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SummaryThe Alpine region is warming fast, and concurrently, the frequency and intensity of climate extremes are increasing. It is currently unclear whether alpine ecosystems are sensitive or resistant to such extremes.We subjected Swiss alpine grassland communities to heat waves with varying intensity by transplanting monoliths to four different elevations (2440-660 m above sea level) for 17 d. Half of these were regularly irrigated while the other half were deprived of irrigation to additionally induce a drought at each site.Heat waves had no significant impacts on fluorescence (F v /F m , a stress indicator), senescence and aboveground productivity if irrigation was provided. However, when heat waves coincided with drought, the plants showed clear signs of stress, resulting in vegetation browning and reduced phytomass production. This likely resulted from direct drought effects, but also, as measurements of stomatal conductance and canopy temperatures suggest, from increased high-temperature stress as water scarcity decreased heat mitigation through transpiration.The immediate responses to heat waves (with or without droughts) recorded in these alpine grasslands were similar to those observed in the more extensively studied grasslands from temperate climates. Responses following climate extremes may differ in alpine environments, however, because the short growing season likely constrains recovery.

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Abstract. Alpine soils are expected to contain large amounts of labile carbon (C) which may become a further source of atmospheric carbon dioxide (CO 2 ) as a result of global warming. However, there is little data available on these soils, and understanding of the influence of environmental factors on soil organic matter (SOM) turnover is limited. We extracted 30 cm deep cores from five grassland sites along a small elevation gradient from 2285 to 2653 m a.s.l. in the central Swiss Alps. Our aim was to determine the quantity, allocation, degree of stabilization and mean residence time (MRT) of SOM in relation to site factors such as soil pH, vegetation, and SOM composition. Soil fractions obtained by size and density fractionation revealed a high proportion of labile C in SOM, mostly in the uppermost soil layers. Labile C in the top 20 cm across the gradient ranged from 39.6-57.6 % in comparison to 7.2-29.6 % reported in previous studies for lower elevation soils (810-1960 m a.s.l.). At the highest elevation, MRTs measured by means of radiocarbon dating and turnover modelling, increased between fractions of growing stability from 90 years in free POM (fPOM) to 534 years in the mineral associated fraction (mOM). Depending on elevation and pH, plant community data suggested considerable variation in the quantity and quality of organic matter input, and these patterns could be reflected in the dynamics of soil C. 13 C NMR data confirmed a relationship of SOM composition to MRT. While low temperature in alpine environments is likely to be a major cause for the slow turnover rate observed, other factors such as residue quality and soil pH, as well as the combination of all factors, play an important role in causing small scale variability of SOM turnover. Failing to incorporate this interplay of controlling factors into models may impair the performance of models to project SOM responses to environmental change.

Summary1. Elevated atmospheric carbon dioxide (CO 2 ) concentrations have often been reported to increase carbon allocation below-ground, particularly to fine root production. However, for trees these responses have primarily been studied in young expanding systems while the evidence for late successional systems that have reached steady state above-and below-ground is very limited. 2. At the Swiss Canopy Crane (SCC) experimental site, we assessed whether elevated CO 2 affects fine root biomass, fine root expansion and fine root C and N concentration under mature deciduous trees ( c . 100 years) exposed to 7 years of free air CO 2 enrichment (FACE) in a typical near-natural central European forest. 3. After 5 and 6 years of CO 2 enrichment, both, the soil core and ingrowth core method yielded similar reductions in biomass of c . -30% under elevated CO 2 for live fine roots < 1 mm diameter. In year 7 of the experiment, when fine root biomass was re-assessed at peak season, there was no significant CO 2 -effect detectable. C and N concentrations in newly produced fine roots remained unaffected by elevated CO 2 . Soil moisture under CO 2 -exposed trees was significantly increased during rainless periods. 4. The isotopic label introduced into the system by canopy enrichment with 13 C-depleted CO 2 allowed us to trace the newly assimilated carbon. After 6 years of growth at 550 ppm CO 2 , recent fine roots (< 1 mm, ingrowth cores) of CO 2 -enriched trees consisted of 51% new carbon, suggesting a rather slow root turnover and/or slow mixing of old and new carbon in these trees. 5. Reduced tree water consumption under elevated CO 2 and resultant soil water savings might cause these trees to reduce their fine root investments in a future CO 2 -enriched atmosphere. 6. Our findings and those from other multi-year experiments indicate that fine root mass in late successional systems may also be unaffected or even suppressed instead of being stimulated by elevated CO 2 .