Our study area is localised in the Mediterranean Basin, one of the 25 biodiversity hotspots (Myers et al. 2000), where the flora is diversified, with a high level of endemism (Médail and Verlaque 1997). Moreover, the Mediterranean Basin is composed of several islands. Due to their insular nature, these islands contain much of the species richness of the Mediterranean area (Kier et al. 2009, Médail and Diadema 2009, Schatz 2017). Flora richness in Corsica, where our study area occurs, is higher than in metropolitan France (0.29 taxon per km² in Corsica and 0.09 taxon per km² in France) and endemism is important, with 5.9% strictly endemic plants and 13.5% sub-endemics (Gamisans and Jeanmonod 1995).

Our study site is the airbase called n°126 Solenzara, a French Air Force base, located at Ventiseri (Fig. 1) and was created in 1952 by the North Atlantic Treaty Organisation (NATO) (Giannini 2019). NATO chose this area because it is airspace with little traffic and marshy areas are suitable for test firing. Major military development work was undertaken after 1956, including the drying out of wetlands. For the airbase, the draining of these wetlands appears to have benefited from a significant input of topsoil from major road infrastructure works carried out before 1973 (Bonfond 2018, Giannini 2019).

Figure 1.  

Study site and localisation in Corsica.

This airbase is about 2 km wide (east-west axis) and about 3.3 km long (north-south axis) and it is located approximately 40 km north of Porto-Vecchio, on the east coast of Corsica. The Palo Pond (Ramsar site) surrounds the airbase in the north, the Travo River in the south, a dune site and the Mediterranean Sea in the east and a national road and an urbanised area to the west. The inventory takes place in the entire airbase, including the wetland area east of the beach (this second part corresponding to a site protected by the Conservatoire du Littoral). The total area surveyed is about 550 ha. This study area is a military site, which implies that it is closed to the public. It is also an airport area. Airports are interesting for biodiversity; lawns around strips are particularly favourable to orchids (Seitre 2017). Indeed, most runways are bordered by open grasslands, managed by regular mowing for aeronautical safety reasons.

There are three main parts to the airbase. The westernmost part consists of buildings and low grasslands, with some woodland. In the central area, the landing strips are surrounded by low vegetation, regularly maintained by mowing. Finally, the easternmost part consists of wetlands and scrub. A de-sodding exercise was carried out in 2019 in this third part. The airbase is located in the coastal and thermomediterranean belts. These belts are rich in plant species, particularly the thermomediterranean belt (Schlüssel et al. 2014).

Several floristic inventories have been carried out and are compiled here to give an almost exhaustive list of the flora present on the airbase. Since 2017, the Ecotonia Consultancy has regularly intervened on the airbase to carry out floristic inventories. From 2017 to 2019, the inventories focused on the central area, around the runways and the northeast wetland (These inventories took place in March, April, May and October). In 2020, a complete inventory of the wetland was conducted (March to June). In 2021, an inventory took place in the northwest and another in the southwest of the airbase (March to June). This completed the data acquired since 2017.

Then, we added floristic data acquired by the Corsican National Botanical Conservatory (CBNC), following inventories conducted in 2018 and 2019 (May to July) (Delage and Hugot 2019). These data complement the inventories of Ecotonia, covering a large part of the airbase and especially along the sea.

More targeted inventories have been previously conducted: in 2010 (April) by the Association of Friends of the Corsican Regional Natural Park on areas around Palo Pond, focused on remarkable flora (Massoni et al. 2010) and, in May 2012 and April 2017, by the Cyrno-Mediterranean Orchidology Association (ACMO), focused on Orchidaceae (Denis Allemand, com. pers.).

The objective of the inventories was to conduct a systematic sampling to determine all species present in the study area. This allowed us to determine a precise list of taxa present. We then compared this list to the protected and threatened species list (decree of 20 January 1982, modified by the decree of 31 August 1995; decree of 24 June 1986 in French law) (Delage and Hugot 2015, UICN France et al. 2018). We noted the degrees of endemism and rarity for each species given by the Flora Corsica determination key (Jeanmonod and Gamisans 2007, Jeanmonod et al. 2009). For several species, the subspecies is not specified in the list we have established. We have chosen to take the lowest degrees of rarity in the Flora Corsica determination key: we likely contacted the most common subspecies. We have taken the corresponding degree of endemism. To find out the proportion of endemic species, we removed non-native species. Finally, we compared the list with the invasive alien species list of Corsica (Petit and Hugot 2019).

Amongst inventoried species, we found a large population of Serapias neglecta, a nationally protected species of orchid (Fig. 2). Its range is limited to the extreme south-east of mainland France, Corsica (mainly around the south coast) and Sardinia, Sicily, the extreme south-east of Italy and along the east coast of the Adriatic (Croatia, Albania and Greece) (Delforge 2016). We estimated the size of the population. Firstly, we defined by cartography using the software QGIS the favourable areas for S. neglecta. We defined five large homogeneous zones:

Figure 2.  

Serapias neglecta on Solenzara airbase (April 2021).

• The edges of runways: the vegetation is regularly mowed; the environment is very favourable to Serapias.
• Wetlands: Serapias grow in smaller quantities, but are present.
• Lawns near buildings: these lawns are regularly maintained. There is little competition and Serapias are numerous.
• The south-western zone: vegetation is high. Serapias are present in a few places, but in reduced quantity.
• The north-western zone: Serapias are present, but scattered. The vegetation is closing in.

We chose a representative area of at least 600 m² in each homogeneous zone, where we made an exhaustive count. We then extrapolated the number of S. neglecta per area.

The 552 taxa found in the study area belong to 279 genera and 74 plant families (Suppl. material 1). Amongst these taxa, there are 45 sub-species, 503 species, four varieties and one hybrid. The families with the most species are Fabaceae (90 species), Poaceae (69 species) and Asteraceae (52 species) (Fig. 3). The genera with the most species are Trifolium (28 species), Carex (13 species), Juncus (13 species) and Vicia (13 species).

Figure 3.  

Distribution of the most contacted families on the airbase.

Only 2.5% of species are sub-endemic and none is strictly endemic. Many protected species are present on the base and 19 species benefit from national protection, representing 3.4% of the plant taxa on the airbase. There are also 15 invasive alien species, representing 2.7% of the taxa present.

More than 70% of the species in the database are common (C) or very common (CC) (Fig. 4). Only 8.6% are rare (R) or very rare (RR).

Figure 4.  

Proportion of taxa by rarity category. CC: Very Common, C: Common, LO: Localised, PF: Uncommon, R: Rare, RR: Very Rare.

A large part of the base is favourable to S. neglecta: the population extends on both sides of the runways and the lawns near the buildings and wetlands. There are also several other orchid species: S. cordigera L., S. parviflora Parl., S. lingua L., Anacamptis morio (L.) R.M.Bateman, Pridgeon & M.W.Chase, A. laxiflora (Lam.) R.M.Bateman, Pridgeon & M.W.Chase and A. papilionacea (L.) R.M.Bateman, Pridgeon & M.W.Chase. However, the case of S. neglecta is particularly remarkable because this species benefits from a national protection status and it is a sub-endemic species with a very localised distribution worldwide. This species is classified as near threatened on the World and European Red Lists of the IUCN (Rankou 2011). In Corsica, this species is present and often abundant. However, no other population of comparable size is known. Note here the rather shallow tuber depth of the orchids present in this area (on average 4.33 cm in the base vs. 7.36 cm outside the base) due to the shallow soil on a pebble bed (due to ancient variations in the course of the Travo River Delta) (see more information in Suppl. material 2).

Concerning the risks of confusion with related species, we confirm the presence of Serapias neglecta subsp. neglecta De Not. on our study site and not that of Serapias neglecta subsp. apulica Landwehr which is rather a subspecies of Serapias orientalis (Greuter) H.Baumann & Künkele (Lorenz 2001, Delforge 2016), located in the Apulian coast in Italy, with a more compact inflorescence with fewer flowers, a more lanceolate epichile and an earlier flowering (Delforge 2016). Similarly, we also confirm the presence of Serapias lingua subsp. lingua L. on our study site and not that of Serapias lingua subsp. tunetana B.Baumann & H.Baumann which is only present in Tunisia, with a lower number of flowers, a shorter, less wide and more lanceolate epichile, a lighter colour and a later flowering (El Mokni and Domina 2019).

The estimated area favourable to S. neglecta is 160 ha on the entire airbase. We assessed the density on different locations: 0.12 S. neglecta/m² on the edge of the runways, 0.009 S. neglecta/m² in the wetlands, 0.2 S. neglecta/m² on the lawns near the buildings, 0.007 S. neglecta/m² in the south-western zone and 0.05 S. neglecta/m² in the north-western zone. Extrapolating, we estimate that the population of S. neglecta is at least 155,000 individuals on the entire airbase.

This estimate is probably underestimated due to the nature of the species. Indeed, the individuals do not flower every year. We, therefore, see only a fraction of the population present. Depending on the year, densities of Serapias have been very variable. In 2019, the winter was arid (especially during the previous winter and spring), which was not favourable to orchids; during this year, we only observed hundreds of Serapias. Almost no individuals were visible in the spring on the airbase. Between 2020 and 2021, we compared the density of an area particularly rich in Serapias of about 900 m². The number of individuals was 2.5 times higher in 2021 than in 2020 (0.28 S. neglecta/m² in 2020 versus 0.81 in 2021).

Some species are remarkable for their status: for example, Gratiola officinalis L. is classified as vulnerable (VU). There are 13 species classified as near threatened (NT). Ranunculus revelierei Boreau have national protection and is NT on theCorsican IUCN Red List. This species is also sub-endemic and rare in Corsica. Four nationally-protected species and very rare in Corsica are found: Trifolium cernuum Brot., Gratiola officinalis L., Ranunculus lingua L. and Anemone coronaria L. Finally, Salix apennina A.K.Skvortsov is very rare and Serapias olbia Verg., Sagina subulata var. gracilis Foucaud & Simon and Ranunculus revelierei are rare. These three species are also sub-endemic.

One subspecies is to be considered as potential on the airbase; Bromus hordeaceus subsp. thominei (Hardouin) Braun-Blanq. This subspecies is present in Corsica, but challenging to determine. We consider it as potential, but it has not been taken into account during the analyses.

Sagina subulata (Sw.) C.Presl belongs to the huge family of Caryophyllaceae. Two subspecies exists: S. subulata subsp. revelierei (Jord. & Fourr.) Rouy & Foucaud, an endemic Corso-Sardinian orophyte and S. subulata subsp. subulata (Sw.) C.Presl, a southern and western European species. The latter is currently divided into two varieties, one of which is present throughout its range (var. subulata (Sw.) C.Presl), but the other is not known: var. gracilis Foucaud & Simon. It is only reported in France (Provence and Corsica), although it is potentially present elsewhere (Sardinia, Tuscany, Liguria, Spain). It is found in a singular environment: shallow temporary ponds with siliceous substrates in the Mediterranean climate. This environment, which is highly stressful for a plant, has led this taxon to adopt a therophyte biological type – its life is limited to a few weeks or months, whereas var. subulata is hemicryptophytic. This adaptation is typical of Mediterranean environments and seems to be an important evolutionary event that may justify this variety being treated at a higher taxonomic rank soon. If specific research is carried out, it could one day lead to the description of a new high-ranking taxon (species or subspecies), whose worldwide distribution area would at best be restricted, or even endemic, to the French Mediterranean region. In Solenzara, this tiny plant has found unusual secondary habitats: the ruts made by vehicles driving on the base. Other annual species accompany it with similar ecologies, some of which are rare: Ranunculus revelierei, Lotus conimbricensis Brot., Solenopsis laurentia (L.) C.Presl, Lysimachia minima (L.) U.Manns & Anderb. and Lythrum portula (L.) D.A.Webb.