Tree Physiology 28, 1317–1323
© 2008 Heron Publishing—Victoria, Canada
Vertical foliage distribution determines the radial pattern of sap flux
density in Picea abies
ALESSANDRO FIORA1,2 and ALESSANDRO CESCATTI1,3
1
Centro di Ecologia Alpina – Viote del Monte Bondone, 38100 Trento, Italy
2
Corresponding author ([email protected])
3
EC, Joint Research Centre, Institute for Environment and Sustainability, Climate Change Unit, 21020 Ispra, Italy
Received May 15, 2007; accepted July 16, 2007; published online July 1, 2008
Summary Understanding the causes determining the radial
pattern of sap flux density is important both for improving
knowledge of sapwood functioning and for up-scaling sap flow
measurements to canopy transpiration and ecosystem water
use. To investigate the anatomical connection between whorls
and annual sapwood rings, pruning-induced variation in the radial pattern of sap flux density was monitored with Granier
probes in a 35-year-old Picea abies (L.) Karst tree that was
pruned from the crown bottom up. Modifications in the radial
pattern of sap flux density were quantified by a shape index
(SI), which varies with the relative contribution of the outer and
inner sapwood to tree transpiration. The SI progressively diminished during bottom up pruning, indicating a significant reduction in sap flow contribution of the inner sapwood. Results
suggest that the radial pattern of sap flux density depends
mainly on the vertical distribution of foliage in the crown, with
lower shaded branches hydraulically connected with inner
sapwood and upper branches connected with the outer rings.
Keywords: foliage removal, heat dissipation method, hydraulic architecture.
Introduction
Many studies have shown large radial variability in sap flux
density in conifers. Most commonly, the radial pattern is either
Gaussian, with a peak between sapwood depths of 10 and
30 mm (Èermák et al. 1992, Nadezhdina et al. 2002, Ford et al.
2004a, 2004b) or it follows a monotonic decrease toward the
stem center (Hatton et al. 1990, Köstner et al. 1996, Phillips et
al. 1996, Lundblad et al. 2001, Delzon et al. 2004, Irvine et al.
2004).
Although the decline in radial sap flux density with increasing sapwood depth has been the focus of many studies, the reason for this depth-dependent variation has not been clearly
identified (Wullschleger and King 2000). Mark and Crews
(1973) showed that low sap flow in the outermost sapwood
could reflect an undeveloped conducting system in the outer
xylem, whereas low sap flow in the innermost sapwood could
reflect a progressive loss of functionality in the inner xylem,
caused by bordered pit membranes becoming progressively
blocked. Based on published information on tracheid length
and lumen diameter, Phillips et al. (1996) speculated that the
lower hydraulic conductivity of juvenile xylem in pine, 40%
less than that of mature sapwood, was the cause of radial variation in sap flux density with sapwood depth. Evidence of age
dependency of hydraulic conductivity of sapwood and sap flux
density was obtained by Spicer and Gartner (2001) and by
Domec and Gartner (2003) and Domec et al. (2005), who
showed significant variation in specific conductivity in the
trunks of conifers, with the maximum occurring in the outermost rings. Barbour and Whitehead (2003) demonstrated that
sap flux density was inversely related to wood density in
Dacrydium cupressinum Lamb., but only in dominant trees.
In contrast, Dye et al. (1991) interpreted the decrease in sap
flux density with sapwood depth on the assumption that transpiration of older branches in the lower crown, which are anatomically connected with the inner sapwood, may decrease
progressively because of increased shading. This phenomenon
is evident in older stands with high stem densities, where natural pruning of the lower crown occurs. Köstner et al. (1998) asserted that radial variation in sap flux density depends on sapwood thickness and the number of growth rings. Jiménez et al.
(2000) reported that sap flow in the inner xylem of two laurel
forest trees supplies the lower, deeper and partially shaded part
of the tree crown, whereas sap flow in the outer sapwood reflects the high transpiration rates of the well-illuminated upper
canopy. Furthermore, these authors concluded that the position and amplitude of sap flow peaks were influenced by the
foliage distribution and the social position of trees. Nadezhdina and Èermák (2003) observed that the outer sapwood often connects surface roots with sun-exposed foliage. Nadezhdina et al. (2002) reported high temporal variations in sap flow
velocity among sapwood layers, the maximum rates occurring
in layers connected to sun-exposed leaves. Fiora and Cescatti
(2006) speculated that different radial sap flux density patterns
in six coniferous trees differing in diameter were related to social position and vertical crown distribution of single trees.
Identification of the causes underlying the radial pattern of
sap flux density is important for understanding sapwood functioning, the physiological mechanisms regulating daily sap
1318
FIORA AND CESCATTI
flow, and for scaling up sap flow measurements to canopy
transpiration and forest water use (Wullschleger and King
2000). Linking the radial pattern of sap flux density with biomass parameters could clarify the anatomical and functional
relationships among annual rings, whorls and shoots, and improve estimates of forest stand transpiration. At present, only
empirical correction factors, resulting from expensive multiple probes inserted at different sapwood depths, are used to extrapolate whole-tree sap flow from measurements at a single
depth (Zang et al. 1996, Köstner et al. 1996, Wullschleger and
King 2000, Lundblad et al. 2001, Delzon et al. 2004) or to
evaluate daily or seasonal radial sap flux modifications (Ford
et al. 2004a).
Based on the studies cited, we hypothesized that the radial
pattern of sap flux density is controlled by crown architecture
and, primarily, by the vertical distribution of leaf biomass. We
tested this hypothesis by monitoring changes in the radial pattern of sap flux density in Picea abies (L.) Karst. trees in response to progressive branch pruning from the crown bottom
up. Responses to foliage removal were quantified based on a
daily index of the pattern of radial sap flow (shape index, SI),
where relative variations indicate the most active portion of the
sapwood involved in sap flow.
Materials and methods
Site description and meteorological measurements
The experiment was performed in a 35-year-old Norway
spruce (P. abies) stand, located near the Center for Alpine
Ecology at an elevation of 1550 m (north-eastern Italian Alps,
Trento; 46°00′56″ N, 11°02′48″ E). The climate is characterized by a mean annual temperature of 5.5 °C and a mean annual precipitation of 1189 mm. The forest is dominated by
Norway spruce, with mean diameter at breast height (dbh) of
25 cm and a dominant height of 20 m at the time of the study.
A meteorological station located near the experimental site
measured ambient air temperature, relative humidity (Rotronic
MP103A), photosynthetically active radiation (PAR, Li-Cor
Li-190SA) and rainfall (Young 52202). Vapor pressure deficit
(VPD = esat – e) was computed from ambient vapor pressure
(e; kPa), derived from relative humidity, and saturation vapor
pressure (esat; kPa) calculated from air temperature.
Meteorological conditions were quite uniform during the
study (only data obtained during sunny and partly cloudy days
were included in the statistical analysis), with a mean PAR of
52.6 ± 10.6 mol m – 2 day –1 and a mean daytime VPD of 0.47 ±
0.17 kPa, (Figure 1). Rainfall was well distributed during the
study (Figure 1).
Foliage removal experiment
Two 35-year-old Norway spruce trees were chosen for study
from among well-developed canopy trees at the forest edge
and located on a river bank to: (1) facilitate access to the tree
crowns with the platform; (2) study the role of the lower part of
the crown on sap flux density; and (3) exclude the effects of
soil water limitation on radial sap flux density (Phillips et al.
Figure 1. Mean daily photosynthetically active radiation (PAR; 䊊),
vapor pressure deficit (VPD; 䊉) and total rainfall (bars) during the
pruning study (day of the year 156–209). Dashed lines identify the
layers of foliage removed during each pruning.
1996) and relative wood content (Domec et al. 2005). During
the summer of 2005, a foliage removal experiment was carried
out on one of the selected trees (hereafter EX), the canopy of
which was accessed by means of a truck-mounted platform.
Beginning 2 m above the ground, all branches in 4 × 2 m and
1 × 2.5 m canopy layers were harvested from the bottom up, at
about weekly intervals (see Figure 2). The other selected tree
served as the control (hereafter CO). Biometric data of the EX
and CO trees are shown in Table 1.
Biomass and sapwood measurements
To describe the distribution of biomass by age class and vertically at each step of the pruning experiment, we recorded fresh
mass, diameter and insertion height of the harvested branches
of the EX tree in the field. In addition, three branches from
each crown layer were transported to a laboratory and analyzed in detail. In the laboratory, branches were separated in
1-year sections for the last 5 year-classes and the masses of the
sections (including twigs and needles) before drying, and of
the needles and branches separately after oven-drying for at
least 48 h at 80 °C were recorded. The fractions of needle biomass in the branch biomass were used to estimate regression
models based on branch diameter and insertion height as predictors. Partitioning of needle biomass among age classes was
computed according to the mean percentages observed on the
Table 1. Biometric characteristics of the pruned and control trees. Abbreviations: EX = pruned tree; CO = control tree; and dbh = over-bark
diameter at breast height.
Tree characteristics
EX
CO
dbh (cm)
Sapwood radius (cm)
Tree height (m)
Tree age at height 1.5 m (years)
Crown length (m)
28
8.8
18.3
24
16
40
11
18
26
16.4
TREE PHYSIOLOGY VOLUME 28, 2008
PRUNING INDUCED VARIATION IN RADIAL SAP FLUX DENSITY
sample branches in each canopy layer. Remaining biomass on
the EX tree was estimated by regression models with diameter
and insertion height of the remaining branches as predictors.
The vertical distribution of biomass was calculated by summing the leaf mass of each canopy layer. Mean foliage height
(MFH) and mean foliage age (MFA) of the pruned tree were
determined at every step of the pruning process by weighting
foliage biomass by branch insertion height and needle age,
respectively.
Sapwood thickness was determined on the basis of xylem
water content (Kravka et al. 1999). At the beginning of the experiment, EX was cored in the morning with a Pressler’s borer
(Suunto, Finland) on the opposite side of the trunk to where
the sap flow probes were installed. The core was immediately
placed in a tube, which was quickly sealed and transported to
the laboratory. The core was sliced into yearly rings, which
were measured with a caliper to the nearest 0.01 mm and then
weighed to ± 0.001 g. The water content of each ring was determined after oven drying at 80 °C for 48 h.
Sap flux density measurements
–2
nored in the following analysis (Do and Rocheteau 2002a,
2002b).
Shape index and statistical analyses
Sap flux density was converted to sap flow (F; g s – 1 ) by
weighting each sap flux density measurement with the sapwood area adjacent to each probe (Hatton et al. 1990) and
adopting the zero-average technique (Pausch et al. 2000) for
sapwood depth without sensors. A shape index (SI) of the radial pattern of sap flow was calculated for the EX and CO trees
as described by Delzon et al. (2004), using measurements
from the outer sensor (0–10 mm) as a reference (Fiora and
Cescatti 2006). Sap flow ratios (Ri; Equation 1) (Delzon et al.
2004) at each depth, obtained from the ratio of sap flow of the
inner probe (Fi ) to sap flow of the reference probe (F0–10 ), were
multiplied by an area-weighted mean corresponding to the
cross-sectional sapwood area (Ai ) sampled at each position i,
divided by the entire cross-sectional conducting area (A):
Ri =
Fi
F0 −10
SI =
∑ Ri
–1
Sap flux density (g m s ) was measured by the heat dissipation method (Granier 1985) with sensors similar in design to
that originally proposed by Granier (1985, 1987), but the effective measuring element was reduced to 10 mm in length
(James et al. 2002, Irvine et al. 2002, 2004). Each handmade
sensor consisted of two cylindrical probes ranging in length
from 3 to 6.5 cm, with an outer diameter of 1.3 mm, and a copper-constantan (Cu-Cn) thermocouple surrounded by a constantan wire. Sensors were installed at five depths in the sapwood of the EX and CO trees: 0–10, 10–20, 20–30, 35–45
and 55–65 mm (hereafter the sensors are identified as EXi, and
COi, where subscript i indicates probe depth, from outer, 1, to
inner, 5).
Sensors at successive depths were installed about 1.5 m
above the ground (Köstner et al. 1996), 10 cm apart along the
circumference, on the sun-exposed side of the tree. Each
probe, coated in thermally conductive paste, was installed into
2.1-mm drilled holes, located 10 cm apart vertically, into
which 11-mm aluminum tubes (2 mm outside diameter) had
previously been pushed. All probes were sheltered from direct
sunlight and rainfall by aluminum foil applied over an insulating foam layer and attached to the tree stem with silicon adhesive. For each sensor, the upper probe was heated continuously
with a constant power of 0.14 W, and the lower probe was left
unheated to measure wood temperature. The system was powered by a 12-V power pack, and the current was adjusted to
0.120 A. Signals from the sap flow sensors were measured at a
1-min time-step and 15-min means were recorded on a data
logger (CR23X, Campbell Scientific, Logan, UT). The temperature difference between the probes (∆T ) was converted to
sap flux density as described by Granier (1985).
At the beginning of the experiment, measurements were
made without heating the upper probes to detect natural temperature gradients. Because the natural temperature gradients
were less than 0.15 °C for all sapwood depths they were ig-
1319
n
i =1
(1)
Ai
A
(2)
To assess the daily variation in radial sap flow profile,
15-min SI (SIi ) values between 0830 and 1900 h were calculated for sunny and partly cloudy days, because in conditions
of low evaporation, differences may not be easily identified
(Oren et al. 1999). The daily SI was obtained by weighting SIi
by the corresponding tree instantaneous sap flow. To remove
variability caused by seasonality or climate from this signal
(Fiora and Cescatti 2006), the index was normalized with SI
values obtained for the CO tree. The same procedure was applied to compare the effect of pruning on sap flow. Variation in
SI from the initial state indicated which portion of the sapwood was activated in response to pruning: decreasing SI indicates a higher sap flux density of the reference probe compared with the inner probes, and vice versa for increasing SI
(Fiora and Cescatti 2006).
We evaluated differences in the ratio between daily sap flow
of the control and pruned trees and differences in daily SI.
Modifications in the radial pattern of sap flow induced by foliage removal were evaluated from the ratio of the daily sap
flow of each probe (fi = vAi ; where v = sap flux density) to total
daily tree sap flow (F ) (i.e., the contribution of the single sapwood layer to total flux), and finally normalizing the daily ratio with the mean fi in the initial state (hereafter referred to as
normalized flux). Portions of the sapwood that were involved
in sap flow modifications in response to pruning were identified based on means of daily ratios between sap flow at
0–10 mm depth in CO and sap flows recorded at different depths in EX (hereafter abbreviated as sap flux CO1/
EX1 to 5).
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FIORA AND CESCATTI
Results and discussion
Foliage removal and variation in sap flow
The vertical distribution of foliage in EX peaked in the 9–11 m
layer, corresponding to whorls with a mean age of 10–13 years
(Figure 2). Each of the first two prunings removed about 6.5%
of total foliage biomass (of which 60% belonged to the most
recent 5 years of needle age classes), the third pruning removed 11.7% of total foliage biomass (more than 65% comprising the last 5 years of needle age classes), whereas the last
two prunings together removed more than 50% of total needles
biomass (64 and 78%, respectively, of the last 5 years of needle
age classes) (Figure 3). At the end of the experiment, EX had
about 20% of its initial foliage biomass, of which most belonged to the last 5 years of needle age classes (Figure 3).
Pruned branches were between 8 and 23 years old.
Pruning decreased daily sap flow in EX, as found in previous studies (Whitehead et al. 1996 on Pinus radiata D. Don;
Pataki et al. 1998 on Pinus taeda L.; Pepin et al. 2002 on Thuja
plicata Donn.). Daily sap flow in EX decreased linearly with
foliage removal, with a sharp decline after the first and last
prunings. The EX/CO sap flow ratio decreased after the last
three prunings (Figure 3), with sap flow declining linearly
with needle biomass removal rate. At the end of the study, sap
flow in EX had decreased by 65% compared with the initial
value (Figure 3).
Figure 3. Effects of successive pruning on mean sap flow ratios (ratio
of sap flow in pruned tree (EX) to sap flow in control tree (CO)). Error
bars represent standard deviations.
lem, but with a secondary peak at a depth of 20–30 mm beneath the cambium (Figure 4a).
Pruning affected the radial pattern of sap flux density. Interactions between the amount of foliage removed and probe
depth were significant, indicating a strong interaction between
Modification of the radial pattern of sap flux density
Mean radial sap flux density profiles of the pruned tree during
each phase of the pruning experiment are shown in Figure 4a.
Before pruning, the radial pattern of sap flux density in EX
was similar to that observed in previous studies of Norway
spruce (Swanson 1967a, 1967b, 1971, Mark and Crews 1973,
Èermák et al. 1992, Lundblad et al. 2001, Fiora and Cescatti
2006), with sap flux density decreasing with depth in the xy-
Figure 2. Vertical distribution of foliage biomass (䊊) and whorl age
(䊉) for the pruned tree (EX). Dashed lines identify the layers of foliage removed during each pruning.
Figure 4. Effects of successive pruning on (a) mean radial patterns of
sap flux density and (b) normalized sap flux (ratio of the daily sap
flow of each probe, fi, to the total daily tree sap flow normalized with
mean fi at the initial state). Symbols represent mean values and vertical bars represent + SE. Legends indicate mean shape index values for
each pruning step.
TREE PHYSIOLOGY VOLUME 28, 2008
PRUNING INDUCED VARIATION IN RADIAL SAP FLUX DENSITY
the shape of the radial sap flux density profile and the distribution of foliage in the crown (Figure 4b). Successive prunings
caused pronounced modifications of the radial pattern of sap
flux density (Table 2). Moreover, there were marked reductions in the sap flow contributions of EX4 and EX5 (Table 2,
Figure 4b); however, after the last two prunings, sap flow
showed a pronounced decline at all sapwood depths precluding assessment of which portion of the sapwood was mainly
affected by these prunings.
In response to progressive pruning, the SI decreased continuously from an initial mean value (± standard deviation) of
0.57 ± 0.02 to 0.46 ± 0.01 (Figure 5), indicating a progressive
shift of a large fraction of total sap flow toward the cambium
and a decline in sap flux density in the inner sapwood. During
the period corresponding to the first three prunings of EX, the
CO SI did not change significantly, whereas during the period
of the last two prunings of EX, CO showed a decrease in
SI. The declining trend in the ratio between SI EX and SI CO
can be ascribed exclusively to pruning (Figure 5), because the
effect of seasonality and climate on the radial pattern of sap
flux density is removed. Contrary to previous observations on
Norway spruce (Fiora and Cescatti 2006), we found no significant correlation between SI EX or SI CO and either PAR or
VPD during the control period before pruning (day of the year
1321
Figure 5. Mean shape index (SI) for a pruned tree (EX; 䊊) and a control tree (CO; 䊐), and the ratio of the two (SI EX/SI CO; 䉬) during
the pruning experiment. Vertical bars represent standard deviations.
150–171, data not shown).
Pruning resulted in significant variations in mean foliage
age (MFA) and mean foliage height (MFH) in the crowns of
EX trees. Because variations in MFA and MFH were closely
correlated (Figures 6a and 6d), it was not obvious which pa-
Table 2. Effects of pruning on mean values (± standard deviation) of daily ratios of sap flow between 0 and 10 mm depth in the control tree to sap
flow at different sapwood depths in the pruned tree (CO1/EX1 to 5).
Pruning
CO1/EX1
CO1/EX2
CO1/EX3
CO1/EX4
CO1/EX5
Unpruned
<4
<6
<8
< 10
< 12.5
1.36 ± 0.11
1.54 ± 0.10
1.36 ± 0.13
1.45 ± 0.06
1.65 ± 0.14
3.32 ± 0.43
1.73 ± 0.13
2.30 ± 0.22
1.99 ± 0.23
2.04 ± 0.05
2.62 ± 0.25
5.52 ± 0.59
1.76 ± 0.11
1.94 ± 0.10
1.86 ± 0.17
2.03 ± 0.11
2.50 ± 0.13
4.99 ± 0.76
2.32 ± 0.26
2.79 ± 0.16
3.16 ± 0.40
4.08 ± 0.36
5.34 ± 0.36
9.46 ± 0.77
4.99 ± 0.46
6.52 ± 0.27
6.45 ± 0.30
6.37 ± 0.40
8.53 ± 0.83
19.05 ± 3.76
Figure 6. Relationship between mean foliage age (MFA) and mean foliage height
(MFH) (symbol sizes are proportional to
(a) normalized SI values), (b) trend of normalized SI (SI NORM) with mean foliage
age and (c) with mean foliage height during the pruning experiment.
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1322
FIORA AND CESCATTI
rameter is controlling the radial pattern of sap flux density.
Therefore, we performed simple and multiple linear regression analyses between normalized SI and MFH and MFA
(Figures 6b–d). Mean foliage height alone explained 72.5%
of the variance in SI (Figure 6c), whereas MFA explained only
49.1% (Figure 6b). Together MFH and MFA explained 96.5%
of SI variance. Despite the strong correlation between MFA
and MFH, these results suggest that the vertical distribution of
foliage biomass has a stronger control on the radial pattern of
sap flux density than foliage age.
The importance of vertical branch distribution on the radial
pattern of sap flux density has been investigated by Dye et al.
(1991), who hypothesized that the innermost sapwood is the
primary conduit for supplying water to older branches. When
these branches become shaded and lose needles their demand
for water decreases, and sap flux density in the inner sapwood
declines accordingly. This hypothesis is supported by the results of our experiment: removal of lower branches induced a
marked decrease in SI, because of the reduced contribution of
the inner sapwood. According to this hypothesis, stems can be
divided into functional areas connecting particular roots to
crown portions (Larson et al. 1994), and sap flow in a given
growth ring should be determined largely by the rate of transpiration from needles attached to that ring (Domec et al.
2005). As a consequence, the Gaussian radial pattern should
be typical of isolated plants with long crowns, in sparse or discontinuous forests (Èermák et al. 1992, Anfodillo et al. 1993,
Nadezhdina et al. 2002), with inner peaks related to the most
active intermediate crown layer. In contrast, decreasing radial
profiles should be characteristic of even-aged stands (Köstner
et al. 1996, Delzon et al. 2004, Irvine et al. 2004), where transpiration is mainly sustained by young branches (and so outer
rings) given the shading of lower canopy layers. However, the
demonstration of this hypothesis is complicated by the occurrence of lateral flow in stems and branches (Brooks et al. 2003,
Schulte and Brooks 2003) and reestablishment of trace junctions between old needles and outer growth rings (Maton and
Gartner 2005), and by the natural diurnal variation in radiation
(Martin et al. 2001) and in profile patterns (Ford et al. 2004a,
Fiora and Cescatti 2006). Moreover, reductions in sap flow
during the experiment may hide inter- and intra-sensors differences (Oren et al. 1999). Further rigorous tests of this hypothesis are therefore required, ideally with smaller sensors installed in single sapwood rings to facilitate correlation between sap flux density and needle-age classes. In addition,
leaf-level gas exchange measurements could be used to investigate the transpiration rates of different needle-age classes before and after foliage removal and to couple processes occurring at different dimensional scales.
Acknowledgments
The authors are grateful to Mauro Cavagna for assistance during foliage removal experiments and processing foliage material, Giovanni
Manca for sensor installation and Mirco Rodeghiero and Federico
Salvagni for field support. We also thank the Forest Service of the Autonomous Province of Trento for giving permission to conduct the
pruning experiments.
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