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In order to develop a general model of litter decomposition rates suitable for the prediction of regional variations in decay rates, and to determine the relative control by macroclimate and litter quality on decomposition rates, data were selected from 5 locations ranging in climate from subpolar to warm temperate. Actual evapotranspiration (AET) was selected as an index of the climatic (energy and moisture) forcing function of the specialized decomposers which is superior to temperature and precipitation. Lignin concentration was selected as an index of litter quality and may be treated as a mediator of climatically (AET) regulated decay rates. In a stepwise, multiple linear correlation—regression, using AET, lignin concentration (%) and AET/lignin concentration (interaction), AET alone accounted for 51% of the variance in observed decay rates, AET/lignin concentration (interaction) added 19% and lignin concentration added 2% of the total (72%) variance accounted. Simple correlation of the five locations between lignin concentration and decomposition rate ranged from r = .32 to r = .95, however, the regression lines for each of the 5 locations indicated that these slopes progressively declined with AET. Moreover, the slope decline was not parallel, indicating a climatically variable control by lignin concentration on decay rates. In low—AET (not arid) climates, litter with high and low lignin, will decay at more nearly similar rates, but as the AET environment increases, the difference in decay rates becomes progressively greater than the increase in AET alone would seem to warrant. A general model of the interaction control by AET and lignin concentration on decomposition rates was formulated which overcomes the restraints of the multiple regression model. At the scale of subpolar to warm—temperate climates, the climate as indicated by AET is several orders of magnitude more important as a predictor of decay rate than is litter quality. This importance is evident in spite of the fact that the data on lignin concentration used in this analysis had a 12—fold range while the AET values had a 2.3—fold range.