3.3.2 Ben Slimane forest

The Ben Slimane forest extends in the rural municipalities of Moualine El Ghaba and Ain Tizgha covering a surface of 12,262ha, out of which 10,500 are in the undulating plateau.

Following the Emberger approach, this area was classified as semi-arid. In fact the pluviometric quotient Q₂ obtained in the station of Ben Slimane is 63.7. Since Quercus suber fits well in areas with 43<Q₂<190 (Sauvage, 1961), Ben Slimane area can be considered suitable for the cork oak forest.

Not only climatic factors but also pedologic characteristics influence the diffusion of Quercus suber. In fact this species is not suitable to calcareous and hydromorphic soils, as they prefer the soils that lie on quartzite and schists alternating with sandstone. Most of the soils in the Ben Slimane forest and in the south part of study area are formed by colluvial deposits on schist and quartzite, therefore resulting very suitable for the cork oak. According to Sauvage (1961), these climatic and pedological factors are favourable to the diffusion of ‘climax’ cork oak forests constituted, in the tree layers, mainly by Quercus suber, whose extended superficial root system does not allow the development of mixed forests. The shrub layer is quite irregular, often absent, and characterised by the presence of Cistus salvifolius and Cytisus arboreus spp. baeticus. Because of this, the herbaceous layer is very rich and variable depending on where it grows, under closed tree coverage or in large gaps. These two main different areas also form, according to Sauvage, the main ecological groups of the semi-arid cork oak forest, in which he describes several associations.

A) Closed tree coverage associations

1. Briza maxima association

This association presents different sub-associations all characterised by Briza maxima. The best conditions
are represented by a very thick litter of oak leaves.

• 1a. Briza maxima (proper Briza maxima association): where litter is very thick.
• 1b. Dactylis glomerata sub-association
• 1c. Pulicaria odora sub-association: it is typical in forest where undergrowth is absent. Pulicaria odora and Origanum compactum are the representative species. Other species: Hyparrhenia hirta.
• 1d. Cistus salviifolius group: more frequent than the one with Pulicaria. Other species: Lavandula stoachas spp. L., Astragalus lusitanicus, Cytisus arboreus spp. baeticus.

The above mentioned sub-associations are strictly related to the closed forest, but Cistus salvifolius can also be present outside of the tree cover.

2. Pistacia lentiscus and Brachypodium phoenicoides association.

B) Gap associations

1. Helianthetum guttatum and Trifolium bocconei association

• 1a. Helianthetum guttatum and Trifolium bocconei properly said: this association is related to the grazing. The soil is compacted and quite impermeable.
• 1b. Poa bulbosa sub-association: this association is related to over grazing and degradation areas.
• 1c. Agrostis salmantica group: also related to degradation.
• 1d. Brachypodium distachyum marginal group

2. Isoetes hystrix and Ornithogalum unifolium association: this association is related to humid zones as some low areas in the Ben Slimane forest, regularly water-logged during wintertime. It is remarkable to note that the cork oak can stand temporary flooding, but does not like heavy soils.

3. Cistus monspeliensis group: it is a scarcely variable group deriving from cork oak forests.

In fact, according to the floristic composition noted during the relevés, the vegetation of the study area presents many differences to the groups of associations described by Sauvage. All the forest is being in far from a "climax" condition and it is in a degradation stage of its successional series. Moreover some areas once covered by vegetation have been destroyed in the last 20 years. In fact, by observing the 1985 and 2000 satellite images of the study area, We can note that the zone located in the northeastern part along the Oued Cherrat was covered by vegetation in 1985 whereas now it is utilised as quarry. There are several reasons for these changes: increased animal grazing, especially by goats and sheep, the long period of drought, intense cork harvesting and diseases.

Because most of the forest is public and not fenced, many sheep, goats and cows graze in it. The overgrazing causes problems to the oak regeneration, poor variety in floristic composition and the weakening of the standing trees. The scarce regeneration, even if it has a quite big stem diameter, can not grow in height because of the animals’ bite. Moreover the trampling causes soil sealing, so the seeds under the soil surface are not able to emerge. Another important negative factor for the forest is the drought: in the last four years the dry season has been very long so the plant do not produce acorns anymore and are very stressed and susceptible to insects attack, especially Limantria dispar which causes the complete lost of oak leaves. The long dry period together with the cork harvesting are causing a slow regression of Quercus suber forest in the Ben Slimane province. The cork harvesting provokes a physiologic shock for the tree, but if made during the dry period causes even more problems because the internal woven is suddenly exposed to climatic factors, and looses an important amount of water (Le Houérou, 1979).

For this reason and because many anthropic and microclimatic factors interfere with the growth of the oaks, causing many little differences among the vegetation groups; the identification of the associations could not be carried out in the vegetation study. Therefore the vegetation was described according to a physiognomic study.

According to this kind of study three forest physiognomic types cab be distinguished (Table 22):

• open to closed forest without undergrowth;
• open forest with undergrowth mainly constituted by Cistus monspeliensis;
• open forest with undergrowth constituted by mediterranean shrubs (Pistacia lentiscus, Phillirea latifolia, Rhus pentaphylla).

The first type covers few little spots mostly located in proximity to the agricultural lands; therefore the absence of undergrowth depends, probably, on intense grazing which does not even allow shrubs to grow. Nevertheless these areas are rich in graminoids species (see relevé 221 in the Table 22). In this spot the density of Quercus suber is around 55%, representing one of the most closed area of all the forest.

The second physiognomic type is the most common. It is characterised by an open tree layer of cork oak, between 3 and 18% of density, and a closed layer of Cistus monspeliensis. Other characteristic species are Pistacia lentiscus and Asphodelus microcarpus. The overgrazing and the high density of the second layer do not allow floristic variability and high density of herbaceous species (see relevés 115, 309, 311, 312, and 412 in Table 22). Even if the soil conditions in this area are very suitable for the cork oak, being constituted by colluvial deposits lying on schists, plants appear very stressed. This is due to the two main causes: intense cork harvesting and drought, so the weakened plants are strongly attacked by Limantria dispar.

The third type is composed by a very open cork oak layer mixed with high mediterranean shrubs like Pistacia lentiscus, Phillirea latifolia, Rhus pentaphylla, Arbutus unedo, and Myrtus communis (see relevés 106, 114, 215, 305, 319, 409, 410, and 411 in Table 22). The presence of these species is probably due to better microclimatic conditions of soil and air humidity. In particular relevé 215 was done in a burned area, therefore the kind of vegetation could be recognised like a degradation stage of Quercus suber forest after a fire.

In the North part of Ben Slimane forest there are plantations of Eucaliptus camaldulensis and E. gomphocephala. These are the most important species utilised in Morocco for afforestation because of their drought-resistance characteristics. They are planted at a spacing of 3x3 or 3.5x3.5m, on ploughed land with nursery-grown pot-plants. These plantations are managed like coppice with clear felling at the ages of 12, 20, and 30 years; in some areas high forest of Eucalyptus spp. and little spots afforested with Pinus halepensis and P. canariensis are present.

Table 22 - Quercus suber forest floristic list: cover percentage per relevé ("a"=<1%; "r"=rare)

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