2-1 Sap flow measurement
Thermal dissipation method was adopted in this study for field long-term sap flow measurement and for indoor calibration experiment on Japanese cedar (Cryptomeria japonica). The thermal dissipation sensors used in this study were handmade which has
the same specification with the sensor in Granier (1987). One thermal dissipation sensor set consists of two probes which have copper-constantan thermocouple junction each other to sensor temperature; one is named heater probe that contains a heating and a temperature-sensing device, and the other one is called referenced probe that contains only a temperature-sensing device. These two probes are about 2 cm long. Before inserting sensors, the bark of the position which sensor will be inserted should be removed to reveal sapwood, and to ensure that the temperature which measured by sensor represented the average temperature of the 2cm long probe in wood. These two probes are inserted into xylem of trees, which one in upper side is heater probe and that in lower side is referenced probe (Fig. 1). Heater probes are provided 0.2W constant heating energy from electricity. Through the temperature difference between the two probes, sap
flow density can be calculated by the empirical formula from Granier (1987). The Granier’s empirical formula was used to calculate the sap flow density (cm3cm-2s-1):
Sap flow density (cm3m−2s−1)= 119 × K1.231 (1)
K =∆TM− ∆T
∆T (2)
where K is the dimensionless flow index; ∆𝑇𝑀 is the value ∆𝑇 obtained under zero sap flow condition; ∆𝑇 is the difference of temperature between heater and reference probe.
Small temperature difference represents that sap flows quickly in xylem, and vice versa.
Thermal dissipation sensors were connected to data logger which recorded every 30 seconds and calculated mean temperature difference every 30 minutes from the field data.
For calibration experiment, data logger recorded every 1 second and calculated mean temperature difference every 10 seconds. Because the sap flow density calculation formula of the thermal dissipation method required the temperature difference under zero sap flow condition, for field data, ∆𝑇𝑀 was different every day depending on the highest
∆𝑇 in each day. For calibration experiment, after installing sensors, started providing
heat and recorded temperature difference without water uptake till the difference of temperature became stable, and took the highest ∆𝑇 as the ∆𝑇𝑀 for each segment.
If the sapwood depth was less than 2 cm, a correction formula from Clearwater would be applied to recalculate sap flow density to reduce the effective of inactive xylem.
Because thermal dissipation method assumed that probes integrate temperature and sap
flow density along the probe length (2cm) (Lu et al., 2004), the ideal case is when
sapwood depth equal to the length of the probes. But, when sapwood depth was smaller
than 2 cm, the sap flow density which sensor measured always underestimated true mean
sap flow density. It is because that when some part of the probe is in inactive sapwood,
the temperature integrated in the probe will be higher, and then the difference of
temperature becomes higher to make the sap flow density underestimated. Therefore,
Clearwater et al. (1999) provided a method to calculate sap flow density that just in active
xylem to deal with this problem of underestimation when active sapwood depth is below
2 cm:
∆T = a∆TSW + b∆TM ,
∆TSW = ∆T − b∆TM
a (3)
This formula considered that the temperature which probe measured contains two parts,
one is the temperature measured in sapwood called ∆TSW, and the other is the
temperature measured in inactive xylem called ∆TM. While a is the proportion of probe
in sapwood and b is the proportion of probe in the inactive xylem. Under the condition
that sapwood depth is known and lower than 2 cm, corrected sap flow density for the
active xylem can be calculated by replacing ∆𝑇 in formula 2 with ∆𝑇𝑆𝑊 in formula 3.
However, because 𝑎 and 𝑏 are the proportions of sapwood and inactive xylem
respectively, under the condition that the value of 𝑏 higher than 0.5 (i.e. the width of
inactive xylem is higher than 1.0 cm), the value of ∆𝑇 may be lower than the value of 𝑏∆𝑇𝑀 or just a little bit higher than it. Therefore, the value of ∆𝑇𝑆𝑊 may be lower than 0 or may be higher than 0 but close to 0. If the value of ∆𝑇𝑆𝑊 is lower than 0, the value
of 𝐾 will also be lower than 0, so that sap flow density cannot be calculated. On the
other hand, if the value of ∆𝑇𝑆𝑊 is higher than 0 and close to 0, the value of 𝐾 will be
very high, so that sap flow density will be extremely high which is distinct from that in
usual. Consequently, to prevent the abnormal phenomenon, since the value of 𝑏 is higher
than 0.5, the correct formula 3 from Clearwater et al. (1999) will not be adopted in this
study.
Fig. 1 Diagram of Granier thermal dissipation method.
2-2 Calibration experiment
2-2-1 Samples and tree segment preparation
Six sample trees of Japanese cedar (Cryptomeria japonica) were harvested from Xitou tract, experimental forest of National Taiwan university. The stem diameter at breast high of these six samples were ranged from 15 to 20 cm. Twelve stem segments were cut from these six sample trees. Three sample trees were cut on 6 March 2016, and the other three trees were cut on 27 August 2016. In the first three sample trees, one segment was cut out from each tree (i.e., total three segments). In the last three sample trees, three segments were cut out from each tree (i.e., total nine segments). The height of segments should be lower than under branch height, and the north side and upside of the segment were recorded on each segment in the field. The tree height and diameter at breast height of each tree were measured and recorded. Also, the height of segments was recorded for the last three trees (9 segments). Segments were about 50 to 60 cm long and were covered with wet towels in two sides to avoid stem dehydration. Finally, segments were brought to laboratory to do experiments.
Before doing calibration experiment, following sample preparation must be carefully done as it had greatly impact on the success of the experiment. Each stem segment was recut in both sides in laboratory before doing experiments to assure conductivity of the segments. The bark on the up side of segments was removed about 3-5 cm strip to make
sure that water passed segments only through xylem; also, removing bark can make attachment much easier to stick on the segment because of its smooth surface. Then, adhesive and silicone were applied to the inside of the attachment (a plastic cylinder with a cover) and the position which bark was removed from the segment. The attachment was putted on the tree segment and adjusted to fit the tree segment to tightly bounded avoiding leaking water or air. After the adhesive and silicone were dry, the segment was prepared to do a calibration experiment (Fig. 2).
Fig. 2 Photo of tree segment preparation.
2-2-2 Process of calibration experiment
The calibration experiment (Fig. 3, Fig. 4) using twelve Japanese cedar stem segments were conducted as follow:
1) Scaffold (75cm high) were set up for hanging the tree segments.
2) A flask was equipped with two plastic tubes, which one was connected to pump and the other one was connected to attachment that stick on tree segment.
3) Tree segment was hung on the scaffold; a bucket with water was placed under a tree segment and the bottom side of the segment (about 4-5 cm) was under water.
4) Thermal dissipation probes were inserted into the tree segments to collected data.
5) By adjusting the pump pressure, water was pumped from bottom to top; also, constant water flow rates at different level were generated to test the accuracy of thermal dissipation methods-based sap flow under different sap flow rates.
6) A volumetric cylinder which was equipped with one plastic tube that connected to the bucket under tree segment was set up. Based on the principle of Pascal, the volume of water which took out from tree segment could be measured from the cylinder.
7) After the sap flow rate became stable under each pump pressure, the volume of water took out from tree segment was recorded every 1 minute in a 10 minutes period or every 30 seconds in a 5 minute period. Then the measurements with the values derived from Granier probe were compared with it during the period.
8) Safranin stain solution was used to dye the tree segment to get the active sapwood area.
After dying, tree segment about 1cm, 5cm and 10cm far from the bottom side were cut and photos were taken. Sapwood area and sapwood depth data were got from image processing using the photo which was about 1cm far from bottom side of each segment.
Fig. 3 Diagram of calibration experiment construction used to test the accuracy of thermal dissipation methods-based sap flow.
Fig. 4 Photo of calibration experiment construction used to test the accuracy of thermal dissipation methods-based sap flow.
2-2-3 Sap flow measurement-sensor arrangement
In the calibration experiment, four sets of thermal dissipation sensors were inserted into four directions (north, east, south and west) for each stem segment. This was in order to prevent that the position where sensor inserted was a nod or without sap flowing, and
to get more representative data of thermal dissipation sap flow for each segment. Finally, the sap flow rate calculated by Granier’s empirical formula (formula 1 and 2) in four
directions were averaged, and this averaged sap flow density represented the value that measured by Granier sensor for each segment, which was compared with sap flow density calculated by real water uptake.
As the calibration experiment compared sap flow densities that calculated from thermal dissipation sensor and from real water uptake, there were two kinds of sap flow densities were calculated. One was measured from thermal dissipation sensor and the
other one was measured from the volumetric cylinder measurement system. For thermal dissipation sensor, the Granier’s empirical formula (formula 1 and 2) was used to
calculate sap flow density (cm3cm-2s-1). If the sapwood depth was less than 2 cm, a correction formula from Clearwater (formula 3) would be applied to recalculate. For volumetric cylinder measurement, the sap flow density (cm3m-2s-1) was calculated by the formula:
Sap flow density (cm3m−2s−1) = V
A × T (4)
where V is the volume (cm3) of water uptake in the measurement period; A is the sapwood area (m2) of the segment; and T is the time (s) of the measurement period.
2-2-4 Determining of sapwood area and sapwood depth
In order to determine active xylem area of each segment, 0.1 % safranin stain solution was used to dye each segment after calibration experiment for about 1 hour. After dying tree segments, about 1 cm long at the bottom side of segment was cut as a disk, in which appearance of active xylem can be identified. Then a photo was taken for image processing to get sapwood area and sapwood depth in four azimuths. Sapwood area and depth were calculated by image analysis software Image J and GNU Image Manipulation Program (GIMP). Sapwood depth for each azimuth was averaged from five point measurements. One point was at the azimuth, two points were 0.5 cm apart from the azimuth, and two points were 1.0 cm apart from the azimuth.
2-3 Long-term measurement of sap flow and meteorological factors
2-3-1 Experiment site and samples
A long-term sap flow measurements plot is located at Xitou, which is situated in Nantou in central Taiwan. The area of our plot is 20*20 m (400 m2). In Xitou, the average annual temperature is about 16.6 ⁰C, and the average annual rainfall is about 2,600 mm (Wey et al., 2011). The monthly precipitation and average temperature from Sep. 2010 to Mar. 2017 were shown in Figure 5, and air temperature and rainfall were high in summer and low in winter.
In our plot, sap flow measurements for Japanese cedar (Cryptomeria japonica) trees have been conducted since August 2010. In this study, the period of sap flow data used to estimate stand transpiration was from 1 September 2010 to 31 March 2017, totally 6
Fig. 5 Monthly precipitation and temperature from Sep. 2010 to Mar. 2017.
years plus 7 months. In this long-term period, the number of sample trees with sensors and the number of trees in the plot were not constant. In the period from August 2010 to 23 March 2012, sample size for sap flow measurements was 19 trees. While in the period 24 March 2012 to 31 March 2017, the sample size was 15 trees. On the other hand, for the number of trees in the plot, from August 2010 to 31 December 2014, there were total 25 trees; from 1 January 2015 to 31 June 2016, there were 24 trees; and from 1 July 2016 to 31 March 2017, there were 23 trees. The more details was shown in Tseng (2011).
2-3-2 Sapwood depth and sapwood area measurement
The sapwood depth of trees at breast height on east and west sides were measured in June 2010 by using increment borer and then determined sapwood depth visually. In August 2016, the sapwood depth of trees at breast height on east and west sides were measured again.
However, to improve the accuracy of stand transpiration estimation, the sapwood depth for each tree should be measured more carefully. To obtain sapwood depth accurately, dye injection method could be used (Gebauer et al., 2008; Meinzer et al., 2001). Therefore, the dye injection method was adopted in this study to measure sapwood depth in 17 trees covering sap flow measurement samples. The dye injection experiment in this study was conducted as follow (Fig. 6). First, a cup was fastened on tree stem and
filled with water. Second, a hole was drilled which diameter was 8 mm into tree xylem under water. Third, water was replaced with 0.1 % safranin stain solution. Finally, about 3-5 hours later, increment borer was used to get increment core above the hole about 1 to 2 cm, and then sapwood depth which the portion had been dyed was measured; also, the sapwood depth visually determined was recorded. Sapwood depth measured by dye experiment was compared with that determined by eye, and the relation between these two was established in this study. This relationship was applied to sapwood depth which was determined by eye in June 2010, to convert to the more accurate sapwood depth based on dye experiment. Sapwood area estimation was performed based on the accurate sapwood depth:
Sapwood area (m2) = π[(DBH 2 )
2
− (DBH
2 − sapwood depth)
2
] (5)
Fig. 6 Picture of dye injection experiment.
2-3-3 Biometric parameters measurement
In order to understand the basic information of all trees in the plot, tree height and diameter in the breast height were measured in June 2010. For long-term tree growth measurements, in August 2016, the increment cores of trees at breast height on east and west sides were obtained by using an increment borer. The width of growth rings of each increment core in each year for 7 years was measured under a magnifier. Then the width of growth rings in east and west side were averaged, and the double of the values represented the diameters of tree growth in each year.
2-3-4 Meteorological factors
Several meteorological factors were used in this study, which were provided from the Experimental Forest of National Taiwan University. Environment data contain solar radiation, relative humidity, air temperature, precipitation and soil water content (weighted averaged from 5 cm, 20cm and 50 cm depth under soil), which have been continuously measured by Xitou flux tower and Xitou agricultural weather station. Lack of air temperature data were filled with other data which use the relationship between these two places data. The period of data was from 1 September 2010 to 31 March 2017.
Vapor pressure deficit was calculated from air temperature and relative humidity.
2-3-5 Sap flow measurement-sensor arrangement
From 10 August 2010 to 23 March 2012, there were 19 trees that inserted sensors.
In these 19 trees, there were three kinds of sensor arrangement. One is that 5 sensors were inserted in each tree, another one is that 4 sensors were inserted in each tree, and the other one is that 2 sensors were inserted in each tree. For the first kind, there were totally 6 trees. Among these 5 sets of sensors, 4 sets were inserted into four directions (north, east, south and west) and in the deep of 0-2 cm, while 1 set was inserted into the deep of 2-4 cm. For the second kind, there were 2 trees only. Four sensors were inserted in four directions and were in the deep of 0-2 cm. For the third kind, there were 11 trees. Two sensors were inserted in the directions of east and west with the deep of 0-2 cm. The more detail information was shown in Tseng (2011).
From 24 March 2012 to 31 March 2017, there were 15 trees for sap flow measurements. Two sets of sensors were inserted into tree xylem in the directions of north and south at the depth of 0-2cm in the 15 trees.
For stand scale transpiration estimation in this study, data of 15 trees with two sets of sensors from Sep 2010 to Mar 2017 were used. For radial variation examination, data of 2 trees with five sets of sensors from 10 Aug 2010 to 10 Feb 2011 were used.
2-4 Data processing for long-term stand scale sap flow
2-4-1 Estimation of stand scale transpiration
Transpiration from one tree can be scaled up to stand scale (Clausnitzer et al., 2011;
Chiu et al., 2016; Kume et al., 2010; Oishi et al., 2008; Shinohara et al., 2013). The
number of sample trees with sap flow measurements; As_tree is the sapwood area of each
tree; x is the total number of trees in the plot. Fd0−2 is sap flow density in 0-2cm;
Fd2−4 is sap flow density in 2-4cm; As0−2 is sapwood area in 0-2cm; As2−4 is sapwood area in 2-4cm.
The data (0-2cm and 2-4cm sap flow density) in the period (from 8/10/2010 to
2/10/2012) were used to calculate the ratio between 0-2cm and 2-4cm, and then this ratio
was used to estimate 2-4cm sap flow density from 0-2cm sap flow density for all data.
2-4-2 Effect of sapwood depth and growth on stand transpiration estimates
In order to know the change of sapwood depth during about 6 years, in August 2016, the sapwood depth of trees at breast height on east and west sides were measured again, and sapwood depths were determined by eye. Besides, after conducting dye injection experiment, sapwood depth determined by dye were compared with that determined by eye, and the regression line between them was used to correct all sapwood depth determined by eye in 2010. Also, the radial variation was examined by comparing sap flow rate in 0-2 cm and 2-4 cm with the corrected sapwood depth and Clearwater formula (formula 3); the ratio of sap flow rate between 0-2 cm and 2-4 cm was calculated.
In order to estimate long-term stand transpiration, this study examined effect of tree growth on stand transpiration. To investigate tree growth, the measurement of growth ring width was conducted in August 2016 using increment cores, which were derived from east and west side of the individuals. The DBH change for each year was double of growth ring width (i.e. average of east and west), so DBH in each year can be calculated by accumulating DBH change to DBH which was measured in 2010. Then, sapwood area for each year was estimated according to the formula 5.
To identify the effect of sapwood depth and tree growth to stand transpiration estimation, sap flow data in 2015 was used. Here, three kinds of estimations were tested, one was “original”, another was “consider new sapwood depth” and the other was
“consider tree growth”. The “original” one did not consider corrected sapwood depth and
“consider tree growth”. The “original” one did not consider corrected sapwood depth and