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

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.