The carbon balance of a forest ecosystem may be a net storage, as is the case in young forests in the growth cycle, or it may also represent a net carbon loss for certain, ageing or declining forests, or for those suffering the consequences of disruption (storms, fires) (Valentini et al., 2000). This balance is subject to major spatial and temporal variations. In addition to current forestry operations (soil preparation, drainage, fertilisation or clearings), it is sensitive to the stand age and climate (Chen et al., 1999). The respective role of these different factors, which all interact, is particularly difficult to elucidate. The short and long term impact of forestry is unfortunately often ignored, in spite of its obvious importance.
Differing approaches and methods of measurement are available to study the processes concerned and their respective role in the carbon content of an ecosystem on varying spatial scales: measurements of turbulent fluxes, measurements of stocks, calculations based on data from repeated inventories.
Even though the recent progress in measuring the flows of carbon exchanged on the scale of ground cover allow us to correctly evaluate the methods of change of scale from leaf to ground cover, we have, on the other hand, a poor understanding of the link between the functioning of an ecosystem at the stand scale and the calculation of a stock of carbon and its variations at the regional or national level. Calculations based on woodland inventories and remote sensing are currently the only possible and verifiable way of detecting and measuring variations in national stocks. Despite the fact that these numbers are required in international protocols, we are only able to make a crude estimation of the changes in national stocks, and to quantify the respective impacts of land use changes, forestry and agronomical activities, indirect effects of human activities and non anthropogenic causes (e.g natural disturbances).
One of the main impacts of climatic changes will be to profoundly alter the potential ecological niche of species whose distribution is wholly or partly limited by climate. Such movements have already been observed for remarkable rare species which are clearly limited by climate. An awareness of these changes is particularly important in the field of forestry, where the species planted or naturally regenerated are for temporal horizons in the order of a century. The forestry choices made today are therefore crucial for the longevity of the forest in a changing climatic context.
Providing a long-term assessment of the future of the functioning of forest ecosystems raises various questions on their vulnerability to biotic and abiotic stresses. This prediction supposes the use of meteorological data from a climatic model with high spatial resolution, together with data associated to the ecosystems under consideration (species, fertility).
Amongst numerous studies devoted to the impact of climate change, very few concern the diseases of forest trees (Coakley et al., 1999; Chakraborty et al., 1998). However, parasites are a major limiting factor for forestry production, in quantity and quality, and may affect the survival of a species, or even an entire ecosystem (Weste and Marks, 1987). It is even more essential to predict the impact of global change on the parasitic risk to the forest when the management of woodland parasites is essentially based on a preventive approach, as recourse to curative methods of combating the problem is often unrealistic and not actually desirable. Forecasting parasitic risks is one of the elements of forest planning.
In terms of research needs, it is therefore important to:
• quantify the stocks and fluxes of carbon in the large forest ecosystems.
• simulate the future of carbon sequestration in these major forest types based on a climatic scenario with high spatial resolution.
• inventory the various forestry practices which have a significant impact on the carbon stocks and fluxes and estimate the impact of various forestry managements on the sequestration of carbon in these ecosystems and their harvested products.
• assess the vulnerability of woodland species to allow alternative proposals to be made: replacement of species, fire prevention methods.
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