Long-term eddy covariance-based direct forest-atmosphere CO₂ flux studies have been proven useful not only for quantitatively evaluating the role of forest ecosystems in the carbon cycle, but also for allowing closer examination of the interaction between biotic and environmental factors that drive the observed CO₂ fluxes. This poster presents results from eight years (1996-2003) of eddy covariance-based ecosystem CO_2, water vapor and energy exchange at the Borden forest. The site represents a mid-latitude (44^o19’N, 79^o 56’ W), 100 year old, mixed deciduous and coniferous forest dominated by red maple, aspen and white pine species.

Results/Conclusions Analysis of long term data using light response curves of net ecosystem CO₂ exchange (NEE) shows a distinct seasonal and inter-annual variation in forest assimilation and ecosystem respiration (R) patterns. The effect of temperature and, to a lesser extent vapor pressure deficit, on CO₂ assimilation was evident when the annual and long-term growing season data was systematically pooled and correlated to the seasonal and year to year patterns of these variables. Differences in the start of the growing season and precipitation patterns were also related to inter-annual variation in forest activity. R values estimated from daytime flux data using light response function were consistently lower by ~15% compared to R estimated by an empirical respiration model relating nighttime CO₂ flux and soil temperature. However, the two independently determined values of R were highly correlated indicating that spatial heterogeneities and fetch problem are insignificant at this site. Over the 1996-2003 period, the forest on average sequestered 126[r1]  ± 146 gC m^-2 y^-1. The corresponding gross carbon gains and losses over this period were 904 ± 228 gC m^-2 y^-1  and -1028 ± 189 gC m^-2 y^-1, respectively. The Borden forest acted as a weak source of CO₂ to the atmosphere only in 1996 and 2001, releasing respectively -93 and -68 gC m^-2 y^-1. For the remaining six years, net carbon sequestration by the forest ranged from as low as 48 gC m^-2 y^-1 in 2002 to as high as 226 gC m^-2 y^-1 in 2000. The cumulative annual NEP in tons of carbon pre hectare (t C ha^-1) was estimated as: -0.9 (1996), 2.1 (1997), 1.5 (1998), 2.3 (1999), 2.6 (2000), –0.7 (2001), 0.5 (2002), 2.7 (2003).  These values are within the range of values reported for other mixed deciduous forests.