The role of Italian coastal dunes as carbon sinks and diversity sources. A multi-service perspective
Introduction
Coastal dunes are dynamic systems which provide essential benefits to society, some of which have a considerable socio-economic impact (Everard et al., 2010, Jones et al., 2011, McLachlan and Brown, 2006, p. 357; MA, 2005). These systems play a major role for recreation and tourism, being highly valued as a place of escape and isolation and as a source of mental well-being (Doody, 1997, Houston, 1997, Nordstrom, 2000). In addition, they provide unique habitat assemblages due to a strong environmental sea-inland gradient, which supports a highly specialized flora and fauna sharing relatively few species with other terrestrial ecosystems (Acosta et al., 2009, Martínez et al., 2004). While services such as coastal defence, groundwater storage and water purification are clearly recognized and integrated into the coastal management of many sites (French, 2001, Rhymes et al., 2015, Van Dijk, 1989), rather less is known about supporting ecosystem services such as nutrient cycling, soil formation and climate regulation (Barbier et al., 2011, Jones et al., 2008). Being an early successional ecosystem, coastal dunes have a high soil carbon accumulation rate (Jones et al., 2008, Olff et al., 1993, Rohani et al., 2014), a feature in common with other coastal environments (Sevink, 1991). There is increasing interest in the role of “blue carbon” in climate regulation (Donato et al., 2011, Donato et al., 2012, Mcleod et al., 2011, Nellemann et al., 2009), and sequestration by marine and coastal ecosystems has been globally quantified as ca. 2 Gt C yr−1 (Chmura, Anisfeld, Cahoon, & Lynch, 2003). Yet, the specific role of carbon storage in dune habitats has been little explored to date, except in the UK, where both annual CO2 sequestered and the stock of carbon in vegetation and soil were estimated for the whole country and changes in value of the carbon sequestration service were projected under different scenarios of coastal change alteration (Beaumont, Jones, Garbutt, Hansom, & Toberman, 2014). While carbon accumulation rates are very high, the gross contribution of dune habitats to climate regulation is relatively small due to their low area. However, in the context of widespread coastal habitat loss and land-use change at fine scale, and within a wider context of habitat management for multiple benefits, their role in regulating greenhouse gas emissions is worth taking into consideration (Everard et al., 2010).
Despite the high biodiversity value and numerous benefits provided by coastal dunes, this ecosystem is among the most threatened both globally (Schlacher et al., 2007) and in the Mediterranean (Rossi et al., 2013). Human activities in European littoral areas have been intensifying in the course of the 20th century (Cori, 1999); consequently, sand dunes across Europe had lost on average 25% of their extent by 1998, compared to 1900 (EUCC, 1998), with peaks of 80% area loss in some Mediterranean countries. In order to prevent these and other endangered habitats from further degradation, European Member States adopted the Council Directive 92/43/EEC (Habitats Directive from now onwards), which lists the habitats of European interest (EU habitat types) and establishes across Europe an extended network of sites of ecological importance, called Natura 2000.
In Italy, 86.7% of EU coastal habitats currently have an unsatisfactory (bad or inadequate) conservation status, having suffered a drastic reduction in both extent and ecological quality, mainly due to urban sprawl (Genovesi et al., 2014). Of the Italian 3000 km coastline, the Northern and Central Adriatic sector is probably the most developed and industrialized (with more than 70% of its seaside urbanized), hosting several international tourist resorts and important port cities, as well as an intense transportation network (Highway A14, State Road No 16 and railway line) which have destroyed the natural coastline in many points (Romano & Zullo, 2014). Therefore, in order to preserve the last intact coastal landscapes in this area, there is a need to study in more detail the role of Adriatic Natura 2000 sites both for biodiversity protection and their capacity to provide additional ecosystem services and, in particular, the interplay between those potentially conflicting functions. In this study we focus on two coastal services: biodiversity protection and carbon storage and sequestration. We selected carbon storage first because a consistent approach to measure and assess carbon storage service in coastal dunes is still lacking (Laffoley and Grimsditch, 2009, Beaumont et al., 2014); and secondly, because of the potential for conflicts with biodiversity provision, since carbon storage requires stabilised systems while much of the unique dune biodiversity relies on natural dune dynamics.
Thus, the aims of this work were i) to quantify soil carbon storage and sequestration provided by a set of coastal dune EU habitat types within Natura 2000 network along the Northern and Central Adriatic Sea; ii) to compare their relative contribution and to create the first inventory of carbon stocks for the Adriatic Natura 2000 sites; iii) to characterise coastal dune biodiversity value, using various metrics of plant species richness as a proxy; iv) lastly, to explore the trade-offs between carbon storage and biodiversity value for the selected habitats and to discuss their relative value in a multi-service perspective.
Section snippets
Selection of EU coastal dune habitat types
For data collection and upscaling we adopted the EU habitat types classification, as it entails spatial data at sufficient detail to distinguish between habitats but at an appropriate spatial scale and consistency required for upscaling. The use of more detailed classifications of dune habitats creates difficulties because dunes usually occur as long, narrow strips following the coastline but they are mapped at a coarse resolution, which makes it problematic to define fine variation in plant
Soil characteristics
All habitats showed a high pH, ranging from 8.35 (wooded dunes) to 8.85 (mobile dunes) on average (Table 3). The carbonate results suggest Adriatic sand dunes have high carbonate content, averaging 33% in bare sand. The percentage of carbonate in wooded dunes showed a greater variability than in bare sand (Table 3).
Results for bulk density, soil %C and soil C density are reported in Table 3. Kruskal-Wallis tests applied on these parameters revealed a clear statistical difference (P < 0.001)
Discussion
This study has enhanced our understanding of the ecological role of an important sector of the Italian coast. In total, 36 Natura 2000 coastal dune sites were characterized for soil carbon storage service and biodiversity, based on four representative dune habitat types. The figures reported in Table 6 represent the first inventory of soil carbon stocks for dune systems in the Mediterranean and are based on survey data, rather than on the literature (for instance by means of “benefit transfer”
Conclusions
This study produced valuable data concerning soil carbon storage and sequestration service provided by biodiverse and fragile ecosystems, which are in need of high-priority protection. A complete and updated inventory of soil carbon values for four representative coastal dune habitats within the Adriatic Natura 2000 network was compiled, and their biodiversity value was compared and discussed. Wooded dunes had greater carbon density and a greater area, thus storing greater soil carbon stocks.
Acknowledgements
This study was partially supported by the School of Ocean Science, Bangor University and by the LIFE project ENVEUROPE (LIFE08/ENV/IT/000399). We are grateful to P. Kennedy and C. Mortimer for their assistance with CSN analysis. We wish to thank A.T.R. Acosta, G. Buffa, M. de Chiro, A.R. Natale, R. Pizzutti, I. Prisco and M. Tomasella for the valuable information provided. For their help in the fieldwork, we are grateful to G. Di Paola, F. Fonzari, F. Genero and G. Valenti. C. Giampieri was
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