Effects of grazing ban of olive plots on floristic richness and diversity and plant biomass in the Zloul Valley (Morocco)

Khalid Benarchid, Mohammed Khatori and Said Hilali

Eco-Design, Energy, Environment and Innovation Laboratory
Faculty of Sciences and Techniques, Hassan First University, Settat– Morocco
k.benarchid@uhp.ac.ma

Abstract

In the past half-century, the forest of Zloul Valley has been severely deforested. Olive trees have been planted on deforested land for 40 years, and now it is close to 5 000 hectares. Therefore, to protect the olive culture, large-scale grazing in the valley is prohibited. The purpose of this study is to determine the impact of the grazing ban on floristic richness and diversity and above-ground biomass. Floristic research has been conducted on 50 surveys, of which 25 were conducted in protected olive fields and 25 were conducted in free grazing areas.

The grazing ban had a positive effect on above-ground biomass and species richness. Compared with those produced in free grazing, the protected olive plots produced significantly (P < 0.001) higher above-ground biomass. The floristic analysis revealed that the prohibition of grazing resulted in a higher species richness compared to that of the free grazing areas with respectively 185 and 119 species. However, it led to a decline in plant diversity, the Shannon-Weaver index values were 4.81 and 4.52, respectively. Furthermore, the floristic composition of the grazing ban olive plots has been more unbalanced compared with the free grazing areas, with evenness index values of 0. 679 and 0.587, and perturbation index values of 76.4 % and 63.2 %, respectively. Grazing ban had resulted also in a high therophytisation that reached 62.4 %.

It seems that long-term grazing prohibition is not recommended. Although it has a positive impact on species richness and above-ground biomass, It could not be considered as a better solution for the sustainable management of deforested areas.

Keywords: floristic diversity, grazing exclosure, rangelands, restoration, therophytisation

Introduction

Rangelands cover a large area of the Mediterranean region and constitute the main land use type (Le Houérou 1980). They represent the most degraded land use type in the world, particularly in arid and semiarid areas, as a result of improper human activities such as overgrazing coupled with drought (Papanastasis 2009). They have important ecological and socio-economic roles. The main causes of rangelands degradation are conversion of natural ecosystems to farmland, exploitation through selective grazing, fuelwood removal, charcoal production, and livestock overgrazing (Reyers 2004). The rangelands degradation is more accentuated in the southern part of the Mediterranean region that is characterized by arid or semi-arid climate, shallow fertility soils, and poor plant cover.

Grazing has a considerable effect on community structure and plant composition as well as traits and plant community patterns (Floret and Pontanier 1982; Le Floc’h 2001). Regular overgrazing may lead to extreme scarcity, see the disappearance of species (Aidoud 1994), and the reduction of above-ground biomass (Lin et al 2010). Therefore, to restore and protect degraded and threatened pastures, grazing prohibitions have been implemented in different areas. A grazing ban is one of the most effective techniques for restoring degraded rangelands by modifying the composition, abundance, and diversity of species (Fenetahun et al 2021). Several studies have proven the effectiveness of the grazing ban in improving pastures (Aidoud et al 2011), the ecological restoration of ecosystems such as perennial plant regeneration, soil protection, and the improvement of plant biodiversity (Abourouh et al 2005; Floret and Pontanier 1982; Foudil et al 2015; Maatougui et al 2013; SER 2004). However, other studies have shown that a grazing ban has a negative impact on plant diversity, especially when the ban is long (Van Coller and Siebert 2019; Yao et al 2019). Excluding domestic animals from such ecosystems may cause a variety of ecological problems such as loss of biodiversity and destructive wildfires (Montalvo et al 1993; Papanastasis 2009; Papanastasis et al 2017). More than that, complete restrictions of open grazing meet neither conservation goals nor socio-economic goals (Ingty 2021). Nonetheless, when the grazing pressure is moderate and the time is evenly distributed, livestock can help soil improvement and increase plant diversity (Gamoun et al 2012; Le Floc’h 2001; Rakotoarimanana et al 2008; Tarhouni et al 2017). Grazing effects on biodiversity depend on the complexity and a variety of situations and consequences of grazing methods, such as herbivore species, stocking rate and periodicity of grazing, invasion risk of certain plant species, grazing history, site productivity, plant palatability, and plant regeneration requirements (Gamoun et al 2012; Habte et al 2020; Le Floc’h 2001; Lunt 2005; Menard et al 2002; Papanastasis et al 2002; Papanastasis et al 2017; Sternberg et al 2000).

The Zloul Valley used to be almost entirely woodland and has undergone extensive deforestation, especially at the end of the last century. The ecosystems of the Zloul Valley demonstrated alarming levels of degradation (Benarchid et al 2020). Grazing is an ancient tradition in the valley. The majority of inhabitants lived mainly on the proceeds of livestock production. The olive culture is relatively new in the Zloul valley. The first orchards were planted in the 1970s. They were first planted on the gentle slopes at the bottom of the valley, about 800 meters, and gradually reached an altitude of 1300 meters. Thus, olive culture occupies currently an important area of 5 000 Ha. To protect their young olive plantations, the residents of the valley have prohibited grazing in the olive plots, but also the surrounding land since most of the land is not fenced. The widespread ban on grazing in the valley has caused many herders to convert into arborists. The study area presents an especial interest. It had undergone an important degradation during the last century, followed by a restoration by the prohibition of grazing during a long period of more than 30 years. The purpose of this study is to determine the impact of grazing prohibition on the floristic richness and diversity and above-ground biomass of herbaceous layer. To achieve these goals, we analyzed the flora of the valley by conducting floristic surveys and measuring the dry weight of the above-ground biomass.

Materials and methods

The study was conducted in the Zloul Valley which is located in the northern Middle Atlas. It is bounded by geographical coordinates of -4°11' and -4°21' in the west and 33°47' and 33°52' in the north. It is also bounded on the north by Jbel Lakraa, which rises to 1621 m and to the south by Jbel Ramouz, which rises to 2329 m. The altitude decreases to 798 m at the level of the streams at the bottom of the valley and rises to 1300 m (Figure 1; Photo 1). The study area is classified by Emberger's climagram as the subhumid bioclimatic stage in cool winter (Emberger 1955). The average temperature of the hottest month reaches 35.46°C and in the coldest month, the average temperature decreases to 2.45°C. The average annual rainfall is 550 mm. During the winter and early spring months, severe frosts can occur. The chergui (hot and dry southeast winds) blow frequently, especially in the spring and summer. The main types of soils are poorly evolved, raw mineral, calcimagnetic, isohumic and fersiallitic (SEI 2014). Livestock is composed essentially of sheep goats and cattle. The analysis of the state of study area pastures revealed that in general, the pastures are little traveled by livestock and did not show any major degradation (SEI 2014).

Figure 1. Map of study area with digital elevation model

Floristic survey

The floristic surveys were carried out along two parallel transects, which intersect the valley perpendicularly. 25 surveys were conducted on 100 m ² plots in the grazing ban olive plots and 25 surveys were in the free grazing and wooded area on plots of 400 m². These surveys were selected in consideration of the topography, geology, geomorphology, and plant diversity used to cover the terrain. They were performed during the spring season which coincided with the full development of the herbaceous and shrub layer and at the time that led to easy identification of species plants and where peak biomass was recorded. The plants were determined using the practical flora of Morocco (Fennane et al 1999; Fennane et al 2007; Fennane et al 2014). Each species was assigned a coefficient of Braun-Blanquet abundance–dominance (Braun-Blanquet 2013). The systematics taxonomy used was the APG IV classification (Chase et al 2016).

Photo 1. Study area

Floristic indices

To analyze the floristic composition, we used Shannon-Wiener Index (H'), Pielou's evenness index (E), perturbation index (PI), and biological spectrum. The Shannon diversity Index combines the two aspects of species richness and evenness and increases with the increase in the number of species and the uniform distribution of individuals among species. It has been completed by the Pielou's evenness index (E) which provides information about the relative abundance structure of the species or its distribution. They are calculated as follows:

ni: recovery of species i in the survey

N: sum of recoveries for all species S

S: total number of species.

The floristic disturbance has been determined by calculating the perturbation index (PI) (Loisel and Gamila 1993), which was estimated using the following formula:

The species inventoried by the floristic surveys were classified according to the biological types of Raunkiaer (1934). The biological spectrum was determined by the relative importance of each of these biological types and allowed the characterization of the physiognomy and structure of plant formations.

Pant biomass

The harvest of the plant biomass of the herbaceous layer was carried out by a semi-destructive method, which involves cutting the above-ground plant material on the ground and performing ten repetitions on randomly selected 2 m² quadrats. Dry matter has been determined after oven-drying at 60 °C up to a constant weight (Floret and Pontanier 1982).

The statistical processing of the data concerns the comparison of the dry weight of the above-ground biomass of the grazing ban olive plots and the free grazing areas. We proceeded first in verifying the conditions of normality and homoscedasticity of the two variables by the Shapiro-Wilk and Levene tests. Tests showed that these variables did not meet these criteria even after their logarithmic transformation, so they were analyzed by the non-parametric Mann Whitney U test. The statistical treatments were carried out using IBM SPSS Statistics 22 software.

Results

Floristic research revealed important species richness in the study area. There were 232 species of vascular plants belonging to 161 genera and 44 families of which 185 species were recorded in the protected plots and only 119 species were recorded in the free grazing areas. The distribution of plant families in the protected olive plots was characterized by a large dominance of Asteraceae at 22.2 %, which exceeded the Fabaceae and Poaceae, which represented 13.0 % and 8.65 %, respectively, and 6 families represented by a single species. While for the free grazing, Fabaceae and Asteraceae were the most representative families accounting for 15.1 % and 14.3 %, respectively, followed by Poaceae accounting for 11.8 %, and 16 families represented by a single species (Table 1).

However, the plant diversity recorded in free grazing areas was higher than that of protected olive plots, with Shannon-Weaver index values of 4.81 and 4.52, respectively. As well, the Pielou evenness index value of the free grazing areas was 15.3 % higher than that of the grazing ban plots with 0.679 and 0.587 respectively. Similarly, the floristic analysis showed that the prohibition of grazing caused a high degree of disturbance in the composition of the flora. The high disturbance index recorded in the grazing ban plots was 76.4 %, while only 63.2 % was recorded in the free grazing areas (Table 2).

The biologic spectrum revealed a clear dominance of therophytes, especially in protected plots, accounting for 62.4 %, while free grazing only accounted for 40.4 % (Figure 2). However, all the others biologic types were relatively more represented in the free grazing areas comparison with the grazing ban plots. The relatively large representation of chamaephytes, phanerophytes, and nanophanerophytes in free grazing areas was due to the persistence of species of the shrub and tree strata. As for hemicryptophytes and geophytes, their importance was relatively almost similar or the prohibited and free grazing areas.

Figure 2. Impact of grazing ban on biological spectrum

The measurement of above-ground biomass showed that the average dry weight recorded in the grazing prohibited olive plots approximately tripled that of the free grazing areas, which was 490 (g/m²) and 138 (g/m ²), respectively (Table 3). The non-parametric Mann Whitney U test showed a significant difference (P < 0.001) between the free grazing and the protected areas (U= 8.000; P ≤ 0.001) (Photos 2A and 2B).

Photo 2. A-Grazing ban olive plot; B- Free grazing area

Discussion

The grazing ban has affected the distribution of plant families in the grazing ban olive plots (Table 1). The grazing prohibition allowed a notable increase of Asteraceae and Fabaceae. In contrast, the latter two families are at a disadvantage under the influence of grazing, which allows the Poaceae to conquer the environment. Similar results were obtained from the study by Abourouh et al (2005) in the rangelands of the Maamora forest in Morocco. While, a positive impact has been recorded by prohibiting grazing on grass species’ abundance and increased significantly in the long-term grazing exclosure areas (Fenetahun et al 2021). The abusive exploitation of the rangelands leads to the scarcity of palatable plants, mainly grasses which are appreciated in all circumstances (Foudil et al 2015).

The Floristic study has shown that compared with free grazing, the richness of species in the grazing prohibited olive plots was more important, with 189 and 115 species, respectively.

Similarly, the fenced plots in the Maamora forest in Morocco recorded 61 plant species compared to only 47 in the unfenced plots (Abourouh et al 2005). Likewise, the enclosure plots in the steppe rangelands in Algeria recorded a significant increase in species richness compared with unprotected plots. Aidoud (1994) reported also the extreme scarcity, seeing the disappearance of species following regular overgrazing. In general, as the stocking rate increases, the species richness decreases significantly, especially in arid low productivity systems than in subhumid and humid systems(Herrero-Jáuregui and Oesterheld 2018). However, opposite effects of overgrazing were found depending on the nature of the terrain, compared to the protected sites, overgrazed ones produced significantly lower species richness in grasslands but rather slightly higher species richness in the phryganic ecosystems (Papanastasis et al 2002). The impact of grazing exclusion on species richness depends also on the duration of prohibition, it remarkably increased species richness in short-term (≤ 5 years) (Xiong et al 2016) and 5-7 years (Foudil et al 2015), but not significantly in the long run. On the other hand, a meta-analyze of 30 studies carried out by Proulx and Mazumder (1998) showed that plant species richness decreases with high grazing pressure in nutrient-poor ecosystems, while it increases with high grazing pressure in nutrient-rich ecosystems.

Our Results displayed that the Grazing ban led to a decline in plant diversity, the Shannon-Weaver index value was slightly higher in the free grazing than the protected olive plots sites with 4.81 and 4.52, respectively. Similar results have been also found in Spain, where it was observed that four years after protection from grazing species diversity was lower in the ungrazed than in the grazed sites (Montalvo et al 1993). Likewise, the works of Papanastasis (2017) demonstrated that grazing significantly contributes to greater plant species diversity, thus degraded rangelands can be restored by moderating the grazing pressure rather than completely banning livestock grazing. However, species biodiversity indicators, including the Pielou evenness index and the Shannon–Weave diversity index did not significantly change under grazing exclusion conditions (Yan and Lu 2015). In contrast, others works showed that grazing exclosure increased species diversity. Grazing fences increased significantly the plant diversity of grassland on the Qinghai-Tibet Plateau, and the Shannon-Weaver index of fenced sites was 25.8% higher than that of open grazing pastures (Wang et al 2019). Naveh and Whittaker (1979) have found a higher diversity of herbaceous plants and particularly annuals and geophytes under moderate grazing than under complete protection and heavy grazing in Mediterranean grasslands and woodlands of Israel.

The impact on species diversity may vary depending on the age of the fence. The species diversity recorded in the fenced plots increased with the extension of the grazing prohibition period (Abourouh et al 2005). While in the Ethiopian grasslands, compared with long-term grazing areas and open grazing areas, the Shannon index value recorded in short-term grazing areas was the highest (Fenetahun et al 2021). Nevertheless, in the semi-arid pastures of southern Ethiopia, the abundance and diversity of herbaceous plants did not change with the age of the fence (12-30 years) (Angassa et al 2010).

The floristic composition was more balanced in the free grazing areas than that of protected olive plots where the evenness index values were 0.679 and 0.587 respectively. Knowing that a value close to 1 indicates that the species distribution is equitable, and a value close to 0 indicates that the species distribution is dominated by a single species. Similarly, The effect of grazing exclusion on species evenness was not significant but did decrease slightly within the enclosure (Wang et al 2019; Yan and Lu 2015). Grazing exclusion may lead to a decrease in species richness and biodiversity because species that are highly adapted to grazing are replaced by strong competitors who have increased abundance due to the cessation of grazing (Mayer et al 2009). Indeed, Grime (1973) suggested that a high concentration of resources favor species that tend to outcompete others and dominate the ecosystem. Therefore, in resource-rich or nutrient-rich ecosystems, diversity tends to decrease as resource availability or concentration increases (Huston 1979; Rosenzweig 1971).

The biologic spectrum of our study area showed a greater therophytisation in the protected areas than in the free grazing, with 62.4 % and 40.4 %, respectively (Figure 2). The dominance of annual species (therophytes) can be explained by the elimination of trees and shrubs which took place during the establishment of olive orchards. This also led to an increase in the disturbance of the floristic composition of the prohibition grazing areas that recorded a high disturbance index, reaching 76.4 %. Likewise, overgrazing has led to large regression of perennial species and promoted the annual species (Abourouh et al 2005; Zhang et al 2021). Therophytisation could provide information on the state of ecosystem degradation. Quézel (2000) considered that this is the final stage of vegetation degradation. In fact, protected olive lands included more unpalatable species than free grazing areas such as Atractylis cancellata, Galactites tomentosa, Scolymus hispanicus, Carduus pycnocephalus, and Pallenis spinosa (Le Houèrou 1980). Similarly, Fırıncıoğlu et al (2007) showed that Complete protection from grazing for a prolonged period of time after a long history of grazing disturbance may not lead to an increase in desirable plant species with a concomitant improvement in range condition.

Grazing prohibition led to a significant increase in above-ground biomass (P < 0.001), the dry weight recorded in the protected plots was 255 % higher than the dry weight recorded in the open grazing plots. Our results are in full agreement with several studies. Above-ground biomass was significantly higher in the grazing exclusion grasslands than in the free grazed grasslands (Angassa et al 2010; Fenetahun et al 2021; Foudil et al 2015; Lin et al 2010; Wang et al 2019 ; Yan and Lu 2015). Similarly, the biomass of above-ground plants increased by 74% after 16 years of Grazing exclosure in the dry bushes of Balochistan, Pakistan (Qasim et al 2017) and it increased by 74 % after 8 years of Grazing exclosure in the desert grassland in northwestern China (Rong et al 2014). This increase could be explained by the decrease in livestock grazing, which allowed the recovery of species and accumulation of biomass (Wang et al 2019). The total biomass increases with the increase of defense time. It is very important in the first five years, reaching more than 300 % and increasing by 60% in the next five years (Abourouh et al 2005). The average above-ground biomass measured in the exclosures was more than twice that of the adjacent grazing area, and the young exclosures produced more biomass than the old exclosures (Yayneshet et al 2009). Likewise, Aidoud and Touffet (1996) noted that the increase in total biomass by grazing exclosure became less important from the fourth year on. They added that this reduction can be explained by the cessation of moderate grazing, which may be a factor that stimulates the production of most perennial species. While the older enclosures (30 years) had no superior benefits over the younger (12-30 years) in terms of herbaceous production (Angassa et al 2010). On the other hand, more biomass was accumulated within seasonal exclosures than in continuously grazed areas, but the decline in species richness with the age of exclosures indicates that long-term exclusion of grazing may not necessarily increase species richness in arid-zone grazing lands (Oba et al 2001).

Conclusion

• Our research aimed to determine the impact of grazing prohibition on the floristic richness and diversity and above-ground biomass of the herbaceous layer. Over a long period of more than 30 years, the effects of grazing ban seem to be mixed. It has a positive effect on the species richness and above-ground biomass. 185 plant species have been recorded in the grazing exclosure and only 119 plants species have been recorded in the free grazing area. Furthermore, grazing ban caused a significant increase in above-ground biomass (P < 0.001) by 255 %.
  
  
• Grazing prohibition has also a negative impact on plant diversity and floristic composition. It led to a decline in plant diversity. Shannon-Weaver and evenness Pielou index values were relatively lower in the grazing ban plots than the free grazing areas. The Floristic composition of the protected plots was more disturbed, showing high therophytisation of 62.4 % and perturbation index of 76. 4 %.
  
  
• The long-term prohibition on the graze seems to be mixed. Although it has a positive impact on species richness and above-ground biomass. It did not seem to lead to a sustainable restoration of deforested areas. Thus, areas, where olive trees are large enough and so cannot be affected by livestock, could be moderately grazed.

Acknowledgements

We sincerely thank Professor Mohamed Ibn Tatou and Professor Mohamed Fennane of the Rabat Institute of Science, and Professor Cyrille Chatelain of the Geneva Conservatory and Botanical Garden for their contributions to the species identification of this study.

Conflict of Interest Declaration

No potential conflicts of interest to declare

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