BOREAS TE-07 Sap Flow Data Summary The BOREAS TE-07 team collected data sets in support of its efforts to characterize and interpret information on the sap flow of boreal vegetation. The heat pulse method was used to monitor sap flow and to estimate rates of transpiration from aspen, black spruce, and mixed wood forests at the SSA-OA, MIX, SSA-OBS, and Batoche sites in Saskatchewan, Canada. Measurements were made at the various sites from May to Oct 1994, May to Oct 1995, and Apr to Oct 1996. A scaling procedure was used to estimate canopy transpiration rates from the sap flow measurements. The data were stored in tabular ASCII files. Analyses to date show a tendency for sap flow in aspen to remain remarkably constant over a wide range of environmental conditions VPD from 1.0 to 4.8 kPa and solar radiation >400 W/m2). For forests with high aerodynamic conductance, the results would indicate an inverse relationship between stomatal conductance and VPD, for VPD >1 kPa. A possible interpretation is that stomata are operating to maintain leaf water potentials above a critical minimum value, which in turn places a maximum value on the rate of sap flow that can be sustained by the tree. Table of Contents 1 Data Set Overview 2 Investigator(s) 3 Theory of Measurements 4 Equipment 5 Data Acquisition Methods 6 Observations 7 Data Description 8 Data Organization 9 Data Manipulations 10 Errors 11 Notes 12 Application of the Data Set 13 Future Modifications and Plans 14 Software 15 Data Access 16 Output Products and Availability 17 References 18 Glossary of Terms 19 List of Acronyms 20 Document Information 1. Data Set Overview 1.1 Data Set Identification BOREAS TE-07 Sap Flow Data 1.2 Data Set Introduction Sap flow and transpiration of trees were measured in the southern boreal forest and aspen parkland of western Canada as part of the BOReal Ecosystem-Atmosphere Study (BOREAS) during the years of 1994, 1995, and 1996. These data sets contain data on the rate of flow of sap (water) in tree stems during the growing season (beginning April to June and ending in October). Also included are a limited number of environmental measurements (air temperature and relative humidity, and in some instances, solar radiation and wind speed). Measurements were made every 3 hours in 1994 and hourly in 1995-96. 1.3 Objective/Purpose The objectives of this research were to: 1) Examine how forest transpiration at different time scales (diurnal and seasonal) is affected by environmental conditions, including vapor pressure deficit (VPD), solar radiation, temperature, wind, and soil moisture. 2) Use sap flow measurements, coupled with environmental measurements, as a means of examining changes in canopy (stomatal) conductance in aspen and spruce. 3) Compare transpiration responses of aspen under two contrasting climates (boreal site at Southern Study Area (SSA) Old Aspen (SSA-OA), drought-prone aspen parkland site at Batoche, located 100 km south of SSA-OA. 4). Ultimately, through collaboration with other BOREAS investigators, determine what is controlling rates of forest transpiration at different time scales. 5) On a long-term scale, use this information as part of the foundation for the development of carbon-based, daily time-step models of forest growth responses to changes in climate. 1.4 Summary of Parameters Air temperature Relative humidity Sap flux density (cm3 per cm2 of sapwood area per hour) Soil moisture (10 and 30 cm depths, Batoche site only) Vapor pressure 1.5 Discussion In the western Canadian interior, regional climatic moisture gradients appear to be closely linked to forest distribution and to hydrological differences in ecosystem functioning (Hogg, 1994, 1997). Thus, an understanding of the factors controlling forest transpiration is especially important in this region, and is critical to predicting how the productivity and hydrology of the boreal forest would respond to a warmer and drier future climate. One of the major processes being studied in the BOREAS project is water vapor flux from different vegetation types and its control by environmental and ecophysiological factors (Sellers et al,. 1995). A major component of the research within BOREAS has been to monitor xylem sap flux (upward flow of water) within individual trees during the growing season. The attempt is then made to "scale up" these results to give transpiration estimates at the stand level, using measurements and estimates of stand and sapwood characteristics. The accuracy of the scaling procedure can then be assessed through comparisons with eddy correlation measurements made above and below the tree canopy. The research described here was conducted by two members of the Canadian Forest Service (CFS), Edmonton, who are members of the Terrestrial Ecology (TE) group, team TE-07, within BOREAS. This work was conducted as part of the CFS Climate Change program, and is also a contribution to the CFS Boreal Forest Transect Case Study (BFTCS; see Price and Apps, 1995). The heat pulse method was used to monitor sap flow continuously during the growing season from May to October (for details, see Section 3.0, Theory of Measurements). All measurements were made in the SSA. At the OA site, sap flow was measured on six aspen (Populus tremuloides) trees near the flux tower in 1996 and near the TE tower in 1994. At the Batoche site, 9 to 12 aspen trees were measured during 1994-96. At the Mixed wood site (MIX), four aspen, two white spruce (Picea glauca), and two black spruce (Picea mariana) were measured in 1994. At the Old Black Spruce SSA-OBS site, six black spruce were measured in 1996. Sap flow was measured every 3 hours at all sites in 1994, and hourly in 1995 and 1996. Hourly meteorological data were obtained from the 18-m tower and at each of four sap flow stations at Batoche. Limited environmental measurements were made from sap flow stations at BOREAS sites. In the data files, the column called MEASUREMENT_DESCR gives a coded description of the measurements taken. Each measurement taken is denoted by a single letter of code. The description code is: “a” to “h” means that trees called “a” to “h” gave good data. “N” denotes trees receiving new probes. “x” denotes missing or bad data taken from that tree. 1.6 Related Data Sets BOREAS TE-11 Sap Flow Data BOREAS TE-11 Leaf Gas Exchange Measurements BOREAS RSS-17 Xylem Flux Density Measurements at the SSA-OBS Site 2. Investigator(s) 2.1 Investigator(s) Name and Title E.H. (Ted) Hogg, Research Scientist Climate Change Network, Canadian Forest Service 2.2 Title of Investigation Climate Change Effects on Net Primary Productivity of Productivity of Aspen and Jack Pine at the Southern Limit of the Boreal Forest. 2.3 Contact Information Contact 1: E.H. (Ted) Hogg Northern Forestry Center, Canadian Forest Service Edmonton, Alberta (403) 435-7225 (403) 435-7359 (fax) thogg@nofc.forestry.ca or Contact 2: P.A. (Rick) Hurdle Northern Forestry Center, Canadian Forest Service Edmonton, Alberta (403) 435-7263 (403) 435-7359 (fax) rhurdle@nofc.forestry.ca Contact 3: Andrea Papagno Raytheon STX Corporation NASA GSFC Greenbelt, MD (301) 286-3134 (301) 286-0239 (fax) apapagno@pop900.gsfc.nasa.gov Contact 4: Shelaine Curd Raytheon STX Corporation NASA GSFC Greenbelt, MD (301) 286-2447 (301) 286-0239 (fax) shelaine.curd@gsfc.nasa.gov 3. Theory of Measurements Sap (water) flow was measured by the heat pulse method (Marshall, 1958; Swanson, 1983, 1994; Hogg et al., 1997), using two thermocouples spaced symmetrically 0.75 cm above and below a heater 5-cm long with a power output of 2 W/cm. All probe elements had a diameter of 1.6 mm, and holes of the same diameter were drilled using a steel template. Thermocouple depth in the sapwood was 1.5 cm for large (>15 cm diameter) trees and 1.0 cm for smaller trees. At the SSA-OBS site, a depth of 7.5 mm was used because of narrow (4 to 12 mm) sapwood under a bark width of about 3-4 mm. Probes were located at a height of approximately 1.3 m on the north side of the tree and were thermally insulated by wrapping with them white polyethylene packing material. At each station, a datalogger (Campbell Scientific, Model 21X, powered by batteries and a solar panel) was used to record the temperature increase at the upper (T_u) and lower (T_l) thermocouples, 60 s following a 4 s heat pulse. On two dates in 1994 (boreal site, Julian Day of Year (DOY) 192 and 209; parkland site, DOY 190 and 205), one or two probes per station were removed and replaced with new probes at another location on the same tree (all probes were replaced once). In 1995 and 1996, probes were replaced only as needed, i.e., following heater failure or abnormal wounding. 4. Equipment 4.1 Sensor/Instrument Description The sap flow instrumentation was designed and constructed by P.A. Hurdle at the CFS. The instrumentation has evolved from earlier implementations used by P.A. Hurdle and R.H. Swanson, former research scientist at CFS (retired 1992). The system operates using Campbell Scientific dataloggers (Model 21x). Measurements of temperature and relative humidity were made using shielded Model HMP35C Vaisala probes. Volumetric water content of the soil was measured using Barber-Colman fibreglass sensors that had been precalibrated in the laboratory using soil samples collected from the site (Batoche only). 4.1.1 Collection Environment Measurements were made with ambient environmental conditions of forest understories, from April or May to October. Temperatures ranged from -10 to 35?C. Occasional damage to sap flow probes was caused by bears, deer, and other wildlife, which usually resulted in complete malfunction of probes in affected trees. Lightning and rain events did not adversely affect the operation of the instruments. 4.1.2 Source/Platform Probes were mounted at a height of 1 to 1.5 m on the north side of trees. The datalogger was in a weatherproof box, anchored either to a wooden fence or to an adjacent tree. Environmental measurements were made either from a tripod or from a horizontally mounted aluminum boom extending from a fencepost or pole. 4.1.3 Source/Platform Mission Objectives The platform objective was to provide a place from which accurate and consistant measurements could be made. 4.1.4 Key Variables sap flux density (cm3 per cm2 of sapwood area per hour) sapwood area to ground area ratio For analysis (including collaborations with other investigators): hourly in-canopy (derived from air temperature and relative humidity) hourly solar radiation daily precipitation and soil moisture leaf area index aerodynamic conductance (to assess degree to which VPD versus net radiation is controlling transpiration using the Penman-Monteith approach) 4.1.5 Principles of Operation A probe placed in the sapwood released a pulse of heat. One minute later, the increase in sapwood temperature 7.5 mm above and below the heater was measured using thermocouples. If no sap (water) is flowing, the temperature increase is symmetrical; as sap flow increases, more heat is transported to the upper heater and less heat reaches the lower heater. Sap flow is proportional to the natural logarithm of the ratio between the temperature increase of the upper thermocouple and that of the lower thermocouple. The calculation also required estimates of sapwood bulk density and moisture content, and a correction factor needed to be applied to account for interruption of the sap stream by probes. 4.1.6 Sensor/Instrument Measurement Geometry The heater provided a linear heat source across the full sapwood depth. The thermocouples were located at an appropriate sapwood depth (0.75 to 1.5 cm, depending on sapwood thickness). 4.1.7 Manufacturer of Sensor/Instrument Barber-Colman Fibreglass Sensors Barber-Colman Company 103 High St. P.O. Box 1067 Sutton West ON L0E 1RO Canada 905-722-9881 905-722-4260 (fax) bc@ils.com Campbell Scientific Dataloggers Model CR21x Campbell Scientific, Inc. 815 West 1800 North Logan, UT 84321-1784 (435) 753-2342 (435) 750-9540 (fax) support@campbellsci.com Sap flow Instrumentation Custom-designed by P.A. Hurdle (address given in Section 2.3) VAISALA Oyj Probe Model HMP35C P.O. Box 26 FIN-00421 Helsinki FINLAND (+358 9) 894 91 (+358 9) 894 9227 (fax) 122832 vsala fi (telex) 4.2 Calibration Calibrations of earlier designs have been conducted on small trees in the laboratory by Cohen et al. (1981) and Swanson (1983), but field calibration was difficult (e.g., on 20-m-tall aspen trees). A heat transfer model, similar to that used by Swanson and Whitfield (1981), was used to determine the degree to which sap flux densities were underestimated through local interruption of sap flow by probes. The primary means of calibration at the stand level was to compare results against other sap flow methods and against eddy correlation measurements of latent heat flux (Hogg et al., 1997; see also Black et al., 1996). 4.2.1 Specifications Freezing and thawing of xylem during early spring and late fall result in "spiking" of sap flow outputs. The sap flow calculation was insensitive to the quantity of heat released by the probe (although signal-to-noise ratio can be affected if power output of heater decreases, e.g., by weak solar panels and batteries). 4.2.1.1 Tolerance As shown by Hogg and Hurdle (1997), TE-07's implementation was highly sensitive to slow rates of sap flow (as would be recorded at night); sap flow rates corresponding to 5 to 10% of the typical midday maximum can be easily resolved. The spacing of thermocouples relative to the heater (0.75 cm) allowed measurements of sap flow rates (S) of up to about 12 cm/h. If sap flow was greater than this, there would be no detectable temperature increase at the lower thermocouple. Such over-ranging was never observed in aspen or spruce using the 0.75 cm spacing, but over-ranging was observed in early trials using a spacing of 1.0 cm. A slight nonlinearity of sensitivity may have occurred as a result of sap flow. This was largely covered by the inflow interruption by the probes; based on the heat transfer modeling, sap flow may be underestimated by up to 10% at the maximum measurable rate. Laboratory comparisons by Swanson (1983, p. 177) using lysimetry with small aspen showed little evidence of nonlinearity, even though heat transfer simulations by Swanson and Whitfield (1981) predicted a significant nonlinear response for some implementations of the heat pulse method. 4.2.2 Frequency of Calibration Inspections and maintenance of instrumentation were conducted every 2 to 4 weeks (more frequently during Intensive Field Campaigns (IFCs). There were no means by which to calibrate sap flow estimates or transpiration estimates, except by comparisons against other methods. 4.2.3 Other Calibration Information Not applicable. 5. Data Acquisition Methods At each station, data from three or four trees were stored every 3 hours (1994) or every hour (1995 and 1996) by a Campbell Scientific datalogger (Model 21X). For sap flow, the temperature increase above and below the heater was reported 1, 2, and 3 minutes following the heat pulse, for each tree being measured. With the amount of data reported and the datalogger used, up to 27 days of hourly time-step sap flow results for three trees could be stored. A cassette tape was also used as backup data storage, in the event of a power loss to the datalogger or in case the period between field visits extended beyond this amount of time. Data were downloaded from the datalogger onto a laptop computer. 6. Observations 6.1 Data Notes In the data files, the column called MEASUREMENT_DESCR gives a coded description of the measurements taken. Each measurement taken is denoted by a single letter of code. The description code is: “a” to “h” means that trees called “a” to “h” gave good data. “N” denotes trees receiving new probes. “x” denotes missing or bad data taken from that tree. See Section 11.4. 6.2 Field Notes See Section 11.4. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage All sap flow measurements were made on individual trees. Three or four trees, all located within a 10-m x 10-m plot, were measured at each sap flow monitoring station. As part of the scaling procedure used to estimate stand transpiration rates from sap flow results, tallies of the diameters of each tree within the 10-m x 10-m plot were made to estimate the local ratio between stand basal area and ground area (i.e., 100 m2 area). The number of sap flow monitoring stations at each SSA site (Ns), number of trees (Nt) per station, and maximum distance separating stations are given below: Site Year Ns Nt Distance SSA-OA 1994 2 3 ca. 20 m MIX 1994 2 4 ca. 30 m Batoche 1994 3 3 ca. 300 m Batoche 1995 3 3 ca. 300 m SSA-OA 1996 2 3 ca. 100 m SSA-OBS 1996 2 3 ca. 100 m The SSA measurement sites and associated North American Datum of 1983 (NAD83) coordinates are: SSA-OA flux tower, site id C3B7T, Lat/Long: 53.62889° N, 106.19779° W, Universal Transverse Mercator (UTM) Zone 13, N: 5,942,899.9, E: 420,790.5. SSA-OA TE tower, located 100 m from the dirt road, site id C3B7T, Lat/Long: 53.62889° N, 106.19779° W, UTM Zone 13, N: 5,942,899.9, E: 420,790.5. ASP-AUX01 Batoche, site id A1A, Lat/Long: 52.74244° N, 106.13372° W, UTM Zone 13, N: 5,844,222, E: 423,465. MIX TE tower, site id D9I1M, Lat/Long: 53.7254° N, 105.20643° W, UTM Zone 13, N: 5,952,989.7, E: 486,379.7. SSA-OBS flux tower, site id G8I4T, Lat/Long: 53.98717° N, 105.11779° W, UTM Zone 13, N: 5,982,100.5, E: 492,276.5. 7.1.2 Spatial Coverage Map Not applicable. 7.1.3 Spatial Resolution These are point measurements made at the given locations. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage The data were collected from 23-MAY-1994 to 23-OCT-1996. 7.2.2 Temporal Coverage Map Site Measurement Dates SSA-OA 26-May-1994 to 20-Oct-1994 (DOY 146-293) SSA MIX 13-Jun-1994 to 20-Oct-1994 (DOY 164-293) Batoche 23-May-1994 to 19-Oct-1994 (DOY 143-292) Batoche 21-May-1995 to 25-Oct-1995 (DOY 141-298) SSA-OA 19-Apr-1996 to 23-Oct-1996 (DOY 110-297) SSA-OBS 13-Apr-1996 to 22-Oct-1996 (DOY 104-296) The sampling interval was always the same, regardless of IFC. 7.2.3 Temporal Resolution Measurement occurred between 00 and 01 minutes after each hour; 1994 measurements began at 00, 03, 06, 09, 12, 15, 18, 21 hour Greenwich Mean Time (GMT), and 1995 and 1996 measurements began on the hour, every hour. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (te07sapf.def). 7.4 Sample Data Record Sample data format shown in the companion data definition file (te07sapf.def). 8. Data Organization 8.1 Data Granularity All of the Sap Flow Data are contained in one dataset. 8.2 Data Format(s) The data files contain numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe marks. There are no spaces between the fields. Sample data records are shown in the companion data definition file (te07sapf.def). 9. Data Manipulations 9.1 Formulae 9.1.1 Derivation Techniques and Algorithms Sap flux density (S) (cm/h) was calculated using the following equation derived from Marshall (1958): S=3600 k_sw ln (T_u/T_l)/(x Cp_w rho_w) (1a) where: x is the spacing between the heater and each thermocouple (0.75 cm). Cp_w is the specific heat (i.e. value for water is 4.18 J/(g °C)). rho_w is the density of sap (i.e., value for water is 1 g/cm3). The thermal conductivity of active sapwood (k_sw) was estimated at 0.0038 J/(cm s °C), based on the following (based on Swanson 1983): k_sw = (k_w M rho_b/rho_w) + k_dw (1 M rho_b/rho_w) (1b) where: k_w is the thermal conductivity of water (0.0060 J/(m s °C)). rho_b is the typical live bulk density of sapwood (0.37 g/cm3 for both aspen and spruce). M is the typical sapwood moisture content of 1.0 g moisture per g dry mass. The reported variation in aspen moisture content from 0.75 to 1.25 g/g, i.e., 100 ± 25%, would lead to ± 9% error in k_sw and resultant estimates of S. The longitudinal thermal conductivity of dry wood (k_dw) was taken to be 0.00248 J/(cm s °C) based on rho_b and the resultant void volume (Siau, 1971). From equation (2a), conditions of zero sap flow can be identified when T_u = T_l. Ideally, this is applicable if probe spacing is exactly symmetrical and if wood heat transfer characteristics are precisely homogeneous above and below the heater. In practice, it was necessary to assume that zero sap flow occurred when the minimum value of mean S in each clone was reached (nights when VPD was near zero, excluding frost events). Thus, a zero offset (z) was applied to obtain the "zeroed" sap flux density (S'), where S' = z + S. The magnitude of z was constant, and normally within ± 0.5 cm/hr; it was recalculated, however, when probes were replaced. The heat transfer theory indicates that the interruption of the sap stream by the probes results in an underestimation of the true sap flux density. For this implementation, wound width was estimated to be 2.0 to 2.2 mm, including a 0.2 to 0.3-mm zone of disturbed sapwood on each side of the holes following drilling (Swanson and Whitfield, 1981; Barrett et al., 1995). Simulations using a two- dimensional numerical model similar to Swanson and Whitfield (1981) indicated that with this implementation, a correction factor (a) of approximately 2.0 is warranted to obtain the actual sap flux density (S*), where S* = aS'. A similar underestimation of 45% (i.e., a=1.8) was observed by Cohen et al. (1981) for their implementation of the heat pulse method. At each station, measurements of S* were multiplied by the stand sapwood area to ground area ratio (SA) to give estimates of canopy transpiration (E). SA was determined as the product of BA and FSA, where BA is the basal area to ground area ratio of aspen stems (at 1.3-m height) in one 10-m x 10-m plot centered on each station, and FSA is the fraction of BA occupied by sapwood. FSA was calculated using two radial increment cores from each of four trees per station. The zone of active sapwood within increment cores was recognized from staining by a solution of methylene blue in methyl hydrate. Measurements of BA and SA for each site monitored are described in Section 3 and given in Section 11.4. 9.2 Data Processing Sequence Sap flow results were calculated using a custom-designed utility program (written in BASIC). The output file was then imported into a Quattro Pro spreadsheet, and bad data were flagged. Mean sap flux densities (S) for up to three properly functioning trees per datalogger station were then calculated. 9.2.1 Processing Steps 1) Uncorrected sap flux density (S) from raw datalogger output was calculated based on equation 1a using a BASIC utility program. 2) The output file was imported into a Quattro Pro spreadsheet. 3) Data were graphed, and bad, anomalous, or missing data points were flagged. 4) Average uncorrected sap flux density (S) for each group of trees was calculated (normally two or three trees of a given species at each sap flow monitoring station). Data points flagged in step 3 were not included in the mean. 5) For each portion of thedata set where there were no changes in the number of properly functioning trees and no probe changes, the zero offset was determined as the difference between zero and the minimum S when the temperature was >0 °C. 6) Zero offset was applied to the entire portion of the data set to give S'(see Section 9). 7) Corrected sap flux density (S*) was obtained from S' by applying a multiplier (a=2.0) to account for interruption of sap flow by probes (see Section 9). 8) Transpiration at the stand level was estimated as follows: a) Sapwood area to ground area ratio (SA) was determined for each species in each 10-m x 10-m plot. b) SA of minor tree species (if present) was combined with SA of related major species (e.g., SA of balsam poplar, a minor species, was considered to be trembling aspen, the major species, for purposes of scaling). c) For each species group, transpiration rate was estimated in each 10-m x 10-m plot as the product of S* and SA. d) For each species group, the transpiration rate for the site as a whole (e.g., SSA-OA) was determined as the average of the transpiration estimates from each 10-m x 10-m plot (i.e. datalogger station). e) Total transpiration was estimated as the sum of the transpiration rates for all tree species. 9.2.2 Processing Changes Since production of the previous data set in November 1994, the analysis method has been modified by incorporating the zero offset and the correction for sap flow interruption by probes. The method of flagging bad data has also been modified slightly. 9.3 Calculations 9.3.1 Special Corrections/Adjustments For each period encompassing no probe changes and no loss of function by probes, a "zero offset" was determined by examining the deviation from zero of minimum flow on nonfreezing nights with negligible VPD. This resulted in a linear correction (usually <0.5 cm/h) to all values of S to obtain S' (as described in Section 9). 9.3.2 Calculated Variables S Uncorrected sap flux density (cm/h) S' Zeroed sap flux density (cm/h) S* Estimated actual sap flux density (cm/h) TRANS Estimated stand transpiration rate (mm/h) 9.4 Graphs and Plots See BOREAS special issue publications (Hogg et al., 1997; Hogg and Hurdle, 1997). 10. Errors 10.1 Sources of Error Potential errors in quantitative determinations of true sap flux density include: a) Freezing and thawing of xylem causes spiking of reported sap flow (including negative spikes). This can be readily recognized in the datasets by reference to air temperature data. b) Sapwood moisture content was generally not measured since it is a destructive procedure and most of the work was conducted in national parks. However, it can be reasonably assumed that it is near 1.0 (weight of moisture/weight of wood). Deviations from this assumption should normally lead to ? 10% error in S' (see Section 9). Diurnal variation in sapwood moisture content is assumed to be small and should not significantly affect the relative variation in S' (see Hogg et al., 1997). c) Sapwood bulk density can have a possible error of ?10%, but this does not affect relative changes in S'. d) Problems in defining zero flow may lead to error of ca. 0.3 cm/h in S'. e) Variation in sap flux density with depth in xylem or by direction (all trees were probed on the north side to avoid thermal gradients caused by sun flecks) can lead to errors. This implementation partially integrates flow rates at different depths, but "spillage" of heat from sapwood into nonconducting heartwood may lead to underestimates of Sí in trees with a very narrow sapwood zone (e.g., black spruce at SSA-OBS). f) The correction factor (a = 2.0, see Section 9) for sap flow interruption by sensors was dependent on a consistent width of xylem disruption following drilling and assumes that there was no progressive wounding response (see items h) and i) below). The appropriate correction factor was also dependent on the time interval between the heat pulse and when the temperature changes in sapwood were recorded (the correction would be reduced if the time interval were increased from 1 minute). However, a longer time interval would also reduce the signal-to-noise ratio. Also, because of this flow interruption, a reduction in sensitivity of up to 10% may occur at the maximum flow rates recorded (see Section 4.2.1.1). For these reasons, the correction factor may be in error by ?10% (i.e., a = 1.8 to 2.2). g) Rapid, uneven heating or cooling of different regions of sapwood can lead to baseline drift. An attempt was made to minimize this by mounting probes on the north side of trees and by insulating the area in the vicinity of probes. This type of error can generally be recognized by the appearance of spikey (or incalculable) data, which were removed from the analysis. The cause is usually attributable to a visible symptom (removal of insulation by bears or bucks), direct sunlight on tree at a certain time of day, or a weak heat pulse, which amplifies the effect of any baseline drift). h) If moisture was trapped beneath the insulating layer around the probes, there was a tendency for reported sap flow values to decrease beause of changes in heat transfer characteristics and/or wounding from fungal infections. i) Long-term implantation of probes in aspen (>2 to 3 months) usually resulted in a gradual, slight decline in sensitivity, typically in the range of 10 to 20% (possible wounding response). Thus, each probe was generally changed at least once during the growing season. For spruce, wounding responses and/or excessive resin from holes containing implanted probes may result in a more rapid reduction in sensitivity, although some trees maintain sensitivity over long time periods (3 to 4 months). j) Estimations of stand-level transpiration from sap flow data were also subject to statistical error (variation among trees and among patches of trees within the same stand). k) At SSA-OBS, the narrow (3 to 12 mm) width of sapwood probably led to significant underestimates of sap flux density, because of lateral heat transfer from the water-conducting sapwood into adjacent, nonconducting tissues. This would also lead to underestimates of stand transpiration rates based on sap flow measurements. 10.2 Quality Assessment 10.2.1 Data Validation by Source The following two cross-comparisons are relevant here: a) A comparison of stand transpiration rates at SSA OA for the entire 1994 growing season, based on sap flow and tower-based eddy correlation measurements of latent flux above and below the aspen canopy (with Tower Flux (TF)-1, TF-2; Hogg et al., 1997; see also Black et al., 1996). With this comparison, total estimated transpiration of aspen (mid-June 1994 to mid-August 1994) using the scaled-up sap flow measurements was 15% lower than the most recent tower-based estimates (see Blanken et al., 1997). A good correspondence was obtained when examining the overall seasonal pattern of transpiration by the two methods, and day-to-day variations in transpiration rates were similar (Hogg et al., 1997). On a mean diurnal basis, changes in sap flow lag behind changes in transpiration by about 1 hour, becasue of changes in water storage in the aspen stems and crowns. b) The constant power method of determining sap flow (TE-11, Bernard Saugier) was operated alongside the heat pulse method at the TE mixed wood site, from 28- Jul-1994 to 6-Aug-1994. The comparison was made for aspen, black spruce, and white spruce (n=2 trees per species for the constant power method; n=4 for aspen and n=2 for black and white spruce for the heat pulse method). The magnitude of daily mean sap flow (S*) by both methods was always within 15% for each of the 9 days, for both aspen and spruce (black and white spruce were combined in the analysis because of small sample sizes). The relative day-to- day pattern was also similar. Differences in the diurnal pattern of sap flow were noted, however. The constant power method was relatively more "sluggish" for early to mid-morning values, and showed relatively greater maximum daytime values compared to the heat pulse method. This difference could be partly attributed to diurnal changes in heat storage with the constant power method. 10.2.2 Confidence Level/Accuracy Judgment In general, TE-07 is confident that this method can accurately determine relative changes in sap flow over periods of time ranging from 1 hour to a few weeks. Errors in the relative changes should be within ?10%. This also applies to relative changes in the transpiration estimates, bearing in mind that there is typically a 0.5-to 1-hour diurnal lag between stem sap flow (at 1- to 1.5-m height) and transpiration. Over longer time periods, wounding responses can introduce additional errors, which TE-07 attempted to minimize by periodic change in probes, regular maintenance, and removal of low-quality data. Nevertheless, relative changes on a full-season basis could be in error by as much as ?25%. Error in quantitative estimates of the actual sap flux density is difficult to assess, but based on the comparison with TE-11 (Section 10.2.1), error is probably ?15% for aspen and spruce. However, progressive wounding responses can result in serious underestimations of sap flux density (such responses are usually evident from the data, and are normally corrected by changing probes). From Section 10.1, it will be evident to the reader that there are many possible sources of error in estimating stand transpiration rates from sap flow measurements. In the 1994 comparison with eddy correlation measurements at SSA- OA, the estimates of transpiration were similar within 15%. This could indicate that much of the error listed in Section 10.1 is of minor consequence, but TE-07 nevertheless urges caution in applying these quantitative estimates of stand transpiration. Tower-based eddy correlation is clearly a superior means of obtaining seasonal totals of water vapor flux at the stand level, since results can be rigorously validated by examining the degree of energy closure. However, sap flow measurements can have the following advantages over tower-based methods: 1) they are much less sensitive to meteorological conditions, resulting in more stable diurnal values; 2) all of the flux is attributable to water flux within a given tree; thus, they can provide a better means of examining transpiration responses for a given species. TE-07 experience indicates the benefits of operating sap flow measurements alongside eddy correlation, since the potential sources of error are different and generally independent. 10.2.3 Measurement Error for Parameters See Section 10.2.2. 10.2.4 Additional Quality Assessments None given. 10.2.5 Data Verification by Data Center Data were examined for general consistency and clarity. 11. Notes 11.1 Limitations of the Data Potential sources of error are noted in Section 10. All of the sap flow data are for trees only; thus, transpiration estimates are not directly comparable to measurements of water vapor flux made above the canopy (which also include evaporation of intercepted water and transpiration by the understory and soil). 11.2 Known Problems with the Data In some instances, sample sizes (number of trees giving good measurements) was reduced because of animal damage, heater failure, thermal drift, and other causes. Discontinuities may occur in the data set when there is a loss of function for one or more trees at a given site, or following probe changes. 11.3 Usage Guidance Readers are advised to examine all sections of this document to determine the suitability of this data set for their purposes. Any constructive feedback that users may have about the contents of this document and associated data sets is welcome. Scathing criticism will be cheerfully rebutted. "Let he/she who is without error in their own measurements throw the first stone." Data may be viewed, transformed, or skewed in any fashion that suits the user's whim or needs. However, contact TE-07 (see Section 2.3) before using these data for presentations, publications, and related venues for the dissemination of erudite information. 11.4 Other Relevant Information In the data files, the column called MEASUREMENT_DESCR gives a coded description of the measurements taken. Each measurement taken is denoted by a single letter of code. The description code is: “a” to “h” means that trees called “a” to “h” gave good data. “N” denotes trees receiving new probes. “x” denotes missing or bad data taken from that tree. The format design of the BOREAS data plan template seems to have been optimized for fluxers and remote sensors rather than for researchers in terrestrial ecology and physiology. In lieu of humor and apologies, this section is hereby used to sneak in some ecological information relevant to these data sets. Section A: Description of Aspen Sites The two aspen sites are both situated in stands of trembling aspen in central Saskatchewan, Canada. The boreal site is the BOREAS SSA-OA tower site, located in the southern boreal forest of Prince Albert National Park (PANP), elevation 600 m. The parkland site is located at Batoche National Historic Park, in the aspen parkland about 100 km to the south, elevation 500 m), midway between Saskatoon and Prince Albert. It is a CFS research site that forms part of the BFTCS (Price and Apps, 1995). The two aspen sites differ dramatically, in terms of climate, vegetation, tree growth form, and productivity (see Hogg and Hurdle, 1995). Based on the 1951-80 climate normals (Environment Canada, 1982), the Batoche site receives considerably less precipitation (375 mm/yr at Rosthern, 52?40'N, 106?20'W) compared to the SSA OA site (462 mm/yr at Waskesui Lake, 53?55'N, 106?05'W). Furthermore, mean growing season temperatures at the Batoche site are about 2 °C warmer at the parkland site, resulting in a much drier long-term climate (see Hogg, 1994). SSA-OA Site: The vegetation of the SSA-OA site consists of an extensive, even-aged stand of trembling aspen, 70 to 80 years old and 18 to 22 m tall. The understory is dominated by beaked hazelnut (Corylus cornuta Marsh.) about 2 m tall. Aspen clones (patches of genetically identical trees) are readily distinguishable because of differences in bark characteristics and phenology. The stand is relatively homogeneous except for a few small openings and occasional patches of balsam poplar (Populus balsamifera L.). The site is gently rolling and generally well drained, with a predominantly clay loam soil texture. In 1994, sap flow was monitored at a location about 1 km east of the main BOREAS flux tower. Two adjacent clones were monitored. They are referred to as the early-leafing (EL) and late-leafing (LL) clones, because of differences in phenology. In 1996, sap flow was monitored at two locations near the main BOREAS flux tower. The first clone (PAA) was located near Hut B, where Remote Sensing Science (RSS)-17 monitored sap flow by the Granier method in 1994. The second clone (PAB) was situated near the canopy access tower located about 100 m east of the main flux tower. Batoche Site CFS site and Auxiliary BOREAS site) The parkland site is in one of the few remaining areas of native aspen parkland in the region that was never cultivated for agriculture. The terrain is rolling and underlain by stabilized sand dunes. The vegetation is highly heterogeneous, ranging from open native grassland with small, stunted clones of aspen to more productive lowland forests of aspen and balsam poplar with an understory of red osier dogwood (Cornus stolonifera Michx.). Trees are stunted in relation to diameter, and are often crooked, leaning, or forked, with heights ranging from <5 m to 15 m (see Hogg and Hurdle, 1995). Within the aspen stands, trees from two or more age classes are usually found (maximum age about 85 years). Dense bands of beaked hazelnut occur on north- and east- facing slopes, but in most areas, the forest understory consists of sparse grasses, herbs, and low shrubs (e.g., Saskatoon, Amelanchier alnifolia Nutt., choke-cherry, Prunus virginiana L., and prickly rose, Rosa acicularis Lindl.). At the Batoche site, four stations were established along a 400-m transect extending from grassland to the forest interior, and three different trees were monitored each year at each station. One station was located in a small clone of stunted aspen surrounded by grassland (GL clone), two stations were located at the edge and in the center of a large (200 m x 75 m) clone in upland forest next to the grassland (UF clone), and one station was located in the center of a more mesic, lowland forest (LF clone). Stand Characteristics Of Aspen Sites Where Sap Flow Measurements Were Made: General Stand Characteristics: SSA-OA Batoche Stand volume (cubic m per ha): 144* 32* Leaf Area Index: 2.3** 1.4*** Height (m): 18-22 5-14 Age (years): 70-80 30-80 Notes: * biometry surveys by Halliwell and Apps (1996) ** from Blanken et al. (1997) *** estimate based on litter traps SSA-OA: Characteristics of clones monitored: 1994 1996 clone name: EL LL PAA PAB diameter at 1.3 m ht (cm): 21-27 17-22 19-23 20-22 basal area ratio (sq. m per sq. m): 0.0039 0.0063 0.0046 0.0048 sapwood area ratio (sq. m per sq. m): 0.0020 0.0032 0.0032 0.0030 Batoche: Characteristics of clones monitored: Batoche 1994-1995 clone name: GL UF UF LF edge center height (m): 5 6- 8 12-14 7-10 diameter at 1.3 m ht (cm): 6-9 8-12 20-23 8-14 age (years): 30-45 30-45 70-80 30-55 basal area ratio (sq. m per sq. m): 0.0015 0.0015 0.0035 0.0017 sapwood area ratio (sq. m per sq. m): 0.0009 0.0010 0.0010 0.0011 Note: Data from UF (edge) were excluded in 1994 because of problems with wounding responses. Data from UF (center) were excluded from the 1995 analysis because of problems with thermal drift. Section B: Description Of Mixed Wood Site This site is located near Candle Lake, Saskatchewan (53? 43.8'N, 105? 12.0'W), in the BOREAS SSA. It was used as an auxiliary site and had a canopy access tower for measurements by the TE-04 team (Joe Berry, principal investigator). It is located on Saskatchewan forest inventory map E43 N595, on the NW side of Hwy 120 about 3 km east of southernmost turnoff into Candle Lake. The stand designation is 819/933, H15b ta a94/S5d BS a94. It is a small (180 m x 180 m) site of young (1940's origin) aspen-black spruce-white spruce mixed wood surrounded (to the north and west) by fen. Characteristics of Trees at Mixed Wood Site Where Sap Flow Measurements Were Made in 1994: Species: Black White Trembling Balsam Total Spruce Spruce Aspen Poplar height (m): 7-8 7-8 11-13 diameter at 1.3 m ht (cm): 11 11 12-16 stem density* number per ha: 1400 300 1800 500 4000 Overall basal area ratio* (sq. m per sq. m): 0.00071 0.00020 0.00202 0.00045 0.00341 Overall sapwood area ratio* (sq. m per sq. m): 0.00040 0.00016 0.00158 (0.00035) (0.0025) Notes: *Based on average of four 10-m x 10-m plots Section C: Description of SSA-OBS Site The SSA-OBS site (53?59'N, 105?07'W) is a BOREAS tower site located NE of Candle Lake, Saskatchewan, in the BOREAS SSA. It is dominated by an extensive stand of black spruce (Picea mariana) >100 years old. There is an understory of labrador tea (Ledum groenlandicum) and other sparse shrubs, and a surface moss layer dominated by Pleurozium schreberi, other feather mosses, and occasional hummocks of Sphagnum spp. The black spruce is highly variable in size, ranging up to ca. 15 m in height and about 15 cm in diameter. The forest is dominated by black spruce, with occasional large tamarack (Larix laricina) trees in the wettest areas and a few jack pine (Pinus banksiana) in the drier areas. Stem density is generally high but variable. Characteristics of Trees at SSA-OBS Site Where Sap Flow Measurements Were Made in 1996: Species: Black Spruce Site: BSA BSB height (m): 6-11 7-10 diameter at 1.3 m ht (cm): 5.5-12 7-11 stem density* number per ha: 9000 7000 Basal area ratio* (sq. m per sq. m): 0.0039 0.0028 Sapwood area ratio* (sq. m per sq. m): 0.00105 0.00082 Notes: *Based on 10 m x 10 m plot around each sap flow station ---------------------------------------------------------------- 12. Application of the Data Set This data set can be used to study the sap flow rates, transpiration rates derived from the sap flow rates, and the environmental conditions affecting these rates of the boreal forest. 13. Future Modifications and Plans No plans as of yet. 14. Software A utility program was used to process these data. 14.1 Software Description The utility program that was used to process these data is written in QuickBasic and is presently not optimized for user friendliness. 14.2 Software Access Contact TE-07 if interested in obtaining the QuickBasic program. 15. Data Access 15.1 Contact Information Ms. Beth Nelson BOREAS Data Manager NASA GSFC Greenbelt, MD (301) 286-4005 (301) 286-0239 (fax) Elizabeth.Nelson@gsfc.nasa.gov 15.2 Data Center Identification See Section 15.1. 15.3 Procedures for Obtaining Data Users may place requests by telephone, electronic mail, or fax. 15.4 Data Center Status/Plans The TE-07 sap flow data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The BOREAS contact at ORNL is: ORNL DAAC User Services Oak Ridge National Laboratory (865) 241-3952 ornldaac@ornl.gov ornl@eos.nasa.gov 16. Output Products and Availability 16.1 Tape Products None. 16.2 Film Products None. 16.3 Other Products Tabular ASCII files. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation P.A. Hurdle has a preliminary document outlining the technical configuration of the heat pulse method used here. Please contact TE-07 if interested in this document. 17.2 Journal Articles and Study Reports Barrett, D.J., T.J. Hatton, J.E. Ash, and M.C. Ball. 1995. Evaluation of the heat pulse velocity technique for measurement of sap flow in rainforest and eucalypt forest species of south-eastern Australia. Plant Cell Env. 18:463-469. Black, T.A., G. den Hartog, H.H. Neumann, P.D. Blanken, P.C. Yang, C. Russell, Z. Nesic, X. Lee, S.G. Chen, R. Staebler, and M.D. Novak. 1996. Annual cycles of water vapor and carbon dioxide fluxes in and above a boreal aspen forest. Global Change Biol. 2: 219-229. Blanken, P.D., T.A. Black, P.C. Yang, H.H. Neumann, Z. Nesic, R. Staebler, G. den Hartog, M.D. Novak, and X. Lee. 1997. Energy balance and canopy conductance of a boreal aspen forest: partitioning overstory and understory components. Journal of Geophysical Research, 102 (D24), 28915-28927. Cohen, Y., M. Fuchs, and G.C. Green. 1981. Improvements of the heat pulse method for determining sap flow in trees. Plant Cell Env. 4:391-397. Environment Canada. 1982. Canadian climatic normals 1951-1980. Temperature and precipitation. Prairie Provinces. Atmospheric Environment Service, Downsview, Ontario, Canada, 429 pp. Halliwell, D. and M.J. Apps. 1996. BOREAS biometry and auxiliary sites. Overstory and understory data, Version 2. Canadian Forest Service, Edmonton, Alberta, 256 p. Hogg, E.H. 1994. Climate and the southern limit of the western Canadian boreal forest. Can. J. For. Res. 24:1835-1845. Hogg, E.H. 1997. Temporal scaling of moisture and the forest-grassland boundary in western Canada. Agric. Forest Meteorol. 84: 115-122. Hogg, E.H. and P.A. Hurdle. 1995. The aspen parkland in western Canada: a dry- climate analogue for the future boreal forest? Water Air Soil Pollut. 82:391- 400. Hogg, E.H. and P.A. Hurdle. 1995. Responses of sap flow in aspen(Populus tremuloides) to vapor pressure deficit in the boreal forest and aspen parkland of central Saskatchewan, Canada. Abstracts of presentations at the 80th Annual Ecological Society of America Meeting, 30-Jul-1995 to 3-August-1995, Snowbird, Utah. Hogg, E.H. and P.A. Hurdle. 1997. Sap flow in trembling aspen: implications of stomatal responses to vapor pressure deficit. Tree Physiology, 17:501-509 (BOREAS Special Issue). Hogg, E.H., T.A. Black, G. den Hartog, H.H. Neumann, R. Zimmermann, P.A. Hurdle, P.D. Blanken, Z. Nesic, P.C. Yang, R.M. Staebler, K.C. McDonald, and R. Oren. 1997. A comparison of sap flow and eddy fluxes of water vapor from a boreal deciduous forest. Journal of Geophysical Research, 102 (D24), 28929-28937. Marshall, D.C. 1958. Measurement of sap flow in conifers by heat transport. Plant Physiol. 33:385-396. Price, D.T. and M.J. Apps. 1995. The Boreal Forest Transect Case Study: global change effects on ecosystem processes and carbon dynamics in boreal Canada. Water Air Soil Pollut. 82:203-214. Sellers, P. and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 94). Sellers, P. and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1996-2.0, NASA BOREAS Report (EXPLAN 96). Sellers, P., F. Hall, and K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPSDOC 94). Sellers, P., F. Hall, and K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere Study: 1996 Operations. NASA BOREAS Report (OPSDOC 96). Sellers, P.J., F.G. Hall, R.D. Kelly, A. Black, D. Baldocchi, J. Berry, M. Ryan, K.J. Ranson, P.M. Crill, D.P. Lettenmaier, H. Margolis, J. Cihlar, J. Newcomer, D. Fitzjarrald, P.G. Jarvis, S.T. Gower, D. Halliwell, D. Williams, B. Goodison, D.E. Wickland, and F.E. Guertin. 1997. BOREAS in 1997: Experiment overview, scientific results, and future directions. Journal of Geophysical Research, 102 (D24), 28731-28769. Sellers, P.J., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. Wickland. 1995. The Boreal Ecosystem-Atmosphere Study (BOREAS): An overview and early results from the 1994 field year. Bull. Am. Meteorol. Soc. 76:1549-1577. Siau, J.F. 1971. Flow in wood. Syracuse Wood Science Series 1. Syracuse University Press. 131 p. Swanson, R.H. 1983. Numerical and experimental analyses of implanted-probe heat pulse velocity theory. Ph.D. thesis, University of Alberta, 298 p. Swanson, R.H. 1994. Significant historical developments in thermal methods for measuring sap flow in trees. Agric. For. Meteorol. 72:113-132. Swanson, R.H. and D.W.A. Whitfield. 1981. A numerical analysis of heat pulse velocity theory and practice. J. Exp. Bot. 32:221-239. 17.3 Archive/DBMS Usage Documentation None. 18. Glossary of Terms aerodynamic conductance - In this context, a measure of the degree to which vertical air movement permits the transfer of water vapor and sensible heat between the leaf surface and a reference height above the forest canopy. BA (basal "area") - In this context, the ratio of tree basal area (at 1.3 m height) to ground area, as measured within plots in the forest stand being monitored. boreal forest - Northern forest that occurs at high latitudes in Canada, Alaska, Russia, and Scandinavia. In western Canada, it can be defined as a forested, cold-climate region where there are peatlands (bogs and/or fens) and at least three species of conifers (typically white spruce, black spruce, and tamarack; and usually either jack pine or lodgepole pine). Trembling aspen is often abundant or dominant, but the diversity of other hardwood tree species is low. canopy conductance - In this context, a measure of the degree to which stomata and plant surfaces in the canopy permit passage of water vapor, expressed on a ground area basis. clone - A group of genetically identical trees that occurs in species that reproduce vegetatively, including trembling aspen and balsam poplar. Typically a clone of aspen consists of dozens to hundreds of trees, occupying areas of up to several hectares in the boreal region. eddy correlation - A micrometeorological technique used extensively in BOREAS to measure the exchange of water vapor, heat, carbon dioxide, and trace gases between the forest and the atmosphere. Eddy correlation measurements in BOREAS were made from towers (above or below the forest canopy) and from aircraft. In simple terms, the technique involves simultaneous, high-frequency measurements of vertical air movement and the quantities of interest. FSA - Fraction of total tree basal area that is sapwood, as estimated using increment cores. latent heat flux density - The rate of energy exchange (e.g., from the landscape) caused by evaporation of water (including transpiration), expressed in watts per square meter ground area. This can also be expressed in mm per hour of water vapor loss from the landscape. leaf water potential - A measure of plant water status, inferred as the negative value of the pressure required to restore moisture to the cut surface of xylem in a leaf petiole or twig when the latter is enclosed by a pressure chamber. parkland - A semi-arid vegetation zone in western Canada that is transitional between the boreal forest and prairies to the south. Conifers such as spruce and pine are generally absent, and forest cover is generally restricted to stunted patches of aspen. Peatlands (bogs and fens) are generally absent, and small lakes tend to dry up during periods of drought. This zone is intensively cultivated for agriculture. sap flow - General term referring to the upward movement of liquid water within trees and other plants. sap flux density - Quantitative measure of the volume of liquid water moving upward per unit area of sapwood in trees. sapwood - Water-conducting tissue in trees, consisting of the outermost and youngest region of xylem. For the major boreal tree species, this typically consists of the "wood" formed in the past 10-25 years. SA (sapwood "area") - In this context, the ratio of sapwood area (at 1.3-m height) to ground area, as measured within plots in the forest stand being monitored. sensible heat flux density - The total rate of energy exchange (e.g., from the landscape) by convection or conduction of heat, expressed in watts per square meter ground area. stomata - Little holes in leaves that provide a pathway for water vapor loss from leaves and for the exchange of carbon dioxide with the atmosphere. stomatal conductance - A measure of the degree to which stomata permit the passage of water vapor or carbon dioxide, expressed on a leaf area basis. transpiration - In this context, refers to water vapor loss through the stomata of tree leaves. vapor pressure deficit (VPD) - Difference between the actual vapor pressure and the saturation vapor pressure of air, as measured at a reference height within or above the forest canopy. xylem - Water-conducting tissue in plants; in trees we are referring to secondary xylem or what the lay person would view as "wood," which also serves as structural support for the tree. 19. List of Acronyms ASCII - American Standard Code for Information Interchange Batoche - The study site located in the Batoche National Historic Park BFTCS - Boreal Forest Transect Case Study BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOREAS Information System CD-ROM - Compact Disk-Read Only memory CFS - Canadian Forest Service DAAC - Distributed Active Archive Center DOY - Julian Day of Year EOS - Earth Observing System EOSDIS - EOS Data and Information System GMT - Greenwich Mean Time GSFC - Goddard Space Flight Center HTML - HyperText Markup Language IFC - Intensive Field Campaign MIX - Mixed Wood NAD83 - North American Datum of 1983 NASA - National Aeronautics and Space Administration NOAA - National Oceanic and Atmospheric Administration NSA - Northern Study Area ORNL - Oak Ridge National Laboratory PANP - Prince Albert National Park RSS - Remote Sensing Science S - Sap Flux Density SSA - Southern Study Area SSA-OA - Old Aspen SSA-OBS - Old Black Spruce TE - Terrestrial Ecology TF - Tower Flux T_l - Temperature at the lower thermocouple T_u - Temperature at the upper thermocouple URL - Uniform Resource Locator UTM - Universal Transverse Mercator VPD - Vapor Pressure Deficit 20. Document Information 20.1 Document Revision Date Written: 03-Oct-1996 Last Updated: 08-Oct-1998 20.2 Document Review Date(s) BORIS Review: 10-Oct-1997 Science Review: 20.3 Document ID 20.4 Citation E.H. (Ted) Hogg and P.A. Hurdle, CFS 20.5 Document Curator 20.6 Document URL Keywords Aspen Sap flow Sap flux density Spruce Stomatal conductance Transpiration Vapor pressure deficit TE07_Sapflow.doc 10/09/98