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 Ijnez 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 alluvia] plain. General deforestation, overgrazing and recurring fires in the montane belt (1100-1600 m a.s.1.) 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 "C radiocarbon techniques (AMS), yielding ages between 100 and 115cal 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.
The montane and subalpine belts in European mountains were affected by intense land use/land cover changes during the 20th century. In the case of the subalpine belt, most European mountains were affected by complex deforestation processes from Neolithic times, leading to the expansion of summer grasslands to support grazing by transhumant sheep and goats flocks. This resulted in an altitudinal reduction of the tree line, and the occurrence of severe soil erosion and shallow landsliding. The intense livestock pressure over centuries explains why the landscape remained without major change until the middle of the 20th century. Since then, depopulation, land abandonment, and the disappearance of the transhumant system have resulted in a marked decline in livestock numbers, and subsequent encroachment of shrubs and trees in the subalpine belt. The Urbión Mountains (Iberian Range, northern Spain) provides one of the clearest examples in the Iberian Peninsula, where there was intense deforestation since the Neolithic period to enlarge the area of subalpine summer grasslands. The recent reversal of this situation in the last decades led to (i) a marked trend to dense forest, which resulted in an average 200 m altitudinal advance; and (ii) the spatial contraction of shrublands, which have been replaced by dense forest in the highest areas. Changes will continue in the future, possibly enhanced by global warming.
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