Treelines differentiate vastly contrasting ecosystems: open tundra fromclosed forest. Treeline advance has implications for the climate system dueto the impact of the transition from tundra to forest ecosystem on carbon(C) storage and albedo. Treeline advance has been seen to increaseabove-ground C stocks as low vegetation is replaced with trees but decreaseorganic soil C stocks as old carbon is decomposed. However, studiescomparing across the treeline typically do not account for elevationalvariation within the ecotone. Here we sample ecosystem C stocks along anelevational gradient (970 to 1300 m), incorporating a large-scale andlong-term livestock grazing experiment, in the southern Norwegian mountains.We investigate whether there are continuous or discontinuous changes in Cstorage across the treeline ecotone, and whether these are modulated bygrazing. We find that vegetation C stock decreases with elevation, with aclear breakpoint between the forest line and treeline above which thevegetation C stock is constant. C stocks in organic surface horizons of thesoil were higher above the treeline than in the forest, whereas C stocks inmineral soil horizons are unrelated to elevation. Total ecosystem C stocksalso showed a discontinuous elevational pattern, increasing with elevationabove the treeline (8 g m?2 per metre increase in elevation), butdecreasing with elevation below the forest line (?15 g m?2 per metreincrease in elevation), such that ecosystem C storage reaches a minimumbetween the forest line and treeline. We did not find any effect ofshort-term (12 years) grazing on the elevational patterns. Our findingsdemonstrate that patterns of C storage across the treeline are complex, andshould be taken account of when estimating ecosystem C storage with shiftingtreelines.
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