This paper describes the detennination of sheet erosion rates by using dendrogeomorphological methods on exposed tree roots, Two sites on the northern slope of the Guadarrama Mountains, Central Spain, were studied: a popular trail in a Scots pine forest (Senda Schmidt, Valsain) growing on granites and gneisses, and an open holm-oak forest on granitic slopes (Monterrubio), These sites were selected because they showed high denudation morphologies due to accelerated soil-erosion processes caused by human influence (trampling by continuous trekking and overgrazing), resulting in exposed roots, The method applied is based on the morphological pattern of roots, defined by the growth-ring series of the sampled roots, In order to confinn the validity of the criteria used and to make the estimations of erosion more accurate, several anatomical indicators of exposed and non exposed Pin us sylvestris roots were characterized, The study entailed a statistical analysis of exposure time and erosion depth. The influence of environmental factors affecting the variation in velocity of the erosion processes was also examined. With a significance level of 95%, the mean erosion rates were in the range of 1.7 2.6 mm/year (29 44 Mw/year) on Senda Sclnnidt over the last 101 years, and 1.1 1.8 mm/year (19 31 t/halyear) in Monterrubio over the last 42 years. Using a multifactor analysis of variance, we observed a change in the erosion rates as a fimction of position on the path along Senda Schmidt. In Monterrubio, however, we reached no significant conclusions, apart from an inverse relationship between erosion and slope gradient that was difficult to interpret.Climate conditions in Senda Sclnnidt and the accuracy of dating Scots pine indicate that the evaluation on P sylvestris roots is fairly reliable, which is not the case for oak roots. Although this paper is based on the application of an existing method, its novelty lies in being the first attempt in Spain to estimate 'accelerated' sheet erosion rates (due to recreational activities and overgrazing) using dendrogeomorphological techniques, supplemented by anatomical indicators for P sylvestris.

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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.