Understanding the diverse elements of the global carbon cycle has been the focus of much recent research [Prentice et al. 2001, Schimel et al. 2001, Gurney et al. 2002, House et al. 2003]; research that is vital to our understanding of the missing sink, future atmospheric carbon dioxide concentrations, and future climate [Fan et al. 1998, Houghton et al. 1998]. Much research has concentrated on carbon dynamics of the large ocean basins [Lee et al. 1998, Le Quéré et al. 2003] and terrestrial exchange in North America and Eurasia [Pacala et al. 2000, Schimel et al. 2000]. Despite representing 20% of the global land mass, Africa has thus far been largely neglected in these studies. We will examine current understanding of carbon stocks and fluxes within Africa and discuss how uncertainty in global carbon dynamics arises in part from uncertainty in the African components. We outline areas where new measurements and research in Africa can contribute to understanding at both continental and global scales.

The mixing ratio and the surface-atmosphere exchange of carbon dioxide have been monitored at different elevations on a tall tower in West Hungary (Hegyhátsál, 46^o57'N, 16^o39'E, 248 m asl) since 1994 and 1997, respectively. The vertical mixing ratio profile measurements along the 115 m tall tower has been completed with occasional aircraft measurements up to 3000 m above the ground. The poster presents the Hungarian tall tower site and the temporal variation of carbon dioxide observed here. We discuss the region of influence determining the mixing ratio variability, the so-called concentration footprint, as well as that of the flux measurements. Methodological problems caused by the elevated monitoring levels, and their solutions, are also given. The environmental factors governing the net ecosystem exchange (NEE) of the vegetation are analyzed by means of a process oriented ecosystem simulation model. It might be used to estimate the future behavior of the region as the climate is changing. On the basis of the measurements at Hegyhátsál a boundary layer model has been developed which can give rough surface-atmosphere carbon dioxide flux estimate for sites where only surface mixing ratio monitoring is available.

Two major disturbances of forests in western North America are fires and insect outbreaks.  Much research has focused on the effects of fires on the carbon cycle; little research to date has occurred about outbreaks.  We are using observations, including field measurements and remotely sensed imagery, together with ecosystem and insect population modeling improve our understanding of these disturbances on carbon cycling. 

Evaluation of the difference between annual net CO₂ ecosystem exchanges (NEE) from the open- and the closed-path data is important for site intercomparison studies. However, long-term measurements of NEE using both systems have been limited. We report the comparison of eddy CO₂ fluxes measured with open- and closed path systems for three years from 2001 through 2003. The annual GPP estimated from closed-path data was 8–10% less negative than that from the open-path data, whereas the annual RE was 11–16% more positive for closed-path data. Consequently, the annual NEE from the closed-path data was less negative by 301–333 gC m^-2 y^-1. The bias of NEE between two systems is large and an extremely important issue. Ecophysiological approaches are needed to validate of the eddy covariance technique.

The long-term net flux of carbon between terrestrial ecosystems and the atmosphere has been dominated by two factors: changes in the area of forests and per hectare changes in forest biomass resulting from management and regrowth. While these factors are reasonably well documented in countries of the northern mid-latitudes as a result of systematic forest inventories, they are uncertain in the tropics. Recent estimates of carbon emissions from tropical deforestation have focused on the uncertainty in rates of deforestation [Achard et al., 2002, 2004; DeFries et al., 2002; Houghton, 2003]. By using the nearly the same data for biomass, however, these studies have underestimated the total uncertainty of tropical emissions and may have biased the estimates. In particular, regional and country-specific estimates of forest biomass reported by three successive assessments of tropical forest resources by the FAO [FAO/UNEP, 1981; FAO, 1995; FAO, 2001] indicate systematic changes in biomass that have not been taken into account in recent estimates of tropical carbon emissions. The ‘changes’ more likely represent improved information than real on-the-ground changes in carbon storage. In either case, however, the data have a significant effect on current estimates of carbon emissions from the tropics and, hence, on understanding the global carbon balance.

The isotopic composition of the ecosystem respiration (d¹³C_ER) and the isotopic discrimination of the ecosystem (D_Eco) were retrieved from intensive campaigns (1998 to 2000) and from weekly diurnal sampling (2003) at a boreal forest site (Fraserdale, Canada, 49°53'N, 81°34'W). The results show that d¹³C_ER was less sensitive to temperature (T) variation compared with D_Eco, suggesting that the photosynthesis CO₂ flux was likely more sensitive to temperature than the ecosystem respiration CO₂ flux during the same period of time at the study site.

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.

The CO₂ flux from coarse woody debris (R_CWD) in a deciduous broad-leaved forest was measured using chamber measurements. The relationships between R_CWD and environmental factors, such as temperature (T) and the water content (θ) of the coarse woody debris (CWD), were determined from long-term continuous measurements. Measurements of the R_CWD of many CWD samples revealed relationships between R_CWD and CWD characteristics, such as wood density (ρ) and diameter (D). A field survey conducted in 2003 estimated the mass of CWD as 9.30tC·ha^-1, with snags amounting to 60% of the total CWD mass. Scaling R_CWD to the ecosystem while considering environmental factors according to the type (snag or log) of CWD and CWD characteristics, we estimated that the annual R_CWD in the forest was 0.50tC·ha^-1·y^-1 in 2003. This came to 13-19% of the total heterotrophic respiration in the forest. The mean annual CWD input mass from 2000 to 2004 was 0.61tC·ha^-1·y^-1. Therefore, 0.11tC·ha^-1·y^-1 were sequestered by CWD, which amounted to 7% of the net ecosystem production (NEP) in the forest. In a younger forest, it is difficult to assume that the CWD input and decomposition are balanced, so the R_CWD and CWD input mass should be quantified to evaluate the forest carbon cycle and NEP.