Simulations of the global carbon cycle are strongly dependent upon model representations of the exchange of carbon, energy, moisture and momentum between the atmosphere and terrestrial biosphere. The carbon flux produced by these biophysical models is subsequently dependent on the method used to produce respiration and photosynthesis within the model on both spatial and temporal scales. We use an updated version of the Simple Biosphere Model (SiB3) to simulate global carbon flux between atmosphere and land surface, and compare model results to flux tower and flask network observations. SiB3 assumes no annual net source or sink of carbon in each gridcell, but the spatial pattern and seasonality of carbon flux and atmospheric concentration can be strongly influenced by parameterization of heterotrophic and autotrophic respiration and the representation of vegetation phenology.

Understanding the geographical distribution of carbon uptake by the terrestrial biosphere is critical for predicting future trends of atmospheric CO₂.  With inverse techniques, atmospheric CO₂ measurements can be used to estimate this uptake. The results from this approach, however, depend on the accuracy of the transport model(s).  Because of the covariance between the seasonally-varying biosphere exchange and the strength of vertical mixing (the rectifier effect), using only the surface CO₂ observations for this analysis yields an inferred carbon flux that is highly sensitive to the details of the boundary-layer dynamics in the transport model [Gurney et al., 2004]. One possible way to reduce the sensitivity of these inversions to poorly-represented boundary-layer dynamics is to use CO₂ vertical profiles (and/or column CO₂ measurements) in addition to surface observations. In theory, multi-level aircraft CO₂ measurements from several well-positioned sites are capable of improving the estimate of the true annual mean inter-hemisphere CO₂ gradient and thereby improving the estimate of the partitioning of carbon sinks between the two hemispheres.

In a temperate deciduous forest in Japan situated complex terrain, net ecosystem CO₂ exchange (NEE) was estimated using micro-meteorological method, and net ecosystem products (NEP) was estimated by measures of major carbon pools and fluxes using biometric and chamber methods. In this study we evaluate the threshold value of u* for interpolation in case stability was high using estimated NEP acquired by biometric method. And the function which relate temperature to FCO₂ for interpolation was evaluated by the data acquired using automated chamber for soil, CWD, trunk and foliage CO₂ exchanges. Averaged net uptake of CO₂ measured by eddy covariance method from 1999 to 2002 was 3.4 tC yr^-1 ha^-1 without compensation of nighttime underreport. Increase of live under and above ground biomass from 1994 to 1999 was 1.56 tC yr^-1 ha^-1. U* threshold values based on biometric and chamber NEP were respectively 0.28 and 0.35 m sec^-1

Precise CO₂ concentration measurements at marine stations and tall towers are crucial for quantifying global trends in atmospheric CO₂ concentrations. We propose that measurements in the continental planetary boundary layer—the poor cousin of the clean background stations—can be used to understand trends in, and controls, of atmospheric CO₂ concentrations at local and regional scales as well as global scales. The key is choosing appropriate time scales of integration for the data. In the US Southern Great Plains, we are measuring precise CO₂ concentrations continuously at 2–60 m and weekly at 300 and 3300 m above ground level (agl). CO₂ flux is measured in individual crop fields and pastures (4 m towers) and at 60 m. The precise CO₂ concentrations show strong continental influence in both diurnal and seasonal cycles. In continental regions, atmospheric CO₂ profiles are strongly influenced by atmospheric dynamics as well as ecosystem and anthropogenic fluxes. Relating site level measurements or atmospheric profiles to regional CO₂ budgets requires methods to represent or evaluate these influences. We observe inter-annual differences in the climatology of diurnal cycles (seasonal average diurnal cycles). Using the several years’ data for boundary layer concentrations, the annual trend in CO₂ growth nearly matches the value estimated by National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring Diagnostic Laboratory for our latitude band.

Understanding the energy/H₂O/CO₂ exchange processes is very important for evaluating the roles of tropical rain forest in climate change. The sensible heat, latent heat, and CO₂ fluxes above a tropical rain forest in Peninsular Malaysia were measured using the eddy covariance method for the year 2003. The diurnal patterns of energy, H₂O and CO₂ flux were investigated using a multi-layer model that considered patchy stomatal closure. Both bimodal and homogeneous stomatal opening distributions were simulated, and the results indicated that the observed negative relationship between CO₂ absorption under light-saturated conditions and vapor pressure deficit were not sufficiently explained by stomatal closure alone, for homogeneous stomatal opening distributions. For bimodal stomatal opening distributions, however, a greater depression in canopy photosynthesis was found with increased atmospheric vapor pressure deficit. These results strongly suggested that the depression in canopy photosynthesis was caused not only by stomatal closure limitation but also by the patchy (bimodal) stomatal behavior response to the increased atmospheric vapor pressure deficit. Thus, the midday depression in canopy photosynthesis was mainly caused by patchy (bimodal) stomatal closure.

In this study, soil CO₂ effluxes determined from CO₂ concentration gradients were compared to effluxes obtained with automated chamber measurements. The CO₂ concentrations showed a diurnal pattern following the soil temperature the concentrations increasing with increasing soil depth. Both methods gave comparable CO₂ effluxes indicating that the gradient method provides an alternative method for monitoring soil CO₂ effluxes.

There remain large uncertainties in our model estimations of terrestrial CO₂ budget at broad scales. We used two terrestrial carbon cycle models (Sim-CYCLE and SASAI) and three climate datasets (NCEP/NCAR, NCEP/DOE, and ERA40) for the period from 1982 to 2001 and performed cross-comparison, aiming at clarifying the source of uncertainties. Using the same model, different carbon budgets were obtained by the three climate datasets, globally due to the difference in solar radiation and locally due to precipitation. The two models, which differ in canopy processes, estimated different temporal trends and spatial patterns of CO₂ budget during the experimental period. This study exemplified the necessity of developments in both models and datasets.

Simultaneous and continuous measurements of O₂ and CO₂ made in the air around terrestrial ecosystems have the potential to improve our understanding of the biogeochemistry of the ecosystem, and may reduce uncertainties in estimates of terrestrial carbon uptake derived from atmospheric O₂ measurements. Following the design of Stephens et al. [2001], we have constructed an instrument that performs continuous in situ measurements of atmospheric O₂ and CO₂ concentrations. We present design and performance data, along with preliminary results from a deployment at the Environmental Measurement Site at Harvard Forest in central Massachusetts.