Hemlock Tower - Physiological Model of CO2 Exchange by Hemlock Forests

• Investigators: David Foster, Julian Hadley
• Contact: Julian Hadley
• Start date: 1996-08-01
• End date: 2000-06-30
• Location: Prospect Hill Tract (Harvard Forest)
• Latitude: +42.539
• Longitude: -72.180
• Elevation: 355 meters
• Taxa: Tsuga canadensis (eastern hemlock)
• Keywords: carbon dioxide, hemlock, photosynthesis, 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.

• Related datasets: HF004 HF103