Ecosystem Manipulation Experimentation as a Means of Testing a Biogeochemical Model

/ Numerical simulation models have found increased usage in the past decade for projecting the environmental effects of ecosystem perturbations such as atmospheric deposition of sulfur and nitrogen, increased atmospheric CO2 concentration, and global or regional temperature change. Such model projections are frequently used as the basis or justification for public policy decisions and legislation. A substantial need has therefore arisen to test and substantiate the veracity of mathematical model projections. Unfortunately, environmental models can never be truly validated because natural systems are never closed and model solutions are always nonunique. Partial model confirmation is possible, however, and entails demonstration of agreement between prediction and observation. Experimental ecosystem manipulation provides one of the best, and in many cases only, available basis for model confirmation. The use and potential misuse of data from experimental ecosystem manipulations for model testing is explored using examples drawn from the application of an acid-base chemistry model, MAGIC. As model projections provide an increasingly important basis for public policy decisions, and as both the scientific questions and the models become increasingly complex, it will become critical to provide data from a suite of well-designed ecosystem manipulation experiments in order to evaluate the quality and uncertainty of those model projections and the models upon which they are based.KEY WORDS: Modeling; Model testing; Acidification; Ecosystem manipulation