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Harvard Forest Data Archive
Linking Xylem Diameter Variations with Sap Flow Measurements at Harvard Forest 2003-2006Related Publications
- Lead: Sanna Sevanto, N. Michele Holbrook
- Investigators: Michael Daley, Teis Mikkelsen, Eero Nikinmaa, J. Cory Pettijohn, Nathan Phillips, Anu Riikonen
- Contact: N. Michele Holbrook
- Start date: 2003
- End date: 2006
- Status: completed
- Location: Prospect Hill Tract (Harvard Forest)
- Latitude: +42.53 to +42.55
- Longitude: -72.20 to -72.17
- Elevation: 280 to 420 meter
- Taxa: Acer rubrum (red maple), Alnus glutinosa (black alder), Betula lenta (black birch), Fagus sylvatica (european beech), Quercus rubra (red oak), Tilia vulgaris (linden)
- Release date: 2009
- EML file: knb-lter-hfr.129.11
- DOI: digital object identifier
- EDI: data package
- DataONE: data package
- Related links:
- Study type: short-term measurement
- Research topic: physiological ecology, population dynamics and species interactions
- LTER core area: populations
- Keywords: hydraulic conductance, plant physiology, transpiration
Measurements of variation in the diameter of tree stems provide a rapid response, high resolution tool for detecting changes in water tension inside the xylem. Water movement inside the xylem is caused by changes in the water tension and theoretically, the sap flow rate should be directly proportional to the water tension gradient and, therefore, also linearly linked to the xylem diameter variations. The coefficient of proportionality describes the water conductivity and elasticity of the conducting tissue. Xylem diameter variation measurements could thus provide an alternative approach for estimating sap flow rates, but currently we lack means for calibration. On the other hand, xylem diameter variation measurements could also be used as a tool for studying xylem structure and function. If we knew both the water tension in the xylem and the sap flow rate, xylem conductivity and/or elasticity could be calculated from the slope of their relationship. In this study we measured diurnal xylem diameter variation simultaneously with sap flow rates (Granier-type thermal method) in six deciduous species (Acer rubrum L., Alnus glutinosa Miller, Betula lenta L., Fagus Sylvatica L. Quercus rubra L., and Tilia vulgaris L.) for 7-91 day periods during summers 2003, 2005 and 2006 and analyzed the relationship between these two measurements. We found that in all species xylem diameter variations and sap flow rate were linearly related in daily scale (daily average R 2 = 0.61-0.87) but there was a significant variation in the daily slopes of the linear regressions. The largest variance in the slopes, however, was found between species, which is encouraging for finding a species specific calibration method for measuring sap flow rates using xylem diameter variations. At a daily timescale, xylem diameter variation and sap flow rate were related to each other via a hysteresis loop. The slopes during the morning and afternoon did not differ statistically significantly from each other, indicating no overall change in the conductivity. Because of the variance in the daily slopes, we tested three different data averaging methods to obtain calibration coefficients. The performance of the averaging methods depended on the source of variance in the data set and none of them performed best for all species. The best estimates of instantaneous sap flow rates were also given by different averaging methods than the best estimates of total daily water use. Using the linear relationship of sap flow rate and xylem diameter variations we calculated the conductance and specific conductivity of the soil-xylem-atmosphere water pathway. The conductance were of the order of magnitude 10-5 kg s-1 MPa-1 for all species, which compares well with measured water fluxes from broadleaved forests. Interestingly, because of the large sap wood area the conductance of Betula was approximately 10 times larger than in other species.
Species and sites
Sap flow rates and xylem diameter variations were measured on two to three individuals of five deciduous species (Acer rubrum L., Alnus glutinosa Miller, Betula lenta L., Quercus rubra L and Tilia vulgaris L.) and one individual of Fagus sylvatica L. The alder trees were type A. glutinosa forma pyramidalis "Sakari". The measurements were carried out during summers 2003, 2005 and 2006 at the time of full leaves (Jun 1-Aug 30). Acer, Betula and Quercus were studied at Harvard Forest in central Massachusetts USA (42d32m N 72d10m W), Alnus and Tilia in Helsinki, Finland (60d10m N 24d56m E) and Fagus at Soroe measurement station, Denmark (55d29m N 11d38m E). Individuals of Acer, Betula, Fagus and Quercus were mature dominant trees (DBH 8-27 cm) in a hardwood forest, while Alnus and Tilia were juveniles growing in an urban park environment.
The microclimatic data for Harvard Forest was obtained from the nearby meteorological station 300-1,000 m south of the measurement sites. In Denmark the same information was attained from the Soroe micrometeorological station located next to the measurement tree (for detailed information see e.g. Pilegaard et al. 2003) and in Helsinki the photosynthetically active photon flux density (PPFD) and soil water content at 10 and 30 cm depths were measured using Delta-T QS2 type silicon cell PAR quantum sensor and Delta-T ML2x Theta-probes, respectively, in the vicinity of each tree. The weather conditions at these sites during the measurement periods were comparable. The mean daily PPFD was 411 and 412 μmol m-2s-1 at Harvard Forest and Helsinki and 238 μmol m-2s-1 in Denmark (note the long measurement period). The percentage of rainy days during the measurement periods was 31% and 23% at Harvard Forest and 36% and 27% in Helsinki and Denmark, respectively. The amount of precipitation at Harvard Forest, 135 and 80 mm for the two measurement periods, was much higher than 4.9 mm in Helsinki. However, the annual precipitation is also much higher at Harvard Forest (2,790 mm) than in Helsinki (700 mm, 30-year average) and the amount of precipitation in both locations was typical of that time of the year. On the other hand, summer 2003 was exceptionally dry in Europe and there was a clear drought period in Soroe in July, but neither the daily slopes nor the daily degree of explanation of the linear model was a function of radiation forcing (PPFD) or soil water content (data not shown). This indicates that the trees did not suffer from drought (see Sevanto et al. 2005) and our results are representative of wellwatered summer conditions at all sites.
Sap flow measurements
The sap flow rates were measured using Granier-type heat dissipation method (Granier 1985). In this method two thermocouples are mounted on thin needles, one on each. The needles are inserted in the stem above each other about 10 cm apart. The upper sensor is heated with constant power and the sap flow velocity is calculated from the temperature difference between the two needles. The maximum temperature difference occurs when sap flow rate is zero (see Granier 1985).
We measured sap flow rates with two sensors on opposite sides of each stem (primary direction north-south). To minimize the effects of natural temperature gradients, the stems were covered by aluminum foil shades about 30 cm length above and below the sensors. The sap flow rate per sap wood area was calculated as an average of these two and the sapwood area was estimated from core samples taken close to the sensors. The Clearwater-correction (see Clearwater et al. 1999) for needle length exceeding sapwood depth was applied to the data from ringporous Quercus. For Fagus we used two sensors at different depths (0-2 and 2-4 cm) and the sap flux (sap flow rate per sapwood area) was calculated from an average of the readings at the two depths distributed evenly through the 4 cm depth. The zero flow temperature difference was estimated from the average of maximum predawn temperature difference of three to five nights in the beginning of the measurement period. The measurements were carried out well below the first living branch. For Acer, Betula and Quercus the measurement height was at about breast height (1.3 m), for Alnus and Tilia 0.3 m and for Fagus 3 m. The measurement interval for Acer and Quercus was 4 min, for Betula 10 min and for Alnus, Fagus and Tilia 30 min.
Xylem diameter variation measurements Xylem diameter variations were measured about 10-30 cm below the sap flow sensors using LVDTs (Solartron AX/5.0/S; Solartron Inc., West Sussex, UK) attached to a rectangular metal frame mounted around the stem. The frame was attached using metal plates screwed to the stem some 15 cm above the sensors. To eliminate the effects of the living tissue outside the xylem on the diameter variation we inserted two screws through the bark, phloem and cambium so, that they contacted the outer xylem on opposite sides of the stem (see e.g. Sevanto et al. 2005b). The sensor tip rested on one of the screws and the opposite side of the frame on the other enabling us to measure the variation in the distance between the two screws (i.e. the diameter of the stem). Xylem diameter variations were detected one per minute and averaged over 4-30 min periods to adjust to the sap flow sampling rate of the tree. The temperature of each frame was measured using copper-constantan thermocouples and the data was corrected for the thermal expansion of the frame and wood as described by Sevanto et al. 2005b. Xylem diameter variation is a measure of the pressure field affecting the whole xylem tissue at the measurement level. We tested the homogeneity of the field independently by inserting a second sensor perpendicular to the primary one on three maple, two oak and two birch trees. To estimate the possible effects of wood density on differences seen in the pressure field we took core samples from both the sensor side and the opposite side and measured the dry and fresh density of sap wood (dry weight/ fresh volume and fresh weight/fresh volume, respectively). The samples were dried in 60 deg C in an oven for 48 h.
The first data file (hf129-01) contains measurements from three black birch trees at 1.3 m height. Data are corrected for thermal expansion. Tree diameters: black birch 1 = 21.5 cm, black birch 2 = 22.0 cm, black birch 3 = 29.0 cm.
The second data file (hf129-02) contains measurements from 1 maple and 2 oak trees. Tree diameters: maple = 12.7 cm, oak 1 = 18.3 cm, oak 2 = 20.3 cm.
This dataset is released to the public under Creative Commons license CC BY (Attribution). Please keep the designated contact person informed of any plans to use the dataset. Consultation or collaboration with the original investigators is strongly encouraged. Publications and data products that make use of the dataset must include proper acknowledgement.
Sevanto S, Holbrook N. 2009. Linking Xylem Diameter Variations with Sap Flow Measurements at Harvard Forest 2003-2006. Harvard Forest Data Archive: HF129.
- datetime: date and time
- year: year
- doy: day of year (unit: nominalDay / missing value: NA)
- birch1.xylem: xylem diameter variation of birch 1 (unit: millimeter / missing value: NA)
- birch1.stem: whole stem diameter variation of birch 1 (whole stem means that it was measured on the bark) (unit: millimeter / missing value: NA)
- birch2.xylem: xylem diameter variations of birch 2 (unit: millimeter / missing value: NA)
- birch2.stem: whole stem diameter variations of birch 2 (unit: millimeter / missing value: NA)
- birch3.xylem: xylem duameter variations of birch 3 (unit: millimeter / missing value: NA)
- birch3.stem: whole stem diameter variations of birch 3 (unit: millimeter / missing value: NA)
hf129-02: maple and oak
- datetime: date and time
- year: year
- doy: day of year (unit: nominalDay / missing value: NA)
- maple.xylem: xylem diameter variation of the maple (unit: millimeter / missing value: NA)
- maple.stem: whole stem diameter variation of the maple (unit: millimeter / missing value: NA)
- oak1.xylem: xylem diameter variation of oak 1 (unit: millimeter / missing value: NA)
- oak1.stem: whole stem diameter variation of oak 1 (unit: millimeter / missing value: NA)
- oak2.xylem: xylem diameter variation of oak 2 (unit: millimeter / missing value: NA)
- oak2.stem: whole stem diameter variation of oak 2 (unit: millimeter / missing value: NA)