THE DYNAMICS OF BIOMASS AND NUTRIENT ACCUMULATION IN A DOUGLAS-FIR (PSEUDOTSUGA-MENZIESII FRANCO) STAND STUDIED USING A CHRONOSEQUENCE APPROACH

Douglas-fir is presently one of the dominant species used in France for afforestation. It was widely introduced without knowing exactly its effects on soils and superficial waters. The aim of this research was to obtain quantitative data on the biogeochemical functioning of a Douglas-fir ecosystem, and to provide relevant information to forest managers. The results presented here concern nutrient accumulation on a stand according to its stage of development. A chronosequence of stands was used to rapidly obtain information on ecosystem dynamics in particular because it is possible to assimilate stands of different ages to the different stage of development of a single stand. The present results make it possible to evaluate with sufficient accuracy nutrient losses at crop harvest. Several scenarios could be drawn using various rotation lengths and harvesting intensities. These results could be introduced in management models. Nevertheless, a direct relationship between nutrient losses and the immediate or delayed decrease of forest productivity is not easy to predict. It depends more on soil nutrient dynamics than on the present available nutrient pool. Comparison between the two main species used in forestry showed that the nutrient efficiency of Douglas-fir for biomass production was at least the same as for Norway spruce. This important conclusion needs to be confirmed by other measurements.