Anthropogenic effects on plant cover in the Tlemcen region (western Algeria) ()
1. INTRODUCTION
Anthropophytic species are distributed all over the Tlemcen Mountains, particularly on the south-western side. This has been shown in recent studies by the Laboratory of Ecology and Ecosystems Management at the University of Tlemcen, which examined the physiognomic composition of the plant formations [1], the specific effects of pasture land colonization [2], the specific effects and influence of anthropophytic species [3], the state of the flora in the Tlemcen wilaya (province) [1].
We propose that plant cover of anthropophytic species [4] unquestionably plays a more important role in zones which are highly exposed to pasture land. For decades, the forested and pre-forested area of the Tlemcen region has experienced continuous degradation, due to the combined action of climate and man [5].
Climate change (Higher temperatures and less precipitation) [3] and semi-arid (dry summer) of the study area disrupt the delicate balance of vegetation [6]. (For example, increasingly high temperatures in summer have led to replacement of hygrophilous species by xerophytes).
Human actions combined è weak or total lacks of regeneration contribute to the establishment of stands of matorral at which any rapid appreciation seems excluded.
Depending on the nature of this climate zone, soil genesis undergoes a significant change. This is the action of high temperature during the dry season.
Soils in the study area (Table1) are part of Red Mediterranean soils characterized by the presence of some remnants of terra-rossa as Chamaerops humilis and Ampelodesma mauritanicum.
The action of man, and especially cattle, alters species composition by choosing species according to their degree
Table 1. Soil characteristics of the study sites.
of palatability. At this pressure is compounded by the phenomenon of trampling which completely destroys the soil structure and makes it sensitive to the action of erosion.
Man collects wood for various uses, practices cereal land cultivation and raises livestock (sheep and goats) on pasture land. All these actions, with time and expansion, have caused a noticeable destruction of the plant cover [7]. Under this permanent pressure, the forests became matorrals [7]. In turn, the matorrals became fragmented and degraded, giving way to spiny and/or toxic species [8]. In addition, many of the native species populations suffered significantly, even to the point of extinction in some cases [8].
The importance of anthropogenic pressure, in association with the climatic conditions, inevitably causes a reduction in soil fertility [9], which directly affects plant growth.
We used vegetation sampling, correspondence factor analysis and ascending hierarchical clustering to provide a comprehensive assessment of this highly degraded matorral, and develop a management policy for preservation of existing native vegetation.
2. MATERIALS AND METHODS
The study area is situated in the western part of the greater Tlemcen, in the Mansourah municipality between 34˚38′N and 34˚50′N, and 1˚20′W and 1˚25′W.
Three study sites were selected within this area (Figure 1).
Site 1 is located in the north-west of the urban district of Imama, next to the Tlemcen university complex, at an altitude of 720 m. It is characterized by uneven relief, with chalky soil, which is quite shallow in some places. The vegetation is dominated by Urginea maritima, Calycotome spinosa and Ulex boivinii. Plant cover is 60%.
Site 2 is located at an altitude of 750 m, with more visible chalky rock than the first one. Here, recent deforestation is evident by the presence of species remains such as: Quercus ilex, Pistacia lentiscus and Olea europaea. Plant cover is 50%.
Site 3 is located opposite the Sites 1 and 2 at an altitude of 760 m; it is located in a hollow and receives greater rainfall due to the orographic effect. The vegetation is dominated by Urginea maritima, Calycotome spinosa, Ulex boivinii and Olea europaea, with some remnants of Quercus ilex. Plant cover is 65%.
The climate is classified as semi-continental [10], with a temperature range of 25˚C - 35˚C, an annual minimum of 5.4˚C in January and an annual maximum of 30.5˚C in July. The EMBERGER pluviothermal Climagramme [11] ranges the Tlemcen region in the higher semi-arid level with a moderate winter season.
In order to determine the minimal sampling area. The minimum area for sampling varies according to the type of vegetation, but should be at least of the same size as
the minimal area defined by means of the “species-area curve” [6]. We found it difficult to determine a minimal area for the species in our study sites, because of the strong influence of the anthropogenic factor, and also the climatic factor (temperature) which contribute to altering their density on the one hand, and their distribution, on the other. Therefore, we selected a minimal area of 100 m2.
The floral survey was carried out throughout the entire vegetative period from March to August (1997-1999) on a number of plant formations in association with Quercus ilex, Olea europaea, Calycotome spinosa and Chamaerops humilis. We took into consideration the richest data at the floral level. i.e. where there is plenty of vegetation. The survey was conducted using the dominant abundance and sociability factors [12] that define the study sites (location, altitude, inclination, geomorphological substratum, average cover).
3. RESULTS AND DISCUSSION
Table 1 presents the data from the floristic survey. Correspondence factor analysis shows the main gradients differentiating between the three vegetation formations. Figure 2 presents the plot of Factors 1 and 2, and represents an anthropogenic gradient, dividing the vegetation into three groups: group A includes annual therophytic species; group B is dominated by anthropophytic species and finally, on the negative side; group C comprises species of pre-forest formations.