Modelling the long-term effects of acid deposition and afforestation in the Mont-Lozere catchments (southern France)
Introduction. The combined acidifying effects of afforestation and acid deposition are well documented for sites in North-Western Europe and North America. In acid-sensitive mediterranean areas, acidification has been delayed by lower deposition rates and alkaline aeolian dust input to the ecosystems. Here, the evolution of stream water and soil chemistry, from 1845 to 2125, in three small submediterranean catchments of Southern France, is assessed using the MAGIC model. Sites and methods. Three granitic catchments have been monitored since 1981 in the South-Eastern Massif Central. The mean elevation is 1300 m above sea level and the soils are rankers and acid brown earth. The catchments are submitted to heavy atmospheric deposition of industrial acidic oxides (mainly sulphate : more than 20 kg.ha-1•year-1 of S-S04) and of alkaline saharan dust. The land use was similar in all the catchments up to 1930, and consisted mainly of extensive sheep grazing on semi natural grassland. Then one catchment (19.5 ha) was afforested with conifers (spruce), another catchment (54 ha) was abandoned and progressively settled by a beech coppice, while the other catchment (81 ha) remained covered with grassland. MAGIC is a lumped, process-oriented biogeochemical model, where the soil physical and chemical characteristics are described by a single set of mean variables. The processes in soils are modelled by a series of equilibrium equations : Aluminium solubility, cation exchange, sulphate adsorption, C02 and carbonates equllibria, organic acids dissociation. The changes in the stocks of elements are calculated from the main input, output and internal fluxes : atmospheric deposition, river load, biological uptake and weathering. The calibration aims to get the best fit between measured and simulated values to the « target variables », i.e., the present day exchangeable cations amounts in soils and river chemical composition. Hindcast and forecast runs need historical and prospective scenarios for atmospheric deposition, dry deposition factor, nutrient uptake and discharge. Here, the growth of the forest was modelled by increasing dry deposition factor and nutrient uptake and decreasing the discharge according to the field observations. The sequence of sulphate deposition was derived from S02 emission data. Three scenarios were tested in the forecasts : a constant deposition al the present level, a 30 % reduction and a 60 % reduction within 2010. In both cases, the nutrient uptake of the spruce stand and grassland was kept constant, white those of the beech coppice was progressively decreased to simulate a natural ageing of the forest. Results. The model successfully reproduced the chemistry of the catchments. The values of the optimised parameters suggest that : - the soils of the area have a very high sulphate adsorption capacity; - the weathering rates are similar in the three catchments except for the calcium (lower in the beech catchment) and the magnesium (much higher in the spruce catchment); - the initial conditions calculated by the model for 1845 are slightly different; the alkalinity and cations concentrations are lower, and the base saturation higher, in the beech catchment. The simulations show that for the grassland catchment, there has been little change due to increasing of S04 wet deposition over the last 100 years. The simulations for the other two catchments (beech and spruce stands) show that they have been acidifying since 1970 due to the combined effects of air pollution and afforestation. This trend could lead to a severe decline of soil base saturation and streamwater quality by 2050. Concerning the coniferous catchment, only a 60 % reduction of the sulphur input could allow a recovery. The natural ageing of the beech stand, resulting in a decrease of the biological uptake, would permit a recovery even under constant sulphur deposition. Conclusion. This exercise show that although the effects afforestation and acid deposition are synergetical and difficult to distinguish, their relative importance in acidification processes varies according to the vegetation. In the spruce catchment, the enhanced deposition due to the scavenging properties of the canopy is the determining factor. If no emission reduction occurs and no mitigating measures are taken, the decline of soil and water quality is a serious threat. In the beech catchment, the dynamics of biological uptake prevail, because the trees store a large amount of base cations. If the stand is allowed to age, a recovery can be observed even if no sulphur emission reduction occurs. This is also partly due, in this context, to the scavenging of the alkaline aeolian dust.