Width of Riparian Area How Does a Riparian Area Look Like? What Are the Ecological and Legal Requirements Regarding Minimal and Optimal Width of Riparian Areas?

Research Program “River Engineering and Ecology” – Project “Sediment and Habitat Dynamics”

Width of riparian area How does a riparian area look like? What are the ecological and legal requirements regarding minimal and optimal width of riparian areas?

S. Fink, O. Nadyeina, C. Scheidegger; WSL, Birmensdorf

Structure of riparian area Riparian areas show high structural diversity (for an overview see in report of Auenberatungsstelle 2001-2008). A dynamic and sustainable (resilient) riparian area along a river on the Central Plateau of Switzerland consists of shifting mosaics of habitats which are created, shaped and destroyed through the dynamics of the river system (Ellenberg 2010, Scheidegger et al. 2012). Besides the aquatic zone with the river itself, three overlapping terrestrial habitat types are typ- ical for a riparian area (Ellenberg 2010): a gravel zone with pioneering vegetation (e.g. German tamarisks) above the low water level, a second zone at intermediate elevation colonized by shrub and soft wood floodplain forest (e.g willows), and a third zone at higher elevations with hard wood floodplain forest (e.g. elm, alder, ash, cottonwood and oak trees) being only affected by periodically occurring high waters (compare to Figure 1 and see Delarze and Gonseth 2008, Ellenberg 2010). Hard wood floodplain forests are habitats for flood tolerant species since an- nual flooding is required to decrease growth of competing vegetation and to allow for deposition of fine sediments and nutrients (Ellenberg 2010). Once established, hard wood floodplain forests persist over long times (40-150 years, depending on the elevation from the river), while soft wood floodplain forests last 15-40 years due to higher impact of flooding and erosion (see Table 2 in Scheidegger et al. 2012). Similarly, the gravel zone is highly influenced by regular flood events resulting in sediment erosion, re-location and deposition, and shows low persistence (3-8 years, see Table 2 in Scheidegger et al. 2012). A high habitat diversity is found within the region of the floodplain due to the occurrence of river branches, back water and other type of wetland habitats (Alp et al. 2011, Ellenberg 2010).

Figure 1: Schematic representation of a riparian area along a river on the Central Plateau of Switzerland. Three terrestrial habitat types are represented: Hard wood floodplain forest, soft wood floodplain forest and shrubs, and a gravel zone with pioneering vegetation. The water level of the river varies according to season and flood intensity.

Ecological requirements for width of riparian area Historical documents of natural riparian areas along the river Rhine (southern region of Upper Rhine, Germany) reveal that a floodplain was several kilometer wide and included river branches and back water (Tittizer and Krebs 1996). Contrary to that, recent vegetation mapping for the river Rhine (Vorderrhein and Hinterrhein, Switzerland) reported the persistance of riparian zones with high dynamics at a width of less than 0.5 km (Roulier 2005). Still, preserving patches of formerly larger areas is less complex than re-establishing riparian areas, as many riparian species are highly sensitive to e.g. drainage and might only rarely colonize new habitats (Campana et al. 2014, Moraes et al. 2014, Stockan et al. 2014).

Figure 2: Schematic representation of ecological and legal requirements for riparian areas or river bank areas along flowing waters (with a channel bed of 1-5m or >5 m respectively). A riverine zone of 30 m plus the size of the channel bed is legally required for flowing waters of a size >5 m. For the river Rhine, historical documents report riparian areas of 1-5 km including ground water streams, river branches and back waters (see text). Flowing waters with a channel size of 1-5 m legally require a riverine zone of 5 m and the 6- fold size of the channel bed. Some target species require at least 500 m of riparian area. Ecological studies show that the width of a riparian area depends on the habitat requirements of each target species. Riparian area restorations include widening of channel beds (Poulsen et al. 2014, Rhode 2005). This intervention results in the formation of new, mainly gravel zone habitats, which are fre- quently affected by flooding and which are colonized in relatively short time spans (Allan 2007, Bousquin and Colee 2014, Ellenberg 2010, Rhode 2005). Due to limited space availability, a river widening leads more likely to gravel zones with pioneering vegetation and soft wood floodplain areas, and the formation of hard wood floodplain forests is rare (Delarze and Gonseth 2008, Ellenberg 2010, Rhode 2005, Rodriguez Gonzalez et al. 2014). To allow the formation of hard wood floodplain forests, restoration efforts must include areas with a minimal size of 10 ha (see Table 2 in Scheidegger et al. 2012) and elevations of 80 cm above the mean water level (Delarze and Gonseth 2008, Ellenberg 2010). Still, the formation of hard wood floodplain forests is a slow process (approximately 40 years,Ellenberg 2010, Leitgeb et al. 2013, Peter and Schulz 2003, Scheidegger et al. 2012) and requires larger and wider undisturbed stretches than other floodplain vegetation types (e.g. 2 km from the river edge, see discussion in Peter and Schulz 2003). Thus, restoration efforts should protect species rich hard wood floodplain forests, and avoid their reduction by widening of rivers with a focus on promot- ing gravel and soft wood floodplain forest zones. Additionally, restoration efforts should ideally include river branches and back water if sufficient space is available. Some restoration efforts focus on riparian forest buffer stripes of both soft and hard wood floodplain forests along rivers (Aslan and Trauth 2014, Lee et al. 2004, Richardson et al. 2012). These stripes of undisturbed vegetation form barriers from cultivated land and corridors which allow species to disperse along rivers (Richardson et al. 2012). A literature review on restoration projects shows that the width of a forest buffer should exceed 30 m to promote functional ecosystems (Sweeney and Newbold 2014). This is confirmed by previous literature claiming a buffer width of 10-100 m (Darby and Sear 2008), although there’s no distinction between soft and hard wood floodplain zones and theoretical tools to model the river width are still missing (Bollati et al. 2014, Fremier et al. 2014). New modelling approaches try to gain insight in the connectivity between aquatic insects’ habitats at the river’s edge to terrestrial habitats further away from the river (Muehlbauer et al. 2014, Sabo and Hagen 2012). For streams of median channel width of 3.5 m, models show that aquatic insects affect terrestrial food webs 1.5 m to 550 m away from the river bank itself (Muehlbauer et al. 2014). Similar requirements for connectivity between aquatic and terrestrial habitats come from data on terrapins (Emys orbicularis), where females lay their eggs 150-600 m away from the stream (Rust-Dubié et al. 2006). (Rust-Dubié et al. 2006). Ecological requirements for the width of a riparian area always depend on the ecological requirements of the target species.

Legal requirements for width of riparian area Swiss legislation states the need to protect riparian areas of national importance and to protect riparian species and river dynamics , without giving detailed information on the width of a riparian area (Article 4, see in Bundesgesetz über den Natur- und Heimatschutz 1966, Verordnung über den Schutz der Auengebiete von nationaler Bedeutung (Auenverordnung) 1992). Bank vegetation (including riparian vegetation) must be conserved, bank areas must be protected and habitats should be sufficiently large (without detailed instructions, see article 18 and 21 in Bundesgesetz über den Natur- und Heimatschutz 1966) Federal regulations classify the width of a riparian area by the size of the channel bed, with riverine zones of at least 11 m for small streams, and of the size of the river and additional 30 m for larger rivers (Articles 41a and b, see in Gewässerschutzverordnung (GSchV) 1998). Guidelines for flood prevention mention a 1:2 ratio of the river bank elevation in correspond- ence to the channel bed (Hochwasserschutz an Fliessgewässern, Wegleitung 2001). This slope ensures various, but steep habitats over short distances from the river’s edge, only theoretically providing habitat for hard wood floodplain forests (compare size requirements above as references in Peter and Schulz 2003, Scheidegger et al. 2012). To maintain the species rich hard wood floodplain zones despite reduced space, natural river dynamics alone might not be sufficient and need to be fostered by tending strategies to ensure habitat quality. Legal requirements for the width of the riverine zone depend highly on the size of the river and are generally smaller than ecological requirements as shown for a stream with a channel bed of 3.5 m: 150-500 m would be necessary for an ecological connectivity or at least 30 m to function as a ecological buffer, whereas only a width of 26 m is legally required (compare to Gewässerschutzverordnung (GSchV) 1998). Still, the regulations might allow for habitat formation for flood resistant target species such as pioneer plants (e.g. skeletonweed, Chondrilla chondrilloides, bur-reed, Sparganium emersum or dwarf cattail, Typha minima, see in Ellenberg 2010), birds (habitat and nesting sites for e.g. common sandpiper, Actitis hypoleucos or common tern, Sterna hirundo see in Rust-Dubié et al. 2006) and beetles (e.g. ground beetles Blethisa multipunctata, Bembidion modestum or Bembidion velox , see in Rust-Dubié et al. 2006).

Conclusion • The width of a resilient (self-regulating and sustainable) riparian area allows the formation of various diverse habitat types. • The width depends on the habitat requirements of each target species. • Literature suggests a minimal ecological width from the river edge of several hundred meters, with minimal requirements of over 30 m. • Swiss legislation for minimal width is dependent on the size of the river, but generally smaller (minimal 5 m) than ecological requirement. • Preserving patches of formerly larger riparian areas is less complex than re-establishing riparian areas, especially for hard wood riparian forests. Thus, high quality riparian areas must be maintained and promoted. • Restoration efforts show that gravel and soft wood floodplain zones can be induced suc- cessfully by widening of channel beds, while the maintenance and promotion of hard wood floodplain forests are limited due to land availability and require additional tending strategies(e.g. by increasing connectivity to river dynamics or to ground water as well as by promotion of target species).

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