Nutrient dynamics in a chronosequence of Douglas-fir ( Pseudotsuga menziesii (Mirb). Franco) stands on the Beaujolais Mounts (France). 2. Quantitative approach

Abstract

Nutrient fluxes were determined in a chronosequence of Douglas-fir stands in the Beaujolais Mounts (France). Annual and seasonal variations occurred during the 3 years of investigation; fluxes were generally highest in autumn-winter. Atmospheric inputs were among the mean values from a monitoring network of forest ecosystems in France. Nutrient outputs from the soil profile were higher than average and occurred mainly during vegetation dormancy. Mean input-output budgets were negative for N, S, K, Ca and Mg, characterising an imbalance of the site chemistry dynamics. The P budget was positive. Most of the nutrient output from the ecosystem occurred as losses in the drainage water. These losses were related to excess nitrification and consecutive cation mobilisation throughout the soil profile. Surface water, however, had a neutral pH and very low nitrate and aluminium contents, which may have been buffered by the subsoil. Budgets differed between stands and tended to be more negative in the youngest stand. Part of this behaviour was related to stand age and part to the former land use of plots. Theoretical budgets were calculated for forest rotation lengths of 20, 40 and 60 years; it was concluded that shorter rotations would increase nutrient losses. The trend of a decrease in budget deficits with stand age suggests that the effect of vegetation change will be reduced at the next rotation but the impact of stand development may remain. Predicted nutrient budgets for a second 60 year Douglas-fir rotation suggested that available Ca in the soil would be depleted and that this depletion would be even more drastic if whole tree harvesting were adopted. In conclusion, Douglas-fir stands introduced changes in soil function that may impoverish the soil if present trends remain the same over the next forest rotations. The maintenance of sustainability will require nutrient input by fertilisation.