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Full carbon budget accounting under both present and future atmospheric carbon dioxide conditions

Forests account for more than 75% of the carbon stored in terrestrial ecosystems and approximately 40% of the carbon exchange between the atmosphere and the terrestrial biosphere each year. Elevated CO2 is reported to stimulate tree growth and net primary production (NPP) in intact forest ecosystems. In the EUROFACE project, the different carbon pools of above and belowground ecosystem processes were determined by the different partners during the period 2002-2004. We integrated the different pools to calculate the NPP and net ecosystem productivity (NEP). As elevated CO2 increased both above- and belowground carbon pools, and soil C content increased after an initial decline (i.e. priming effect), we hypothesize that the net carbon storage capacity of this poplar plantation may increase in a future CO2 enriched atmosphere.

From the different teams of the EUROFACE consortium, data were gathered and structured into lists of input parameters for the ANAFORE model describing the climate, the EUROFACE stand, species characteristics, and the soil under control conditions.

As the model simulates ecosystem processes on a daily bases, and integrates the pools and fluxes to a yearly scale, data collected in the field were structured on a yearly base, showing the differences between CO2 and fertilisation treatments. Preliminary runs show that the ANAFORE model is able to simulate the effects of future atmospheric CO2 levels on the above and belowground carbon partitioning in a poplar coppice plantation. However, as the data from the three growing seasons are only becoming available now after finalisation of the project, the final parameterisation is under way.

The different carbon pools in the high density stand of three poplar species were calculated using data from the different teams collected during the second rotation cycle of the EUROFACE plantation. Elevated CO2 increased the standing biomass pools, both above (stem + branch biomass) and below (coarse and fine root biomass)-ground. Fertilisation did not significantly increase biomass production indicating nutrients were sufficient. Remarkable was the strong leap elevated CO2 gave during the first year after coppice: trees showed an accelerated growth under high atmospheric CO2, but during the third growing season (2004), although the standing biomass under FACE was stimulated and significantly larger, control trees gained more biomass during that year. We postulate that the initial stimulation declined when the canopy reached a climax. This pattern was confirmed by the leaf litter production: during the first and second year, poplar trees produced more leaves under elevated CO2, resulting in a larger yearly litter production. Fertilisation did not affect the litter pool. Litter production increased after the first growing season after coppice, and stabilized by the end of the second cycle.

The fact that our climate is changing is no longer an issue of debate, and predictions of the impact of rising atmospheric CO2 levels on the stocking of carbon on both regional and global scale are crucial to take the necessary precautions and actions in future. After all, an increased stocking of carbon in terrestrial biomes (forests), above and/or belowground, may slow down the present atmospheric CO2 increase and the greenhouse effect. Since decades, the effects of elevated CO2 have been studied, known by a gradual broadening of the eco-physiological processes and species investigated, and amelioration of the technique to simulate a future high CO2 atmosphere. We are now able to study, thanks to the FACE technique (Free air Carbon dioxide Enrichment), whole ecosystems under quasi natural circumstances. Apart from the focus of many scientists to understand the CO2 effects on processes at the cellular, leaf, or tree level, it is mainly important to integrate the data to the scale of the canopy or the ecosystem, to be able to validate current ecosystem models, and extrapolate predictions to a regional or even global scale. The full carbon budget of the EUROFACE plantation, the result of 2 rotation cycles of growth under elevated CO2, is the finalization of the EUROFACE project, and will gain insight in the long term response of the carbon sequestration in a bio-energy poplar short rotation coppice.

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