Wood storage within the active zone of a large European gravel-bed river

Abstract

Wood storage within the active zone of the dynamic, gravel-bed, Fiume Tagliamento, Italy, was investigated at eight sites along the river's main stem. The quantity, nature, and mode of wood storage revealed a number of trends related to active zone morphology, cover type, and distance from the river's source. Relatively small quantities of wood were stored on open-gravel surfaces (estimates ranged from 1 to 21 t ha −1), intermediate quantities were associated with established islands (24–186 t ha −1), and large quantities were associated with pioneer islands (293–1664 t ha −1). Thus, variations in the geomorphological style of the river, which are associated with changes in these three cover types, are reflected in variations in the amount of wood that is stored in different reaches. In addition, although wood was found in many locations within the active zone, it was preferentially stored in three specific locations: (i) bar crests (the main open-gravel location for wood accumulations and pioneer islands); (ii) the margins and (iii) surfaces of established islands. The proportion of the stored wood that was living (sprouting) increased downstream and was higher on the open gravel than in association with established islands. There was a downstream gradient in the dominant type of wood accumulation. Individual logs predominated at the most upstream site. Thereafter, on the open gravel, whole shrubs and trees dominated the more confined sites in the headwaters and middle reaches, whereas, jams were the most frequent form of accumulation in the downstream reaches. Jams were the most frequent type of accumulation associated with established islands throughout the river. In contrast to small streams, where debris dams constitute the major type of wood accumulation, complex patterns and trends of wood storage were revealed along the Tagliamento. Although further studies are needed, it is clear that erosion of woody vegetation, its subsequent transport and deposition, play a major role in structuring the geomorphological and ecological character of this relatively natural, large European river-system. Insight into the mechanisms underlying the observed spatial patterns will contribute to a better understanding of the dynamic processes involved, and is essential for more effective management of river ecosystems.