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Harvard Forest Data Archive


Physiological Model of CO2 Exchange by Hemlock Forests at Harvard Forest 1996-2000

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  • Lead: Julian Hadley
  • Investigators: David Foster
  • Contact: Information Manager
  • Start date: 1996
  • End date: 2000
  • Status: completed
  • Location: Prospect Hill Tract (Harvard Forest)
  • Latitude: +42.539
  • Longitude: -72.180
  • Elevation: 355 meter
  • Taxa: Tsuga canadensis (eastern hemlock)
  • Release date: 2003
  • Revisions:
  • EML file: knb-lter-hfr.63.22
  • DOI: digital object identifier
  • EDI: data package
  • DataONE: data package
  • Related links:
  • Study type: short-term measurement
  • Research topic: forest-atmosphere exchange
  • LTER core area: primary production
  • Keywords: carbon dioxide, hemlock, photosynthesis, physiology, respiration
  • Abstract:

    A physiological model of carbon (C) exchange for a mature hemlock forest was developed, with separate component models for net photosynthesis (Pn), leaf respiration (Rl) , woody tissue respiration (Rw) and soil respiration (Rs). The model estimated that about 1.2 Mg C/ha was stored above and below ground between November 1, 1997 and October 31, 1998. This was generally a wet year with a wet and cloudy summer, except during August, which probably influenced the model output significantly. The whole-forest C exchange model estimated that most C storage in the forest occurred in spring. Warm temperatures with high soil moisture caused whole-forest respiration to exceed Pn during the summer, leading to a net C loss from the ecosystem. Leaf-level light-saturated Pn reached a maximum at about 20 deg C, then remained stable up to about 30 deg C, but at lower light levels Pn decreased above 20 deg C. This contributed to the lack of carbon storage during the summer, when the warmest days reached 30 to 32 deg C. Soil respiration was estimated at 60 to 75% of total ecosystem respiration, and during summer Rs increased exponentially with soil temperature with a Q10 of 3.8, so that from July through September, monthly Rs alone was 73 to 88% of total canopy Pn (Estimated monthly Rs ranged from 1.14 to 1.68 Mg/ha and estimated monthly Pn was 1.29 to 2.06 Mg/ha in July through September).

    A second major control on carbon storage by the hemlock forest was daily minimum temperature in spring and fall. There was no measurable Pn after daily minimum temperatures of -5 deg C or lower, although no effect of minimum temperature on Pn was observed for temperatures above 0 deg C.

  • Methods:


    Physiological characteristics of a mature to old eastern hemlock forest and its physiological responses to environmental factors were measured, in order to develop a model predicting carbon exchange by this type of forest. Four large hemlock trees (40 to 85 cm dbh, 23 to 27 m tall) were measured from a 22m canopy access tower placed between them. Shoot-level photosynthesis and respiration, as well as woody tissue respiration and total soil respiration (root plus microbial) were measured under ambient conditions. In addition, net photosynthesis was measured with controlled temperature and light, to determine short-term photosynthetic responses. Photosynthesis measurements were made approximately every two weeks during the growing season. In winter, measurements were made only on days with above-freezing air temperatures. In addition, we measured photosynthetically active radiation (PAR) above the forest canopy and at shoot surfaces in the upper, middle and lower canopy (30 locations). Air temperature and relative humidity above the canopy, sapwood temperatures in branches and tree boles, and soil temperature at 10 cm depth were also measured, all at 30-second intervals with averages calculated every 30 minutes.

    For modeling purposes, tree canopies were divided into three levels, and representative shoots from the highest and lowest levels were selected for photosynthesis measurements. Branches from all three levels were selected to measure woody tissue respiration, which was quantified on the basis of sapwood volume within or beneath a respiration chamber. Regression equations predicting sapwood volume and leaf area from branch diameter were derived from destructive sampling of other nearby large hemlock trees. Photosynthetic responses of middle-canopy foliage were estimated as the average of upper and lower canopy photosynthesis.

    In running the photosynthesis and respiration models, all trees in the forest were assumed to have the same leaf area index as the group of four sample trees. Leaf area and sapwood volume throughout the forest were assumed to be distributed between three canopy layers in the same proportions as in the sample trees. PAR levels in each of the three canopy layers throughout the forest were also assumed to be similar to the four trees with PAR sensors installed.

    Detailed Methods

    This data set includes microclimatic measurements and estimates of all easily distinguishable forms of carbon exchange in an old-growth eastern hemlock (Tsuga canadensis L.) forest in the Harvard Forest, Petersham, Massachusetts, USA. Maximum tree age is about 220 years, and the data presented here are from four hemlocks 126 to 195 years old and 26 to 30 m tall, which surrounded a 22m canopy access tower. Eastern hemlock composed over 90% of the basal area in eight randomly located plots within 50m of the tower.

    Air, wood and soil temperatures, atmospheric humidity, windspeed and photosynthetically active radiation (PAR) above the canopy were measured every 10 to 60 seconds and averaged at ten minute and hourly intervals. PAR near leaf surfaces at 9-12 locations at each of three levels in the canopy was measured every 10 seconds and also averaged at ten minute and hourly intervals.

    Respiration was measured using a portable infrared gas analyzer with suitable chambers in a closed-system configuration. All easily separable types of respiration in the forest (night respiration by foliage plus wood, soil, and seed cone respiration) were measured separately. Exponential functions of air, sapwood, and soil temperature explained most of the variation in respiration rates. However, seed cone respiration decreased with increasing PAR (indicating that the cones were photosynthetic) and therefore PAR was included in the seed cone respiration model. An unusually large seed cone crop in 1998 resulted in estimated total seed cone respiration comparable to estimated total wood and foliage respiration. Beginning late spring of 1998 soil moisture was included as a covariate in the soil respiration model. Separate wood respiration models were developed for tree boles and lower, middle and upper canopy branches. Within each of these, wood respiration models during wood production in summer were different from models for the rest of the year. Separate foliar night respiration models were developed for upper and lower canopy foliage because of inherent differences in respiration rate.

    Photosynthesis was measured on attached foliage at ambient temperature and light and also at controlled leaf temperature and PAR levels. Temperature and light responses were measured once or twice during each season of the year and combined with more frequent measurements in ambient conditions to develop photosynthesis models. These models used PAR, leaf temperature, atmospheric humidity, time of day, daily minimum air temperature and stomatal conductance and water potential of foliage (for which separate models were developed), and to predict photosynthesis. All or most of these variables were needed in spring, summer and fall models, while winter photosynthesis was often controlled only by PAR and daily minimum air temperature. Photosynthesis of upper and lower canopy foliage was modeled separately, and middle canopy foliage was assumed to follow the average of upper and lower canopy model predictions. Foliage respiration and photosynthesis were measured primarily on the youngest mature foliage, but some measurements of older foliage showed that maximum net photosynthesis and foliage respiration declined roughly 10% per year. This rate of decline was used to estimate carbon exchange of older foliage. Over 95% of the total leaf area was composed of foliage three years old or younger.

    All forms of respiration were estimated from models using hourly average data. Average photosynthetic rate in each canopy layer was modeled using hourly temperature and humidity data combined with a logarithmic frequency distribution of PAR measurements near leaf surfaces every ten seconds. This enabled the model to take into account rapid photosynthetic responses to 'light flecks' which pass quickly across leaf surfaces, particularly in the middle and lower canopy.

    Estimated leaf area index (LAI) and foliage age distribution were used to scale foliage respiration and photosynthesis to an ecosystem level. Total leaf area and age distribution were estimated from leaf biomass on branches of nearby hemlocks similar to the study trees. The leaf area estimates were regressed against branch basal diameter, and the regression equation was applied to all branches of the four trees used in model development. Dividing all estimated foliage areas by the ground surface directly beneath the canopies of the study trees gave estimates of LAI by canopy layer and age class. Total LAI was ~4.4 during July and August (after maturation of new foliage but before abscission of older foliage) and ~3.5 from September through June.

    Seed cone respiration on a ground area basis was estimated from a ratio of seed cone number to foliage area within each canopy layer, derived from random samples collected in August 1998.

    Wood respiration of branches was scaled up by a similar process to leaf respiration and photosynthesis, using regressions of branch basal diameter versus sapwood volume in cut branches. Bole sapwood volume was calculated from sapwood thickness in tree cores taken on the N and S sides of the four primary study trees at 1.5, 10, 13, 16, 19 and 22 m above the ground. Total estimated wood respiration of the four trees used in modeling was divided by the ground surface directly beneath their combined canopies; this produced a ground area-based ecosystem wood respiration rate.

  • Use:

    This dataset is released to the public under Creative Commons CC0 1.0 (No Rights Reserved). Please keep the dataset creators informed of any plans to use the dataset. Consultation with the original investigators is strongly encouraged. Publications and data products that make use of the dataset should include proper acknowledgement.

  • Citation:

    Hadley J. 2003. Physiological Model of CO2 Exchange by Hemlock Forests at Harvard Forest 1996-2000. Harvard Forest Data Archive: HF063 (v.22). Environmental Data Initiative:

Detailed Metadata

hf063-01: CO2 exchange

  1. datetime: date and time
  2. year: year
  3. doy: Julian date or day of year (unit: nominalDay / missing value: NA)
  4. hours: local time (24-hour clock) at end of averaging interval
  5. doy.dec: Julian date or day of year with HOURS converted to a decimal fraction (unit: nominalDay / missing value: NA)
  6. above-canopy total solar radiation measured with a LI-200S pyranometer sensor. It was unshaded except for brief periods (less than 1 hour) in late evening in spring and fall. (unit: kilowattPerMeterSquared / missing value: NA)
  7. ac.par: Above-canopy photosynthetically active radiation measured by a LI-190S quantum sensor. It was shaded only during the same evening periods as the pyranometer sensor. (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  8. wind: wind speed at 24.5m height, measured by a cup anemometer (RM Young model 12102-D) with a stall speed of 0.2 m s-1. (unit: metersPerSecond / missing value: NA)
  9. airt: above-canopy air temperature measured by a shaded thermocouple (unit: celsius / missing value: NA)
  10. rh: above-canopy relative humidity measured by a shaded Vaisala(r) humidity sensor (unit: number / missing value: NA)
  11. uc.par: average PAR at 12 locations in the upper canopy, measured by Hamamatsu photodiodes (model GAASP-118) affixed parallel to the surface of randomly selected shoots on the N, E, S and W sides of the three trees to the E, S and W of the tower. (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  12. mc.par: average PAR at 9 locations in the middle canopy. Measured similarly to UC PAR except that there were no measurements on the sides of trees closest to the canopy access tower due to shading by the tower. (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  13. lc.par: average PAR at 9 locations in the lower canopy. Measured similarly to MC PAR. (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  14. soilt: average soil temperature at 10 cm depth measured at five randomly chosen locations (unit: celsius / missing value: NA)
  15. grav.soil.water: gravimetric soil water content (unit: dimensionless / missing value: NA)
  16. uc.wood.temp: sapwood temperature at about1 cm depth, measured 20-40 cm from the tree bole on the upper side of 8 branches near the middle of the upper canopy (unit: celsius / missing value: NA)
  17. mc.wood.temp: sapwood temperature at about1 cm depth, measured 20-40 cm from the bole on the upper sides of 8 branches near the middle of the middle canopy (unit: celsius / missing value: NA)
  18. lc.wood.temp: sapwood temperature at about1 cm depth, measured 20-40 cm from the bole on the upper sides of 8 branches near the middle of the lower canopy (unit: celsius / missing value: NA)
  19. bole.wood.temp: sapwood temperature at 4 cm depth in tree boles, measured on both the north and sides at about 1.5 m height (unit: celsius / missing value: NA)
  20. soil.resp: estimated from measurements at 12 randomly chosen locations within 50m of the canopy-access tower (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  21. foliage.resp: rates estimated separately for upper and lower canopy foliage. Middle canopy estimate was the average of these (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  22. seedcone.resp: seed cone respiration estimate (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  23. wood.resp: estimated from wood temperatures and measurements of respiration in all canopy layers and in boles (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  24. total.resp: sum of other respiration estimates (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  25. net.photosynthesis: rates estimated separately for upper and lower canopy foliage. Middle canopy estimate is the average of these. (unit: micromolePerMeterSquaredPerSecond / missing value: NA)
  26. nee: total.resp minus net.photosynthesis (unit: micromolePerMeterSquaredPerSecond / missing value: NA)