Decomposition of mountain birch leaf litter at the forest-tundra ecotone in the Fennoscandian mountains in relation to climate and soil conditions

Litter decomposition is a key process in terrestrial ecosystems, releasing nutrients, returning CO2 to the atmosphere, and contributing to the formation of humus. Litter decomposition is strongly controlled both by climate and by litter quality: global warming scenarios involving shifts in vegetation communities are therefore of particular interest in this context. The objective of the present study was to quantify the role of climatic environment and underlying substrate chemistry for the decomposition of standard mountain birch (Betula pubescens Ehrh. spp. czerepanovii) leaf litter at four sites, spanning the forest-tundra ecotone, in the Fennoscandian mountain range. Litter quality effects were thus held constant, but the study incorporated systematic changes in (i) latitude/altitude, (ii) 'continentality', and (iii) vegetation community at each site, together with (iv) experimental manipulation of temperature using passive warming systems. The study was undertaken during a 3 year period, and forms part of a broader investigation of forest-tundra ecotone dynamics in the Fennoscandian mountains. Our results showed (1) higher decomposition rates in forest sites compared to tundra, (2) that the difference between the two vegetation communities was most pronounced at the more maritime sites, and (3) that chemistry of litter remaining after the three years experiment varied according to site and vegetation community (e.g. at the most southerly site, more lignin had decomposed at tundra communities compared with the forest). (4) Surface temperature explained 58% of the variation in mass loss at forest sites; at tundra sites, however, we hypothesise that litter moisture content was the more important factor. (5) Experimental warming lent weight to this hypothesis by reducing rates of mass loss: this reduction was likely the result of surface soil drying, an artefact of the warming treatment. We conclude that a replacement of tundra by forest would likely accelerate litter decomposition both via changes in surface and near-surface temperature and moisture regimes, although the strength of this response will vary between maritime and continental parts of the mountain range.