Bilans d'�l�ments min�raux dans de petits bassins versants forestiers de Galice (NW Espagne)

Abstract

In order to assess the sustainability of mineral supply to fast growing Eucalyptus globulus and Pinus pinaster stands in Galice (NW Spain), we calculated (input-output) nutrient budgets in three small catchments: the Castrove and Jabali catchments with eucalyptus on a base poor granite and the Arcos catchment with pine on granodiorite. In each catchment, the annual drainage output was lower than the rainwater input for Ca, Mg and K. This particularity is attributed to (a) the relative low content of Ca and Mg of the parent material, (b) the very low concentrations of NO3 and SO4 of drainage water in an area with very low level of atmospheric pollution, and (c) to the strong mineral uptake of fast growing stands. The present management of forests leads to negative budgets, both in the eucalyptus and pine catchments. But the traditional management of pine stands including the harvest of needles and branches was much more impoverishing than the present management of eucalyptus stands, where timber is the only harvested product. Various estimates of the weathering rate suggest that this flux is sufficient to compensate for the losses in the catchment on granodiorite but probably insufficient on granite. Since nitrogen deposition is extremely low, and forest fires are quite frequent, the symbiotic fixation of large amounts of nitrogen is needed in order to ensure the sustainability of forest growth.