Current analyses with respect to the global carbon budget suggest that there is a strong northern hemispheric sink relative to the southern hemisphere. These conclusions are based on the changes in O2/N2 with respect to time that suggest that there is a global land sink. In addition, the O2/N2 data coupled with the meridional gradient in atmospheric CO2 concentrations suggest that the northern hemisphere is a stronger sink relative to the southern hemispheric sink. However, the processes that govern the gross hemispheric interpretations are poorly quantified. Ciais et al. (1995) results indicate that the northern temperate zone was a large terrestrial sink (3.5 GTC) in 1992 and 1993, whereas the biosphere in the northern tropics (from equator to 30N) was a large source (2.01.3 GTC) of carbon. This estimate is considerably higher than the estimate of 0.9GTC by Houghton et al. (1987). Ciais et al. (1995) attributed this difference to either a problem with their intra-hemispheric transport between the tropical and temperate latitudes or that previous deforestation estimates were lower than the actual deforestation. It is also interesting to note that they estimated that the southern tropics were a small terrestrial sink in 1992 and 1993. This is surprising because this latitudinal band contains Brazil and Indonesia, two regions that have gained recent notoriety for extensive deforestation practices. Estimates of CO2 emission from tropical land use rank these two countries as the top two in terms of total emissions due to deforestation. The weak terrestrial sink in the southern tropical zone suggests that either the forests have a positive net ecosystem production (due to fertilization of undisturbed forest and/or secondary growth formation), decadel average rate of deforestation was much higher than actual deforestation in 1992 and 1993, or that large rates of abandonment during this time period coupled with lower deforestation rates further dampened the biogenic source of carbon in the tropics due to land cover change (Skole et al. 1998). If global inversion activities (e.g. TransCom) are to use their models to invert atmospheric CO2 to the appropriate hemispheric sources and sinks, it is critical that the net biospheric carbon fluxes from this important land-use practice are better quantified. Annual, spatially explicit estimates of CO2 emissions due to deforestation and abandonment would help resolve some of the issues and discrepancies resulting from these recent carbon models using annual atmospheric measurements. Annual deforestation data could be used to validate or identify problems with these carbon models, such as intra-hemispheric transport between the tropical and temperate zones or potential positive NEP in the tropical forests (Ciais et al. 1995). The only way to properly validate these annual models is to have contemporaneous data on annual changes in carbon emissions due to land use stratified on the same regional basis as the inversions.

Forest removal for agricultural or pastoral use results in significant losses of carbon from both the soil and plant systems through increased heterotrophic respiration and loss of carbon stored in biomass. It is also commonly accepted that the rates of carbon accumulation during secondary succession are a function for many factors, including type of disturbance (e.g. method of clearing, temperature and frequency of burning), agricultural practices (intensity), size of disturbance, and climate. During succession total biomass and soil organic matter increase toward a climax state, however this climax state may differ from the pre-disturbance state. What is unclear, however, is the interplay between edaphic, climatic and land use controls on the rates of biomass uptake during succession. For instance, average annual rate of above ground biomass accumulation in abandoned pastures near Paragominas in the eastern Amazonia varied from 0.63 to 11.3 tons per hectare per year (Nepstad et al. 1991).

The current IGBP/GAIM land-use/land-cover project led by Rik Leemans is focused on the development of a much-needed database that describes global changes in land cover over the past 200+ years. Along these lines, recent analyses by Ramunkutty and Foley (1999) have provided global cropland maps over the past 100 years. Results from INPE’s PRODES project indicate that the average annual rate of deforestation in the Amazon from 1978 to 1997 was approximately 20,000 km2/yr. However, their results indicate large inter-annual variability in these rates, ranging from 11,000 km2/yr (occurred in 1991, the time period for the Ciais et al. 1995 inversion) to 29,000 km2/yr (in 1995). A major focus of recent LUCC studies in the tropics has been to understand the inter-annaul variability in the rates of deforestation, rates of abandonment, and persistence of secondary forests. Within the framework of land-use and land-cover changes, deforestation and reforestation are important drivers of potential changes to the local and regional biogeochemistry and ultimately in the regional and global climate system.

We would like to propose an IGBP/GAIM workshop that would bring together four generally disparate groups: inversion, dynamic vegetation modelling, direct observation of land cover dynamics (LUCC focus 2), and field observation communities. The atmospheric inversion modellers have raised some interesting questions that highlight our uncertainties about the biospheric contribution to the interannual and zonal distribution of CO2 fluxes. We would like to pose two scenarios that can be used to constrain inversion models:

The overall goals of the workshop would be to bring together the four communities to better understand what the relative importance of deforestation and subsequent land-use practices might have on regional to hemispheric and ultimately, global carbon fluxes. It is thought that if the inversion community could be represented, a better communication between these global transport modelers and the terrestrial community might be fostered. Through this type of workshop, it would also be possible to introduce, and perhaps test and expand the hypothesis proposed by John Grace; that a non-zero NEP for mature forests may be a general rule rather than an exception. In addition, we would like to include members of not only the tropical forest community, but also boreal and temperate forest scientists to explore and test these ideas.