HR: 0830h
AN: B51E-1006    [PDF]
TI: Interannual Variability in Regional-Scale Measurements of ecosystem-Atmosphere CO2 Exchange at the WLEF Tall Tower: A Testbed for Regional Scaling Methodology.
AU: * Ricciuto, D M
EM: ricciuto@essc.psu.edu
AF: The Pennsylvania State University Department of Meteorology, 503 Walker Building, University Park, PA 16802 United States
AU: Davis, K J
EM: davis@essc.psu.edu
AF: The Pennsylvania State University Department of Meteorology, 503 Walker Building, University Park, PA 16802 United States
AU: Bakwin, P S
EM: pbakwin@cmdl.noaa.gov
AF: NOAA/CMDL, 325 Broadway, Boulder, CO 80303 United States
AU: Bolstad, P
EM: bolstad@umn.edu
AF: The University of Minnesota Department of Forest Resources, 115 Green Hall 1530 N. Cleveland Ave., St. Paul, MN 55108 United States
AU: Cook, B
EM: bcook@essc.psul.edu
AF: The University of Minnesota Department of Forest Resources, 115 Green Hall 1530 N. Cleveland Ave., St. Paul, MN 55108 United States
AU: Yi, C
EM: cxyi@essc.psu.edu
AF: The Pennsylvania State University Department of Meteorology, 503 Walker Building, University Park, PA 16802 United States
AU: Wang, W
EM: wang@essc.psu.edu
AF: The Pennsylvania State University Department of Meteorology, 503 Walker Building, University Park, PA 16802 United States
AU: Butler, M
EM: mpbutler@essc.psu.edu
AF: The Pennsylvania State University Department of Meteorology, 503 Walker Building, University Park, PA 16802 United States
AU: Teclaw, R
EM: rteclaw@fs.fed.us
AF: U.S. Forest Service, 5985 Highway K, Rhinelander, WI 54501 United States
AB: We present the interannual variability of net ecosystem-atmosphere exchange (NEE) of CO2 observed from WLEF, a 447 meter tall tower in a mixed forest in northern Wisconsin. NEE data from this site are unique because the flux footprint is 10-100 times larger than the footprints of most Ameriflux towers. This large footprint integrates fluxes from a highly varied ecosystem, including upland forests, wetlands, managed areas, and disturbed areas. Unlike most North American flux towers, the NEE measured at WLEF is consistently positive, indicating a source of CO2 to the atmosphere. High-accuracy CO2 mixing ratio measurements at the site allow us to compare inverse and eddy-covariance methods of estimating NEE of CO2. Such comparisons will aid in our understanding of the spatial representativeness of eddy covariance measurements and the coherence of interannual variability in these measurements over large spatial scales. During the period 1997-2002, the WLEF flux footprint was observed to be an average source of 80 gC m-2 yr-1. The range of interannual variability over this six year period is about 125 gC m-2. This range is larger than the observed random and systematic errors at WLEF. Both disturbances and climate variations are observed to play roles in the variability of NEE, ecosystem respiration (RE) and photosynthesis. For example, a drought in 1998 and a caterpillar infestation in 2001 have significant impacts on NEE during the growing seasons of those years. Although soil temperature and photosynthetically active radiation (PAR) are excellent predictors of NEE on hourly to daily timescales, other climate variables such as precipitation and growing season length become important at monthly to yearly timescales. For the purposes of upscaling studies, it is important to understand why the WLEF region is a source of CO2. Although the WLEF flux tower footprint is a mix of forested wetlands and uplands, we cannot use a linear combination of NEE observations from our nearby wetland and upland small towers to obtain the NEE for WLEF. Both the wetland and upland sites are small sinks of CO2. This failure to upscale the smaller flux tower footprints to the large WLEF footprint is caused by values of RE that are consistently higher at WLEF than at the other towers. These high values of RE may be the result of several factors. Evidence exists that the water table in the region is dropping, possibly exposing organic matter formerly in wetlands to aerobic respiration. Management and disturbance also play significant roles in the WLEF footprint, but may not be important in the footprints of the smaller towers. The comparison of NEE variability at WLEF with NEE variability computed with inverse methods and of nearby sites will yield insight into the scale and the nature of the disturbance causing CO2 release at WLEF. This abstract focuses on an analysis of the causes of interannual variability and long-term net ecosystem release of CO2 at WLEF. Companion abstracts will discuss comparisons to the nearby wetland and upland towers, and experimental inverse approaches that are being attempted in the region.
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting