Marc Gerhard

1 Wood is increasingly used in restoration projects to improve the hydromorphological and ecological status of streams and rivers. However, despite their growing importance, only a few of these projects are described in the open literature. To aid practitioners, we conducted a postal mail survey to summarize the experiences gained in central Europe and compile data on 50 projects. 2 Our results indicated the potential for improvement from an ecological point of view, as the number and total wood volume, and the median volume of single wood structures placed in the streams per project, were low compared with the potential natural state. Moreover, many wood structures were placed nearly parallel to the water flow, reducing their beneficial effect on stream hydraulics and morphology. 3 Restoration success has been monitored in only 58% of the projects. General conclusions drawn include the following. (i) The potential effects of wood placement must be evaluated within a watershed and reach-scale context. (ii) Wood measures are most successful if they mimic natural wood. (iii) Effects of wood structures on stream morphology are strongly dependent on conditions such as stream size and hydrology. (iv) Wood placement has positive effects on several fish species. (v) Most projects revealed a rapid improvement of the hydromorphological status. 4 Most of the wood structures have been fixed, called ‘hard engineering’. However, soft engineering methods (use of non-fixed wood structures) are known to result in more natural channel features for individual stream types, sizes and sites, and are significantly more cost-effective. 5 Synthesis and applications. Large wood has been used successfully in several projects in central Europe, predominantly to increase the general structural complexity using fixed wood structures. Our results recommend the use of less costly soft engineering techniques (non-fixed wood structures), higher amounts of wood, larger wood structures and improved monitoring programmes for future restoration projects comparable with those in this study. We recommend the use of ‘passive restoration’ methods (restoring the process of wood recruitment on large scales) rather than ‘active restoration’ (placement of wood structures on a reach scale), as passive restoration avoids the risk of non-natural amounts or diversity of wood loading developing within streams. Local, active placement of wood structures must be considered as an interim measure until passive restoration methods have increased recruitment sufficiently.

Large wood was added to regulated and straightened reaches of two third-order streams in Central Germany; the Jossklein and the Lüder. In the Jossklein, the wood was a by-product of the forest management in the floodplain and accumulated in the channel during peak floods. In the Lüder, logs were builtin as deflectors in regular intervals and fixed within the stream bank. In the Jossklein, the addition of large wood improved the channel morphology within four years. The variation in channel width and depth was considerably larger than in a regulated section. The extension of the riparian zone, especially of the semi-aquatic gravel and sand bars was strongly correlated with the amount of large wood that accumulated in the single sections. The number of microhabitats and their patchiness on the stream bottom was higher in restored sections, as well as the density of macroinvertebrates and the species number. In the Lüder, some of the observed trends were similar, but not that clear. This differences can be explained by higher amounts of LWD in the Jossklein, organised in dynamic debris dams situated above the water level at low flow, in contrast to the single stacks of logs at the Lüder, situated as stable deflectors within the low flow water level.

Summarized here are ten investigations concerning the volume of coarse woody debris (CWD) in Central European streams. Altogether, 69 stream sections were examined ranging from Northern German lowland streams to brooks in alpine regions. Most of the study streams are according to Central European standards quasi-natural and are bordered by deciduous forest. The geometric mean of CWD volume related to stream length is 1.44 m3 /100 meter reach. Related to stream bottom area, the geometric mean of CWD volume is 0.202 m3 /100 m2 . The mean number of logs (≥10 cm diameter) is 12.5 logs/100 meter reach, and 3.01/100 m2 bottom area (geometric means). Regarding only quasi-natural stream sections (riparian forest currently unmanaged and no removal of CWD for at least 10 years), the geometric mean of CWD standing stock is 0.45 m3 /100 m2 for lowland streams, 0.38 m3/100 m2 for streams in lower mountainous areas and 0.02 m3 /100 m2 for alpine floodplains. From the distribution of size classes and comparison with other studies it is likely, that the current CWD standing stock is considerably less than the potential amount of CWD. For centuries all of the streams have been influenced by man. Historic alterations of the stream, its floodplain and the riparian vegetation may still affect CWD supply and standing stock. We conclude that virtually all streams in Central Europe are highly altered with respect to the amount of CWD, and that the importance of CWD is under-represented in recent assessment principles for streams in Germany.

Abstract This paper investigates the impact of a 100‐year flood in May 1999 on community composition and large woody debris standing stock in an alpine floodplain (Isar, Germany). Detailed pre‐flood data sampled from 1993 to 1998 are compared with the situation directly after the flood. In those parts of the Isar floodplain mainly covered with pioneer vegetation prior to flooding, the coverage of unvegetated gravel bars increased by 22% following the flood. However, the flood did not remove larger amounts of older successional vegetation stages (willow thickets, floodplain forest). No significant changes in the benthic invertebrate fauna were recorded. The lowest densities of riparian ground beetles (Carabidae) within the study were recorded one month after the flood. Two months later, the ground beetle densities increased to the highest values ever recorded, indicating the ground beetle's high potential for recolonization. These results highlight the degree of resilience of both the aquatic and the riparian invertebrate fauna. The flood also caused a significant increase in large woody debris standing stock; in one section the number of logs increased tenfold and the volume increased by a factor of 20, leading to the assumption that most woody debris in alluvial flood‐plains is provided by catastrophic events. Copyright © 2004 John Wiley & Sons, Ltd.