Yi, C., K.J. Davis, P.S. Bakwin, T. Zhou, D.D. Baldocchi, M.P. Butler, B.D. Cook, A. Desai, A. L.Dunn, E. Falge, J.W. Munger, D.M. Ricciuto, W. Wang, K. Wilson, and S.C. Wofsy, submitted. The observed responses of forest carbon exchange to climate variations from daily to annual time scale, Journal of Geophysical Research D: Atmospheres.

The climate factors controlling forest carbon dioxide exchange may vary as a function of time scales. This hypothesis is examined using direct measurements of net ecosystem exchange (NEE) of CO2 and meteorological data obtained over multiple years from four forest sites in North America. We find that: (1) the relationship between nighttime NEE (respiration) and temperature is strong on daily, monthly and seasonal time scales, but breaks down on an annual time scale. Autumn respiration rates are 1 to 3 times higher than spring respiration rates and the temperature sensitivity of soil respiration is lower in summer than in spring and autumn. (2) Unlike temperature, precipitation is poorly correlated with NEE on daily, monthly, and seasonal time scales but is well correlated on an annual time scale. Forest NEE is influenced by annual variability in precipitation because soil moisture during the growing season, which is often associated with forest growth and CO2 uptake, is controlled in part by cold season precipitation. (3) Net radiation is a comprehensive climate variable that is related to temperature, light, water vapor, cloud cover, and albedo; and it is correlated with NEE on all time scales. (4) Although NEE is correlated to photosynthetically active radiation on daily, monthly and seasonal time scales, the relationship breaks down on an annual time scale because the respiration component of NEE is not coupled to light. (5) We observe that drought stress has a very strong effect on forest uptake of CO2. The radiative index of dryness, defined as a ratio of annual net radiation and precipitation, is proposed as a governing parameter for the interannual variability of NEE of CO2.