Channel changes are the consequence of changes in sediment yield from the slopes and in the connectivity between slopes and channels because of distinct land use and climate impacts. In this study, we investigated the characteristics and evolution of a short reach in the headwater of the Ijuez River, central–southern Pyrenees. Assessment of a series of sedimentary and geomorphic structures confirmed major changes to the valley bottom, mainly related to changes in the intensity of human activity. The oldest sedimentary structure is a terrace level located 3 to 4 m above the current alluvial plain. General deforestation, overgrazing and recurring fires in the montane belt (1100–1600 m a.s.l.) have led to increased soil erosion and connectivity, and to the triggering of debris flows that have been deposited on the fluvial terrace. Woody fragments from within the debris flows were dated using accelerator mass spectrometry ¹⁴C radiocarbon techniques (AMS), yielding ages between 100 and 115 cal years bp, which coincides with the period of maximum deforestation and human density in the Pyrenees. Depopulation and farmland abandonment since the beginning of the 20th century has resulted in generalized natural and artificial reforestation, a shrinkage of the eroded areas and a decline in connectivity between slopes and the channel. The most important consequence has been channel incision and narrowing, and the development of a sediment armour layer. Active sediment transport is continuing, although there has been a decrease in sediment yield from the slopes. Copyright © 2014 John Wiley & Sons, Ltd.

Tree lines are supposed to react sensitively to the current global change. However, the lack of a long‐term (millennial) perspective on tree line shifts in the Pyrenees prevents understanding the underlying ecosystem dynamics and processes. We combine multiproxy palaeoecological analyses (fossil pollen, spores, conifer stomata, plant macrofossils, and ordination) from an outstanding ice cave deposit located in the alpine belt c. 200 m above current tree line (Armeña‐A294 Ice Cave, 2,238 m a.s.l.), to assess for the first time in the Pyrenees, tree line dynamics, and ecosystem resilience to climate changes 5,700–2,200 (cal.) years ago. The tree line ecotone was located at the cave altitude from 5,700 to 4,650 cal year bp, when vegetation consisted of open Pinus uncinata Ramond ex DC and Betula spp. Woodlands and timberline were very close to the site. Subsequently, tree line slightly raised and timberline reached the ice cave altitude, exceeding its today's uppermost limit by c. 300–400 m during more than four centuries (4,650 and 4,200 cal year bp) at the end of the Holocene Thermal Maximum. After 4,200 cal year bp, alpine tundra communities dominated by Dryas octopetala L. expanded while tree line descended, most likely as a consequence of the Neoglacial cooling. Prehistoric livestock raising likely reinforced climate cooling impacts at 3,450–3,250 cal year bp. Finally, a tree line ecotone developed around the cave that was on its turn replaced by alpine communities during the past 2,000 years. Synthesis. The long‐term Pyrenean tree line ecotone sensitivity suggests that rising temperatures will trigger future P. uncinata and Betula expansions to higher elevations, replacing arctic–alpine plant species. Climate change is causing the rapid melting of the cave ice; rescue investigations would be urgently needed to exploit its unique ecological information.

Flash floods represent one of the most common natural hazards in mountain catchments, and are frequent\ud in Mediterranean environments. As a result of the widespread lack of reliable data on past events,\ud the understanding of their spatio-temporal occurrence and their climatic triggers remains rather limited.\ud Here, we present a dendrogeomorphic reconstruction of past flash flood activity in the Arroyo de los\ud Puentes stream (Sierra de Guadarrama, Spanish Central System). We analyze a total of 287 increment\ud cores from 178 disturbed Scots pine trees (Pinus sylvestris L.) which yielded indications on 212 growth\ud disturbances related to past flash flood impact. In combination with local archives, meteorological data,\ud annual forest management records and highly-resolved terrestrial data (i.e., LiDAR data and aerial imagery),\ud the dendrogeomorphic time series allowed dating 25 flash floods over the last three centuries, with a\ud major event leaving an intense geomorphic footprint throughout the catchment in 1936. The analysis of\ud meteorological records suggests that the rainfall thresholds of flash floods vary with the seasonality of\ud events. Dated flash floods in the 20th century were primarily related with synoptic troughs owing to\ud the arrival of air masses from north and west on the Iberian Peninsula during negative indices of the\ud North Atlantic Oscillation. The results of this study contribute considerably to a better understanding\ud of hazards related with hydrogeomorphic processes in central Spain in general and in the Sierra de\ud Guadarrama National Park in particular