It is often assumed that daytime patterns of ecosystem carbon assimilation are mostly driven by direct physiological responses to exogenous environmental cues. Under limited environmental variability, little variation in carbon assimilation should thus be expected unless endogenous plant controls on carbon assimilation, which regulate photosynthesis in time, are active. We evaluated this assumption with eddy flux data, and we...

Phenology, by controlling the seasonal activity of vegetation on the land surface, plays a fundamental role in regulating photosynthesis and other ecosystem processes, as well as competitive interactions and feedbacks to the climate system. We conducted an analysis to evaluate the representation of phenology, and the associated seasonality of ecosystem‐scale CO 2 exchange, in 14 models participating in the N orth A merican C a...

Vegetation albedo is a critical component of the Earth's climate system, yet efforts to evaluate and improve albedo parameterizations in climate models have lagged relative to other aspects of model development. Here, we calculated growing season albedos for deciduous and evergreen forests, crops, and grasslands based on over 40 site‐years of data from the AmeriFlux network and compared them with estimates presently used in th...

Interannual variation of carbon fluxes can be attributed to a number of biotic and abiotic controls that operate at different spatial and temporal scales. Type and frequency of disturbance, forest dynamics, and climate regimes are important sources of variability. Assessing the variability of carbon fluxes from these specific sources can enhance the interpretation of past and current observations. Being able to separate the va...

Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activit...

Tower‐based eddy covariance measurements of forest‐atmosphere carbon dioxide (CO 2 ) exchange from many sites around the world indicate that there is considerable year‐to‐year variation in net ecosystem exchange (NEE). Here, we use a statistical modeling approach to partition the interannual variability in NEE (and its component fluxes, ecosystem respiration, R eco , and gross photosynthesis, P gross ) into two main effects: v...

Annual budgets and fitted temperature response curves for soil respiration and ecosystem respiration provide useful information for partitioning annual carbon budgets of ecosystems, but they may not adequately reveal seasonal variation in the ratios of these two fluxes. Soil respiration ( R s ) typically contributes 30–80% of annual total ecosystem respiration ( R eco ) in forests, but the temporal variation of these ratios ac...

The δ 13 C values of atmospheric carbon dioxide (CO 2 ) can be used to partition global patterns of CO 2 source/sink relationships among terrestrial and oceanic ecosystems using the inversion technique. This approach is very sensitive to estimates of photosynthetic 13 C discrimination by terrestrial vegetation (Δ A ), and depends on δ 13 C values of respired CO 2 fluxes ( δ 13 C R ). Here we show that by combining two independ...

Seven years of carbon dioxide flux measurements indicate that a ∼90‐year‐old spruce dominated forest in Maine, USA, has been sequestering 174±46 g C m −2 yr −1 (mean±1 standard deviation, nocturnal friction velocity ( u * ) threshold >0.25 m s −1 ). An analysis of monthly flux anomalies showed that above‐average spring and fall temperatures were significantly correlated with greater monthly C uptake while above‐average summer...

Summary Carbon dioxide, water vapour, and sensible heat fluxes were measured above and within a spruce dominated forest near the southern ecotone of the boreal forest in Maine, USA. Summer, mid‐day carbon dioxide uptake was higher than at other boreal coniferous forests, averaging about – 13 μmol CO 2 m –2 s –1 . Nocturnal summer ecosystem respiration averaged ≈ 6 μmol CO 2 m –2 s –1 at a mean temperature of ≈ 15 °C. Significa...