HR: 0800h
AN: B51A-0937    [Abstracts]
TI: Carbon Exchange of Central New England Deciduous Forests: Variability Related to Age and Topography
AU: * Hadley, J L
EM: jhadley@fas.harvard.edu
AF: Harvard University, Harvard Forest PO Box 68, Petersham, MA 01366 United States
AU: Kuzeja, P S
AF: Harvard University, Harvard Forest PO Box 68, Petersham, MA 01366 United States
AU: Schedlbauer, J L
EM: sche4619@uidaho.edu
AF: University of Idaho, Dept. of Forest Resources, Moscow, ID 83844 United States
AU: Munger, J W
EM: jwm@io.harvard.edu
AF: Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA 02138 United States
AB: Forests in much of the northeastern U.S. occur in hilly or mountainous terrain and vary widely in age, due to forest harvesting and natural disturbances. Sites in the NE U.S. with relatively long-term C exchange records represent two very different major tree species associations (boreal coniferous forest and oak-maple dominated deciduous forest) but cover relatively little variation in topography and age. All of the forests measured are in somewhat low-lying areas and are fairly mature, ranging from 65 to $>$120 years in age. Data are needed from younger forests and forests with higher slope position in order to accurately estimate forest C storage in the NE U.S. In May 2002 we began the first eddy covariance (EC) measurements in a higher deciduous forest, about 1.1 km from the Harvard Forest Environmental Measurement Site (HFEMS), where C exchange has been measured since 1991. The higher site has similar tree species composition to HFEMS, but most trees within 300 m of the higher eddy covariance tower (and some beyond) originated after a fire in 1957. Wind direction and nocturnal turbulence strongly affect EC data at the higher site. With wind between 30 and 210 $^{o}$ from N, we observe large apparent C effluxes ($>$30 æmol m$^{-2}$ s$^{-1}$) at night, and sometimes during the day. Such large C effluxes have very seldom been observed at HFEMS, and at the higher site we interpret them as artifacts generated by lee-slope turbulence, due to airflow over forest that is 20-30 m higher than the point of EC measurements. With other wind directions, nocturnal C flux at the higher site increases with increasing turbulence. We attribute this to cold air drainage on the long approximately 10% slope to the W and NW. This inference is supported by very low measured C fluxes when air 20 cm from the ground is $>$ 1.5 $^{o}$C colder than air above the canopy. Accordingly, at the higher site we only accept C flux data if wind direction is between 215 and 360$^{o}$ and u* $>$ 0.35 m/s. Under these conditions, nocturnal C fluxes measured at the higher site were within the range measured at the HFEMS throughout 2002 and 2003, except during NW winds. With NW winds, the footprint of HFEMS includes a bog, and measured C fluxes were significantly higher. When the wind direction and turbulence criteria were not met, we estimated C flux using statistical models derived from the acceptable EC data. A combination of valid data and models indicates that in the summers of 2002 and 2003 C storage at the higher site was nearly equal to C storage at HFEMS. Peak monthly C storage of 1.9-2.1 Mg/ha occurred in July each year. However, during December through March estimated monthly C loss at the higher site was only about half as great as at HFEMS (0.20-0.28 versus 0.38-0.53 Mg//ha), perhaps due to frequent NW winds in winter. In sum, our first two years of data show that the younger, higher deciduous forest had similar annual net ecosystem exchange (NEE) (about 2.0 to 2.5 Mg C ha$^{-1}$ y$^{-1}$) as the 65-100 year old forest at the HFEMS site. Lower average ecosystem respiration at the higher site, possibly caused by a lack of wetland areas, may allow the higher forest to maintain approximately the same NEE with a smaller annual GEE.
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting