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Over the last two and half decades, strong evidence showed that the terrestrial ecosystems are acting as a net sink for atmospheric carbon. However the spatial and temporal patterns of variation in the sink are not well known. In this study, we examined latitudinal patterns of interannual variability (IAV) in net ecosystem exchange (NEE) of CO2 based on 163 site-years of eddy covariance data, from 39 northern-hemisphere research sites located at latitudes ranging from ∼29°N to ∼64°N. We computed the standard deviation of annual NEE integrals at individual sites to represent absolute interannual variability (AIAV), and the corresponding coefficient of variation as a measure of relative interannual variability (RIAV). Our results showed decreased trends of annual NEE with increasing latitude for both deciduous broadleaf forests and evergreen needleleaf forests. Gross primary production (GPP) explained a significant proportion of the spatial variation of NEE across evergreen needleleaf forests, whereas, across deciduous broadleaf forests, it is ecosystem respiration (Re). In addition, AIAV in GPP and Re increased significantly with latitude in deciduous broadleaf forests, but AIAV in GPP decreased significantly with latitude in evergreen needleleaf forests. Furthermore, RIAV in NEE, GPP, and Re appeared to increase significantly with latitude in deciduous broadleaf forests, but not in evergreen needleleaf forests. Correlation analyses showed air temperature was the primary environmental factor that determined RIAV of NEE in deciduous broadleaf forest across the North American sites, and none of the chosen climatic factors could explain RIAV of NEE in evergreen needleleaf forests. Mean annual NEE significantly increased with latitude in grasslands. Precipitation was dominant environmental factor for the spatial variation of magnitude and IAV in GPP and Re in grasslands. [ABSTRACT FROM AUTHOR]

Abstract: Hourly measurements of O3, NO, NO2, PAN, HNO3 and NO y concentrations, and eddy-covariance fluxes of O3 and NO y over a temperate deciduous forest from June to November, 2000 were used to evaluate the dry deposition velocities (V d) estimated by the WRF-Chem dry deposition module (WDDM), which adopted scheme for surface resistance (R c), and the Noah land surface model coupled with a photosynthesis-based Gas-exchange Evapotranspiration Model (Noah-GEM). Noah-GEM produced better V d(O3) variations due to its more realistically simulated stomatal resistance (R s) than WDDM. V d(O3) is very sensitive to the minimum canopy stomatal resistance (R i) which is specified for each seasonal category assigned in WDDM. Treating Sep-Oct as autumn in WDDM for this deciduous forest site caused a large underprediction of V d(O3) due to the leafless assumption in ‘autumn’ seasonal category for which an infinite R i was assigned. Reducing R i to a value of 70sm−1, the same as the default value for the summer season category, the modeled and measured V d(O3) agreed reasonably well. HNO3 was found to dominate the NO y flux during the measurement period; thus the modeled V d(NO y ) was mainly controlled by the aerodynamic and quasi-laminar sublayer resistances (R a and R b), both being sensitive to the surface roughness length (z 0). Using an appropriate value for z 0 (10% of canopy height), WDDM and Noah-GEM agreed well with the observed daytime V d(NO y ). The differences in V d(HNO3) between WDDM and Noah-GEM were small due to the small differences in the calculated R a and R b between the two models; however, the differences in R c of NO2 and PAN between the two models reached a factor of 1.1–1.5, which in turn caused a factor of 1.1–1.3 differences for V d. Combining the measured concentrations and modeled V d, NO x , PAN and HNO3 accounted for 19%, 4%, and 70% of the measured NO y fluxes, respectively. [Copyright &y& Elsevier]