Analysis of the floristic diversity in a southern Mediterranean ecosystem. Case of Bissa forest, Chlef (Algeria)

The forest of Bissa is a Mediterranean ecosystem, characterized by a high floristic diversity. In order to assess the floristic diversity in this forest, 133 floristic readings were conducted in this area between 2013 and 2017. As a result, it was possible to identify 151 species, 125 genus and 54 botanical families, with a clear dominance of Asteraceae, Fabaceae and Poaceae.  Biologically, this ecosystem was mainly dominated by therophytes (37%). Physiognomically, the phanerophytes were the most dominant types, whereas in term of phytogeography this forest was dominated by the Mediterranean type (50%). According to Shannon-Winner index, this area was characterized by a relatively low diversity (1.3 bit / ind), with a clear dominance of a few species. Finally this forest was relatively disturbed (42%) and under a strong anthropozoic influence.


Introduction
Biodiversity is a hot topic that is receiving more and more attention, it refers to diversity within species, between species and between ecosystems (Baillie & Upham 2012). The recent worldwide decline in the biodiversity is considered by many specialists as an irreversible process which puts at risk the future of the humanity (Abdelguerfi & Ramdane 2003). In light of this, the assessment of this natural wealth has been the subject of several studies during the last decades (Sedjar 2012;Abdourhamane et al. 2013;Merioua et al. 2013;Benkhettou et al. 2015;Larbi 2015;Slimani & Aidoud 2018). In terms of biodiversity, the ARTÍCULOS Mediterranean ecosystems are characterized by minor bioclimatic differences leading up to very diverse forms of vegetation, whereas the variation in the Mediterranean landscape translates into high genetic, specific, and ecosystem diversity. In this context, the Mediterranean is known by an extraordinary floristic richness, with 25000 known species according to Medail & Quezel (1997). Algeria, the largest country in the Mediterranean basin, is also characterized by a very significant diversity. Indeed, according to Quezel & Santa (1962) the Algerian floristic richness estimated is of 3139 species, with 700 endemic species. Unfortunately, apart from the great work of Quezel & Santa (1962, 1963, this floristic richness remains highly unknown and is currently under a strong anthropic and climatic influence.
In this context, the present work aims to study the floristic diversity in an exceptionally woody area in the north of Algeria, namely the cork oak forest of Bissa, in order to assess the floristic diversity, the degree of disturbance in this forest and to ensure its long-term protection and maintain its floristic diversity. Indeed, despite its importance, only few ecological studies have been conducted in this forest.

Study area
In opposition to most of the cork oak woodlands located in the humid and sub-humid areas in the north-eastern part of Algeria, the forest of Bissa is one of very few cork oak forests in the north-western part of this country, characterized by its high floristic diversity compared to the surrounding areas. The forest of Bissa is located east of Dahra Mountains, in the north of the state of Chlef, it extends over an area of 1487 hectares (Figure 1), between 36° 25' and 36° 28' of north latitude and between 1° 25'-1° 30' of east longitude. It is a mountainous area with a rugged topography, characterized by altitudes ranging from 940 to 1152 meters and a main ridge line running along the forest from east to west over 4,5 km and a secondary north-south ridge.
In terms of climate, the area is sub-humid (Berchouche & Sedaki 2014), marked by ample rainfall around 700 mm/ year, mainly recorded between autumn and spring, and an average annual temperature of 16°C. According to the temperature/rainfall ratio (Bagnouls & Gaussen 1953), the dry period extends over a period of 4 months, from June to September. Biogeographically the forest of Bissa belongs to the North African Mediterranean domain (Quezel & Santa 1962, 1963, whereas according to the new phytogeographical nomenclature of Meddour (2010) this forest belongs to the Maghreb-Tellian domain.

Sampling
Vegetation ecology surveys were recorded according to a subjective sampling, in homogeneous and representative areas (Gounot 1969;Prodon & Lebreton 1994) between 2013 and 2017, adding up 133 inventories. Each inventory was taken over a minimum area of 100 m² (Quezel 1965). In addition to the floristic data, we collected the altitude, latitude, longitude, exposure as well as the Anthropozoic (human-animals) action in each site.

Biodiversity assessment
The study area has been evaluated floristically, biologically and in terms of phytogeography (Quezel & Santa 1962, 1963Dobignard & Chatelain 2010, 2011a, 2011b, 2012. The floristic diversity was evaluated in terms of species richness, perturbation index, Shannon-Weaver diversity index and equitability.

Disturbance index
The forest degradation was assessed using the perturbation index (PI) as defined by Hebrard et al. (1995)

Shannon-weaver diversity index "H"
The biological diversity is most commonly assessed using the Shannon index (H), this index helps to understand the distribution of species that compose a given habitat. This index is given by equation 2.
Where p i : is the proportion of individuals of one particular species i (n) divided by the total number of all individuals (N).
The diversity index (H) was calculated based on the relative cover of each species in each group, not on the probability of occurrence. This index is expressed in bit per individuals (bit/ind.) with an interval ranging from 0 to 5 bits (Frontier 1983;Frontier & Etienne 1990), the maximum value is reached when all species individuals are distributed evenly (Frontier 1983).
Where: H' is the Shannon-Weaver Index and N is the total number of species.
Equitability (E) ranges between 0 and 1 (Liyod & Gheraldi 1964; Legendre & Legendre 1984), it is maximal when all the species are evenly distributed and minimal in case of a dominant species. Results

Floristic composition
Through this survey 54 families have been identified in the study area (Figure 2), the most dominant botanical families were respectively the Asteraceae with 15% of the total flora, adding up 22 species and 19 genera, the Fabaceae with 14 species and 11 genera, the Poaceae represented by 11 species and 10 genera and Lamiaceae with 10 species and 9 genera, whereas the remaining families were represented by only 1 to 3 species.

Biological spectrum
Throughout the study area, therophytes were the most dominant among all the biological types with 37%, followed by phanerophytes and hemicryptophytes with 24 and 23% respectively. The least represented biological types were geophytes and chamaephytes with 11% and 5% respectively (Figure 3).

Weighted biological spectrum
In contrast to the biological spectrum, the species cover weighting revealed a clear preponderance of phanerophytes; up to 88% of all the biological types present in this forest, while the remaining biological types were insignificantly represented with 4% of geophytes, 3% of therophytes and hemicryptophytes and only 2% of chamaephytes ( Figure 4).

Phytogeographic spectrum
The phytogeographic spectrum ( Figure 5) showed a strong preponderance of the Mediterranean type up to 50%, followed by 31% of multi-regional taxa and 15% of Nordic types (Paleo temperate, Eurasian, European), finally 4% of the species were endemic to Algeria, Algeria-Tunisia and Algeria-Morocco.

Weighted Phytogeographic spectrum
The real species cover revealed a high abundance of Mediterranean species up to 84% of all the Phytogeographic types, 14% were of multi-regional nature, Endemic and Nordic taxa accounted only 1% of the encountered flora ( Figure 6).

Disturbance index
This index increases when the encountered flora is dominated by therophytes and chamaephytes. Based on this index, the forest of Bissa was found to be averagely disturbed with a disturbance index of 42%.

Biodiversity index
Despite its very restricted surface (almost 0,06% of the Mediterranean surface), the forest of Bissa showed a relatively high floristic diversity, indeed the 151 species recorded in this forest accounted for 6,04% of the total Mediterranean flora. However, the Shannon's diversity index (H) of this forest was found to be relatively low (1,3 bit/ind.), the equitability index showed a value of 0,31% indicating the predominance of few species (Table 1).

Floristic diversity
Despite the conclusions drawn by Mathez et al. (1985), stating that the Algerian forest has undergone during the last decades a very significant floristic decline, 151 species belonging to 125 genera and 53 botanical families were recorded in the very limited surface, occupied by the forest of Bissa, which is an indicator of a relatively high floristic diversity, this finding was consistent with the finding of Ajbilou (2007) in the Tingitane peninsula in Morocco, according to this author, among several forest formations the highest specific richness was observed in the cork oak forests. Due to its limited surface leading to a climatic and topographic uniformity, the species richness in Bissa forest was less than that observed in areas under similar conditions, but extending on larger surfaces such as the species richness revealed by Sedjar (2012) in Setif, with 367 species belonging to 56 botanical families, Rebbas (2014)  The specific and very favorable environmental conditions (climate, topography and pedology) characterizing the forest of Bissa, resulted in a very important species richness (151 species) even though the Shannon-Weaver Diversity Index was relatively low (1,3 bit/ind.) indicating an inequality in the abundance and dominance of species in the forest of Bissa, indeed, Quercus suber L. covers by itself alone a 17,27%, Arbutus unedo L. and Quercus ilex L. cover respectively 11,62% and 11,52%. In this forest ecosystem, the specific environmental conditions were highly favorable to the appearance of over a hundred different species, nevertheless the evenness index was relatively low (0,31%), which indicates the dominance of a limited number of species such as: Quercus suber L., Arbutus unedo L., Quercus ilex L., Cistus monspeliensis L.
and Erica arborea L., while the remaining taxon were marginally represented. In contrary to Bissa forest, the evenness index recorded by Stiti et al. (2012) in a Tunisian cork oak forest was very high (0,89%), indicating a strong species equitability, while the Shannon-Weaver diversity index was low (1,44 bit/ind.), almost similar to that recorded in Bissa.

Functional diversity
In terms of biological types, Barbero et al. (1990) reported that the human disturbances are diverse and leading to increasingly severe situations, ranging from therophytisation (abundance of therophytes) to shrub encroachment (a step of forest degradation) of the forest, this degradation process is the case of most of the Algerian forests. Indeed according to Le Houerou (1980, 1995), Quezel (2000, Quezel & Medail (2003), Benabadji et al. (2009) most of the Mediterranean forest ecosystems are subject to the phenomena of therophytisation. In the forest of Bissa, and the therophytes, which constitute a form of plants adaptation in arid conditions, appeared to be clearly predominant (37%) compared to the remaining biological types when considering the biological spectrum. Whereas according to the weighted biological spectrum the forest of Bissa was dominated by phanerophytes (88%) mainly for two reasons, firstly because it is a forest ecosystem, secondly the important cover of the phanerophytic species such as oaks. However, compared to adjacent and comparable ecosystems the forest of Bissa showed a slightly lower therophytisation (37%). Indeed, according to Sedjar (2012) the forest of Jebel Boutaleb in Setif, showed 42% of therophytes, in Beni Saf forest, Merioua et al. (2013) listed 40% of therophytes, in the Southern matorral of Tlemcen, Belhacini (2011) recorded 47,5% of therophytes, while in the forest of Djurdjura, Larbi (2015) mentioned a strong predominance of hemicryptophytes (47%), according to Floret et al. (1990) this situation was mainly due to the abundance of rainfall and the winter cold prevailing in this area.
The increase in therophytes and chamaephytes in a given area leads to a higher perturbation index, in this context the forest of Bissa was averagely disturbed (DI = 42%), this disturbance was mainly caused by fires and Anthropozoic action, indeed, the forest of Bissa is known to be under the influence of repeated and frequent fires leading up to the proliferation of specific species such as Cistus monspeliensis, whereas, the influence of animals, especially the excessive grazing (goats in particular), resulted in the appearance of characteristic species such as Asphodelus microcarpus Viv., Iris planifolia (Mill.) Fiori & Paol. and Poa bulbosa L. Compared with some similar and surrounding areas in arid and semi-arid climates the DI of Bissa remains well below that recorded by Belhacini (2011) in Tlemcen (76%), Merioua et al. (2013) in Beni Saf (68%) and Benkhettou et al. (2015) in Nador massif in Morocco (63,6%), while, in humid regions such as the cedar forest of Djurdjura (Tikjda) Larbi (2015) recorded a relatively low DI (31,65%), even though the author reported a clearly visible degradation (fire, clearing, grazing) caused by the human action.
Phytogeographically, as a Mediterranean area, the forest of Bissa showed a notable phytogeographic diversity, but largely dominated by the Mediterranean type. According to Quezel (1985), Medail & Quezel (1997), the multiple and simultaneous paleogeographic events and the contrasting climate cycles around the Mediterranean basin, led to the emergence of high phytogeographic diversity. Indeed comparably to the forest of Bissa, all the phytogeographical studies around the Mediterranean (Ajbilou 2007;Mejías et al.2007;Messaoudène et al. 2007, Belhacini 2011Sedjar 2012;Merioua et al. 2013;Babali 2014;Rebbas 2014;Benkhettou et al. 2015;Larbi 2015;Celep & Dirmenci 2017) have highlighted a high phytogeographic diversity dominated by the Mediterranean types, this abundance was reported by Quezel (2000) in all the North African countries, however, Bonnet et al. (1999) in a 60 years diachronic study of the Mediterranean flora of Frioul archipelago in Marseille (France), reported a slight decrease in the Mediterranean type, while remaining the predominant. It was also noted that the multiregional species were largely represented, indicating a strong anthropozoic pressure, whereas, the endemic species were very limited when compared to the finding of Gavira-Romero et al. (2016) in Malaga, who pointed out26 regional endemic out of 351 collected taxa, among which 7 taxa exclusive to Malaga.

Conclusion
This investigation contributed to complement the efforts of all botanical researchers around the Mediterranean, and most importantly to expand the Mediterranean floristic database, in the perspective of protecting and valuing the natural resources around the Mediterranean Basin.
Following this survey, it was possible to notice a very important floristic richness in the forest of Bissa, mainly dominated by Asteraceae, Fabaceae, Poaceae and Lamiaceae representing 40% of the total flora. Biologically, therophytes (37%), phanerophytes (24%) and hemicryptophytes (23%) prevail in this ecosystem; however, the weighted biological spectrum revealed a strong dominance of phanerophytes (88%), phytogeographically most of the species are of Mediterranean type.
Despite the high species richness, the forest of Bissa was modestly diverse (H=1,3) and dominated by a limited number of species as expressed by a low equitability index (0,31%). Finally, this forest was moderately disturbed (DI = 42%) under the influence of anthropozoic factors, the frequent fires, illicit tree cuttings, clearing and grazing are the main factors causing the degradation.
Unfortunately, these cork oak forests are very rare and cover very limited areas in the southern Mediterranean, particularly in Algeria. It is very important to multiply the protection measures in order to preserve these valuable and endangered ecosystems.