Hemlock Tower - Net Carbon Exchange of an Old-Growth Hemlock Forest
HF103 Overview Data EML Archive- Investigators: David Foster, Julian Hadley, Jessica Schedlbauer
- Contact: Julian Hadley
- Start date: 2000-11-02
- End date: 2001-11-01
- 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)
- Keywords: carbon dioxide, forest age, forest type, photosynthesis, respiration
- 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.
- Related datasets: HF004 HF063 HF072