Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.

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On the basis of cluster analysis of phytosociological releves, the mire vegetation in the Rhaetian Alps was arranged in sixteen plant communities syntaxonomically in cluded in four classes. The class Potametea was characterised by two different plant communities of perennial hydrophytes rooting in the bottom sediment of lakes (commu nities of Potamogeton alpinus and Potamogeton natans, respectively). The class L i tt 0 re 1letea included two plant communities characteristic of shallow water bodies (Callit richo-Sparganietum and community of Ranunculus reptans). The class Scheuchz erio-Caricetea nigrae included small-sedge swards and quaking rafts spanning a broad range of hydrochemical conditions with 8 plant communities and 5 alliances. The alliance Rhynchosporion albae included the vegetation of hollows and shallow ponds in nutri ent-poor mires, with the Caricetum limosae and a community of Eriophorum angusti fo lium. The alliance Caricion lasiocarpae, including vegetation of hollows and pools in intermediate mires, was represented by an only association, the Caricetum rostra tae. The alliance Caricion nigrae included sedge-swards in acid fens, with the Cari cetum nigrae, the Caricetum pauperculae, the Menyantho-Sphagnetum ter etis and the Eriophoretum scheuchzeri. The alliance Caricion davallianae in cluded rich fen vegetation with an only association, the Drepanoclado-Trichopho return cespitosi. The Caricion atrofusco-saxatilis included only the Caricetum microglochinis, a carbonate fen association in the alpine vegetation belt. The class Oxycocco-Sphagnetea corresponded to the vegetation of Sp hagnum hummocks in ombrogenous mires, with three associations: the Sphagnetum magellanici, the Erio phoro-Trichophoretum cespitosi and the Pinetum rotundatae. The habitat in all plant communities was defined on the basis of pH and electrical conductivity of pore water, reflecting the degree of inflow of geogenous water. The role of elevation in affecting the distribution of plant communities was particularly clear for the associations of the class Oxycocco-Sphagnetea, which were mainly developed in the lower sub-alpine vegetation belt, and for the Eriophoretum scheuchzeri, confined to the alpine vege tation belt.