993, a clear
summer day in eastern Siberia. Two data points are labeled with the
local time of day (hour).
changes in irradiance, air saturation deficit and xylem sap flow
following the passage of a cloud at midday on July 23 (Figure 3). There was less consistency in the time of xylem sap
flow cessation which occurred between 2200 and 0100 h local
time or up to 2 h after sunset. Although this might suggest some
xylem refilling from stem water reservoirs, we believe that this
would be incorrect because of the small sapwood thickness of
10 mm (Schulze et al. 1995). Xylem sap flow rates were
generally negligible (< 0.1 l h −1) during hours of darkness.
Daily plot-level F′ ranged from 1.8 ± 0.4 to 2.3 ± 0.6 mm day −1
(Table 3). These values were generally 0.6--0.7 mm day −1
(about 30%) larger than the energy balance and understory
lysimeter estimates of tree canopy transpiration rate. However,
ignoring sap flow from trees of stem area < 0.01 m2 (Figure 2),
comprising 60% of the plot tree population (15% of the plot
stem area), yielded daily F′ values only 5--10% larger than the
Et values. Values from this second F′ calculation thus fell
within the bounds of our sap flux/stem area data. This much
better agreement with independent measurements of tree canopy transpiration rate suggests that there may be a different
relation between sap flux and stem area for the smallest trees
in the plot.
Nevertheless, given the 10--20% coefficients of variation for
F′ values and the likelihood of similar errors associated with
(Ra − H − Eu) calculations, the differences between the two
estimates of tree canopy transpiration rates are probably not
statistically significant. Even so, it is clear that scaling up tree
measurements and combining a series of micrometeorological
and lysimeter measurements can be associated with some
uncertainty. This may be especially important for the estimation of tree transpiration rates for an entire growing season
based on data obtained during intensive, short-term measurements.
Table 3. Daily sap flux density of Larix gmelinii trees in a 204-m2 plot (F′) and estimates of tree canopy transpiration rate (Et ) calculated from the
difference between the available energy (Ra), sensible heat flux (H) and understory evaporation rate (Eu ) (Et = Ra − H − Eu ) on six clear summer
days in eastern Siberia. All terms are expressed as water depth equivalents ± standard deviation.
July
Ra
Eu
(mm day −1)
H
(mm day −1)
Et
(mm day −1)
F′
(mm day −1)
21
22
23
24
25
26
6.1
4.4
4.2
5.9
5.4
5.0
1.1 ± 0.1
1.0 ± 0.1
1.0 ± 0.2
1.0 ± 0.3
1.0 ± 0.2
0.6 ± 0.1
3.1
2.0
2.1
3.3
3.2
2.9
1.9 ± 0.10
1.4 ± 0.10
1.1 ± 0.20
1.6 ± 0.30
1.2 ± 0.20
1.5 ± 0.10
2.1 ± 0.23
2.0 ± 0.20
2.3 ± 0.56
2.2 ± 0.44
1.8 ± 0.38
2.2 ± 0.43
Table 4. Tree-dependent factors, describing the boundary function for the relationship between total conductance (Gt ) and visible irradiance (Q):
maximum total conductance (Gtmax ), Q when Gt was 50% of Gtmax (Q50) and 85% of Gtmax (Q85), and the coefficients a and b for the boundary
function between Gt and air saturation deficit (D). Further explanation of the measurements and calculations is given in the text.
Tree
July
Gtmax
(mm s − 1)
Q50
(µmol m −2 s −1)
Q85
(µmol m −2 s −1)
a
b
19
32
34
26
16
27
9
13
23--27
23--27
21--27
21--27
21--27
21, 24--27
23--27
21, 22, 24--27
8.5
10.7
10.4
15.6
10.8
6.2
9.9
7.3
138
99
97
304
290
99
125
162
550
550
600
920
850
550
800
750
2.4
2.9
2.9
3.4
2.9
2.4
3.0
2.4
−0.6
−0.6
−1.1
−1.3
−1.1
−0.9
−0.7
−0.4
9.9 ± 2.8
164 ± 85
695 ± 151
Average ± SD
XYLEM SAP FLOW AND TOTAL CONDUCTANCE OF LARCH IN SIBERIA
Figure 5. The relationship between half-hourly total tree conductances
for water vapor transfer (Gt) and above-canopy visible irradiance (Q)
(panel A) in eastern Siberia for emergent Tree 34 during seven consecutive summer days, July 21--27, 1993. The line in panel A is fitted
by the function Gtmax (Q/[Q + Q50]) for 10 equidistant maximum
values, where Gtmax was the maximum value of Gt (10.4 mm s − 1) and
Q50 = 97 µmol m − 2 s − 1 when Gt = Gtmax/2. Panel B shows the
relationship between Gt and above-canopy air saturation deficit (D) for
nonirradiance-limiting conditions when Q > 600 µmol m −2 s −1 (Q85),
and when Gt = 85% of Gtmax in panel A. The line in panel B is fitted
by the function Gt = (a + blnD)2 for 10 equidistant maximum values
(see Table 4).
Total tree conductance for water vapor transfer
The occurrence of maximum total tree conductance (Gtmax )
was generally around 0800--0930 h, at least 3 h after sunrise
(Figure 3). Based on the eight trees with continuous data sets,
the saturating upper-boundary function was Gt = Gtmax(Q/[Q +
Q50 ]), where Q50 = 164 ± 85 µmol m −2 s −1 when Gt = Gtmax/2
253
and Gtmax = 9.9 ± 2.8 mm s −1 (Table 4, Figure 5A). Data for Q
from 0 to 1400 µmol m −2 s −1 was divided into equal-sized
steps, and maximum values of Gt in each step were used to
calculate Q50 and Gtmax. The value of Q50 depends on plant
species, plant nutrition and irradiance during growth (Kelliher
et al. 1995). From comparable coniferous forest studies, Q50
was 25, 170 and 340 mmol m −2 s −1 for Pinus radiata D. Don,
P. sylvestris L. and P. pinaster Ait., respectively (Kelliher et al.
1993).
The average value of stomatal conductance (gsmax ) for a tree
canopy may be calculated by dividing Gt by the tree’s leaf area
index (tree leaf area divided by projected crown area). For the
eight trees for which there were continuous data sets, average
calculated gsmax was 6.1 ± 3.3 mm s −1. A comparable average
value of gsmax = 4.3 ± 1.5 mm s −1 for three other Larix species
was derived from published leaf-scale porometer measurements (Table 5). The difference between these two averages
was not statistically significant.
The gsmax data were further compared with two other estimates. First, we determined leaf nitrogen content for four of
the 13 trees (emergent Trees 5 and 19, and canopy Trees 25 and
26; see Table 1) used in the xylem sap flow measurements. For
these four trees, the average leaf N content was 14.6 ± 0.3 mg
g −1 and, on July 26 (Table 1), the corresponding gsmax was 3.2
± 1.6 mm s −1. This was within 27% of an independent estimate
of gsmax = 4.4 mm s −1, calculated from the linear relationship
between leaf N content and gsmax of slope 0.3 mm s −1 per mg
N g −1 (Schulze et al. 1994). Finally, an average woody species
value of gsmax = 5.5 mm s −1 from the comprehensive review of
Körner (1994) was within 11% of our value for L. gmelinii.
Accounting for Q by excluding data for Q < Q85 when Gt was
at least 85% of Gtmax, the decrease in Gt with increasing D was
determined using a similar approach as before; namely that the
maximum values of Gt within each of 10 equal-sized D steps
defined the upper-boundary function (Table 4, Figure 5B). The
variability of Gt at low D values was substantial and probably
precludes accurate model predictions (e.g., Kelliher et al.
1993, Leuning 1995). The variability in Gt increased significantly as D decreased below about 2 kPa, as has been observed
elsewhere (Granier and Loustau 1994). In Siberia, this may
have resulted from variable self-shading of tree crowns early
Table 5. Maximum stomatal conductances (gsmax , mm s −1, expressed on a one-sided leaf area basis) of four Larix species derived from
measurements on field-grown trees. The value for this study is the mean (± standard deviation) obtained by dividing the maximum total
conductance per tree in Table 4 by the tree’s leaf area index. Porometer data from the literature were converted from molar units (mmol m − 2 s −1),
by dividing by 40, and expressed on an all-surfaces leaf area basis by multiplying by 2.5 (the all-surfaces to one-sided leaf area ratio for Larix
(Oren et al. 1986)).
Species
Location
gsmax
(mm s − 1)
Source
L. gmelinii
L. decidua
L. decidua
L. decidua
L. decidua × leptolepis
L. laricina
L. laricina
Eastern Siberia
Germany
Austria
New Zealand
Germany
Canada
Canada
6.1 ± 3.3
3.6
3.1
4.4
3.9
3.5
7.3
This study
Benecke et al. (1981)
Benecke et al. (1981)
Benecke et al. (1981)
Schulze et al. (1985)
Dang et al. (1991)
Lafleur (1992, personal communication)
254
ARNETH ET AL.
conditions that characterize its short, hot, dry summer growing
season.
Acknowledgments
Figure 6. Daytime estimates of half-hourly total tree conductances for
water vapor transfer and above-canopy visible irradiance (Q, panel A)
and air saturation deficit (D, panel B) for emergent Tree 19 on July 24,
1993, a clear summer day in eastern Siberia. Two data points are
labeled with the local time of day (hour).
E.-D. Schulze acknowledges support of the German Bundesminister
für Ernährung Landwirtschaft und Forsten and the Humboldt Foundation (Max-Planck Forschungspreis). The Manaaki Whenua----Landcare Research team was funded by a long-term grant for atmospheric
research from the New Zealand Foundation for Research, Science and
Technology. A travel grant for the journey from New Zealand to
Germany was awarded to D.Y. Hollinger under the New Zealand/Federal Republic of Germany Agreement for Scientific and Technological
Cooperation. We also acknowledge the help of Herbert Koch, Kira
Kobak, Vela Kuznetsova, Fyodor Tatarinov and Sascha Issajev with
measurements. Finally, we express our sincere thanks to Rudolf Meserth for his tremendous support during the preparation of the measurements and Inge Schulze for keeping us supplied with good food
during our month living in a Siberian forest.
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© 1996 Heron Publishing----Victoria, Canada
Environmental regulation of xylem sap flow and total conductance of
Larix gmelinii trees in eastern Siberia
A. ARNETH,1,2 F. M. KELLIHER,3 G. BAUER,1 D. Y. HOLLINGER,3,4 J. N. BYERS,3
J. E. HUNT,3 T. M. MCSEVENY,3 W. ZIEGLER,5 N. N. VYGODSKAYA,6 I. MILUKOVA,6
A. SOGACHOV,6 A. VARLAGIN6 and E.-D. SCHULZE1
1
Universität Bayreuth, Lehrstuhl für Pflanzenökologie, Box 101251, 95440 Bayreuth, Germany
2
Present address: Lincoln University, Department of Soil Science, P.O. Box 84, Lincoln, New Zealand
3
Manaaki Whenua----Landcare Research, P.O. Box 69, Lincoln, New Zealand
4
Present address: U.S. Forest Service, P.O. Box 640, Durham, NH 03284, USA
5
Comenius University, Department of Biophysical and Chemical Physics, Mlynska Dolina F1, CS-842 15, Bratislava, Slovak Republic
6
V.N. Sukachov Laboratory of Biogeocenology, Russian Academy of Sciences, Leninsky Prospect 33, 117071 Moscow, Russia
Received March 28, 1995
Summary Xylem sap flow and environmental variables were
measured on seven consecutive midsummer days in a 130-yearold Larix gmelinii (Rupr.) Rupr. forest located 160 km south of
Yakutsk in eastern Siberia, Russia (61° N, 128° E, 300 m asl).
The site received 20 mm of rainfall during the 4 days before
measurements, and soil samples indicated that the trees were
well watered. The tree canopy was sparse with a one-sided leaf
area index of 1.5 and a tree density of 1760 ha −1. On a clear
day when air temperature ranged from 9 to 29 °C, and maximum air saturation deficit was 3.4 kPa, daily xylem sap flux
(F) among 13 trees varied by an order of magnitude from 7 l
day −1 for subcanopy trees (representing 55% of trees in the
forest) to 67 l day −1 for emergent trees (representing 18% of
trees in the forest). However, when based on xylem sap flux
density (F′), calculated by dividing F by projected tree crown
area (a surrogate for the occupied ground area), there was only
a fourfold range in variability among the 13 trees, from 1.0 to
4.4 mm day −1. The calculation of F′ also eliminated systematic
and large differences in F among emergent, canopy and subcanopy trees. Stand-level F′, estimated by combining halfhourly linear relationships between F and stem cross-sectional
area with tree size distribution data, ranged between 1.8 ± 0.4
(standard deviation) and 2.3 ± 0.6 mm day −1. These stand-level
F′ values are about 0.6--0.7 mm day −1 (30%) larger than daily
tree canopy transpiration rates calculated from forest energy
balance and understory evaporation measurements.
Maximum total tree conductance for water vapor transfer
(Gtmax, including canopy and aerodynamic conductances), calculated from the ratio of F′ and the above-canopy air saturation
deficit (D) for the eight trees with continuous data sets, was 9.9
± 2.8 mm s −1. This is equivalent to a leaf-scale maximum
stomatal conductance (gsmax ) of 6.1 mm s −1, when expressed
on a one-sided leaf area basis, which is comparable to the
published porometer data for Larix. Diurnal variation in total
tree conductance (Gt ) was related to changes in the above-can-
opy visible irradiance (Q) and D. A saturating upper-boundary
function for the relationship between Gt and Q was defined as
Gt = Gtmax (Q/[Q + Q50 ]), where Q50 = 164 ± 85 µmol m −2 s −1
when Gt = Gtmax /2. Accounting for Q by excluding data for Q
< Q85 when Gt was at least 85% of Gtmax , the upper limit for the
relationship between Gt and D was determined based on the
function Gt = (a + blnD)2, where a and b are regression
coefficients. The relationship between Gt and D was curvilinear, indicating that there was a proportional decrease in Gt with
increasing D such that F was relatively constant throughout
much of the day, even when D ranged between about 2 and 4
kPa, which may be interpreted as an adaption of the species to
its continental climate. However, at given values of Q and D,
Gt was generally higher in the morning than in the afternoon.
The additional environmental constraints on Gt imposed by
leaf nitrogen nutrition and afternoon water stress are discussed.
Keywords: boreal forest, larch, stomatal conductance, transpiration.
Introduction
The eastern Siberian taiga is dominated by Larix gmelinii
(Rupr.) Rupr. with a forested area of 1 × 106 km2 (Shridenko
and Nilssen 1994). The species forms pure stands regenerated
by recurrent fire (Schulze et al. 1995). A common habitat
feature of Larix is an extremely cold winter when physiological activity ceases (Gower and Richards 1990). The trees thus
complete needle development, growth, flowering and seed
development during relatively short summers. During the approximately 90-day snow-free growing season, average rainfall is only 110 mm. This is about half the annual precipitation
of 210 mm, whereas the corresponding potential evaporation
rate is 370 mm (Müller 1982), indicating that there must be
severe environmental regulation of the forest evaporation rate.
248
ARNETH ET AL.
However, despite the possible influence of this ecosystem’s
energy balance on the regional and global climate (Shukla and
Mintz 1982), we are not aware of any published research about
Larix forest evaporation.
The objective of this paper was to examine the effects of
environmental variables on xylem sap flow of L. gmelinii
during a 7-day period in midsummer. We calculated total tree
conductances for water vapor transfer (Gt ) from xylem sap flux
density and above-canopy air saturation deficit measurements,
and developed a multiplicative environmental constraint Gt
model for diagnosis, rather than strictly for prognosis (Granier
and Loustau 1994). Adaptation of L. gmelinii to its continental
climate is discussed.
Materials and methods
Site description
Measurements were made in a 130-year-old L. gmelinii forest
located 160 km south of Yakutsk in the extreme continental
part of eastern Siberia, Russia (61° N, 128° E, 300 m asl).
Characteristics of the vegetation were determined by measurement and destructive sampling in a representative 0.5-ha plot
(Schulze et al. 1995). Average tree height was 12 m, although
some canopy emergents were nearly 20 m tall, average stem
diameter was 110 mm, stand density was 1760 ha −1, and
one-sided leaf area index was 1.5 (Schulze et al. 1995). The
evergreen, broad-leaved understory vegetation was about 0.05
m tall with a one-sided leaf area index of 1.0. Understory plant
species included Arctostaphlos uva ursi, Vaccinium vitis idaea
and V. uliginosum.
According to the USDA taxonomic classification, the soil
was an inceptisol known as Pergelic Cryochrept. There was a
30-mm deep leaf litter horizon, and the top 50-mm deep
mineral soil horizon was enriched with humus (carbon content
= 10%). The texture of the underlying soil horizons was silt
loam. Because of underlying permafrost, the soil temperature
at a depth of 700 mm was only 1--2 °C. Soil water release
measurements were made in the laboratory on undisturbed
cores. Roots were generally confined to the upper 300 mm that
had an available soil water storage capacity of about 45 mm.
Xylem sap flow measurements
Xylem sap flow was measured at a height of 1.5 m in 13 trees
on a second 204-m2 plot (Table 1). The plot was a polygon
determined by connecting bisects of outer plot trees and their
nearest neighboring trees outside the plot (Figure 1). Twenty
small L. gmelinii and two Betula platyphyllos Sukachev trees
were present in the polygon plot (Table 2). There was about
twice as many large (> 160 mm diameter) trees on the polygon
plot (23% of total) than on the representative 0.5-ha plot.
Mass flow rate of water through the xylem of trees (xylem
sap flux, kg s −1, Fi) was estimated from half-hourly steadystate energy balances of undisturbed sapwood (to a depth of
50 mm) heated by electrodes to a constant 2 K (Th) above the
temperature of unheated sapwood (Tu) (UP Umweltanalytische Produkte, Munich, Germany), located at the same height
but 0.2 m away, as:
Fi =
k iP i
,
cw(Th − Tu)
(1)
where Pi is the power input to tree i (W, for an average time of
30 min), cw is the specific heat of water, and ki = Ci /[d(e − 1)]
with Ci, d and e being stem circumference of tree i, distance
between electrodes and number of electrodes, respectively
(Èermák et al. 1973, Pearcy et al. 1989). Power input is the
difference between power supplied to the stem and losses
Table 1. Some dimensional characteristics of 13 Larix gmelinii trees, and their associated xylem sap fluxes and maximum total tree conductances
(Gtmax ) on July 26, 1993, a clear summer day in eastern Siberia. Xylem sap flux densities may be calculated by dividing xylem sap fluxes by
projected crown area or sapwood area. Tree social position is indicated by the canopy emergence class as emergent, canopy and subcanopy.
Tree
Height
(m)
Stem diameter
(mm)
Projected crown area
(m2)
Sapwood area
(m2 × 10 −3)
Daily sap flux
(kg day −1)
Maximum sap flux
(kg h −1)
Gtmax
(mm s −1)
Emergent
19
29
34
32
5
2
19.5
18.5
16.0
19.5
19.0
19.5
253
244
178
255
204
221
28.4
15.4
15.8
6.7
10.3
10.1
7.64
5.51
6.34
8.58
5.51
6.60
66.9
41.9
37.1
23.0
44.4
43.2
6.3
3.7
3.1
2.2
3.9
1.1
3.1
3.5
3.5
5.5
7.2
7.3
Canopy
9
27
1
13
25
26
17.5
15.5
18.5
13.5
16.0
13.0
140
178
170
129
146
138
3.5
12.2
13.2
5.2
3.1
4.7
4.81
5.03
4.82
3.63
4.22
3.75
14.3
12.1
20.1
14.2
10.3
13.7
1.3
1.1
2.0
1.4
1.6
1.6
5.2
1.7
1.7
3.3
9.0
4.1
Subcanopy
16
11.0
116
4.3
2.72
6.8
0.8
3.0
XYLEM SAP FLOW AND TOTAL CONDUCTANCE OF LARCH IN SIBERIA
249
Figure 1. Map of projected crown areas of
the 35 trees in the representative 204-m2
(polygon) plot. The tree numbers appear at
the stem locations and the additional symbol B indicates a birch tree. The crowns of
the measured trees are shaded. Also shown
are a scale, north arrow, and locations of
the 15 neighboring trees whose crowns
reached the plot boundary.
Table 2. Some average (± standard deviation) dimensional characteristics of the 33 Larix gmelinii trees in a representative 204-m2 plot in eastern
Siberia. Tree social position refers to the canopy emergence class as emergent, canopy and subcanopy.
Social position
Number of trees
Height
(m)
Stem area
(m2 × 10 − 2)
Projected crown area
(m2)
Crown depth
(m)
Emergent
Canopy
Subcanopy
6
9
18
18.2 ± 1.4
14.6 ± 3.2
7.3 ± 2.4
4.03 ± 1.03
1.59 ± 0.51
0.28 ± 0.20
14.5 ± 9.7
6.9 ± 6.2
2.9 ± 2.8
7.7 ± 2.9
5.0 ± 2.2
2.3 ± 2.2
caused by convection by xylem sap flow and also by conduction and radial heat loss to the surrounding air.
Conduction and radial heat losses were constant because of
the constant heated--unheated temperature difference, and
were equal to the minimum power input obtained during darkness. Forest evaporation, determined from micrometeorological measurements, was zero during the night (Kelliher et al.
1994). Heat losses, which were determined each night for
every tree, were generally constant for each tree throughout the
2--5-day measurement period (see below). For a 1-m vertical
band centered at the height of instrumentation, the stems of
measured trees were surrounded by 200-mm thick fiberglass
insulation and covered with aluminized mylar sheeting. The
electrodes were inserted on branchless stem sections at about
1.5 m, facing west, north or east. This arrangement avoided
possible sap flow irregularities caused by branches and direct
sun exposure during most of the day.
Within 2 to 5 days after installation of a sap flow measurement system, resin production increased the resistance be-
tween electrodes such that we could not generate enough
power to maintain (Th − Tu) = 2 K. Consequently, measurement
systems were regularly shifted throughout the study. The number of trees measured varied between 7 and 13.
Comparison of sap flow for trees differing in size requires
normalization on an area basis (sap flux density, F′, kg m −2
s −1). We used a vertical projection of the crown perimeter for
each tree, determined from anascope measurements (Figure 1),
as an estimate of the tree’s occupied ground area. For comparison with tree canopy evaporation, we estimated F′ for the
polygon plot based on the tree size distribution data and the
half-hourly relations between Fi and stem cross-sectional area.
The relations were linear and passed through the origin with
coefficients of determination (r 2) ranging from 0.40 in the
early morning before sap flow had started in all trees to 0.75
during the rest of the day.
Plot level F′ estimates were scrutinized by comparison with
tree canopy transpiration rates (Et) determined from a combination of energy balance and understory evaporation rate (Eu)
250
ARNETH ET AL.
measurements described by Kelliher et al. (1994). Briefly, Et
was estimated as (Ra − H − Eu), where Ra is the available energy,
and H is the sensible heat flux density. We determined Ra from
measurements of the above-canopy net radiation and ground
heat flux after Kelliher et al. (1992). We measured H with a
three-dimensional sonic anemometer. Five lysimeters, thinwalled plastic sleeves (370 × 370 × 150 mm deep) encasing
undisturbed understory vegetation--forest-floor--soil cores,
were weighed at 2-h intervals to determine Eu.
Estimating total tree conductance for water vapor transfer
Total tree conductance for water vapor transfer (Gt ) was estimated from the ratio of F′i and air saturation deficit (D),
measured above the forest at a height of 22.5 m with an
ultraviolet absorption hygrometer (Model KH2O, Campbell
Scientific, Logan, UT). When F′i and D are expressed in units
of mm h −1 and kPa, respectively, Gt = k(F′i/D), where k is a
weakly temperature-dependent conversion factor (= 38 at
20 °C; Köstner et al. 1992). The conductance Gt includes
stomatal, leaf boundary layer (gb) and eddy diffusive aerodynamic (gaM ) conductance components because we are accounting for conductances from the average height of stomata in the
tree canopy to the height of a D measurement made above the
forest (Thom 1972). The sonic anemometer measurements
indicated gaM = 0.1u, where u is the wind speed above the
canopy (Jarvis et al. 1976). Daytime wind speeds during the
measurement period were generally low, between 1 and 2 m
s −1, so gaM = 0.1--0.2 m s −1. We set gb = 0.17 m s −1, an average
for several coniferous species given by Jarvis et al. (1976). For
the tree canopy, we multiplied by the leaf area index to estimate an average gb = 0.26 m s −1. The series sum of gaM and gb
is therefore at least 10 to 20 times larger than Gt. Consequently,
Gt closely resembles the tree’s canopy conductance (Köstner
et al. 1992).
We examined the optimal response of Gt to above-canopy
visible irradiance (Q, measured with a calibrated galiumarsenide photodiode) and D by a combination of boundary-line
analysis (e.g., Livingston and Black 1987) and a multiplicative
constraint-function model (e.g., Granier and Loustau 1994).
We first fitted a hyperbolic saturating function to the upper
boundary of a plot of Gt and Q as Gt = Gtmax (Q/[Q + Q50]),
where Q50 is the value of Q when Gt = Gtmax /2. We then
examined the upper limit of the decrease in Gt with increasing
D under nonirradiance-limiting conditions (Q > 85% of the
saturating value) as Gt = (a + blnD)2, where a and b are
regression coefficients.
We did not include the effects of soil water deficit in our Gt
model. However, the site had 20 mm of rain in the 4 days
preceding the study, and gravimetric forest floor and soil
samples, which were collected daily to a depth of 700 mm,
indicated that the trees were well watered throughout the study.
Results and discussion
Xylem sap flow rate
On a clear day (July 26, 1993) when air temperature ranged
from 9 to 29 °C, and maximum air saturation deficit was
3.4 kPa, daily xylem sap flux (F) among 13 measured trees
varied by an order of magnitude from 7 to 67 l day −1 (Table 1).
The lowest value of F was for an 11-m tall subcanopy tree that
was representative of 55% of the trees in the forest in terms of
size, based on an inventory of 875 L. gmelinii trees in the
0.5-ha plot (data not shown). The highest daily xylem sap flux
was for a 20-m tall emergent tree representing 18% of the plot
trees. For emergent, canopy and subcanopy trees, respectively,
average values of F could be normalized with respect to emergent tree values as 1.00, 0.33 and 0.16.
There was, however, substantial variation in the daily xylem
sap flux and dimensional characteristics of trees within each of
the three canopy emergence classes (Tables 1 and 2). Adopting
a pipe model of xylem sap flow (Shinozaki et al. 1964) and
assuming a constant depth of physiologically active sapwood,
because of the generally small ring of sapwood (Table 1) and
the adequate root water supply during measurements, we
found a statistically significant linear relation between stem
area and sapwood area (r 2 = 0.4, Figure 2A), and between stem
area and daily F (r 2 = 0.6, Figure 2B). The slope was interpreted to indicate the stem growth requirement for water transport (Köstner et al. 1992). Stem area increased by about 0.001
m2 for each additional liter of water transpired per day during
the summer growing season.
When F was converted to xylem sap flux density (F′),
Figure 2. The relationship between stem cross-sectional area (m2 ×
10 −2) and sapwood area (m2 × 10 − 3, panel A) and daily xylem sap flux
(l day −1, panel B) in eastern Siberia on July 26, 1993, a clear summer
day. The lines are regressions through the origin with slopes of 0.18
(r 2 = 0.4, panel A) and 9.6 (r 2 = 0.6, panel B).
XYLEM SAP FLOW AND TOTAL CONDUCTANCE OF LARCH IN SIBERIA
obtained by dividing F by projected tree crown area, there was
only a fourfold range in variability among the 13 trees, from
1.0 to 4.4 mm day −1 (Figure 3, Table 1). Variation in F′
appeared to be normally distributed with a median (2.7 mm
day −1) nearly equal to the mean (2.9 mm day −1) and a standard
deviation (1.1 mm day −1) equivalent to about one third of the
range. Conversion to F′ also significantly decreased the systematic and large differences in F among emergent, canopy
and subcanopy trees. Average values of F′ for the three canopy
emergence classes were normalized as 1.00, 0.80 and 0.52,
respectively.
Comparable F and F′ measurements were made in eight
Picea abies (L.) Karst. trees in a Danish plantation (K.
Schelde, 1995, personal communication). For these approximately 20-m tall trees, F ranged from 11 to 37 l day −1 on a clear
summer day. Calculation of F′, based on an estimation of
projected crown area, reduced the variability to 1--1.8 mm
day −1.
Measurements of F and calculations of F′ were also made
for 14 representative trees in a vertically structured 36-m tall,
natural broad-leaved forest with a leaf area index of 7 located
near Maruia, New Zealand (Kelliher et al. 1992, Köstner et al.
251
1992). On a clear late-summer day, F′ ranged from 0.1 to 1.4
mm day −1. Approximately 50% of daily stand transpiration
emanated from emergent trees representing only 3--4% of the
tree population. Variation in F′ was not normally distributed in
horizontal space because F′ depended on the location and size
of neighboring trees. The low variability of F′ in our study
probably reflects the relatively simple tree canopy architecture
and low leaf area index in eastern Siberia.
Xylem sap flow began at about 0600 h local time or approximately 1 h after sunrise with little difference in the timing
among trees of differing size or canopy emergence class (Figure 3). On clear days, F′ varied sinusoidally reaching a broad
4-h long maximal plateau between the times of maximum
irradiance and air saturation deficit. Although the magnitude
of this plateau varied among trees of different sizes and sometimes among canopy emergence classes (Table 1), a remarkably similar diurnal variation in F′ occurred for all trees.
However, F′ for a given irradiance or air saturation deficit, or
combination of the two, was different in the morning and
afternoon (Figure 4). These hystereses and the accompanying
implications for modeling F′ are discussed in the next section.
Nevertheless, there was a close correspondence between
Figure 3. Half-hourly averages of above-forest visible irradiance (Q) and air saturation deficit (D), xylem sap flux density (F′), and the associated
total tree conductances for water vapor transfer (Gt ) during a 4-day measurement period: emergent tree (e), canopy tree (c) and subcanopy tree
(sc). The numbers in brackets refer to the tree’s position in the sap flow plot as indicated in Figure 1.
252
ARNETH ET AL.
Daily flux densities on the 204-m2 polygon plot
Figure 4. Daytime measurements of half-hourly xylem sap flux density and above-canopy visible irradiance (Q, panel A) and air saturation deficit (D, panel B) for emergent Tree 19 on July 24, 1993, a clear
summer day in eastern Siberia. Two data points are labeled with the
local time of day (hour).
changes in irradiance, air saturation deficit and xylem sap flow
following the passage of a cloud at midday on July 23 (Figure 3). There was less consistency in the time of xylem sap
flow cessation which occurred between 2200 and 0100 h local
time or up to 2 h after sunset. Although this might suggest some
xylem refilling from stem water reservoirs, we believe that this
would be incorrect because of the small sapwood thickness of
10 mm (Schulze et al. 1995). Xylem sap flow rates were
generally negligible (< 0.1 l h −1) during hours of darkness.
Daily plot-level F′ ranged from 1.8 ± 0.4 to 2.3 ± 0.6 mm day −1
(Table 3). These values were generally 0.6--0.7 mm day −1
(about 30%) larger than the energy balance and understory
lysimeter estimates of tree canopy transpiration rate. However,
ignoring sap flow from trees of stem area < 0.01 m2 (Figure 2),
comprising 60% of the plot tree population (15% of the plot
stem area), yielded daily F′ values only 5--10% larger than the
Et values. Values from this second F′ calculation thus fell
within the bounds of our sap flux/stem area data. This much
better agreement with independent measurements of tree canopy transpiration rate suggests that there may be a different
relation between sap flux and stem area for the smallest trees
in the plot.
Nevertheless, given the 10--20% coefficients of variation for
F′ values and the likelihood of similar errors associated with
(Ra − H − Eu) calculations, the differences between the two
estimates of tree canopy transpiration rates are probably not
statistically significant. Even so, it is clear that scaling up tree
measurements and combining a series of micrometeorological
and lysimeter measurements can be associated with some
uncertainty. This may be especially important for the estimation of tree transpiration rates for an entire growing season
based on data obtained during intensive, short-term measurements.
Table 3. Daily sap flux density of Larix gmelinii trees in a 204-m2 plot (F′) and estimates of tree canopy transpiration rate (Et ) calculated from the
difference between the available energy (Ra), sensible heat flux (H) and understory evaporation rate (Eu ) (Et = Ra − H − Eu ) on six clear summer
days in eastern Siberia. All terms are expressed as water depth equivalents ± standard deviation.
July
Ra
Eu
(mm day −1)
H
(mm day −1)
Et
(mm day −1)
F′
(mm day −1)
21
22
23
24
25
26
6.1
4.4
4.2
5.9
5.4
5.0
1.1 ± 0.1
1.0 ± 0.1
1.0 ± 0.2
1.0 ± 0.3
1.0 ± 0.2
0.6 ± 0.1
3.1
2.0
2.1
3.3
3.2
2.9
1.9 ± 0.10
1.4 ± 0.10
1.1 ± 0.20
1.6 ± 0.30
1.2 ± 0.20
1.5 ± 0.10
2.1 ± 0.23
2.0 ± 0.20
2.3 ± 0.56
2.2 ± 0.44
1.8 ± 0.38
2.2 ± 0.43
Table 4. Tree-dependent factors, describing the boundary function for the relationship between total conductance (Gt ) and visible irradiance (Q):
maximum total conductance (Gtmax ), Q when Gt was 50% of Gtmax (Q50) and 85% of Gtmax (Q85), and the coefficients a and b for the boundary
function between Gt and air saturation deficit (D). Further explanation of the measurements and calculations is given in the text.
Tree
July
Gtmax
(mm s − 1)
Q50
(µmol m −2 s −1)
Q85
(µmol m −2 s −1)
a
b
19
32
34
26
16
27
9
13
23--27
23--27
21--27
21--27
21--27
21, 24--27
23--27
21, 22, 24--27
8.5
10.7
10.4
15.6
10.8
6.2
9.9
7.3
138
99
97
304
290
99
125
162
550
550
600
920
850
550
800
750
2.4
2.9
2.9
3.4
2.9
2.4
3.0
2.4
−0.6
−0.6
−1.1
−1.3
−1.1
−0.9
−0.7
−0.4
9.9 ± 2.8
164 ± 85
695 ± 151
Average ± SD
XYLEM SAP FLOW AND TOTAL CONDUCTANCE OF LARCH IN SIBERIA
Figure 5. The relationship between half-hourly total tree conductances
for water vapor transfer (Gt) and above-canopy visible irradiance (Q)
(panel A) in eastern Siberia for emergent Tree 34 during seven consecutive summer days, July 21--27, 1993. The line in panel A is fitted
by the function Gtmax (Q/[Q + Q50]) for 10 equidistant maximum
values, where Gtmax was the maximum value of Gt (10.4 mm s − 1) and
Q50 = 97 µmol m − 2 s − 1 when Gt = Gtmax/2. Panel B shows the
relationship between Gt and above-canopy air saturation deficit (D) for
nonirradiance-limiting conditions when Q > 600 µmol m −2 s −1 (Q85),
and when Gt = 85% of Gtmax in panel A. The line in panel B is fitted
by the function Gt = (a + blnD)2 for 10 equidistant maximum values
(see Table 4).
Total tree conductance for water vapor transfer
The occurrence of maximum total tree conductance (Gtmax )
was generally around 0800--0930 h, at least 3 h after sunrise
(Figure 3). Based on the eight trees with continuous data sets,
the saturating upper-boundary function was Gt = Gtmax(Q/[Q +
Q50 ]), where Q50 = 164 ± 85 µmol m −2 s −1 when Gt = Gtmax/2
253
and Gtmax = 9.9 ± 2.8 mm s −1 (Table 4, Figure 5A). Data for Q
from 0 to 1400 µmol m −2 s −1 was divided into equal-sized
steps, and maximum values of Gt in each step were used to
calculate Q50 and Gtmax. The value of Q50 depends on plant
species, plant nutrition and irradiance during growth (Kelliher
et al. 1995). From comparable coniferous forest studies, Q50
was 25, 170 and 340 mmol m −2 s −1 for Pinus radiata D. Don,
P. sylvestris L. and P. pinaster Ait., respectively (Kelliher et al.
1993).
The average value of stomatal conductance (gsmax ) for a tree
canopy may be calculated by dividing Gt by the tree’s leaf area
index (tree leaf area divided by projected crown area). For the
eight trees for which there were continuous data sets, average
calculated gsmax was 6.1 ± 3.3 mm s −1. A comparable average
value of gsmax = 4.3 ± 1.5 mm s −1 for three other Larix species
was derived from published leaf-scale porometer measurements (Table 5). The difference between these two averages
was not statistically significant.
The gsmax data were further compared with two other estimates. First, we determined leaf nitrogen content for four of
the 13 trees (emergent Trees 5 and 19, and canopy Trees 25 and
26; see Table 1) used in the xylem sap flow measurements. For
these four trees, the average leaf N content was 14.6 ± 0.3 mg
g −1 and, on July 26 (Table 1), the corresponding gsmax was 3.2
± 1.6 mm s −1. This was within 27% of an independent estimate
of gsmax = 4.4 mm s −1, calculated from the linear relationship
between leaf N content and gsmax of slope 0.3 mm s −1 per mg
N g −1 (Schulze et al. 1994). Finally, an average woody species
value of gsmax = 5.5 mm s −1 from the comprehensive review of
Körner (1994) was within 11% of our value for L. gmelinii.
Accounting for Q by excluding data for Q < Q85 when Gt was
at least 85% of Gtmax, the decrease in Gt with increasing D was
determined using a similar approach as before; namely that the
maximum values of Gt within each of 10 equal-sized D steps
defined the upper-boundary function (Table 4, Figure 5B). The
variability of Gt at low D values was substantial and probably
precludes accurate model predictions (e.g., Kelliher et al.
1993, Leuning 1995). The variability in Gt increased significantly as D decreased below about 2 kPa, as has been observed
elsewhere (Granier and Loustau 1994). In Siberia, this may
have resulted from variable self-shading of tree crowns early
Table 5. Maximum stomatal conductances (gsmax , mm s −1, expressed on a one-sided leaf area basis) of four Larix species derived from
measurements on field-grown trees. The value for this study is the mean (± standard deviation) obtained by dividing the maximum total
conductance per tree in Table 4 by the tree’s leaf area index. Porometer data from the literature were converted from molar units (mmol m − 2 s −1),
by dividing by 40, and expressed on an all-surfaces leaf area basis by multiplying by 2.5 (the all-surfaces to one-sided leaf area ratio for Larix
(Oren et al. 1986)).
Species
Location
gsmax
(mm s − 1)
Source
L. gmelinii
L. decidua
L. decidua
L. decidua
L. decidua × leptolepis
L. laricina
L. laricina
Eastern Siberia
Germany
Austria
New Zealand
Germany
Canada
Canada
6.1 ± 3.3
3.6
3.1
4.4
3.9
3.5
7.3
This study
Benecke et al. (1981)
Benecke et al. (1981)
Benecke et al. (1981)
Schulze et al. (1985)
Dang et al. (1991)
Lafleur (1992, personal communication)
254
ARNETH ET AL.
conditions that characterize its short, hot, dry summer growing
season.
Acknowledgments
Figure 6. Daytime estimates of half-hourly total tree conductances for
water vapor transfer and above-canopy visible irradiance (Q, panel A)
and air saturation deficit (D, panel B) for emergent Tree 19 on July 24,
1993, a clear summer day in eastern Siberia. Two data points are
labeled with the local time of day (hour).
E.-D. Schulze acknowledges support of the German Bundesminister
für Ernährung Landwirtschaft und Forsten and the Humboldt Foundation (Max-Planck Forschungspreis). The Manaaki Whenua----Landcare Research team was funded by a long-term grant for atmospheric
research from the New Zealand Foundation for Research, Science and
Technology. A travel grant for the journey from New Zealand to
Germany was awarded to D.Y. Hollinger under the New Zealand/Federal Republic of Germany Agreement for Scientific and Technological
Cooperation. We also acknowledge the help of Herbert Koch, Kira
Kobak, Vela Kuznetsova, Fyodor Tatarinov and Sascha Issajev with
measurements. Finally, we express our sincere thanks to Rudolf Meserth for his tremendous support during the preparation of the measurements and Inge Schulze for keeping us supplied with good food
during our month living in a Siberian forest.
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