APPLICABILITY OF A FOREST SIMULATION-MODEL FOR ESTIMATING EFFECTS OF NITROGEN DEPOSITION ON A FOREST ECOSYSTEM - TEST OF THE VALIDITY OF A GAP-TYPE MODEL

Ecosystem models have been used to compile scattered information on va rious ecosystem processes and to test the hypotheses about ecosystem r esponses to various simultaneously changing environmental factors. In spite of the widespread use of models, there have been comparatively f ew quantitative evaluations of model projections compared to long-term observations under changing environmental conditions (e.g. increased nitrogen deposition). In this study we tested the validity of a gap-ty pe forest simulation model (SIMA) in order to extend the applicability of the model for the prediction of how nitrogen deposition influences the production of a boreal forest ecosystem. The validity of the mode l was tested by comparing the prediction of the model with independent data from long-term fertilization experiments. The predictions provid ed by the SIMA model agreed fairly well with the results of long-term fertilization experiments. Both the experiments and the model simulati ons showed similar increases in stem-wood production and in growing st ock as a consequence of repeated nitrogen fertilization over the 30-ye ar study period. The addition of nitrogen increased the total producti on by 30-53% according to field experiments and by 39-63% according to model computations. In both the model calculations and the field expe riments, organic matter accumulated in the soil after the addition of nitrogen. The increase in the amount of soil organic matter can be exp lained as being due to the increased biomass production and the result ing increase in litterfall. According to the model computations, annua l litterfall of needles on the mesic site varied from 970 kg ha(-1) to 3050 kg ha(-1) and this agreed well with measured litterfall of the s tand. (C) 1998 Elsevier Science B.V.