International audienceA high-resolution sedimentological and geochemical study was performed on a 20 m long core from the alpine Lake Anterne (2063 m a.s.l., NW French Alps) spanning the last 10 ka. Sedimentation is mainly of minerogenic origin. The organic matter quantity (TOC%) as well as its quality (hydrogen (HI) and oxygen (OI) indices) both indicate the progressive onset and subsequent stabilization of vegetation cover in the catchment from 9950 to 5550 cal. BP. During this phase, the pedogenic process of carbonate dissolution is marked by a decrease in the calcium content in the sediment record. Between 7850 and 5550 cal. BP, very low manganese concentrations suggest anoxic conditions in the bottom-water of Lake Anterne. These are caused by a relatively high organic matter (terrestrial and lacustrine) content, a low flood frequency and longer summer stratification triggered by warmer conditions. From 5550 cal. BP, a decrease in TOC, stabilization of HI and higher sedimentation rates together reflect increased erosion rates of leptosols and developed soils, probably due to a colder and wetter climate. Then, three periods of important soil destabilization are marked by an increased frequency and thickness of flood deposits during the Bronze Age and by increases in topsoil erosion relative to leptosols (HI increases) during the late Iron Age/Roman period and the Medieval periods. These periods are also characterized by higher sedimentation rates. According to palynological data, human impact (deforestation and/or pasturing activity) probably triggered these periods of increased soil erosion

Summary1 Relatively few studies have looked for patterns of invasion by non-native species within communities. We tested the hypotheses that: (i) some types of microhabitats within a community are more invasible than others; (ii) microhabitat types that differ in invasion also differ in resource availability; and (iii) invasibility is mediated by effects of these resources on competition between native and non-native species. 2 To test the first two hypotheses, we measured plant cover and soils in a coastal grassland in northern California. Consistent with these hypotheses, cover of non-native plants was consistently high where nitrogen-fixing shrubs had recently grown, in the bottoms and sides of gullies and on deep soils, and these microhabitats tended to have relatively high nitrogen or water availability. 3 Cover and number of native species tended to be lower where cover of non-native species was higher, indicating that non-native species as a group negatively affected native species. However, the number of non-native species also tended to be lower where the total cover of non-natives was higher. This suggests that a few non-native species excluded natives and other non-natives alike. 4 To test the third hypothesis, we grew a common non-native, the annual grass Lolium multiflorum , and a common native, the perennial grass Hordeum brachyantherum , at different levels of water and nitrogen. The relative competitive ability of the native was higher at lower nitrogen availability but not at lower water availability. When 10-weekold native plants were grown with non-native seedlings and nitrogen was relatively low, the native out-competed the non-native. However, the non-native out-competed the native at all resource levels when species were both grown as seedlings. Competition between native and non-native grasses in this system may thus help prevent invasion by non-natives in microhabitats where nitrogen availability is low, but invasion may be relatively irreversible.

International audienceWe review the scientific efforts over the last decades to reconstruct erosion from continuous alpine lake sediment records. We focused both on methodological issues, showing the growing importance of non-destructive high resolution approaches (XRF core-scanner) as well as progresses in the understanding of processes leading to the creation of an “erosion signal” in lakes. We distinguish “continuous records” from “event-records”. Both provide complementary information but need to be studied with different approaches. Continuous regionally-relevant records proved to be particularly pertinent to document regional erosion patterns throughout the Holocene, in particular applying the source to sink approach. Event-based approaches demonstrated and took advantage of the strong non-linearity of sediment transport in high altitude catchment areas. This led to flood frequency and intensity reconstructions, highlighting the influence of climate change upon flood dynamics in the mountain.The combination of different record types, both in terms of location (high vs. low elevation), sedimentology (high vs. low terrigenous contribution) and significance (local vs. regional) is one of the main outputs of this paper. It allows the establishment of comprehensive histories of NW French Alps erosion, but also and consequently, soil dynamics and hydrological patterns throughout the Holocene. We also discuss the influence of glacier dynamics, one of the major agents of erosion in the Alps.A major feature is the growing human influence upon erosion at a local scale since at least the middle of the Bronze Age (3500 cal. BP). However and according to the regional record from Lake Bourget, only few periods of rising erosion at local scales generated a regional record that can be discriminated from wetter climatic periods. Among them, the period between 200 BCE and 400 AD appeared to be marked by a generalised rise in human-triggered erosion at local scales in the northern French Alps.This review highlights the importance of modern high-resolution and interdisciplinary studies of lake sediments, in order to better understand the complex relationships between humans, climate and the Earth system in general. We strongly argue that regional integration of data is now required to move a step further. Such an integration is easier with cost- and time-effective methods as well as after a better definition of approaches and their limits. This should lead to a stronger collaboration between paleo-data producers and modellers in the near future

International audienceFlood hazard is expected to increase in the context of global warming. However, long time-series of climate and gauge data at high-elevation are too sparse to assess reliably the rate of recurrence of such events in mountain areas. Here paleolimnological techniques were used to assess the evolution of frequency and magnitude of flash flood events in the North-western European Alps since the Little Ice Age (LIA). The aim was to document a possible effect of the post-19th century global warming on torrential floods frequency and magnitude. Altogether 56 flood deposits were detected from grain size and geochemical measurements performed on gravity cores taken in the proglacial Lake Blanc (2170 ma.s.l., Belledonne Massif, NW French Alps). The age model relies on radiometric dating (137Cs and 241Am), historic lead contamination and the correlation of major flood- and earthquaketriggered deposits, with recognized occurrences in historical written archives. The resulting flood calendar spans the last ca 270 years (AD 1740-AD 2007). The magnitude of flood events was inferred from the accumulated sediment mass per flood event and compared with reconstructed or homogenized datasets of precipitation, temperature and glacier variations. Whereas the decennial flood frequency seems to be independent of seasonal precipitation, a relationship with summer temperature fluctuations can be observed at decadal timescales. Most of the extreme flood events took place since the beginning of the 20th century with the strongest occurring in 2005. Our record thus suggests climate warming is favouring the occurrence of high magnitude torrential flood events in high-altitude catchments