The possibility of a causal relationship between the parallel increases in atmospheric CO2 concentrations and in global temperature has pinpointed the urgent need of a thorough understanding of the global carbon (C) cycle. If we are to optimise general circulation models and make realistic climate projections, deeper insight is needed in the climate-dependency of processes such as primary productivity of ecosystems or turnover of soil organic matter. Although still far from being perfect, our understanding of the global C cycle has improved considerably over the past decades. The integration of atmospheric, terrestrial and oceanic modelling and experimental studies has significantly decreased uncertainties and has allowed separation of terrestrial and oceanic carbon sinks. Nonetheless, considerable uncertainty remains regarding the continental distribution of the Northern-Hemispheric C sink.
Over very long time periods, C sequestration in terrestrial ecosystems depends primarily on the occurrence of catastrophic events, such as fires, pest outbreaks, hurricanes or floodings. At shorter time scales, the net exchange of C between terrestrial ecosystems and the atmosphere is determined by the difference between photosynthetic C uptake and its release through autotrophic and heterotrophic respiration, and is typically one order of magnitude smaller than these nearly-offsetting terms. Both photosynthesis and respiration are strongly climate-dependent (although the parameters exerting dominant control differ). Because of the large interannual variability in climate, also the interannual variability in the net exchange of C between ecosystems and the atmosphere is very large. At this moment, our understanding of the climate-dependency of the net C exchange is still limited. Temperature-response functions, for example, vary among seasons, as well as among years. Unless we understand which factors determine this seasonal and interannual variability, we cannot further reduce the uncertainty surrounding future climate projections. If we cannot accurately predict how climate change will affect the balance between uptake and release of C from terrestrial ecosystems and oceans, we cannot determine whether natural ecosystems will mitigate or exacerbate global warming. Thus, there is an urgent need of studies that relate net C exchange to climate at both short (heat waves, drought spells, etc) and longer periods (El Nino-Southern Oscillation, North Atlantic Oscillation), but also for a variety of different climatic regions (boreal, maritime-temperate, continental-temperate, mediterranean).
In this project, we will focus on three main objectives that differ in both the spatial resolution as in the carbon-cycle components being studied. The first objective of this research proposal is to quantitatively estimate the net biospheric C sink of the entire European continent. This will be done using a dual approach, namely up-scaling of C inventories and validation of these results with estimates obtained independently with inverse atmospheric tracer-transport models. The second objective of this research proposal is to study in great detail the relationship between the temporal variability in climate and in net C exchange between terrestrial ecosystems and the atmosphere. For this study we will analyse measured fluxes obtained with the 'eddy covariance" technique in a selection of ecosystems from different climatic regions at both short and long time scales. The third objective of this research proposal goes one step further: we will try to deconvolute the net C exchange into the component fluxes (photosynthetic uptake, above-ground respiration, soil C efflux, heterotrophic respiration) and study their climate-dependency. This "case" study is restricted to one ecosystem (a mixed coniferous/deciduous forest at Brasschaat, Belgium), where our research group has been studying in great detail the balances of C, nutrients and water since