Our understanding of the richness and uniqueness of the flora growing on gypsum substrates in Italy has grown significantly since the 19^th century and, even today, new plant species are still being discovered. However, the plants and plant communities, growing on gypsum substrates in Italy, are still a relatively unknown subject.

The main aim of this paper was to elaborate a checklist of the Italian gypsophilous flora, to increase knowledge about this peculiar flora and for which conservation efforts need to be addressed.

Through a structured group communication process of experts (application of the Delphi technique), a remarkable number of experienced Italian botanists have joined together to select focal plant species linked to gypsum substrates. From the results obtained, 31 plant species behave as absolute or preferent taxa (gypsophytes and gypsoclines) and form the ‘core’ Italian gypsophilous flora. The most abundant life forms were chamaephytes and hemicryptophytes, belonging to Poaceae and Brassicaceae; as for chorotypes, the most represented are Mediterranean and narrow endemics. By improving on previously available information about the flora with a clear preference for gypsum in Italy, this undertaking represents an important contribution to the knowledge of a habitat which is today considered a priority for conservation.

Edaphism, Gypsophyte, Habitats Directive, Plant preservation

The relationship between local bedrock types and vegetation cover has long been highlighted. Andrea Cesalpino, in De plantis libri XVI (1583), had already documented the existence of endemic plant species on the Italian serpentines. The term ‘edaphism’ – interpreted as a ‘geobotanical phenomenon giving rise to particular floras on certain substrates’ (Font Quer 1977) or ‘those physical and chemical effects induced on living beings by the soil’ (Sarmiento 2001) – has been used extensively in Europe since the 19^th century (Parsons 1976, Kruckeberg 2002). The species and the plant assemblages growing on gypsum provide a clear example of the strict relationship between soil and vegetation, as many plant species grow exclusively or preferentially on such peculiar substrates. This geobotanical pattern occurs in more than 70 countries worldwide (Pérez-García et al. 2017). Nevertheless, the gypsicolous substrates represent a largely underrated or ignored habitat, with serious consequences for both flora and fauna conservation. These habitats host sparse and scattered vegetation, since the gypsum outcrops often represent geological islands interrupting the uniformity of other surrounding landscapes. Local evolutionary processes have probably been facilitated by the geographical isolation of outcrops, so that the appearance of several plant species with a narrow distribution have been favoured (Moore et al. 2014). This kind of geographic speciation could have been complemented by some selective pressure that might have favoured the survival of certain plant lineages on nutrient-unbalanced, water-limited soils which are unfavourable to the establishment of most plants (Merlo et al. 1998, 2001, Bolukbasi et al. 2016).

There is an abundance of accurate information concerning gypsophilous plant communities in Spain (Mota et al. 2011), but not for those other European countries where such substrata also occur. In Cyprus, for instance, it is only the outcrop located at Rizoelia National Park that are known, but unfortunately it seems that both local flora and vegetation are degraded due to several impact factors (EIONET 2008). The first scholar to study the gypsophilous flora of Italy was Macchiati (1888). There have been some studies in mainland Italy and Sicily which have increased our knowledge about this flora, e.g. Gallo (2014). Such previous studies on both the rate of endemism and risk of extinction amongst these plant communities pointed out that ‘Mediterranean gypsum vegetation’ should be considered a Priority Habitat (*1520) for conservation according to the 92/43 EU Directive (Anonymous 1992). Since no specific national law has yet been enacted in Italy (Fenu et al. 2017, Rossi et al. 2016), this directive theoretically represents the major instrument for plant conservation. Several Italian gypsum outcrops have been identified as SCI (Sites of Community Importance) and SCZ (Special Conservation Zones) e.g. Piano di Gestione Complessi Gessosi M. Conca, SIC ITA050006 (IUCN UNEP-WCMC 2014, Mento 2008). However, information about gypsum vegetation in Italy is still deficient: in fact, neither its current area, nor the recent trends in term of quantity and quality (last 50 years) are known (Loidi 2016). An accurate checklist of Italian gypsophilous flora has not yet been produced; hence, only published investigations concerning these flora and vegetation could be used as a reference guide. When taking into account that conservation policies at any level must be based on scientific assessments about habitats, species conservation status and existing threats (IUCN 2001; Joppa et al. 2013), the exhaustive knowledge about the flora typical of gypsum outcrops in Italy would be a crucial step towards the application of any sort of conservation measure (Mota et al. 2004, 2011, Martínez-Hernández et al. 2011, 2015).

Therefore, the aims of this research were (i) to elaborate a checklist of Italian gypsophilous vascular flora through a structured group communication process of experts; (ii) to expand the knowledge of this flora type to which conservation efforts need to be addressed; (iii) to examine the spectrum of taxonomical groups, life forms and chorotypes of this flora. Through this approach, the comparison between the gypsophilous flora of Italy and that of other countries was carried out in order to detect common phylogenetic, functional and biogeographic patterns that allow a better understanding of the gypsophily phenomenon at European and global levels.

Several approaches have been proposed to elucidate which plant species can be considered as best linked to gypsum substrates (Mota et al. 2016). However, coping with an extremely species-rich flora over a wide territory such as Italy, using the Delphi technique (Hasson et al. 2000) resulted in being the most effective way to build a checklist of gypsophilous flora by using the so-called ‘expert criterion’ (Mota et al. 2008, 2009).

Selection and invitation of a panel of experts

Participants from a great diversity of backgrounds were included (e.g. teachers, scientists, conservationists, non-governmental organisations, policy-makers, environmental managers and technicians) in order to obtain a wide range of perspectives and minimise bias arising from self-interest or information preferences. The participants included the co-authors of this article, i.e. experts situated in the Italian peninsula and Sardinia – hereinafter Italy (9) – and in Sicily (11). The reason why the number of Sicilian botanists involved is greater than those from mainland Italy is due to the considerable extension and importance that gypsum outcrops have in Sicily (Fig. 1).

Figure 1. 

Italian gypsum outcrop presences in 10×10 km UTM grids.

Ad hoc investigations on gypsophily have been performed in only 12 countries and only five of these studies approached a functional perspective (Mota et al. 2016). However, documented gypsophilous flora can be found in at least 75 countries (Pérez-García et al. 2017, 2018).

This work provides the first Checklist of Italian gypsophytes, including 31 taxa showing a great affinity for this substrate, 12 of which can be unequivocally considered as strictly gypsophytes. In addition, a number of further species often found on these substrates is detailed. As mentioned before, although the studies on gypsophilous flora in Italy date as far back as the 19^th century (Macchiati 1888), there is no knowledge of the existence of detailed ecological, functional or phylogeographic studies. After this first analysis of Italian flora, it would not be possible to discard a second evaluation based on the information presented in this work and any new information generated in the future (e.g. Montanari et al. 2016). A new revision should not only take into account the vascular flora, but also the plant assemblages associated with these outcrops. It must be noted that Italy presents a complex natural scenario, with a strong North-South environmental gradient conditioning the composition and dynamics of plant communities. In the case of vegetation associated with gypsum outcrops, this gradient is remarkable and it is impossible to overlook the far greater aridity of the southernmost regions, which exaggerates the gypsophily phenomenon (Merlo et al. 1998, 2001). This probably explains why the largest contingent of gypsophytes in the Italian territory is concentrated in Sicily.

This research reinforces the idea that, provided there is no definitive criterion for establishing whether a species is a gypsophyte or not, the inductive approach based on ‘expert criterion’ is not only plausible, but perhaps the only one possible to establish the groundwork for future research on gypsophily. To further complicate this scenario, the same species may have different affinity levels for gypsum substrates in isolated territories: e.g. Sedum gypsicola subsp. trinacriae shows gypsophily median value of 5 in Italy, but in Spain, the nominal subspecies (S. gypsicola subsp. gypsicola) reached a median value of 4 in a previous study (Mota et al. 2011).

Both the taxonomical and life form spectra concerning the 31 gypsophytes on the Italian Checklist are largely in agreement with the data recorded in other areas of the Mediterranean Basin for this type of substrates (EIONET 2008, Bolukbasi et al. 2016). As far as the most represented families are concerned, the taxonomic spectrum of the strictly gypsophilous flora in Italy is similar. Brassicaceae and Poaceae are amongst the families with a higher number of species, although the latter is slightly over-represented in Italy. Poaceae occur frequently in very stressful environments (Baskin and Baskin 2000) and, consequently, it is easy to understand why there is a high number of them, which can be considered as peculiar to gypsum substrates. Other conspicuous families in the taxonomic spectrum of the Italian gypsophytes are Apiaceae, Asteraceae, Plumbaginaceae and Crassulaceae. All of them are found in arid Mediterranean environments and, therefore, it is not surprising to find them so well represented in Italy.

Although there were variations in the abundance percentages, the most common families with the greatest match with the grouping occurred for the Spanish gypsophilous flora (Mota et al. 2011), where Asteraceae (14.1%), Brassicaceae and Plumbaginaceae (12.68%) correspond to the larger families. Nevertheless, Poaceae and Apiaceae obtained greater representation in the Italian gypsophilous flora. On the contrary, Lamiaceae, Fabaceae and Caryophyllaceae families appear to be less common on Italian gypsum outcrops with respect to other countries (Table 3).

Plant formations linked to gypsum substrates are usually dominated by small plant species such as chamaephytes or hemicryptophytes, similarly to those which occur in other Mediterranean areas, although there are exceptions of woody plant formations growing on gypsum (Pérez-García et al. 2017). The intense exposure to sunshine for these environments, the shortage of water and nutrient imbalance are probably responsible for this scenery. In addition, the abundance of therophytes may be explained by water shortage. Thus, some annual plants can be favoured on gypsum by drought (Merlo et al. 2001).

Efforts to ensure the conservation of Mediterranean gypsophilous vegetation, considered as a Priority Habitat, should be focused on endangered, rare or endemic species, according to the premises established by the EU. All these efforts cannot be easily undertaken unless it is previously determined which species, out of many hundreds, are to be given top priority. The approach adopted in this work may help both to focus on certain species and to detect research and conservation priorities. The high proportion of Italian endemic species and the geographic rarity component of the flora associated with Italian gypsum outcrops is an aspect that makes these outcrops very interesting habitats. However, the degree of threat to the gypsophilous flora in Italy could have been insufficiently assessed. This fact is confirmed since less than 7% of the gypsophytes of the Italian Checklist have been evaluated under IUCN protocol. As there has been a prior effort to establish a network of sites for the conservation of nature (Natura 2000 Network), it would be worthwhile to extend this initiative to areas supporting Italian gypsum outcrops that have biodiversity values worthy of consideration. In order to achieve this purpose, an in-depth review of the conservation status for both the gypsophilous flora and the natural areas where these substrata occur in Italy is crucial. Data generated by experts in conservation, for which threat categories of red-listed species are based, should be incorporated into nature protection Acts (Mendoza-Fernández and Mota 2016) to ensure the preservation of these sites in Italy.

There are numerous SCI and SCZ including gypsum outcrops and their associated flora. The question that remains to be clarified is whether they are sufficient to ensure the conservation of this flora as well as the vegetation linked to this peculiar substrate. In this examination of gaps in conservation, fauna and other plant groups, such as lichens and bryophytes, should be integrated (Mota et al. 2011).

At this moment, the existence of endemic and rare flora with remarkable eco-morphological adaptations and the description of new taxa growing on gypsum outcrops fully justifies the conservation of these outcrops (Gallo 2014). Some Italian gypsum outcrops are currently under protection, but other areas have not yet been included in the network of nature reserves. As Panuccio et al. (2017) and Spampinato et al. (2018) pointed out for Calabria (South-Italy) and Mendoza-Fernández et al. (2014) for Andalusia (southern Spain), arid or semi-arid territories are often under-represented in protected area networks. This is the case for most Italian gypsum sites. Furthermore, the checklist of Italian gypsophytes should contribute to a better understanding of the autoecology and synecology of rare and endemic species and, therefore, to better conservation of the biodiversity associated with gypsum areas in Italy. For example, scrub communities, typical of Italian gypsum substrates, represent one of the major gaps in conservation habitats in the European Union. In order to fill this gap, the peculiar Italian gypsum habitats could be considered by modifying the meaning that habitat 1520* currently has in the manual of habitat interpretation and through the addition of Italian gypsophytes in order to include the gypsum habitat in Italy, amongst those of European interest.

Data provided in this paper denote an important advance in this sense, because only five plant species in Italy have been recognised as characteristic taxa for this habitat on the European Red List of Habitats (Loidi 2016). These are: Brassica villosa subsp. tineoi, Chaenorhinum rupestre, Festuca gypsophila (Ctenopsis gypsophila), Erysimum metlesicsii and Sedum gypsicola subsp. trinacriae. However, according to the information provided in this paper, at least twelve species, more than double, could be indicative plants for this Priority Habitat. Thus, the following should be added to those already mentioned: Petrosedum ochroleucum subsp. mediterraneum, Allosorus persicus, Artemisia pedemontana, Diplotaxis harra subsp. crassifolia and Stipa austroitalica subsp. frentana. In addition, some species typically related to saline soils belonging to the “Serie gessoso-solfifera” of Sicily, such as Reaumuria vermiculata, Limonium catanzaroi and Limonium optimae, may also occur.

The study results from the international collaboration between the “Mediterranea” University of Reggio Calabria (Italy) and the University of Almeria (Spain). The authors Antonio Jesús Mendoza-Fernández and Carmelo Maria Musarella were sponsored by the University of Almeria through the Programme ‘Plan Propio de Investigación y Transferencia’, with a postdoctoral grant (Contrato Puente 2015) and financial assistance for their stay abroad (Estancias en otros Centros de Investigación 2016); and with support for guest teachers (Programa de Profesores Invitados 2017).

This study was carried out under the GYPWORLD Project, funded by the European Commission under the European Union’s Horizon 2020 Programme/MSCA-RISE-2017 Grant Agreement n. 777803.

We thank Beatrice Antolin and Edward Parker for reviewing the English version of the text.

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