Monitoring Natura 2000 habitats: habitat 92A0 in central Italy as an example

The evaluation and the subsequent monitoring of the conservation status of habitats is one of the key steps in nature protection. While some European countries have tested suitable methodologies, others, including Italy, lack procedures tested at the national level. The aim of this work is to propose a method to assess the conservation status of habitat 92A0 (Salix alba and Populus alba galleries) in central Italy, and to test the method using data from the Molise region. We selected parameters that highlight the conservation status of the flora and vegetation in order to assess habitat structures and functions at the site level. After selecting the parameters, we tested them on a training dataset of 22 unpublished phytosociological releves taken from the whole dataset, which consists of 119 releves (49 unpublished releves for the study area, and 70 published releves for central Italy). We detected the most serious conservation problems in the middle and lower course of the Biferno river: the past use of river terraces for agriculture and continual human interventions on the river water flow have drastically reduced the riparian forests of Molise. Our results show that in areas in which forest structure and floristic composition have been substantially modified, certain alien plant species, particularly Robinia pseudoacacia, Amorpha fruticosa and Erigeron canadensis, have spread extensively along rivers. In the management of riparian forests, actions aimed at maintaining the stratification of the forest, its uneven-agedness and tree species richness may help to ensure the conservation status, as well as favour the restoration, of habitat 92A0.


INTRODUCTION
According to European Directive 92/43/EEC, better known as the Habitat Directive, member states are required to preserve, or restore to a favourable conservation status, habitats within the Natura 2000 Network, that is Sites of Community Importance (SCIs) and Special Protection Areas (SPAs). The conservation of such habitats can only be guaranteed by effective monitoring. The Habitat Directive requires the identification and evaluation of the defining characteristics of the habitats, as well as the threats that affect their current status or that may damage them in the future. According to Article 17 of the Directive, the conservation status of a natural habitat is considered favourable when its area of natural distribution is stable or expanding, when the structure and functions specific to its long-term maintenance exist and are not exposed to future threats, and when the conservation status of its typical species is favourable. While some European countries (JNCC 2004, BfN 2006, Calleja 2009, Carnino 2009) have already designed appropriate methods to monitor the conservation status of habitats, others have yet to introduce standard procedures adopted on a national level. The latter group of countries includes Italy, even though evaluations of Italian habitats, based on expert knowledge, have been performed and were published in the 3rd National Report ex-art. 17 Habitat Directive (92/43/EC) (www.sinanet.isprambiente.it/Reporting_Dir_Habitat).
Riparian habitats represent one of the ecosystems threatened most by human activities, particularly by changes in water regimes, the management of riparian vegetation and pollution (Allan & Flecker 1993, Liendo et al. 2015, as recently highlighted also in central Italy by Viciani et al. (2014). Indeed, nearly 20% of the research projects that have been conducted in recent years have focused on freshwater habitats, as highlighted by the EuMon database on monitoring methods and systems of surveillance for species and habitats of community interest (http://eumon.ckff.si/index1.php).
Changes in water regimes, as well as other types of human disturbance, can facilitate the spread of invasive alien species (Liendo et al. 2015). Indeed, riparian environments are, owing to their inherent predisposition to disturbance, among the environments most prone to invasions of non-native species (Stohlgren et al. 1998. Since the publication of the Italian checklist of nonnative flora (Celesti-Grapow et al. 2009, 2010, which raised considerable interest in plant invasions among local botanists, an increasing amount of attention has been paid to alien plant species in Italy. Despite this, the amount of information available in Italy is still incomplete, as highlighted by Assini (2000) for wet areas, and the experience limited, particularly for riparian habitats, if compared with other European and non-European countries (Pyšek and Prach 1993, Richardson et al. 2007, Schnitzler et al. 2007).
Forests dominated by Salix alba or Populus alba are widespread in the majority of Mediterranean EU member states, though their distribution is scattered (http://natura2000.eea.europa.eu/) owing to their ecological requirements. A specific monitoring strategy for Salix alba and Populus alba forests based on plant species and community indicators may prove useful to other member states in the Mediterranean area insofar as such forests are azonal. The aim of this study is to help fill this gap by proposing a suitable method to assess the conservation status of habitat 92A0 (Salix alba and Populus alba galleries) by testing field data from the Molise region, and to shed light on the reasons underlying the expansion of invasive plant species in these environments.
We believe that our case study may be considered a useful example of conservation status assessment of fresh water habitats in central Italy.

MATERIALS AND METHODS
In Table 1, we list the Natura 2000 sites (SCI/SPAs) included in the study area ( Fig. 1), together with the area of the sites and the area covered by the study habitat in each site, derived from the Natura 2000 database (update 2012) (http: //www.eea.europa.eu/data-and-maps/data/natura-2000). Although the majority of the sample plots were selected from Natura 2000 sites, some fall within the Biferno river basin and are located outside of the Natura 2000 Network. Owing to the scattered distribution of residual areas with riparian forests belonging to habitat 92A0, for the sampling design, we identified the sites for the sample plots after selecting potential areas referred to these forests by integrating the map of the Natura 2000 habitats in Molise (http://www3.regione.molise.it/flex/cm/pages/ServeBLOB.php/L/IT/IDPagina/657) and the map of the Nature (http://www.isprambiente.gov.it/en/environmental-services/map-of-the-naturesystem?set_language=en). Although the number of relevés is not particularly high, we believe that this selection process yields a fairly accurate representation of the current situation of riparian forests in the Molise region.
The vegetation sampling was performed mainly by means of the phytosociological method (Braun-Blanquet 1931, Dengler et al. 2008, Biondi 2011. This method, based on relevés conducted in areas with homogeneous vegetation, records the species and their coverage (as of percentage of the relevé area) and describes the local environment (Mucina et al. 2000). These relevés are aimed at characterising plant communities and identifying the habitat (Biondi et al. 2009).
We first carried out 22 relevés, with a mean area of 70 m 2 , located in the courses of the all the main rivers in the region except the Volturno and Fortore rivers. A vegetation database was created in Turboveg 2.0 (Hennekens 1995), starting from these 22 original relevés, which were then integrated using a further 27 unpublished relevés conducted along the Biferno river (B. Paura and collaborators, unpublished data), and 70 from the Adriatic side of the Apennines in central and southern Italy, derived from literature (Pedrotti 1970, 1984, Pedrotti and Cortini-Pedrotti 1978, Pirone 1981, Pirone et al. 1997, Manzi 1988, Biondi et al. 2002, Baldoni and Biondi 1993, CUM 2002, Allegrezza 2003, Allegrezza et al. 2006).
The 49 unpublished relevés were classified by means of cluster analysis using Past 2.1 (algorithm UPGMA, and Ochiai distance on species cover/presence) (Hammer et al. 2001).
The conservation status was assessed in each site by estimating the characteristics of the habitat and the threats it was exposed to. We focused on the type of data that can be collected from flora and vegetation surveys, adopting those parameters proposed in other member states (JNCC 2004, BfN 2006, Calleja 2009, Carnino 2009) that we considered to apply most to our study area. We then obtained the threshold values for the parameters by using our whole dataset (119 relevés), classifying them according to natural breaks (Jenks 1967). We decided to use natural breaks for classification purposes because we considered them to be more representative of the variation of our data. Table 2 summarizes the three types of parameters and the threshold values of the corresponding indicators used to assess the conservation status.
To assess the structure of the forest vegetation, we selected the following indicators: (i) the cover of the native tree layer and of the shrub layer (h 2-5 m) to highlight the stratification of the vegetation (JNCC 2004, Calleja 2009); (ii) the number of diameter classes of the tree trunks, which provide information on the uneven-agedness of the forest and the presence and type of forest management (BfN 2006, Carnino 2009); (iii) the presence of dead wood (relative cover in each relevé of woody debris and/or standing dead wood, and the presence of fallen old trees), to highlight the absence of management or natural forest management (BfN 2006, Carnino 2009).   We selected the following floristic and vegetation indicators: (i) the floristic consistency of the communities detected (JNCC 2004, BfN 2006, Calleja 2009, Carnino 2009) with the vegetation of reference, by comparing the presence of diagnostic and frequent species for the alliances (Biondi et al. 2014; http://www.prodromo-vegetazione-italia.org/); (ii) the presence and cover of exotic species, which reduce the degree of naturalness (JNCC 2004, Carnino 2009); (ii) the native tree species richness, which is particularly marked in Italian old-growth forests ), and known to be a good proxy for the total richness (Abbate et al. 2015); (iv) the presence of species of biogeographic or conservation interest, selected from Red Lists and other lists of protected or rare species (Table 3), which highlight the peculiarities of the site (BfN 2006). We tested the parameters we selected on a training dataset, i.e. 22 relevés that we carried out in the Molise Region. In this way, we only considered the most recent relevés, for conservation status assessment purposes.
When assessing the conservation status of forests in the Mediterranean area, it should be noticed that, owing to the impact of man over the millennia, forest habitats that have either never been used by humans or were only used in very ancient times are extremely rare. We cannot consequently expect the best-preserved context to be represented by a primeval forest (Carnino 2009). This is why we decided to determine the threshold values of the parameters for the best-conserved situations based on our whole dataset.
The resulting synthetic assessment for each relevé is determined by the condition of the worst parameter, as suggested by Article 17 of the Habitat Directive (92/43/EEC).

RESULTS
The cluster analysis led to the identification of two main types of riparian forests, referred to Salicion albae Soó 1930 (55% of the 49 unpublished relevés) and to Populion albae Br.-Bl. ex Tchou 1948 (45%).
The species recorded in more than 40% of the relevés are Salix alba, Rubus ulmifolius, Brachypodium sylvaticum, Salix purpurea, Populus nigra, Cornus sanguinea and Urtica dioica (see Supplementary Table S1 for the complete list).
Fourteen of the 22 relevés were found to have an Unfavourable-Bad (U2) conservation status. The worst parameters, indicating a bad conservation status, were mainly richness of the native trees species and shrub cover. We detected a marked difference between the western and eastern parts of the region (relevés n° 1 and 2 in Table 4). The site with the best conservation status was located in SCI IT7218213, where the native tree species richness and the presence of woody debris, two surrogates for the natural or semi-natural evolution of the forest, are very good. The majority of the relevés in the Campobasso province (in the east) were found to have an unfavourable conservation status. In the surroundings of IT7222287 lies the only riparian forest in which we found a favourable conservation status (relevé n° 3), particularly as regards the native tree species richness and the presence of woody debris, which were comparable to those detected in the aforementioned site n° 2.

DISCUSSION
The Molise region is characterized by a strong altitudinal gradient and by the presence of large river valleys (e.g. Volturno and Biferno) that connect the two sides of the Apennines. These valleys have always allowed the migration of plant species (Lucchese 1995, Paura et al. 2010b). These migrations are now represented by invasive exotic species. Indeed, it is along the rivers that the greatest spread of invasive species has been witnessed in the region (Lucchese 2010). Rivers play an important role in the invasion of plant species insofar as their waters act as important agents of propagule dispersal, just as aquatic birds do. In addition, periodic disturbance events, due to floods, create openings in plant cover that can easily be colonized by alien plant species thanks to the availability of nutrients. Low water periods also provide areas that are exposed to colonization by pioneer annual plants. Lastly, the rivers are subject to anthropogenic disturbance (agriculture, urbanization, water regimentations, etc.), which also promotes the spread of invasive species (Stohlgren et al. 1998, Schnitzler et al. 2007).
Riparian forests are known to be azonal formations that are conditioned mainly by the water level and water regime (Pedrotti and Gafta 1996). Although such forests are very dynamic owing to the natural disturbance to which they are subjected, they remain relatively stable if the hydrogeological conditions do not change. Riparian forests belonging to habitat 92A0 can be divided in two types, as described in the national interpretation manual (Biondi et al. 2012). They differ in dominant tree species and from an ecological point of view. Willow groves are located on the lower terraces, which are affected regularly by the ordinary flooding of the river, while poplar forests colonize the upper terraces, which are only sporadically affected by extraordinary flooding. The aforementioned manual recognized two different alliances of reference: Populion albae Br.-Bl. ex Tchou 1948and Salicion albae Soó 1930(Biondi et al. 2014 The name of the habitat makes explicit reference to gallery forests, sometimes generating difficulty in recognizing the habitat where riparian forest conservation does not preserve this aspect. In Spain, Calleja (2009) has proposed extending the definition to include the intermediate stages of vegetation dominated by shrubby willows with sparse trees of Salix alba and Populus alba. We agree with this proposal and have included forests of Populus alba, Populus nigra and Salix alba and shore vegetation dominated by shrubby willows, with some willow trees or poplars, in this study.
Our study identified two types of riparian forests, as expected for habitat 92A0: Salicion albae and Populion albae (Biondi et al. 2009(Biondi et al. , 2014. The main difference between these two types of forest lies in the dominant tree species (Salix or Populus species), there being little difference in the understory flora. Our findings are confirmed by data in the literature. Indeed, as shown in previous works on riparian vegetation (Pirone 1981, Pedrotti 1984, Manzi 1988, the general impoverishment of the flora of poplar forests in central Italy, due to the past use of this habitat by humans for agricultural purposes, makes it somewhat difficult to distinguish them from willow forests. Furthermore, the forests of Populus alba, which are affected to a lesser extent by river flooding, are also characterized by species that belong to oak forests, as highlighted in Molise by Paura et al. (2010a). It is likely that ISPRA (Institute for Environmental Protection and Research) included the Mediterranean tall willow galleries (EUNIS code 44.41) in the Italian poplar galleries (EUNIS code 44.614) on account of the floristic similarity between poplars and willows forests in the Apennines (for more information see http://www.isprambiente.gov.it/files/carta-della-natura/catalogo-habitat.pdf).
The rivers in central and southern Italy do not tend to create large floodplains that lend themselves to intensive agriculture. Human activities in these two regions have resulted in substantial changes in the flora and vegetation, particularly as a result of works related to riverbank reinforcement and to the production of electricity. These changes have often promoted the establishment and spread of exotic plant species (Lucchese 2010). The riparian forests of willows and poplars examined in this study revealed a peculiar susceptibility to human intervention and displayed considerably different features if compared with the past Cortini-Pedrotti 1978, Pedrotti 1984).
The most serious conservation problems were detected along the lower course of the Biferno river, in the Campobasso province. The past land use of river terraces and the continual human interventions have led to only small portions of what was once likely to have been the richest lowland forest in Molise being left, such as that near Colle d'Anchise, which lies out of the Natura 2000 Network (relevés n° 3 in Table 3), and where a well-preserved forest still exists. This area has not yet been included as a Natura 2000 site. It is a very rich poplar forest that is well stratified and contains a large amount of woody debris, standing dead wood and fallen old trees. In order to promote the natural evolution of this forest and its conservation, the boundaries of SCI IT7222247 "Valle Biferno da confluenza Torrente Quirino al Lago Guardalfiera -Torrente Rio" should be redrawn in such a way as to include the forest of Colle d'Anchise in the Natura 2000 network.
A better state of preservation is found in the areas that lie in the upper course of the rivers, where human impact is less marked and there are few exotic species. The site with the best conservation status is SCI IT7218213 "Isola Fonte della Luna" (relevés n°2 in Table 4), which has been unmanaged for approximately 30 years. As shown in Table 4, the structure of the forest is fairly well preserved and nine tree species were been found, which is the highest number recorded in the study area.
Exotic species tend to be promoted in areas in which the structure and floristic composition of the forest are substantially compromised. This is particularly evident in relevés n°17 in the Biferno valley (in the proximity of Morgia dell'Eremita), where the poplar layer has been completely replaced by Robinia pseudoacacia. The poor conservation status of the poplar forests in the lower course of the Biferno river may have been caused by the overall reduction in the size of this habitat, following the replacement in many areas of riparian forests by cultivated fields, a trend first observed in the 1970s in central and southern Italy (Pedrotti 1970, 1984, Manzi 1988. It is in the woods in this area that we observed the greatest spread of alien plant species such as Robinia pseudoacacia, Amorpha fruticosa and Erigeron canadensis. A similar trend has been observed for the willow forests in the Po Plain (Poldini et al. 2011). We should not forget that the success of invasive plant species is often the result of a poor conservation status of riparian habitats (Stohlgren et al. 1998, Chytrý et al. 2009, Liendo et al. 2015. The introduction and spread of these invasive species appear to be promoted in areas in which trees and shrub layers are not well developed. Indeed, alien species were not recorded in sites with a favourable conservation status, particularly as regards the structure. In conclusion, we believe that forest management should focus on maintaining the stratification of the forest, its uneven-agedness and tree species richness because a forest can withstand the invasion of alien species only as long as its structure is well preserved and strong. Stohlgren, T.J., Bull, K.A., Otsuki, Y., Villa, C.A & Lee, M. (1998)