HEM Tower - Net Carbon Exchange of an Old-Growth Hemlock Forest

Data

• hf103-01: eddy flux (2000-01)
• hf103-02: model equations (2000-01)
• hf103-03: eddy flux (2004)
• hf103-04: eddy flux (2005-06)

Overview

• Lead: Julian Hadley
• Investigators: David Foster, Liza Nicoll, Jessica Schedlbauer, Mark VanScoy
• Contact: Julian Hadley
• Start date: 2000-11-02
• End date: ongoing
• Location: Prospect Hill Tract (Harvard Forest)
• Latitude: +42.539
• Longitude: -72.180
• Elevation: 355 meters
• Taxa: Acer rubrum (red maple), Betula lenta (black birch), Quercus rubra (red oak), Pinus strobus (white pine), Tsuga canadensis (eastern hemlock)
• Research topic: atmosphere
• Study type: long-term measurement
• LTER core area: primary production, inorganic nutrients
• Keywords: carbon dioxide, forest age, forest type, photosynthesis, respiration
• Release date: 2003
• EML version: knb-lter-hfr.103.11
• Revisions: data updated 2009-06-16
• Related links:
• EMS - Canopy-Atmosphere Exchange of Carbon, Water and Energy
• Hemlock Tower - Physiological Model of CO2 Exchange by Hemlock Forests
• LPH Tower - Net Carbon Exchange of a Young Upper-Slope Deciduous Forest
• Abstract:

  This project estimates carbon exchange rates of multiple forest types at Harvard Forest (see HF072) and compares them to long-term ongoing carbon exchange measurements at the EMS, which is located in a mesic, 60-90 year old red oak and red maple dominated forest on abandoned farmland (HF004). Measurements in each forest type are used to investigate climatic influences on carbon exchange.

  This mesic hemlock-dominated forest with most trees 100-200 years old on undisturbed soils stored only about 3 Mg/ha of carbon in 2001, compared to over 4 Mg/ha in the 60-90 year old oak/maple stand. However, both sites stored more carbon in 2001 than was measured in the oak/maple stand in any previous year since 1991 (see HF004). The hemlock forest behaved very differently from the oak-maple stand in that the highest rates of carbon storage occurred in spring, while there was very little carbon storage in mid to late summer. Statistical models of carbon exchange in the hemlock forest showed that carbon storage was positively related to daily minimum air temperature in spring, but negatively correlated with soil temperature in the summer. The first effect was attributable to a positive influence of above-freezing minimum temperatures on photosynthesis by hemlock foliage. The negative relationship of soil temperature to carbon storage by hemlock forest in summer was due to exponentially increasing soil and ecosystem respiration, accompanied by a neutral or negative effect of high air temperature on photosynthesis by hemlock trees (see HF063). These effects indicate that carbon storage in the hemlock forest could be strongly affected by climate warming, but the effects will probably be in opposite directions in spring and summer.

• Methods:

  Carbon exchange above the forest canopy is measured by the eddy covariance procedure, in which CO2 concentration and three-dimensional wind are measured simultaneously at high frequency (Goulden et al. 1996). The sampling frequency in this study is 5 hz, and CO2 flux is calculated every 30 minutes. When atmospheric turbulence is too low for valid eddy covariance measurements, or wind direction was unsuitable for measuring the forest type of interest, multiple regression models are used to estimate carbon exchange from major environmental controllers. These controllers are photosynthetically active radiation (PAR), air temperature, soil temperature, daily minimum air temperature, and atmospheric humidity. Statistical models of CO2 exchange are created from measurements of these controlling variables taken simultaneously with eddy covariance data, when turbulence and wind direction are adequate.

  At this site, multiple regression models were used to estimate carbon exchange during the majority of hours in all months, because SW winds and fairly high turbulence (u* or friction velocity greater than 0.4) were needed for valid eddy covariance measurements. Simultaneously with eddy covariance measurements, we record air temperature above the canopy, PAR, soil temperature at multiple locations, and atmospheric humidity every 30 seconds and average these measurements every half-hour. Soil respiration is measured at 1 to 2 week intervals in spring, summer and fall and monthly during the winter, using chambers placed on the soil surface and a portable CO2 analyzer.

• Use:

  This dataset is released to the public and may be freely downloaded. 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. For more information on LTER Network data access and use policies, please see: http://www.lternet.edu/data/netpolicy.html.

• Citation:

  Hadley J. 2003. HEM Tower - Net Carbon Exchange of an Old-Growth Hemlock Forest. Harvard Forest Data Archive: HF103.

Detailed Metadata

hf103-01: eddy flux (2000-01)

1. Year: year (YYYY)
2. Day_of_year: day of year, with hour of day converted to a fraction of a day (number )
3. u: horizontal windspeed measured by the sonic anemometer at 27 m (5 m above average canopy surface) (metersPerSecond )
4. u*: friction velocity, or square root of momentum flux (metersPerSecond )
5. wdir: wdir (degree )
6. PAR: photosynthetically active radiation at 24 m (2 m above average canopy surface) (micromolesPerMeterSquaredPerSecond )
7. Tair: air temperature measured by shielded thermocouple at about 22 m (average canopy surface height) (celsius )
8. daily_min_Tair: minimum Tair during the 24 hours preceding noon (celsius )
9. Tsoil: soil temperature measured 10 cm below soil surface. (Average of 5-6 measurements randomly located between 0 and 50 m from the eddy covariance tower, with one measurement on each of 6 transects spaced 60 degrees apart in compass orientation.) (celsius )
10. VPD: water vapor pressure deficit of air, based on water vapor concentration provided by CO2/H2O analyzer, and Tair (kilopascal )
11. Measured_CO2_flux: value measured by the eddy covariance system. Valid as data point only if wind is from SW (between 180 and 270 degrees) and u*> 0.4 (micromolesPerMeterSquaredPerSecond )
12. Estimated_CO2_flux: estimate based on a mathematical model of carbon flux created at the same time of year as the missing data point, using hour of day plus measured values of PAR, Tair, Tsoil, VPD, and daily_min_Tair (micromolesPerMeterSquaredPerSecond )
13. Eddy_covariance_data_(D)_or_estimate_(E): Indicates whether the flux value used for the half-hour is a measured_CO2_flux data point or an estimated_CO2_flux
    • D: measured_CO2_flux data point
    • E: estimated_CO2_flux

hf103-02: model equations (2000-01)

1. day or night model: day or night model
   • day: day model
   • night: night model
2. Period of model - application: period of model application
3. Period of model - derivation: period of model derivation
4. Physical variables: physical variables
   • HR: time of day
   • PAR: photosynthetically active radiation
   • Ta: air temperature
   • Tm: daily minimum air temperature
   • FROST: daily minimum air temperature class: 0= Tm>0 oC, 1= Tm less than or equal to 0 oC but greater than -5 oC, and 2= Tm less than or equal to -5 oC
   • Ts: soil temperature at 10 cm depth
   • VPD: water vapor pressure deficit of the air
5. Interactions: interactions.Models used to estimate carbon flux during periods with wind not from the SW and/or friction velocity (u*) less 0.4 m/s. For night models, lin indicates a linear function and exp is an exponential function. The natural log of PAR and the square of HR are also present in some models, however to save space only the raw variables from which ln(PAR) and HR2 were derived are shown in the table. Wintertime models have very low r2 values because of very low biological activity and low carbon flux in winter, compared to random CO2 fluctuations resulting from atmospheric and CO2 analyzer noise.
6. n: number
7. r2: r squared (dimensionless )
8. P: p value (dimensionless )

hf103-03: eddy flux (2004)

1. Year: year (YYYY)
2. Day.of.year: day of the year with hours and minutes converted to a decimal fraction of a day (DDD)
3. CO2: carbon dioxide concentration of the air drawn into the eddy covariance system at 28 m above ground or about 5 m above the average tree canopy top (micromolesPerMeterSquaredPerSecond )
4. H2O: water vapor concentration of the air drawn into the eddy covariance system at 28 m above ground or about 5 m above the average tree canopy top (millimoles per mole) (number )
5. u: wind speed measured by the sonic anemometer at 28 m or 4.5 m above the average tree canopy top (metersPerSecond )
6. ustar: friction velocity measured by the sonic anemometer at 28 m. Friction velocity is the square of momentum flux from the atmosphere above the sonic to the air layers below the sonic, and is a measure of atmospheric turbulence (metersPerSecond )
7. wind.direction: compass direction in degrees of the average wind vector at 28 m, with 0 and 360 degrees indicating geographic north (degree )
8. H: sensible heat flux from the forest to the atmosphere, calculated by the sonic anemometer from the covariance of air temperature and the vertical component of wind velocity (wattsPerSquareMeter )
9. LE: Flux of latent heat (heat used in evaporating water) from the forest to the atmosphere, calculated by multiplying FH2O by the heat of evaporation of water. (wattsPerSquareMeter )
10. FCO2: Measured carbon dioxide (CO2) flux from forest to atmosphere. Includes all data collected, some of which do not represent hemlock-dominated forest or are invalid (micromolesPerMeterSquaredPerSecond )
11. Hemlock.FCO2.ustar.gt.0.4: CO2 flux data exclusively for hemlock-dominated, which occurs primarily to the SW of the tower (compass directions of 180 to 270 degrees), and after removal of data that are invalid due to low atmospheric turbulence (u* less than 0.4 m/s) (micromolesPerMeterSquaredPerSecond )
12. NEE.hemlock(data.or.estimate): best estimate of CO2 flux, using either valid measurement from the column to the left, or a model estimate (micromolesPerMeterSquaredPerSecond )
13. NEE.data.point(1)or.NEE.estimate(0): binary variable identifying whether measured FCO2 or an estimate was used
    • 0: estimate
    • 1: measured FCO2
14. R.estimate: estimated ecosystem respiration. This is equal to Hemlock.FCO2.ustar.gt.0.4 when there is a value for this at night. Otherwise, it is an estimate of CO2 production by the ecosystem, based on a statistical model that uses soil and air temperatures and valid nighttime FCO2 values to predict FCO2 under other circumstances. (micromolesPerMeterSquaredPerSecond )
15. R.estimate(min.0): estimated ecosystem respiration with a minimum value of 0 since respiration is generally defined as positive. (micromolesPerMeterSquaredPerSecond )
16. GEE.estimate: estimate of gross carbon fixation by the forest, calculated difference between NEE and R (micromolesPerMeterSquaredPerSecond )
17. FH2O: Measured water vapor flux from forest to atmosphere. Includes all data collected, some of which do not represent hemlock-dominated forest (millimolesPerMeterSquaredPerSecond )
18. Hemlock.FH2O: Water vapor flux data exclusively for hemlock-dominated forest, which occurs primarily to the SW of the flux tower. Low turbulence data are not removed as the H2O flux is primarily from the canopy, which therefore does not act as a barrier to movement of H2O, as it does for the large amount of CO2 produced by soil and forest-floor litter. (millimolesPerMeterSquaredPerSecond )
19. Sonic.Tair: air temperature estimated from the speed of sound measured by the sonic anemometer. This estimate is based on air density, which is directly related to the speed of sound. The sonic air temperature estimate can differ from actual air temperature by a few degrees due to variation in the concentration of water vapor, which lowers air density. (celsius )
20. Tair.above.canopy: air temperature measured at 23 m height, or about 6.5 m above the average tree canopy top, by a Campbell Scientific HMP35C sensor (celsius )
21. RH.above.canopy: relative humidity measured at 23 m height by a Campbell Scientific HMP35C sensor (same location as Tair.above.canopy) (number )
22. Tair.above.canopy(TC): Air temperature measured by a shaded thermocouple mounted at 22m, just below the top platform of the scaffolding tower to which the mast holding the sonic anemometer and air intake port for CO2 and H2O measurements is mounted at 27 m. This air temperature is used in making flux estimates only if the HMP35C temperature sensor is not working correctly. (celsius )
23. VPD.above.canopy: water vapor pressure deficit (equals saturation water vapor pressure at Tair.above.canopy minus actual water vapor pressure) (kilopascal )
24. Tsoil.10cm: Soil temperature measured at 10 cm depth. The average of 3 to 4 values at randomly located points within 15 m of the flux tower base. (celsius )
25. PAR: photosynthetically active radiation measured at approximately the top of the tree canopy at about 22 m height. (micromolesPerMeterSquaredPerSecond )

hf103-04: eddy flux (2005-06)

1. Year: year (YYYY)
2. Day.of.year: day of the year with hours and minutes converted to a decimal fraction of a day (DDD)
3. Day.Night: Indicates whether data was collected during a 30-minute period when average PAR was greater than 5 µmol m-2 s-1 (Day) or less than 5 µmol m-2 s-1 (Night). All carbon and water fluxes at night are considered to represent ecosystem respiration because PAR is too low for photosynthesis.
   • Day: day
   • Night: night
4. CO2: carbon dioxide concentration of the air drawn into the eddy covariance system at 28 m above ground or about 5 m above the average tree canopy top (measured by Licor LI7000 gas analyzer). Height raised to 29 m in November 2006. (micromolesPerMeterSquaredPerSecond )
5. H2O: water vapor concentration of the air drawn into the eddy covariance system at 28 m above ground or about 5 m above the average tree canopy top (measured by Licor LI7000 gas analyzer). Height raised to 29 m in November 2006. (number )
6. u: wind speed measured by the sonic anemometer at 28 m or 5 m above the average tree canopy top. Height raised to 29 m in November 2006. (metersPerSecond )
7. ustar: friction velocity measured by the sonic anemometer at 28 m. Friction velocity is the square of momentum flux from the atmosphere above the sonic to the air layers below the sonic, and is a measure of atmospheric turbulence (metersPerSecond )
8. wind.direction: compass direction in degrees of the average wind vector at 28 m, with 0 and 360 degrees indicating geographic north (degree )
9. H: sensible heat flux from the forest to the atmosphere, calculated by the sonic anemometer from the covariance of air temperature and the vertical component of wind velocity (wattsPerSquareMeter )
10. LE: flux of latent heat (heat used in evaporating water) from the forest to the atmosphere, calculated by multiplying FH2O by the heat of evaporation of water. (wattsPerSquareMeter )
11. FCO2: Measured carbon dioxide (CO2) flux from forest to atmosphere. Includes all data collected, some of which do not represent hemlock-dominated forest or are invalid (micromolesPerMeterSquaredPerSecond )
12. Hemlock.FCO2.no.ustar.filter: CO2 flux data exclusively for hemlock-dominated forest, which occurs primarily to the SW of the tower (compass directions of 180 to 270 degrees). (micromolesPerMeterSquaredPerSecond )
13. Hemlock.FCO2.ustar.filtered: CO2 flux data exclusively for hemlock-dominated forest, which occurs primarily to the SW of the tower (compass directions of 180 to 270 degrees), and after removal of data that with friction velocity (ustar) below 0.4 m/s in which the measured nighttime CO2 flux appeared to be turbulence-limited and not representative of ecosystem CO2 production. (micromolesPerMeterSquaredPerSecond )
14. Hemlock.FCO2(data.or.estimate): best estimate of CO2 flux, using either valid measurement from the column to the left, or a model estimate (micromolesPerMeterSquaredPerSecond )
15. Hemlock.FCO2.data(1)or.estimate(0): binary variable identifying whether measured FCO2 or an estimate was used
    • 0: estimate
    • 1: measured FCO2
16. R.estimate: estimated ecosystem respiration. This is equal to Hemlock.FCO2.ustar.filtered at night, if this is available. Otherwise, it is an estimate of CO2 production by the ecosystem, based on a statistical model that uses soil and air temperatures and valid nighttime FCO2 values to predict FCO2 under other circumstances. By definition, R is greater than zero. (micromolesPerMeterSquaredPerSecond )
17. GEE.estimate: estimate of gross carbon fixation by the forest, calculated difference between NEE and R. By definition, GEE is a negative number. (micromolesPerMeterSquaredPerSecond )
18. FH2O: Measured water vapor flux from forest to atmosphere. Includes all data collected, some of which do not represent hemlock-dominated forest (millimolesPerMeterSquaredPerSecond )
19. Hemlock.FH2O: Water vapor flux data exclusively for hemlock-dominated forest to the SW of the flux tower. Low turbulence (low ustar) data are not removed as the H2O flux is primarily from the canopy, which therefore does not act as a barrier to movement of H2O, as it does for the large amount of CO2 produced by soil and forest-floor litter. (millimolesPerMeterSquaredPerSecond )
20. Hemlock.FH2O.data.or.estimate: best estimate of H2O flux, using either valid measurement from the column to the left, or a model estimate. For most periods nighttime FH2O estimates were not made, because average measured nighttime H2O flux was very close to zero. (millimolesPerMeterSquaredPerSecond )
21. Sonic.Tair: air temperature estimated from the speed of sound measured by the sonic anemometer. This estimate is based on air density, which is directly related to the speed of sound. The sonic air temperature estimate can differ from actual air temperature by a few degrees due to variation in the concentration of water vapor, which lowers air density. (celsius )
22. Tair.above.canopy: Air temperature measured above the canopy at 24 m height by a Campbell Scientific HMP35C sensor. (celsius )
23. RH.above.canopy: Relative humidity measured above the canopy at 23 m height by a Campbell Scientific HMP35C sensor (same location as Tair.above.canopy). (number )
24. VPD.above.canopy: Water vapor pressure deficit. (Equals saturation water vapor pressure at Tair.above.canopy, minus actual water vapor pressure calculated as saturation vapor pressure times relative humidity). (kilopascal )
25. Tsoil.10cm: Soil temperature measured at 10 cm depth. The average of 3 to 4 values at randomly located points within 15 m of the flux tower base. (celsius )
26. PAR: photosynthetically active radiation measured above the canopy at 24 m height by a Licor 190S quantum sensor. (micromolesPerMeterSquaredPerSecond )