Integrative conservation of riparian zones

BIOC-07015; No of Pages 10 Biological Conservation xxx (2016) xxx–xxx

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Integrative conservation of riparian zones Eduardo González a,b,⁎,1, María R. Felipe-Lucia c,1, Bérenger Bourgeois d, Bruno Boz e, Christer Nilsson f, Grant Palmer g, Anna A. Sher b a

EcoLab, Université Paul Sabatier, Institut National Polytechnique de Toulouse, Centre National de la Recherche Scientifique, 118 Route de Narbonne Bâtiment 4R1, 31062 Toulouse Cedex, 9, France Department of Biological Sciences, University of Denver, F. W. Olin Hall, 2190 E Iliff Ave., Denver, CO 80208-9010, United States Institute of Plant Sciences, University of Bern., Altenbergrain 21, CH-3013 Bern, Switzerland d Département de Phytologie, Faculté des Sciences de l'Agriculture et de l'Alimentation, Université Laval, 2425 rue de l'agriculture, Québec, Québec G1V 0A6, Canada e Italian Centre for River Restoration, Viale Garibaldi 44/a, 40123 Mestre, Venice, Italy f Landscape Ecology Group, Department of Ecology and Environmental Science, Umeå University, SE-901 87 Umeå, Sweden g Centre for Environmental Management, Faculty of Science and Technology, Federation University Australia, PO Box 663, Mt Helen 3353, VIC, Australia b c

a r t i c l e

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Article history: Received 2 August 2016 Received in revised form 20 October 2016 Accepted 24 October 2016 Available online xxxx Keywords: Conservation strategies Floodplain Frodo effect Riverine Small ecosystem features

a b s t r a c t Riparian zones are the interface between aquatic and terrestrial systems along inland watercourses. They have a disproportionate ecological role in the landscape considering their narrow extent, which makes them a good example of small natural features (sensu Hunter, this issue). Characteristically, riparian zones increase species richness in the landscape and provide key services to society, such as soil fertility, water purification, and recreation. Despite the recognized importance of riparian zones for ecological, economic and social reasons, and the vast amount of scientific literature exploring measures for their conservation, current management is still failing at enabling a proper ecological functioning of these areas. The best practices for conservation of riparian zones have mostly focused on manipulating biotic and physical components (e.g. renaturalizing flow regimes, improving channel mobility, and controlling invasions of exotic ecosystem engineer species). However, these strategies face important technical, socio-economic, and legal constraints that require a more integrative approach for effective conservation. In this paper we summarize the main problems affecting riparian zones and their current management challenges. Following Hunter et al. (this issue), we review novel approaches to conservation of riparian zones, complementary to manipulating processes that reflect contemporary management and policy. These include (1) investing in environmental education for both local people and technical staff, (2) guaranteeing qualitative and long term inventories and monitoring, (3) establishing legislation and solutions to protect riparian zones, (4) framing economic activities in riparian zones under sustainable management, and (5) planning restoration of riparian zones at multiple and hierarchical spatio-temporal scales. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Riparian zones are ecosystems created at the interface between terrestrial and freshwater habitats along flowing waters. They represent only a narrow portion of the landscape while contributing disproportionately to the biodiversity of the region as a whole (Sabo et al., 2005) and providing many ecosystem services (“Frodo effect”, Primack and Sher, 2016), mainly due to the dynamic “edge effect” of the aquatic/terrestrial transition zone following flooding pulses (Junk et al., 1989). Therefore, riparian zones are small natural features (SNFs) with an ecological role extending beyond their area (Hunter, this issue). The conservation of riparian zones, as for many other SNFs, ⁎ Corresponding author at: EcoLab, Université Paul Sabatier, Institut National Polytechnique de Toulouse, Centre National de la Recherche Scientifique, 118 Route de Narbonne Bâtiment 4R1, 31062 Toulouse Cedex, 9, France. 1 equal contribution.

is acutely threatened by human activities. Flow regulation by dams, diversions and other infrastructures to reduce flood risk, and the conversion of riparian zones by agriculture, forestry, industrial and urban development are responsible for the deterioration and loss of riparian ecosystems worldwide (Hughes and Rood, 2003; Nilsson and Berggren, 2000). Up to 90% of North American and European floodplains, for example, are considered ecologically dysfunctional following human occupation (Schillinga et al., 2015; Tockner and Stanford, 2002). In Europe, the combined effect of conversion to agriculture and regulation has resulted in the disappearance of up to 88% of floodplain forests (Hughes and Rood, 2003). Unlike other SNFs such as temporary streams (Acuña et al., this issue) or large old trees (Lindenmayer, this issue), conservation of riparian ecosystems has been the object of much research. Many studies have focused on how to manipulate riparian ecosystems to enable their conservation or restoration (e.g. renaturalization of flow regimes (Poff et al., 1997; Rood et al., 2003); restitution of channel migration

http://dx.doi.org/10.1016/j.biocon.2016.10.035 0006-3207/© 2016 Elsevier Ltd. All rights reserved.

Please cite this article as: González, E., et al., Integrative conservation of riparian zones, Biological Conservation (2016), http://dx.doi.org/10.1016/ j.biocon.2016.10.035

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(Rohde et al., 2005; Jähnig et al., 2009); and control of species invasions (Richardson et al., 2007; Stromberg, 2001). However, conservation strategies purely based on manipulating ecological processes face important technical–ecological, socio-economic, and legal constraints. First, after decades of impact, some rivers may have lost their capacity to positively respond to conservation actions (Cooper and Andersen, 2012; Johnson et al., 2015). Secondly, society may prioritize extractive uses and particular ecosystem services such as flood prevention or recreation over key ecological functions like wildlife habitat or nutrient filtering (Gumiero et al., 2013; Rohde et al., 2005). Thirdly, legislation may subordinate environmental goals to other interests. In Europe, for example, there are a number of socio-economic reasons (such as “overriding public interest” or “no other significantly better option”) to be exempted from meeting the environmental objectives of the Water Framework Directive (European Commission, 2009). The conflict between different interests and directives usually leads to the adoption of uncoordinated measures in the management of riparian zones. For instance, in Italy, in order to save public money in flood risk prevention, private companies are allowed to remove vegetation from riparian zones for biomass production (WWF, 2016). However, this increases flood risk downstream and may generate dramatic effects on riparian wildlife (Anderson et al., 2006). Thus, conservation of riparian zones will not be possible if technical–ecological, socio-economic, and legal issues are not addressed holistically. Hunter et al. (this issue) have proposed a matrix of key conservation activities (educate, inventory, protect, sustainably manage, restore, and create) that may include incidental, voluntary, incentive, or restrictive approaches relevant to the conservation of SNFs, which we can apply to riparian zones. The main goal of this paper is to identify and discuss conservation measures for riparian zones complementary to manipulation of biotic and physical processes that reflect management and policy directions. To achieve this goal, we (i) briefly introduce riparian zones (Section 2) and their ecological, economic, and social importance (Section 3), (ii) describe the main human impacts on riparian zones (Section 4) and current management challenges (Section 5), and, following Hunter et al. (this issue) (iii) review key measures for conserving riparian zones that will facilitate SNF conservation (Section 6).

2. What are riparian zones? Riparian zones are the interface between terrestrial and aquatic ecosystems along inland watercourses (Naiman and Décamps, 1997). They encompass the space between the flowing water at low levels and the highest water mark where vegetation is influenced by floods, elevated water tables, and soil type. In the landscape, they function as a dendritic network of narrow-shaped corridors with flowing energy, matter and biodiversity (Gregory et al., 1991; Naiman and Décamps, 1997). Riparian zones are present in all biomes from tropical rainforests to arid and arctic deserts, and range from large floodplain–river systems draining millions of cubic meters of water annually at a continental scale (Ward et al., 2002), to small temporary streams (Acuña et al., this issue) (Fig. 1). The main abiotic and biotic characteristics of riparian zones are: (i) a flooding regime with high temporal and spatial variability that creates a landscape mosaic of both vegetated and bare fluvial landforms functioning as habitats organized hierarchically (Gurnell et al., 2016), and (ii) unique biotic communities with species that benefit from a high water and nutrient availability but that also must tolerate shear stress and temporary submersions (Naiman and Décamps, 1997). The dependence of riparian zones on the flooding regime—along four dimensions: longitudinal (upstream–downstream), lateral (hillslope-channel), vertical (hyporheic-channel bed), and temporal (Ward, 1989)—is the main singularity that makes them functionally distinct from purely terrestrial or aquatic lentic ecosystems (Tockner et al., 2000).

3. Ecological, economic, and social importance of riparian zones Riparian zones perform multiple ecological functions, including refuge for regional biodiversity, climate regulation, flood buffering, water and nutrient filtering, shading stream channels and high primary productivity (Naiman and Décamps, 1997; Palmer and Bennett, 2006). These ecological functions are directly related to key ecosystem services provided to society (Felipe-Lucia et al., 2014; Vidal-Abarca Gutiérrez and Suárez Alonso, 2013). Many of these functions have direct economic relevance, including flood control; support for agriculture, forestry, industry and urbanization; and several outdoor recreational activities, such as visits to waterfalls and gorges, hiking, canoeing, and fishing. For instance, the economic value of the ecosystem services provided by riverine wetlands and riparian buffers in three Canadian rivers was estimated to be ca. 6000 Canadian dollars per hectare and year (Buffin-Bélanger et al., 2015). 4. Impacts on riparian zones Riparian zones have faced profound anthropogenic modifications since the rise of civilization (Feld et al., 2011), which have been shown to affect trophic networks at every level (Mensing et al., 1998). Human activities have been centered along rivers and riparian zones because of their position in the landscape. Agriculture exploits the nutrient-rich substrates of wide floodplains, while rivers in canyons or open valleys are dammed to store water for agricultural, domestic, and/or industrial use (Graf, 2006; Hughes and Rood, 2003). Furthermore, streams have been adapted to serve as corridors for transportation, facilitating forestry, industrial, and urban development in riparian zones. While some free-flowing rivers remain in remote headwaters, nature reserves and less populated regions (e.g. Tagliamento river in Italy, Ward et al., 1999; Merced River in Yellowstone National Park, U.S., Yochim and Lowry, 2016), most rivers and associated riparian zones in the world are severely impaired by altered flow regimes (Nilsson et al., 2005). Dams disrupt longitudinal connectivity and dramatically alter the structure and composition of riparian vegetation (Graf, 1999; Ward and Stanford, 1995). Downstream of dams, peak flows are attenuated, summer water levels are kept abnormally low (prolonged droughts following water storage in reservoirs) or high (water releases for irrigation, navigation, energy production, and recreation; Graf, 2006). Often riparian plant and animal communities cannot adjust their life cycles to such disturbance of the natural flow regime and may suffer from severe declines in their populations (e.g. collapse of riparian trees (Rood and Mahoney, 1990); decline in colonial waterfowl breeding events Kingsford and Auld, 2005) and be replaced with non-strictly riparian communities (e.g. dryland plants; Dixon et al., 2012), which may paradoxically increase alpha and beta diversity in riparian habitats as a temporary stage within an overall process of ecological degradation and homogenization (Gumiero et al., 2015). Upstream of dams, raised water tables also alter riparian habitats (e.g. replacement of woody riparian by meadow type vegetation Tombolini et al., 2014). This impact is global as virtually all rivers in industrialized regions are dammed and more dams are planned to be built in developing countries (Nilsson et al., 2005). Laterally, changes in sediment and flow regime, geomorphological alterations (e.g. channel incision), dikes, levees, ripraps, and other structures prevent channel migration (Magdaleno and Fernández, 2011), essential for vegetation regeneration (Scott et al., 1996). Vertically, river bed incision by channel embankment and gravel extraction, and groundwater overexploitation induce water tables lowering and leading to loss of plant species less adapted to water scarcity (Stromberg et al., 1996; Gumiero et al., 2015) and may favour particular functional groups (e.g. swallows and kingfishers taking advantage of exposed banks, Silver and Griffin, 2009). In addition to stream flow alterations, pollution is a significant threat to viability of riparian zones across the globe. While measures to

Please cite this article as: González, E., et al., Integrative conservation of riparian zones, Biological Conservation (2016), http://dx.doi.org/10.1016/ j.biocon.2016.10.035

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Fig. 1. Examples of different riparian zones (river, country, photo credit): a) Diamond Creek, Australia, Grant Palmer; b) Green River, U.S., Anna A Sher; c) Vindel River, Sweden, Christer Nilsson; d) Piedra River, Spain, María Felipe-Lucia; e) Sabana River, Puerto Rico, Deborah K Kennard, f) Tana River, Kenya, Francine MR Hughes.

improve water quality have had some successes in industrialized areas (Plum and Schulte-Wülwer-Leidig, 2013), pollution is worsening in developing countries (Müller et al., 2008; Yang et al., 2012). 5. Management challenges 5.1. Establishing reasonable goals for the effective conservation of riparian zones Returning degraded riparian zones to pre-disturbance or low human impact conditions is rarely possible. Riparian zones are especially dynamic and evolving systems subjected to non-linear, often unpredictable trajectories and are in permanent change. Conservation of riparian zones must aim at establishing a state able to adapt to changing conditions (Dufour and Piégay, 2009; Hughes et al., 2005). In order to define such a state representing desirable yet reasonable conservation goals, an accurate diagnosis of the past, present, and future

(expectations) for the riparian zone is paramount. Any evaluation should include: (i) formal designations for ecological components organized by scale, from river catchment to elements of a reach (e.g. sediment, vegetation, etc.) (Gurnell et al., 2016), and within these: (ii) the specific ecological functions and ecosystem services of interest to improve (Dufour and Piégay, 2009; Shafroth et al., 2008); (iii) the biotic (e.g. presence of ecosystem engineer species) and abiotic (e.g. alterations in water and sediment regime) constraints for achieving these goals; (iv) the socio-economic and legal context of any action to be taken (e.g. Shafroth et al., 2008); and (v) the uncertainty associated with the diagnosis (Hughes et al., 2005). Analytical tools such as the BACI methodology (Before-After Control-Impact; Stewart-Oaten et al., 1986) and the DPSIR framework (Driver-Pressure-State-Impact-Response; Smeets and Weterings, 1999) are specifically designed to identify constraints and envisage reasonable conservation goals (Feld et al., 2011; Vidal-Abarca et al., 2014). Another possibility is modelling future scenarios based on long term

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data series (Carpenter et al., 2015) or new scientific evidence (Fernandes et al., 2016). Other analytical frameworks have been specifically designed for river management, such as the REFORM project (Gurnell et al., 2016), and can still be improved with increasing understanding of riparian functioning and developing technologies. Finally, methods such as Delphi (technique based on successive rounds of anonymous questionnaires and feedback; Dalkey and Helmer, 1963) facilitate expert discussion reducing social pressures and have been successfully applied to plan conservation actions in riparian zones (Rohde et al., 2005). 5.2. Dealing with legal frameworks There is a myriad of national, regional, and local legislation affecting riparian zones; however, the authors have not encountered any that integrates all of the activities involving riparian zones, and few explicitly recognize the ecological functions and ecosystem services. Most of the legislation affecting riparian zones focuses on water quality (e.g. the European Water Framework Directive and the U.S. Clean Water Act, National Environmental Protection Act and Farm Bill), but usually neglects the ecological status of the non-aquatic components. An exception to this trend and a possible model may be the U.S. National Wild and Scenic Rivers Act (WSR Act, 1968), which protects rivers with “outstanding and remarkable natural, cultural, or aesthetic qualities” to preserve water quality and other vital conservation purposes. Although the WSR Act was written too early to incorporate modern ecological jargon, it has successfully protected over 12,700 miles of 208 rivers in 40 U.S. states and the Commonwealth of Puerto Rico from energy development or other physical modifications that would change the flow of the river (National Wild and Scenic Rivers System, http://www.rivers.gov). These protections generally include land a quarter mile from each bank, except for in Alaska, where the protected riparian zone is half a mile on either bank. However, this seemingly impressive extent of protection represents less than 0.25% of U.S.'s rivers and corresponding riparian zones. One of the problems associated with the lack of formal recognition of riparian zones is that the application of individual policies can have antagonistic effects. For example, in Europe, young cohorts of poplar and willow trees are frequently removed under the Flood Risk Directive to avoid vegetation encroachment and increase stream conveyance capacity (Geerling et al., 2008), while these same species are being promoted by the Habitats Directive to preserve alluvial forests (Hughes and Rood, 2003) and create ecological networks along river corridors (Jongman et al., 2004). The creation and maintenance of alluvial forests are also supported by the European Agricultural Funds for Rural Development (EAFRD, 2005) and enforced by the cross-compliance regulation as one strategy to achieve vegetated buffer zones along rivers (Gumiero et al., 2016). In the southwestern U.S., non-native tamarisks (Tamarix spp.) were extensively removed from riparian zones following a federal bill in 2006 calling for floodplain restoration (USA House of Representatives 2720 and USA Senate 177: Salt Cedar and Russian Olive Control Demonstration Act) but currently those restoration efforts are quarantined in some river reaches where the southwestern willow flycatcher (Empidonax traillii extimus, listed in the U.S. Endangered Species Act) has been shown to nest on tamarisk-dominated riparian zones (Hultine et al., 2010). In the northwestern U.S., measures to conserve cottonwood forests were proposed by federal agencies to protect the bald eagle (Haliaeetus leucocephalus, federally listed as threatened), but once the species was delisted in 2007, conservation of cottonwood forests lost its political urgency despite their dramatic decline in recent decades (Dixon et al., 2015). 5.3. Uncertainties for future riparian conservation The future of riparian zones will be affected by both natural and anthropogenic forces. First, climate change will alter stream flow regimes

by modifying precipitation and runoff patterns and increasing the frequency of extreme weather events, such as floods and droughts (IPCC, 2007). Consequently, riparian zones will experience major structural and functional changes (Garssen et al., 2014; Kominoski et al., 2013). Secondly, climate change combined with the intensification of worldtrade is expected to increase the likelihood of biological invasions leading to biotic homogenization to the detriment of ecosystem resilience (Diez et al., 2012; Richardson et al., 2007). Riparian zones are inherently vulnerable to invasions as naturally disturbed areas (Kominoski et al., 2013). Confounding direct and indirect effects of anthropogenic impacts makes the diagnosis of riparian states and their response to conservation actions difficult. There are often long time-lags between human impacts and river adjustments (Johnson, 1998; Katz et al., 2005) that jeopardize a prompt diagnosis of the status of riparian zones necessary to establish reasonable conservation goals (see Section 5.1). Similarly, the outcome of conservation actions often takes time. For example, significant recruitment of Salicaceae riparian trees may take years or even decades after the renaturalization of flow regimes (Mahoney and Rood, 1998). A review of restoration projects where native trees and plants were reintroduced in riparian zones found that on average, planted species cover had not markedly increased until nine years post-restoration (Bay and Sher, 2008). Moreover, it is expected that the potential response of riparian ecosystems to conservation actions be attenuated over time, with a progressive loss of natural resilience after decades of anthropogenic disturbance (Dixon et al., 2015; Johnson et al., 2015). In addition, societal needs and perception of ecological problems also vary over time. For instance, the abovementioned invasive tamarisks in western U.S. riparian zones were maligned for decades but now the ecosystem services they provide have been recognized, resulting in more careful consideration of when and where control is desirable (Stromberg et al., 2009). Finally, the lack of studies in some regions of the world increases the uncertainty of eventual outcomes of conservation actions. For instance, González et al. (2015) only found one evaluation of restoration efforts for riparian vegetation in Central and South America, five in Africa (i.e. South Africa), compared to ten in Oceania, 24 in Asia, 26 in Europe and 103 in North America. Despite riparian zones being ubiquitous across the world, their conservation in some biomes such as the tropics remains overlooked by the scientific community. 6. Key measures to conserve riparian zones In this section, we expand on some important aspects of conservation that have received less attention in the literature. These solutions are grouped in five key activities (education, inventory, protection, sustainable management, and restoration) that respond to one or more approaches: incidental (as a result of conserving the landscapes in which riparian zones are embedded), voluntary (often due to ethical concerns), incentive (due to financial inducements or other benefits), or restrictive (based on legal threat of penalties for harming riparian zones) (Hunter et al., this issue; Fig. 2). 6.1. Educate Environmental education sets the grounds for society to demand and be more receptive to riparian conservation in the long term. For instance, ‘manicured’ riparian zones are often more appreciated than ecologically functioning zones if the values of natural riparian zones are not properly understood by the public. For instance, students representing non-expert views perceived in-stream wood as something negative while trained river managers associated wood with less danger, better aesthetics, and less need of river condition improvement (Chin et al., 2014). Education can, therefore, be targeted to the general public, but is especially important for the technical staff, policy-makers, and

Please cite this article as: González, E., et al., Integrative conservation of riparian zones, Biological Conservation (2016), http://dx.doi.org/10.1016/ j.biocon.2016.10.035

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Fig. 2. Summary of the main conservation activities suggested and the approaches that can be applied to each activity sorted from low (left) to high (right) obligation to conserve.

landowners, as they are able to apply recommended conservation measures and have an important role in ensuring their success. Environmental education can be approached voluntarily by NGOs and through incentives of public policies. For example, the Spanish National Strategy for River Restoration encourages volunteer participation in restoration activities such as trash clean-up by funding NGO projects (González del Tánago et al., 2012). The European Union has some specific programmes to support information campaigns on environmental issues (LIFE Environmental Governance and Information; http://ec. europa.eu/environment/life/funding/life2016/). Universities and schools often organize outreach activities and engage local communities in citizen science projects (e.g. BEMP—Bosque Ecological Monitoring Programme in the Rio Grande, New Mexico, southwestern U.S.; http:// www.bemp.org). Citizen science in particular can be used as a tool to monitor the health of riparian zones and collect long term data (see Section 6.2), while instructing and educating local communities. For example, schools have monitored riparian habitat condition in New Mexico sponsored by the regional government (Fleming, 2003), and trained volunteers have mapped surface flow conditions on the San Pedro River for 12 years (Turner and Richter, 2011). Engaging local communities through environmental education goes in accordance to the principles of sustainable management (see Section 6.4; da Silva Melo et al., 2011). The private sector (e.g. banks, companies) can also fund environmental education programmes through incentives like branding and tax reductions (Primack and Sher, 2016). 6.2. Ecosystem inventories Conducting biotic and physical inventories, though required to design conservation strategies (Groves et al., 2002; Shafroth et al., 2008; Hunter et al., this issue), is particularly complicated in riparian zones due to their inherent spatio-temporal dynamics, extremely low interior-to-edge ratio and extraordinary length. Field-based inventories are labour-intensive, time consuming and expensive. Alternatively, imagebased assessments can help to achieve a systematic mapping and long-term monitoring of riparian zones, especially at the regional scale (Jarnevich et al., 2013; Johansen et al., 2007). Among these techniques, remote sensing by aerial photographs or LiDAR imagery obtained from satellites, airplanes and drones, photogrammetry, and traditional

photointerpretation has provided meaningful evaluation of riparian conditions, even if their results generally require ground truthing. To date, image-based assessment has been successfully used to measure the width of riparian zones (Arroyo et al., 2010), distinguish riparian zones from other types of wetlands (Baker et al., 2006), track the expansion of invasive species (Evangelista et al., 2009; Jarnevich et al., 2013; Michez et al., 2016), and determine the reduction of the spatial extent of riparian zones after anthropogenic disturbances (Apan et al., 2002) and shifts in vegetation patterns in relation to flooding inventories (Džubáková et al., 2015). Inventories of riparian zones generally originate from voluntary initiatives from local stakeholders, conservationists, scientists, or researchers (Bell et al., 2008), and may be combined with indicators to assess riparian zones health or ecological status. Several indicators with specific conservation targets and spatial coverage (from microhabitat to catchment) have been developed to assess the integrity of riparian zones (Appendix S1). Most of these indicators combine measurements of different ecological components such as hydrology, geomorphology, vegetation, or human disturbances to calculate an index of the riparian health, while in some cases, specific taxa are also used to assess riparian condition such as butterflies (Nelson and Andersen, 1994). These integrative indexes can help to design management options, select sites for restoration (Rohde et al., 2006), and are useful to enforce restrictive legislation (e.g. European Water Framework and Habitats Directives; Munné et al., 2003; Rinaldi et al., 2013). Depending on conservation goals, ecological indicators thus provide integrative techniques to valorise inventory data and communicate them more easily. However, these indicators are often site-specific and frequently fail to consider reference ecosystems or spatio-temporal dynamics. Multi-scale hierarchical frameworks are a promising perspective to develop indicators taking into account the geographically nested ecological processes that drive riparian conditions (Gurnell et al., 2016). 6.3. Conserve through protection Riparian zones can be protected directly by their inherent value via restrictive approaches, such as implementing legislation to prevent human activities at a certain river width (e.g. EAFRD, 2005). For

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example, in Spain, a specific regulation of the Public Hydraulic Domain (Real Decreto 9/2008) limits the range of activities within 100 m from the streams in order to both reduce flood risk and protect riparian zones from developments. Legislation may also indirectly benefit riparian zones, for example the U.S. Endangered Species Act and the European Habitat Directive require protection of habitats for listed species, resulting in the protection of riparian zones incidentally because of the presence of endangered species or as the result of protecting a larger natural area where rivers flow through (e.g. through the creation of natural parks or reserves). These activities are not only originated at the governmental level; rather, private owners, NGOs, non-profit foundations may also acquire land containing riparian zones for their protection voluntarily (e.g. land trusts such as The Nature Conservancy, Kareiva et al., 2014). Another way to protect riparian zones is through incentives to abandon economic activities and allow the renaturalization of the riparian zone. These can be applied as conferring compensatory rights (e.g. extra quotas for fishing or land exchange following a “value equalization” mechanism) or economic benefits, such as tax reduction or direct subsidies to the landowners. Rhodes et al. (2002) showed that farmers in New Zealand were more willing to install riparian fencing for livestock if they received financial incentives. A common mechanism to compensate landowners for the loss of incomes is known as Payments for Ecosystems Services (PES) or Payments for Hydrologic Services (PHS). Examples of these are incentivizing the purchase of cultivated lands by governments to protect forest and water services in Madagascar (Wendland et al., 2010), and the removal of invasive alien plants species to increase water availability, carbon sequestration, fire protection, and create job opportunities in South Africa (Turpie et al., 2008). Similarly, agri-environmental schemes set to support the conservation of farmland biodiversity can incidentally protect riparian zones, as by creating buffer strips between the river and the crops (Stoate et al., 2007; Stutter et al., 2012). However, voluntary non-monetary approaches have also been able to implement conservation actions on private land (Santangeli et al., 2016; see Section 6.4). 6.4. Sustainably manage Based on the concept of sustainable development (UNCED, 1992), sustainable management incorporates economic, societal, and environmental needs in ways that will benefit current and future generations. Policies applying this concept can facilitate conserving riparian zones while still meeting human needs (Richter et al., 2003). Unfortunately, incorporating the importance of riparian areas for the supply of ecosystem services has not been traditionally addressed in large management plans. Major initiatives focusing on nature based solutions (using green infrastructure) are now being promoted by the European Commission (EU, 2013), the U.S. EPA (2016), the UNEP, the IUCN and the Nature Conservancy (UNEP, 2014), among others. Making riparian zones (conserved and managed) part of achieving the sustainable development goals in landscape planning is currently the most promising pathway to preserving their larger value (Clerici and Vogt, 2013). In urban and agro-forest areas, conserving a buffer strip of natural riparian forest alongside the riverbanks stabilizes the banks, reduces runoff, and acts as a natural protection from flooding, while increasing aesthetics and property value of adjacent developments. Selective cutting (instead of clear-cutting) to manage for timber production and reducing the inputs of pesticides and fertilizers in adjacent landcrops to preserve wildlife habitat and increase recreational value are other examples of sustainable exploitation of riparian zones (Tilman et al., 2002). Designed paths and other facilities, such as toilets, control and thereby decrease extent of damage by visitors while increasing accessibility. Incorporation and integration of human needs in protected areas management have often proved more effective for conservation in the long-run than exclusion (Primack and Sher, 2016). One of the main challenges of implementing sustainable management of riparian zones is that in most parts of the world, these areas

are predominantly owned by private landowners who may be reluctant to voluntarily participate (Ollero, 2010). One way to overcome this is by actively engaging landowners in the design of sustainable management plans (Feld et al., 2011; Gumiero et al., 2013). For example, in the Sacramento River (U.S.), directly engaging local partners in the design of restoration actions was key to gaining support for the project and led to great improvement of both ecosystem health and services (Golet et al., 2006). In the mid-western U.S., farmers were motivated to engage in riparian conservation practices (e.g. maintaining woody vegetation, notill farming) due to their attachment to the land and the desire to be seen as good land managers (Ryan et al., 2003). Most often, land owners require incentives and practical support to accomplish conservation goals, such as easements in which the owners are paid by the government to sustainably manage their land and maintain ecological functioning (e.g. payments for ecosystem services, agro-environmental schemes; see Section 6.3). Involving stakeholders other than landowners, such as local environmental associations, can help mediating between landowners and policy makers. Engaging stakeholders in riparian zones management is not only key to achieve success, but is also fair (Dufour and Piégay, 2009), as including multiple opinions and needs might reduce the inequalities in the access to riparian ecosystem services (Felipe-Lucia et al., 2015). Finally, restrictive legislation may be necessary, for example to prevent the dumping of toxins or other waste in riparian areas, or to preserve vegetated buffer zones along rivers (Gumiero et al., 2016), and even to expropriate lands (Ollero, 2010; see Section 6.3). 6.5. Conserve through restoration Restoration of riparian zones has mostly evolved over time from targeting reference systems representing pre-industrial, pristine conditions to focus on maximizing ecological functions and ecosystem services (target- vs. process-based restoration, sensu Dufour and Piégay, 2009). If the impacts of regulation are low and recent, removal of flood prevention infrastructures may be enough to restate geomorphic processes as a result of channel widening (González et al., 2016; Jähnig et al., 2009; Rohde et al., 2005), in accordance with the wellestablished international river conservation principle of channel mobility, coined by different authors as: “freedom or mobility space” (Malavoi et al., 1998), “room for the river” (Baptist et al., 2004; Rohde et al., 2005), “erodible corridors” (Piégay et al., 2005), or “fluvial territory” (Ollero, 2010). However, when riparian zones face profound and irreversible anthropogenic modifications, more active measures are usually needed. For example, riparian habitats can be improved by modulating the flow regime through environmental flows, as in the Colorado River (Arizona, U.S.) (Stevens et al., 2001), the Murray-Darling (Australia) (Siebentritt et al., 2004), and the Tarim and Ejina Rivers (China) (Chen et al., 2010; Zhang et al., 2011). Unfortunately, some rivers have lost their resilience after decades of regulation and renaturalization of the flow regime is no longer effective to restore key ecological processes such as recruitment of keystone riparian tree species. In the Missouri River (northwestern U.S.), for example, a spontaneous (non-induced) extraordinary flood of 500-years interval recurrence period was unable to induce channel migration and subsequent recruitment of Salicaceae forests (Dixon et al., 2015; Johnson et al., 2015). When the hydrogeomorphic regime cannot be modulated from channel mobility and environmental flows from dams, other types of active restoration can be applied, such as floodplain excavation (Geerling et al., 2008; González et al., 2016), or using former irrigation channels to reproduce the mosaic of patches typical of riparian forests (Bunting et al., 2011; Taylor et al., 2006). Other restoration actions may include control of exotic species, active introduction of vegetation to accelerate desirable successional trajectories, control of herbivory, and abandonment of human uses followed by spontaneous recovery of vegetation (González et al., 2015). Occasionally, riparian zones can be artificially created as the result of newly created river channels (Lapin et al., 2016). When channel

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mobility cannot be promoted, bioengineering works can improve the ecological status of riparian zones along riverbank protection structures (Cavaillé et al., 2015). Restoration can be motivated or supported by incentives and restrictive approaches. For example, the Spanish National Strategy for River Restoration was created by the Ministry of Environment of Spain as a tool to fund restoration projects (incentive) to fulfil the requirements of the European Water Framework Directive (restrictive, see González del Tánago et al., 2012). The U.S. Department of Agriculture offered incentive payments to private landowners to transform 275,000 ha of non-productive farmlands into bottomland hardwood forests in the Mississippi Alluvial Valley (De Steven et al., 2015). The Grain for Green Programme (also known as Sloping Land Conversion Programme) incentivized the conversion of croplands to forests including natural riparian vegetation in the Yangtze and Yellow rivers in China (Kolinjivadi and Sunderland, 2012). Carbon sequestration credits could be used to incentivize restoration of floodplain forests (Matzek et al., 2015). Other countries like Switzerland also have incorporated the restoration of riparian ecosystems into their legislation (Kurth and Schirmer, 2014). Finally, evaluating the success of restoration activities is still a pending task (Nilsson et al., 2016; Palmer et al., 2005). Post-restoration monitoring is usually not budgeted in restoration projects and when done, this happens in a very short time lag (5–6 years after the project ends; González et al., 2015); and information on the long-term effects of restoration is scarce.

management. Incentives are applied to engage landowners into sustainable management through the integration of ecosystem conservation goals as a part of their activities, economic benefits, or compensatory rights, but also for environmental education, protection, and restoration. Finally, riparian zones are often protected incidentally through specific legislation to safeguard endangered species or through activities promoting the sustainable management of riparian zones. Taken together, these approaches will improve protection of the invaluable ecological, economic, and social values of riparian zones. Supplementary data to this article can be found online at doi:10. 1016/j.biocon.2016.10.035.

7. Conclusions

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Riparian zones are threatened by continuous and increasing anthropogenic impacts since the rise of civilization. Among the main management challenges facing riparian zones we stress: (i) difficulties to establish reasonable conservation goals, (ii) a vast but non-specific and sometimes contradictory legislation, (iii) uncertainty about the response of riparian zones to climate change, biological invasions, and the long time-lags between the application of conservation measures and eventual positive results, (iv) evolving societal needs and values, and (v) unbalanced amount of information to understand the functioning, current status, and conservation plans across different world regions and biomes. Unlike other SNFs, riparian zones have been broadly studied and strategies for their conservation extensively discussed. However, the technical–ecological, socio-economic, and legal aspects are still not fully integrated in conservation plans. Up-to-date, scientific research on conservation of riparian zones has mainly focused on finding technical solutions to deal with the impacts caused by the alterations of the flow and sediment regimes by regulation, and the negative effects of exotic ecosystem engineer species. The most effective solutions, such as the renaturalization of the flow regime or the recovery of channel mobility, are not always possible due to socio-economic and legal constraints. A framework for an integrated conservation strategy including five activities proposed by Hunter et al. (this issue) is presented here: education, inventory, protection, sustainable management and restoration (including creation). We advocate conservation plans that apply all these activities in an integrative manner, i.e. the principles of sustainable management, a better specification and coordination of legislation, a clear definition of restoration goals when this approach is considered necessary, an effective public participation in the decisionmaking and implementation processes supported by environmental education, and high quality information collected through systematic inventories and monitoring. The approaches to implement these activities are not mutually exclusive. Thus, in riparian zones, restrictive approaches are used to inventory, protect, and restore riparian zones while contributing to their sustainable management. Voluntary approaches support education, inventory, and protection, and also contribute to sustainable

Acknowledgements We are grateful for the invitation of Dr. Malcolm Hunter to participate in the symposium and write this paper. Drs. Vicenç Acuña, Aram Calhoun, Monique Poulin and two anonymous reviewers provided helpful comments to improve the manuscript. EG and MFL wrote the paper with contributions from the remaining authors, who are listed in alphabetical order. EG participation in the symposium was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme (ESFFORES project grant number 299044). MFL thanks the Society for Conservation Biology who kindly supported her travel costs to participate in the symposium. References

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