Litter alteration during winter may override the effects of summer warming and drought on early decomposition in snow-dominated ecosystems

Aims Warming and drought are predicted to affect litter decomposition, although the magnitude of their effects in cold ecosystems remains uncertain. Since influences of litter alteration during the winter may be more important than impacts during the short summer, this study aims to assess how early-decomposition is driven by climatic seasonality. Methods We tested the effects of simulated summer warming and/or drought on litter decomposition in an alpine grassland using high- and low-quality standard material (i.e. green and rooibos tea) incubated on the soil surface and belowground. Local litter decomposition was also assessed and linked to that of teabags after one year, considering winter and summer separately. To disentangle the effects of litter alteration and microclimate, a further set of teabags was incubated during the summer. Results Summer warming and drought did not affect teabag decomposition. Decomposition was instead driven by litter quality, incubation depth, and their interaction. The majority of local litter types experienced higher decomposition during winter compared to summer, with decomposability of standard litter being similar to that of leaf litter of graminoids and shrubs. Finally, although summer conditions were more favourable for decomposition than winter ones, this advantage was cancelled out by litter alteration during winter. Conclusions It appears that, at present, early decomposition phases occurring during the long winters characterizing snow-dominated ecosystems buffer subsequent decay against the effects of warmer and/or drier summers. However, climate-induced changes in the quality, quantity, and temporal dynamics of snow, litter, and decomposers will probably alter decomposition processes in the future.