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.

Abstract. The purpose of this study was to relate regional variation in litter mass-loss rates (first year) in pine forests to climate across a large, continental-scale area. The variation in mass-loss rate was analyzed using 39 experimental sites spanning climatic regions from the subarctic to subtropical and Mediterranean: the latitudinal gradient ranged from 31 N to 70 N and may represent the the largest geographical area that has ever been sampled and observed for the purpose of studying biogeochemical processes. Because of unified site design and uniform laboratory procedures, data from all sites were directly comparable and permitted a determination of the relative influence of climate versus substrate quality viewed from the perspective of broad regional scales.Simple correlation applied to the entire data set indicated that annual actual evapotranspiration (AET) should be the leading climatic constraint on mass-loss rates (R2d = 128 0.496). The combination of AET, average July temp. and average annual temp. could explain about 70% of the sites' variability on litter mass-loss. In an analysis of 23 Scots pine sites north of the Alps and Carpatians AET alone could account for about 65% of the variation and the addition of a substrate-quality variable was sufficiently significant to be used in a model.The influence of litter quality was introduced into a model, using data from 11 sites at which litter of different quality had been incubated. These sites are found in Germany, the Netherlands, Sweden and Finland. At any one site most (> 90%/6) of the variation in mass-loss rates could be explained by one of the litter-quality variables giving concentration of nitrogen, phosphorus or water solubles. However, even when these models included nitrogen or phosphorus even small changes in potential evapotranspiration resulted in large changes in early-phase decay rates.Further regional subdivision of the data set, resulted in a range of strength in the relationship between loss rate and climatic variables, from very weak in Central Europe to strong for the Scandinavian and Atlantic coast sites (Rdj = 0.912; AET versus litter mass loss). Much of the variation in observed loss rates could be related to continental versus marine/Atlantic influences. Inland locations had mass-loss rates lower than should be expected on the basis of for example AET alone. Attempts to include seasonality variables were not successful. It is clear that either unknown errors and biases, or, unknown variables are causing these regional differences in response to climatic variables. Nevertheless these results show the powerful influence of climate as a control of the broad-scale geography of mass-loss rates and substrate quality at the stand level.Some of these relationships between mass-loss rate and climatic variables are among the highest ever reported, probably because of the care taken to select uniform sites and experimental methods. This suggest that superior, base line maps of predicted mass-loss rates could be produced using climatic...

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