Document: JUL-3-30-40

Carbon exchange of an old growth eastern hemlock forest: Model predictions versus eddy covariance measurements.

HADLEY, J.L.*

Harvard University Forest, Petersham, MA 01366 USA ¹

Abstract:
Predictions of a carbon exchange model for an old-growth eastern hemlock (Tsuga canadensis L.) forest in central Massachusetts were compared with eddy covariance measurements above the forest in July and August 1999. This forest is about 200 years old, with dominant trees up to 1 m in diameter and 25 to 30 m tall. The model predicts net photosynthesis (Pn) from photosynthetic photon flux density (PFD) throughout the canopy and from air temperature, water vapor pressure deficit and time of day. Respiration of foliage, wood and soil are functions of temperature. There are separate models for wood respiration during wood production and for the rest of the year. During the test period, the model predicted maximum canopy photosynthesis of 15 mol m^-2 s^-1, maximum daytime net ecosystem exchange (NEE) of -10 mol m^-2 s^-1 and maximum night ecosystem respiration (Rn) of 12 mol m^-2 s^-1. Daytime NEE measured by eddy covariance reached 18 mol m^-2 s^-1, but maximum Rn was only 8 mol m^-2 s^-1. Total NEE from the model during daylight was about 40% lower than eddy flux, but modeled Rn was nearly twice the eddy flux. The model predicted carbon release of about 1 g m^-2 d^-1, while eddy flux showed 1.5 g m^-2 d^-1 of C uptake. These differences could have been due to trapping of air beneath the hemlock canopy, especially during calm nights, when there were periods up to 3 hr long without any measured carbon flux. Long-term CO₂ profile measurements at a nearby site showed a strong inverse correlation between CO₂ storage and momentum transfer from the atmosphere to the forest.

Keywords: hemlock,Tsuga canadensis,carbon,photosynthesis,respiration

This abstract is being presented at: 3:15 PM in session:
Oral Session #50: Plant Gas Exchange.