Forest ecosystem degradation and rehabilitation

The degradation of forest ecosystems may be attributed to various natural and anthropogenic factors such as climatical extremes, biotic stresses, selection of tree species, harvesting regimes, litter raking, off-site amelioration measures, former land use, air pollutant deposition and soil acidification, as caused by internal and external processes. An important factor for a loss of tree vitality are nutritional disturbances, eventually leading to declining stand stability and productivity. Therefore, the potential risks of forest fertilization as a major tool for rehabilitation of naturally or anthropogenically degraded ecosystems are discussed. In this respect, both salt-like and lime fertilizers contribute positively to revitalization of nutrient deficient stands. Experience indicates that forest liming may indeed counterbalance the progress of soil acidification as caused by high H deposition. However, specific reactions within the rhizosphere, the enhanced mineralization and loss of organic matter, the mobilization of heavy metals and M-a(+) cations and an increased NO3 leaching are possible risks of forest liming. The results from experiments with salt-like fertilizers do not show any negative influence of dissolved aluminium on root vitality but succeeded in revitalization of nutrient deficient stands. In the case of sulfatic fertilizers, both increased sulfate leaching and storage of sulfate in the bulk soil have been reported, Therefore, selection of fertilizer has to be based on a precise characterization of site specific chemical and physical conditions, including relevant stand parameters. Besides fertilization, forest regeneration with site-adapted tree species may greatly contribute to the rehabilitation of forest ecosystems, since the capacity for storage of C and N and hence, for a closer nutrient cycling, may be improved significantly through this approach. (C) 1998 Elsevier Science B.V. All rights reserved.