1University Museum, Norwegian University of Science and Technology, 7491 Trondheim, Norway
2Department of Environmental Sciences, Norwegian University of Life Sciences, PO Box 5003, 1432 Ås, Norway
3The James Hutton Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
4Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, PO Box 5003, 1432 Ås, Norway
5Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biosciences, University of Oslo, 0316 Oslo, Norway
Received: 04 Sep 2014 – Discussion started: 07 Nov 2014
Abstract. Treelines differentiate vastly contrasting ecosystems: open tundra from closed forest. Treeline advance has implications for the climate system due to the impact of the transition from tundra to forest ecosystem on carbon (C) storage and albedo. Treeline advance has been seen to increase above-ground C stocks as low vegetation is replaced with trees but decrease organic soil C stocks as old carbon is decomposed. However, studies comparing across the treeline typically do not account for elevational variation within the ecotone. Here we sample ecosystem C stocks along an elevational gradient (970 to 1300 m), incorporating a large-scale and long-term livestock grazing experiment, in the southern Norwegian mountains. We investigate whether there are continuous or discontinuous changes in C storage across the treeline ecotone, and whether these are modulated by grazing. We find that vegetation C stock decreases with elevation, with a clear breakpoint between the forest line and treeline above which the vegetation C stock is constant. C stocks in organic surface horizons of the soil were higher above the treeline than in the forest, whereas C stocks in mineral soil horizons are unrelated to elevation. Total ecosystem C stocks also showed a discontinuous elevational pattern, increasing with elevation above the treeline (8 g m−2 per metre increase in elevation), but decreasing with elevation below the forest line (−15 g m−2 per metre increase in elevation), such that ecosystem C storage reaches a minimum between the forest line and treeline. We did not find any effect of short-term (12 years) grazing on the elevational patterns. Our findings demonstrate that patterns of C storage across the treeline are complex, and should be taken account of when estimating ecosystem C storage with shifting treelines.
Revised: 30 Jan 2015 – Accepted: 17 Feb 2015 – Published: 12 Mar 2015
Speed, J. D. M., Martinsen, V., Hester, A. J., Holand, Ø., Mulder, J., Mysterud, A., and Austrheim, G.: Continuous and discontinuous variation in ecosystem carbon stocks with elevation across a treeline ecotone, Biogeosciences, 12, 1615-1627, doi:10.5194/bg-12-1615-2015, 2015.