LITTER MASS-LOSS RATES IN PINE FORESTS OF EUROPE AND EASTERN UNITED-STATES - SOME RELATIONSHIPS WITH CLIMATE AND LITTER QUALITY

The purpose of this study was to relate regional variation in litter m ass-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 subarct ic to subtropical and Mediterranean: the latitudinal gradient ranged f rom 31-degrees-N to 70-degrees-N and may represent the the largest geo graphical 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 c omparable and permitted a determination of the relative influence of c limate versus substrate quality viewed from the perspective of broad r egional scales. Simple correlation applied to the entire data set indi cated that annual actual evapotranspiration (AET) should be the leadin g climatic constraint on mass-loss rates (R(adj)2 = 0.496). The combin ation of AET, average July temp. and average annual temp. could explai n about 70% of the sites' variability on litter mass-loss. In an analy sis 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 sub strate-quality variable was sufficiently significant to be used in a m odel. The influence of litter quality was introduced into a model, usi ng data from 11 sites at which litter of different quality had been in cubated. These sites are found in Germany, the Netherlands, Sweden and Finland. At any one site most (> 90%) of the variation in mass-loss r ates 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-pha se decay rates. Further regional subdivision of the data set, resulted in a range of strength in the relationship between loss rate and clim atic variables, from very weak in Central Europe to strong for the Sca ndinavian and Atlantic coast sites (R(adj)2 = 0.912; AET versus litter mass loss). Much of the variation in observed loss rates could be rel ated to continental versus marine/Atlantic influences. Inland location s had mass-loss rates lower than should be expected on the basis of fo r example AET alone. Attempts to include seasonality variables were no t successful. It is clear that either unknown errors and biases, or, u nknown variables are causing these regional differences in response to climatic variables. Nevertheless these results show the powerful infl uence of climate as a control of the broad-scale geography of mass-los s rates and substrate quality at the stand level. Some of these relati onships between mass-loss rate and climatic variables are among the hi ghest ever reported, probably because of the care taken to select unif orm sites and experimental methods. This suggest that superior, base l ine maps of predicted mass-loss rates could be produced using climatic data. These models should be useful to predict the changing equilibri um litter dynamics resulting from climatic change.