Restoration ecology of river valleys

ARTICLE IN PRESS Basic and Applied Ecology 7 (2006) 383—387

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SPECIAL FEATURE

Restoration ecology of river valleys Kai Jensena,, Michael Trepelb, David Merrittc, Gert Rosenthald a

Biocentre Klein Flottbek, Vegetation and Population Ecology, Hamburg University, Ohnhorststr. 18, 22609 Hamburg, Germany b Schleswig-Holstein State Agency for Nature and Environment, Hamburger Chaussee 25, D-24220 Flintbek, Germany c Watershed, Fish, and Wildlife, U.S.D.A. Rocky Mountain Research Station and the Natural Resource Ecology Laboratory at Colorado State University, Natural Resources Research Center, Bldg. A, Suite 368, Fort Collins, CO 80526, USA d Institute of Landscape Planning and Ecology, University of Stuttgart, Keplerstraße 11, 70172 Stuttgart, Germany Received 2 May 2006; accepted 2 May 2006

¨ kologie. Published by Else& 2006 Gesellschaft fu ¨r O vier GmbH. All rights reserved.

In Central Europe, river valleys have long been subjected to human alterations: 6000 years ago the Neolithic tribes used the course of river beds to reach Central Europe from the Southeast and settled along the edges of the river valleys. River valleys gave them access to water, fishing and rich Corresponding author. Tel.: +49 4042816576;

fax: +49 4042816565. E-mail addresses: [email protected] (K. Jensen), [email protected] (M. Trepel), [email protected] (D. Merritt), [email protected] (G. Rosenthal).

game-hunting areas. Floodplains of the large rivers (e.g. Rhine, Elbe, Weser) offered nutrient rich soils for arable fields, whereas riparian peatlands along small rivers were subjected to moderate grazing and, since the Iron age, clear cutting and hay making (Schwaar, 1990). The ridges along the edges of river valleys gave shelter against flooding and, therefore, were main settlement areas (Ku ¨ster, 1999). Since that time humans have used rivers also for transportation. During Medieval times, humans began building embankments and dikes along large rivers to prevent flooding and began to drain the riparian wetlands. More recently, humans straightened the riverbeds, constructed roads and railways through river valleys, and fragmented rivers through the widespread construction of dams (Nilsson, Reidy, Dynesius, & Revenga, 2005), water

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ARTICLE IN PRESS 384 diversion structures, and navigational locks. During this time, residential areas have expanded from valley margins onto valley bottoms and into river floodplains. With increasing human populations and demands for freshwater, the number and magnitude of these anthropogenic stressors will continue to grow at the expense of the structure and ecological functions of riparian and aquatic ecosystems (Giller, Covich, Ewell, Hall, & Merritt, 2004). Today most European rivers are channel-like structures, much simplified from their natural form, restricted to a narrow riverbed and bordered by rigid dikes (e.g. Dynesius & Nilsson, 1994; EEA, 1999; Nilsson et al., 2005). As a consequence, characteristic plant and animal species of riparian landscapes suffered from habitat changes and losses (e.g. Rosenthal, 2003) and today the functional composition between human-impacted and natural reference communities of European rivers differs significantly (Statzner, Bis, Doledec, & Usseglio-Polatera, 2001). Both (natural) floodplain forests and semi-natural riparian grasslands are considered as one of the most endangered ecosystems in the European cultural landscape (Tockner & Stanford, 2002). Natural and (semi-)natural riparian ecosystems do not only harbour disproportionately high biodiversity, they also have important functions for the water and nutrient budget of landscapes. Functioning floodplains attenuate flood waves and buffer against catastrophic flooding, they perform many important biochemical functions, such as absorbing, uptaking and transforming nutrients, and riparian plants provide food, shelter, and nest and stopover sites for insects, mammals, birds, amphibians and reptiles as well as providing carbon to aquatic ecosystems (Giller et al., 2004; Mant & Janes, 2006). European rivers and their associated valleys differ in ecological conditions. Large rivers like the Oder, Elbe, Danube and Rhine as well as many of their tributaries originate from mid- or high mountain ranges. Their river valleys in the lowlands are characterized by very pronounced water level fluctuations with high water levels after snow melt in spring and low water levels during summer and winter. In contrast, many rivers and brooks originating from the lowlands or from the hilly moraine landscape of Northern Germany exhibit a different flow pattern with high water levels both in winter and summer (Koenzen, 2005). Additionally, upwelling groundwater at the bottom of the ridges along the valleys leads (together with the hydrological effect of the river itself) to more or less constant annual water levels in these river valleys. Here, this stable and reliable water source, combined with an ample supply of organic material supports

K. Jensen et al. extensive riparian peatlands. Like the above-mentioned large floodplains, riparian peatlands have also been altered by humans during the last centuries. Today, most of these peatlands are heavily drained with negative impacts both on the nutrient budget (Trepel & Kluge, 2004) of the landscape and on the flora and fauna (Rosenthal et al., 1998). Increasing ecological and economic problems confined to the heavily altered European riparian landscapes have sharpened the human awareness for a need to restore river valleys. Furthermore, changes of EU-policies and agricultural markets made grassland management on heavily drained riparian peatlands no longer profitable. As a consequence, many sites have been abandoned during the last 20 years followed by a succession leading to tall-grass and tall-herb-dominated vegetation and a further decrease in biodiversity (e.g. Jensen & Schrautzer, 1999). Support for restoration concepts of European river valleys resulted from the extraordinary floods along the Oder (summer 1997) and the Elbe (summer 2002 and spring 2006), which led to considerable economic damages and intensive discussions in the public and among scientists about ways to reduce such high water levels in the future (Platteeuw & Pieterse, 2005). One reasonable way for flood management is the restoration of river valleys as retention (and detention) areas. Another important topic of restoration ecology of European river valleys is the restoration of plant species diversity of riparian wet grasslands, which includes both the recovery of habitats and the reestablishment of target species. The latter came into the focus of restoration ecology when longterm studies on permanent plots in river grasslands had documented a distinct gap between successful habitat restoration and the expected but nonetheless ineffective biodiversity restoration. Bottlenecks of biodiversity restoration result from local and regional species pools (Zobel, Van der Maarel, & Dupre `, 1998) and characteristics of plant reproduction such as reproduction strategies, propagule dispersal, soil seed banks, germination and establishment (Geertsema, Opdam, & Kropff, 2002). Soil seed banks allow for (in principal) ‘dispersal in time’ but their restoration function is still controversially discussed. Although many target species are expected to build up only transient or short term persistent seed banks (e.g. Bekker, Verweij, Bakker, & Fresco, 2000; Bossuyt & Hermy, 2003; Hutchings & Booth, 1996), recent results of burial experiments with seeds of 20 regionally rare or endangered fen grassland species (Jensen, 2004, unpubl. data) showed that seeds of more than 50%

ARTICLE IN PRESS Restoration ecology of river valleys of the investigated species were able to survive in the soil for at least 10 years. Propagule ‘dispersal in space’ might allow the colonization of widely dispersed restoration sites from spatially remote, remnant target species populations. Dispersal efficiency for restoration purposes does not only depend on the fertility of source populations and species-specific diaspore characteristics but also on the availability and efficiency of dispersal vectors which are operating on the landscape scale (Bakker, Poschlod, Strykstra, Bekker, & Thompson, 1996). For the restoration of grasslands in river valleys floods appear to be one of the most efficient natural dispersal vectors. Conservation and restoration management in riparian wet grasslands should, therefore, support the maintenance of wetland specific dispersal vectors such as extensive inundations. All successful restoration measures have to be based on sound ecological principles and an understanding of hydrologic processes. This includes knowledge of the water and nutrient budget of riparian ecosystems as well as the characteristic animal and plant species and their assemblage rules (see e.g. Statzner & Moss, 2004 for a recent review with special emphasis on heterotrophic stream ecosystems). The specialist group for restoration ecology (funded in 2001 within the ‘Gesellschaft fu ¨r ¨ kologie’) tries to integrate these aspects. One O main aim of this group is to bridge the gap between science and application. In April 2003, the annual meeting of the restoration ecology group was held at the University of Kiel with the broad issue ‘restoration ecology of river valleys’ and sessions on dispersal and diversity as well as on river restoration and ecosystem functions. The meeting was visited by ecologists from eleven countries and the programme opened with a talk by David Merritt (U.S.D.A. Forest Service and Colorado State University) on ‘reconnecting fragmented riverscapes’. The special issue presented here is mainly based on presentations given on this meeting and it represents the most important topics. In the opening talk, David Merritt emphasized the importance of restoring and maintaining lateral and longitudinal connectivity along river corridors and enhancing the physical integrity of river channels either passively or through active river restoration (Graf, 2001; Merritt & Wohl, 2006). Active restoration is a direct human intervention aimed at constructing natural-looking engineered channels and simulating processes performed historically in natural channels. Examples of active restoration include mechanical restructuring of river channel form, constructing fluvial features with heavy machinery, reconnecting side-arms and floodplains

385 with the channel, mechanical disturbance to riparian areas to simulate fluvial disturbance through sod-cutting on floodplains, removing or disturbing existing vegetation, and ‘‘new site’’ formation through disturbing soil, as well as controlling exotic species, prescribing fire, and replanting or seeding riparian areas to recover desirable populations and/or enhance species diversity. Passive river restoration (which is becoming increasingly popular on regulated rivers throughout North America) includes restoring key river processes so that restoration can influence ecosystems over larger spatial and temporal scales than are possible through active, reach-scale projects. Examples include restoring key attributes of hydrologic regimes by designing flow release schedules from reservoirs so that flows are strategically used to perform important physical and biological functions. Biologically important flowrelated processes include the magnitude of extreme events (floods and droughts), the timing of high and low flows, the rate of change in flows, and the seasonal and interannual variation in flow regime. Through pairing attributes of the hydrologic regime with the life-history requirements of riparian and riverine species, restoration along entire river segments can be performed very costeffectively. However, such projects are often constrained by legal (water or property rights), societal (property damage, safety), or practical (limited water availability) concerns. David Merritt also presented results of his work on the importance of flow regime and hydraulic processes in the dispersal of plants along channels and throughout river networks and the role of flow in the establishment and maintenance of riparian plant communities (Merritt & Wohl, 2002, 2006) and concluded with some general guidelines for river restoration: (1) Define quantifiable goals and outcomes for restoration projects before they begin as this will assist in monitoring and evaluating progress as well as determining ‘‘yield on investment’’ from restoration. (2) Begin restoration projects with a strong scientific foundation and a solid conceptual framework. (3) Adaptively manage so that mistakes from one project (and lessons learned) become an asset to future projects; refine approaches to restoration so that restoration becomes increasingly effective. (4) Restoration of process is preferable to restoration of form (i.e., restoration towards a fixed endpoint); incorporating flooding and flow

ARTICLE IN PRESS 386 related processes into restoration is desirable (within societal constrains) and understanding that river ecosystems are dynamic is paramount. (5) View river restoration within the context of the entire watershed; restoration at the reachscale is more likely to succeed if consideration of processes occurring upstream, downstream, and laterally are taken into account and the reach is viewed as a segment in a much larger network of stream channels. (6) River restoration is one of the most visible aspects of the river related sciences (Malakoff, 2004). Thus, it is important that successful restoration of ecological services and biological functions is conveyed to the public and policy makers so that society will continue to invest in the restoration of rivers and river valleys (Wohl et al., 2005). In the special issue presented here, Kieckbusch, Schrautzer, and Trepel (2006) focus on the hydrological condition of drained riverine peatlands and show that these ecosystems are not able to fulfil important ecological functions like nutrient retention, flood mitigation and conservation of wetland species. Their study revealed that the hydrology of riparian peatlands is affected by three important pathways: springs with high nitrate concentrations (‘young groundwater’), springs with low nitrate concentrations (‘old groundwater’) and peat drains also with low nitrate, but high ammonium and ortho-phosphate concentrations. Their analysis points to a high spatial heterogeneity of water pathways and to the necessity to consider these water pathways and their spatial distribution for appropriate restoration measures. Rothenbu ¨cher and Schaefer (2006) analyse how arthropods (plant- and leafhoppers, spiders, ground beetles) can cope with regular flooding in floodplain ecosystems. They investigated whether the typical wetland fauna recolonizes the floodplain after each flooding event or survives winter submersion in the habitat. The results show that most species of plant- and leafhoppers tolerated submersion and overwintered in the floodplain, whereas most spiders and carabids immigrated with receding water level. The authors conclude that suitable non-inundated overwintering sites for immigrating species need to be included into restoration measures in river floodplains. In his paper on the effects of seed dispersal, seed persistence and regional abundance on the restoration of riparian grasslands Rosenthal (2006) analyses long-term data from a large-scale restoration project (Borgfelder Wu ¨mmewiesen) in Northern

K. Jensen et al. Germany. The author found that the re-establishment of (target) grassland species during restoration succession depends mainly on population density in the established vegetation, whereas long distance dispersal by means of winter floods and persistence in the seed bank contributed less to the recruitment rates. He concludes that a large species pool (e.g. as a consequence of only moderate human alterations to the river valley) and the spatial interconnection of species-rich (source-) and species-deficient (sink-)habitats are important for the restoration success of riparian grasslands. In the final contribution, Vogt, Rasran, and Jensen (2006) analyse short- and long-distance dispersal processes of plant species during the extraordinary flood of the Elbe River in summer 2002. Drift line material was sampled along a transect of 400 km of the Middle Elbe River from five locations and three habitats each (arable field, grassland, river bank). As species composition of the drift line samples between the studied habitats and locations differed only marginally whereas the vegetation of the analysed habitats differed significantly, it is concluded that seeds from different habitats and locations are transported together during flooding and that floods might thus expand the usual dispersal ranges of plant species. Altogether, this special issue encompasses a wide range of topics, riparian landscapes and ecological disciplines. We hope that this collection of papers will fertilize the current discussion on the restoration of river valleys and that it contributes to bridging the gap between science and restoration, that is, between Basic and Applied Ecology.

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