# Grazing effects on vegetation dynamics in the savannah ecosystems of the Sahel

Haftay Hailu Gebremedhn, Ousmane Ndiaye, Sylvanus Mensah, Cofélas Fassinou, Simon Taugourdeau, Torbern Tagesson, Paulo Salgado

► To cite this version:

Haftay Hailu Gebremedhn, Ousmane Ndiaye, Sylvanus Mensah, Cofélas Fassinou, Simon Taugourdeau, et al.. Grazing effects on vegetation dynamics in the savannah ecosystems of the Sahel. Ecological Processes, 2023, 12 (1), pp.54. 10.1186/s13717-023-00468-3. hal-04506510

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# Grazing effects on vegetation dynamics in the savannah ecosystems of the Sahel

Haftay Hailu Gebremedhn<sup>1,2*</sup> ID, Ousmane Ndiaye<sup>2,3</sup>, Sylvanus Mensah<sup>4,5 ID</sup>, Cofélas Fassinou<sup>6</sup>, Simon Taugourdeau<sup>2,7</sup>, Torbern Tagesson<sup>8,9</sup> and Paulo Salgado<sup>2,7</sup>

**Abstract**

**Background** The savannah ecosystems of Sahel have experienced continuous and heavy grazing of livestock for centuries but still, their vegetation response to grazing pressure remains poorly understood. In this study, we analysed the herbaceous plant dynamics, measured by species diversity, composition, cover, and biomass in response to grazing pressure in the savannah ecosystems of Sahel. In Senegal, we selected four savannah sites represented with high, moderate, light and no grazing intensity levels. Transect survey methods were used for sampling the vegetation data within each of the sites. Species richness and composition were analysed using species accumulation curve and multivariate analyses. Furthermore, we used General Linear Models and a piecewise Structural Equation Model (pSEM) to examine the relationships between grazing intensity, vegetation cover, diversity and biomass.

**Results** The herbaceous species diversity and composition varied significantly among the different grazing intensity levels (p <0.001). The plant species composition shifted from the dominance of grass cover to the dominance of forb cover with increasing grazing pressure. Moreover, the attributes of species diversity, herbaceous biomass, and ground cover were higher on sites with low grazing than sites with high and moderate grazing intensity. Across all sites, species diversity was positively related to total biomass. The pSEM explained 37% of the variance in total biomass and revealed that grazing intensity negatively influenced total biomass both directly and indirectly through its negative influence on species diversity.

**Conclusions** Managing grazing intensity may lead to higher plant production and higher mixed forage establishment in the dryland savannah ecosystems. This information can be used to support land management strategies and promote sustainable grazing practices that balance the needs of livestock with the conservation of ecosystem health and biodiversity.

**Keywords** Biomass, Biodiversity, *Diodella sarmentosa*, Forb, Grazing intensity, Invasive species, Pastoral ecosystem, Senegal, West Africa

<figure><img src="image_2.png" /></figure>

<footnote>*Correspondence:
Haftay Hailu Gebremedhn
hailuhft418@gmail.com
Full list of author information is available at the end of the article</footnote>
# Grazing effects on vegetation dynamics in the savannah ecosystems of the Sahel

Haftay Hailu Gebremedhn<sup>1,2*</sup> ID, Ousmane Ndiaye<sup>2,3</sup>, Sylvanus Mensah<sup>4,5 ID</sup>, Cofélas Fassinou<sup>6</sup>, Simon Taugourdeau<sup>2,7</sup>, Torbern Tagesson<sup>8,9</sup> and Paulo Salgado<sup>2,7</sup>

**Abstract**

**Background** The savannah ecosystems of Sahel have experienced continuous and heavy grazing of livestock for centuries but still, their vegetation response to grazing pressure remains poorly understood. In this study, we analysed the herbaceous plant dynamics, measured by species diversity, composition, cover, and biomass in response to grazing pressure in the savannah ecosystems of Sahel. In Senegal, we selected four savannah sites represented with high, moderate, light and no grazing intensity levels. Transect survey methods were used for sampling the vegetation data within each of the sites. Species richness and composition were analysed using species accumulation curve and multivariate analyses. Furthermore, we used General Linear Models and a piecewise Structural Equation Model (pSEM) to examine the relationships between grazing intensity, vegetation cover, diversity and biomass.

**Results** The herbaceous species diversity and composition varied significantly among the different grazing intensity levels (*p* <0.001). The plant species composition shifted from the dominance of grass cover to the dominance of forb cover with increasing grazing pressure. Moreover, the attributes of species diversity, herbaceous biomass, and ground cover were higher on sites with low grazing than sites with high and moderate grazing intensity. Across all sites, species diversity was positively related to total biomass. The pSEM explained 37% of the variance in total biomass and revealed that grazing intensity negatively influenced total biomass both directly and indirectly through its negative influence on species diversity.

**Conclusions** Managing grazing intensity may lead to higher plant production and higher mixed forage establishment in the dryland savannah ecosystems. This information can be used to support land management strategies and promote sustainable grazing practices that balance the needs of livestock with the conservation of ecosystem health and biodiversity.

**Keywords** Biomass, Biodiversity, *Diodella sarmentosa*, Forb, Grazing intensity, Invasive species, Pastoral ecosystem, Senegal, West Africa

<figure><img src="image_2.png" /></figure>

<footnote>*Correspondence:
Haftay Hailu Gebremedhn
hailuhft418@gmail.com
Full list of author information is available at the end of the article</footnote>
## Background

Two opposing directions of changes in semi-arid rangeland vegetation in response to climatic variables and grazing intensity have been suggested (Behnke 2000; Derry and Boone 2010). First, it is suggested that changes in rangeland vegetation in arid and semiarid areas are primarily driven by rainfall variability (Behnke 2000; Booker et al. 2013), and therefore, moving livestock to cope with periodic droughts is seen as the most appropriate management system for pastoralists (Seid et al. 2016). Second, it is argued that the intensity of livestock grazing should be maintained at a level matching the carrying capacity of the rangelands (Oba et al. 2000; Kassahun et al. 2008; Pricope et al. 2013). Therefore, managing grazing intensity, timing, and distribution can lead to better plant productivity and higher quality mixed forage (Biondini et al. 1998; Egeru et al. 2020). These concepts have strongly influenced the development of rangeland management over the years. Currently, it is difficult to predict which management actions facilitate positive vegetation changes.

Vegetation grazing resistance is a term used to describe the relative ability of plants to survive different grazing conditions (Milchunas and Noy-Meir 2002), and different plant forms and species respond differently to grazing strategies. However, more insights can be gained by considering vegetation grazing resistance as outcome of tolerance (i.e., capable of regrowing more rapidly following defoliation) and avoidance (i.e., species grazed less severely by developing escape mechanisms) or a combination of these two resistance components to realize a competitive advantage within the community (Archer and Pyke 1991). These mechanisms of grazing avoidance include physical characteristics, such as thorns, prickles, and spines that make plants less likely to be grazed by livestock (Milchunas and Noy-Meir 2002). The size, shape, or arrangement of leaves and seeds may also make it difficult for animals to access and feed on the plant (Trlica and Rittenhouse 1993).

Grazing resistance mechanisms and the intensity of grazing shape the species composition and community structure of grazed ecosystems. For example, in heavily grazed rangelands, more palatable plant species are less encountered (Weber et al. 1998; Haftay et al. 2013), while the less or unpalatable species become dominant (Fleischner 1994; Vesk and Westoby 2001; Gemedo-Dalle et al. 2006; Hailu 2017). Moreover, it is common that invasive plant species dominate heavily grazed rangelands, fill the spatial niche left by the suppressed palatable plants, and replace highly diverse native plant communities with uniform communities (Oomen et al. 2016). This makes species composition one of the most important attributes of ecosystems, reflecting the outcomes

of important ecological processes in arid and semiarid rangelands (Rydgren et al. 2020).

Studies showed that forbs (i.e., non-graminoid herbaceous vascular plants) and grasses exhibited different responses to grazing in terms of their vegetation cover (Stahlheber and D'Antonio 2013; Koerner et al. 2018; Siebert and Dreber 2019). Fulbright et al. (2021) noted that cattle selectively forage on grasses, which can reduce competition between forbs and grasses, resulting in an increased abundance of forbs. This increase of forbs' ground cover in response to grazing might be because they have better grazing tolerance than grass. In addition, forb response to grazing in terms of their ground cover may vary depending on the forb species, life form (annual vs. perennial) (McIntyre and Lavorel 2001; Hayes and Holl 2003), origin to the area (native vs. invasive) (Koerner et al. 2018), soil texture (Drawe and Box 1968) and precipitation (Fulbright et al. 2021).

The African Sahel, located at the Saharan desert border, is a region dominated by herbaceous plants and a scattered population of shrubs and trees (Amole et al. 2022). The annual rainfall in the typical Sahel ranges from 400 and 600 mm, and is unimodal with a short rainy season. This limited and unreliable rainfall in combination with poor soils make it difficult to cultivate (Grillot et al. 2018). So for centuries, people have instead support their livelihoods through pastoralism. However, these arid and semi-arid rangeland ecosystems experience different forms of land and vegetation degradation due to external factors such as climate change, drought, desertification (Le Houérou 2002; Tagesson et al. 2015), and grazing is thought to play a major role (Hiernaux et al. 1999; Miehe et al. 2010). A grazing pressure beyond a certain threshold may affect these rangelands and thus the native species composition (Amiri et al. 2008), diversity and biomass production is lost (Cingolani et al. 2005). However, defining the native plant species of African rangelands is problematic, because these ecosystems have been exposed to long evolutionary grazing pressure (Backéus et al. 1994).

The dynamics of herbaceous plant species composition, diversity, and biomass production in response to grazing intensity in the savannah ecosystem of the Sahel remain poorly understood mainly due to the absence of permanent sites protected from grazing. Characterizing the livestock grazing intensities and assessing their effect on vegetation attributes are also challenging in these rangeland ecosystems, because the livestock stocking rates are generally unknown. Moreover, even though forbs are a significant part of the herbaceous layer in the Sahelian savannah, studies on the response of forbs to grazing are limited. Most studies have traditionally focused on maximizing forage productivity rather than considering plant
**Background**

Two opposing directions of changes in semi-arid rangeland vegetation in response to climatic variables and grazing intensity have been suggested (Behnke 2000; Derry and Boone 2010). First, it is suggested that changes in rangeland vegetation in arid and semiarid areas are primarily driven by rainfall variability (Behnke 2000; Booker et al. 2013), and therefore, moving livestock to cope with periodic droughts is seen as the most appropriate management system for pastoralists (Seid et al. 2016). Second, it is argued that the intensity of livestock grazing should be maintained at a level matching the carrying capacity of the rangelands (Oba et al. 2000; Kassahun et al. 2008; Pricope et al. 2013). Therefore, managing grazing intensity, timing, and distribution can lead to better plant productivity and higher quality mixed forage (Biondini et al. 1998; Egeru et al. 2020). These concepts have strongly influenced the development of rangeland management over the years. Currently, it is difficult to predict which management actions facilitate positive vegetation changes.

Vegetation grazing resistance is a term used to describe the relative ability of plants to survive different grazing conditions (Milchunas and Noy-Meir 2002), and different plant forms and species respond differently to grazing strategies. However, more insights can be gained by considering vegetation grazing resistance as outcome of tolerance (i.e., capable of regrowing more rapidly following defoliation) and avoidance (i.e., species grazed less severely by developing escape mechanisms) or a combination of these two resistance components to realize a competitive advantage within the community (Archer and Pyke 1991). These mechanisms of grazing avoidance include physical characteristics, such as thorns, prickles, and spines that make plants less likely to be grazed by livestock (Milchunas and Noy-Meir 2002). The size, shape, or arrangement of leaves and seeds may also make it difficult for animals to access and feed on the plant (Trlica and Rittenhouse 1993).

Grazing resistance mechanisms and the intensity of grazing shape the species composition and community structure of grazed ecosystems. For example, in heavily grazed rangelands, more palatable plant species are less encountered (Weber et al. 1998; Haftay et al. 2013), while the less or unpalatable species become dominant (Fleischner 1994; Vesk and Westoby 2001; Gemedo-Dalle et al. 2006; Hailu 2017). Moreover, it is common that invasive plant species dominate heavily grazed rangelands, fill the spatial niche left by the suppressed palatable plants, and replace highly diverse native plant communities with uniform communities (Oomen et al. 2016). This makes species composition one of the most important attributes of ecosystems, reflecting the outcomes

of important ecological processes in arid and semiarid rangelands (Rydgren et al. 2020).

Studies showed that forbs (i.e., non-graminoid herbaceous vascular plants) and grasses exhibited different responses to grazing in terms of their vegetation cover (Stahlheber and D'Antonio 2013; Koerner et al. 2018; Siebert and Dreber 2019). Fulbright et al. (2021) noted that cattle selectively forage on grasses, which can reduce competition between forbs and grasses, resulting in an increased abundance of forbs. This increase of forbs' ground cover in response to grazing might be because they have better grazing tolerance than grass. In addition, forb response to grazing in terms of their ground cover may vary depending on the forb species, life form (annual vs. perennial) (McIntyre and Lavorel 2001; Hayes and Holl 2003), origin to the area (native vs. invasive) (Koerner et al. 2018), soil texture (Drawe and Box 1968) and precipitation (Fulbright et al. 2021).

The African Sahel, located at the Saharan desert border, is a region dominated by herbaceous plants and a scattered population of shrubs and trees (Amole et al. 2022). The annual rainfall in the typical Sahel ranges from 400 and 600 mm, and is unimodal with a short rainy season. This limited and unreliable rainfall in combination with poor soils make it difficult to cultivate (Grillot et al. 2018). So for centuries, people have instead support their livelihoods through pastoralism. However, these arid and semi-arid rangeland ecosystems experience different forms of land and vegetation degradation due to external factors such as climate change, drought, desertification (Le Houérou 2002; Tagesson et al. 2015), and grazing is thought to play a major role (Hiernaux et al. 1999; Miehe et al. 2010). A grazing pressure beyond a certain threshold may affect these rangelands and thus the native species composition (Amiri et al. 2008), diversity and biomass production is lost (Cingolani et al. 2005). However, defining the native plant species of African rangelands is problematic, because these ecosystems have been exposed to long evolutionary grazing pressure (Backéus et al. 1994).

The dynamics of herbaceous plant species composition, diversity, and biomass production in response to grazing intensity in the savannah ecosystem of the Sahel remain poorly understood mainly due to the absence of permanent sites protected from grazing. Characterizing the livestock grazing intensities and assessing their effect on vegetation attributes are also challenging in these rangeland ecosystems, because the livestock stocking rates are generally unknown. Moreover, even though forbs are a significant part of the herbaceous layer in the Sahelian savannah, studies on the response of forbs to grazing are limited. Most studies have traditionally focused on maximizing forage productivity rather than considering plant
diversity. This approach may result in misinterpretation regarding local species diversity conservation, restoration and management of these ecosystems. Hence, studying the response of both the forbs and grass species to grazing is important to ensure an understanding of the overall ecological dynamics.

Here, we study the response of herbaceous plant dynamics, as measured by species diversity, composition, vegetation ground cover and herbaceous biomass in response to grazing intensity in savannah ecosystems of Senegal. Specifically, we (i) determined the impacts of grazing intensity on species richness and composition; (ii) examined the effects of grazing intensity on percentages of forbs cover, grass cover, total cover, Shannon diversity and total biomass; and (iii) investigated how grazing intensity influenced biomass directly and indirectly through species diversity and vegetation cover.

# Methods

## Study area

The study was carried out in the savannah ecosystem of the Dahra field site (15° 21′ N, 15° 28′ W) in the Senegalese pastoral zone (Fig. 1) which is located in the western part of the Sahel region. Mean annual rainfall over the last 50 years in the current study area was 371 mm, which is typical of the Sahel (Taugourdeau et al. 2022). The rainfall in the study area is unimodal with a rainy season from July to October. The soils are mainly sandy loams (Ndiaye et al. 2015a). All life forms of herbaceous plants in the study area are annuals. *Cenchrus biflorus*, *Chloris prieurii*, *Diodella sarmentosa* and *Zornia glochidiata* are the most dominant herbaceous species, whereas Balanitacea, Combretaceae and Mimosaceae are the most dominant woody families in the study area (Ndiaye et al. 2015b). The land-use system is predominantly pastoral and the pastoralists depend on the livestock, with nomadic in lifestyle and much of the land for grazing.

<figure><img src="image_3.png" /><figcaption>**Fig. 1** Map of Senegal with the location of the Dahra field site in the savannah ecosystem of the Sahel</figcaption></figure>
diversity. This approach may result in misinterpretation regarding local species diversity conservation, restoration and management of these ecosystems. Hence, studying the response of both the forbs and grass species to grazing is important to ensure an understanding of the overall ecological dynamics.

Here, we study the response of herbaceous plant dynamics, as measured by species diversity, composition, vegetation ground cover and herbaceous biomass in response to grazing intensity in savannah ecosystems of Senegal. Specifically, we (i) determined the impacts of grazing intensity on species richness and composition; (ii) examined the effects of grazing intensity on percentages of forbs cover, grass cover, total cover, Shannon diversity and total biomass; and (iii) investigated how grazing intensity influenced biomass directly and indirectly through species diversity and vegetation cover.

# Methods

## Study area

The study was carried out in the savannah ecosystem of the Dahra field site (15° 21′ N, 15° 28′ W) in the Senegalese pastoral zone (Fig. 1) which is located in the western part of the Sahel region. Mean annual rainfall over the last 50 years in the current study area was 371 mm, which is typical of the Sahel (Taugourdeau et al. 2022). The rainfall in the study area is unimodal with a rainy season from July to October. The soils are mainly sandy loams (Ndiaye et al. 2015a). All life forms of herbaceous plants in the study area are annuals. *Cenchrus biflorus*, *Chloris prieurii*, *Diodella sarmentosa* and *Zornia glochidiata* are the most dominant herbaceous species, whereas Balanitacea, Combretaceae and Mimosaceae are the most dominant woody families in the study area (Ndiaye et al. 2015b). The land-use system is predominantly pastoral and the pastoralists depend on the livestock, with nomadic in lifestyle and much of the land for grazing.

<figure><img src="image_3.png" /><figcaption>**Fig. 1** Map of Senegal with the location of the Dahra field site in the savannah ecosystem of the Sahel</figcaption></figure>
placed, yielding 36 quadrats. At the site of the moderate grazing, a 4-km transect was laid out from the east to west, and 1-m² quadrats at 200-m intervals were systematically installed, resulting in 21 quadrats. The light-grazing (enclosure) site was 400 m×500 m. A 400-m transect was laid out to sample the vegetation, covering the periphery from the east to west of the enclosure. 21 quadrats of 1-m² each were placed along this transect at 20-m intervals. The no-grazing site was 80 m×40 m. In the 80-m long direction, we established three parallel transects separated by 10-m intervals. Within these transects, 1-m² quadrats at 10-m intervals were placed, yielding 21 quadrats.

## Data

From each of the 1-m² quadrats, data on species composition and richness, ground cover and biomass were collected. The measurements were taken in September 2022 when the vegetation was at its peak flowering stage. Species composition was estimated by rating the percentage of each species abundance in the quadrats and assigned them to one of two growth forms: forbs or graminoids (henceforth 'grasses'). We did not subdivide forbs into nitrogen-fixing and non-fixing species because of the low number of nitrogen-fixing species in the area. Then, the proportion of all forbs and grass species dominance was summed up for each quadrat across the sites to estimate forbs and grass cover separately. The proportion of soil surface covered by herbaceous plants (vegetation ground cover) was also estimated visually for each quadrat. Species richness was determined as the total number of species encountered in the quadrats. The diversity of species was computed using the Shannon–Wiener index ($H'$) calculated following Krebs (1999):

$$H' = - \sum_{i=1}^{s} pi \ln pi$$

where s = number of species; $p_i$ = proportion of individuals or abundance of the $i^{th}$ species; and ln is the natural logarithm to the base e.

The aboveground herbaceous biomass was estimated by harvesting live and dead material at ground level from each of the quadrats and all the 99 quadrats across the four sites. The harvested samples were weighed in the field to get fresh weight. Thirty percent of the harvested samples from each quadrat were placed in a paper bag for later dry matter analysis. This harvested biomass was dried in an oven at 105 °C for 48 h, and then weighed to obtain the dry matter. Then, the total dry biomass in each quadrat was calculated by multiplying the proportion of each dried sample biomass by the weight of the total fresh biomass.

## Statistical analyses

All analyses were performed with the R Statistical Software version 4.2.2 (R Core Team 2022). We determined the impacts of grazing intensity on species richness and composition using species accumulation curve and multivariate analyses. The grass and forb diversity was quantified based on the species × quadrat abundance matrix, the estimated species richness using the *accuncomp* function of the *Biodiversity R* package (Kindt and Coe 2005). A sample-based rarefaction procedure was used to estimate the 95% confidence intervals and compare the patterns of plant richness among grazing intensities. To determine the impact of grazing on the species similarity, we performed an Analysis of Similarities (ANOSIM) test using the “anosim” function in the *vegan* package (Oksanen et al. 2022). ANOSIM is a non-parametric test of significant difference between two or more groups, based on any distance measure (Clarke and Ainsworth 1993), and used for taxa-in-sample data, where groups of samples are to be compared. Then, a non-metric multidimensional scaling (NMDS) was used to group plots with similar species into separate classes using the Bray–Curtis dissimilarity matrix. The NMDS analysis was performed using the *metaMDS* function in the *vegan* package (Oksanen et al. 2022). Stress value was used as a criterion of efficiency, where stress is the departure from monotonicity in the plot of distance in the original p-dimensional space (dissimilarity) vs. distance in the ordination space (k-dimensional space) (Fasham 1977). A rule of thumb is that stress < 0.05 provides an excellent representation in reduced dimensions, < 0.1 is great, < 0.2 is good/ok, and stress > 0.3 provides a poor representation (Clarke 1993). Moreover, species composition collected from the quadrats was averaged for each of the four grazing areas to determine the relative dominance of each species of the four grazing sites.

We used General Linear Models (GLM) to test for significant ($p < 0.05$) effects of grazing intensities on forb-, grass- and total percentage cover, Shannon diversity and standing aboveground biomass. Specifically, GLM with family binomial was used for forb-, grass- and total percentage cover modeled as percentage data (Zuur et al. 2009), while the Gaussian family was used for Shannon diversity and total biomass due to the normality of their distribution. Mean values and standard errors were represented graphically for better visibility. In addition, we tested for the significance of grazing intensity-dependent effects of Shannon diversity on biomass. Specifically, we assessed both the main and interaction effects of the grazing intensity with Shannon diversity, using linear models (biomass ~ grazing intensity × Shannon diversity).

Finally, we used piecewise Structural Equation Modelling (pSEM) to investigate how grazing intensity
placed, yielding 36 quadrats. At the site of the moderate grazing, a 4-km transect was laid out from the east to west, and 1-m² quadrats at 200-m intervals were systematically installed, resulting in 21 quadrats. The light-grazing (enclosure) site was 400 m×500 m. A 400-m transect was laid out to sample the vegetation, covering the periphery from the east to west of the enclosure. 21 quadrats of 1-m² each were placed along this transect at 20-m intervals. The no-grazing site was 80 m×40 m. In the 80-m long direction, we established three parallel transects separated by 10-m intervals. Within these transects, 1-m² quadrats at 10-m intervals were placed, yielding 21 quadrats.

## Data

From each of the 1-m² quadrats, data on species composition and richness, ground cover and biomass were collected. The measurements were taken in September 2022 when the vegetation was at its peak flowering stage. Species composition was estimated by rating the percentage of each species abundance in the quadrats and assigned them to one of two growth forms: forbs or graminoids (henceforth 'grasses'). We did not subdivide forbs into nitrogen-fixing and non-fixing species because of the low number of nitrogen-fixing species in the area. Then, the proportion of all forbs and grass species dominance was summed up for each quadrat across the sites to estimate forbs and grass cover separately. The proportion of soil surface covered by herbaceous plants (vegetation ground cover) was also estimated visually for each quadrat. Species richness was determined as the total number of species encountered in the quadrats. The diversity of species was computed using the Shannon–Wiener index ($H'$) calculated following Krebs (1999):

$$H' = - \sum_{i=1}^{s} pi \ln pi$$

where s = number of species; $p_i$ = proportion of individuals or abundance of the $i^{th}$ species; and ln is the natural logarithm to the base e.

The aboveground herbaceous biomass was estimated by harvesting live and dead material at ground level from each of the quadrats and all the 99 quadrats across the four sites. The harvested samples were weighed in the field to get fresh weight. Thirty percent of the harvested samples from each quadrat were placed in a paper bag for later dry matter analysis. This harvested biomass was dried in an oven at 105 °C for 48 h, and then weighed to obtain the dry matter. Then, the total dry biomass in each quadrat was calculated by multiplying the proportion of each dried sample biomass by the weight of the total fresh biomass.

## Statistical analyses

All analyses were performed with the R Statistical Software version 4.2.2 (R Core Team 2022). We determined the impacts of grazing intensity on species richness and composition using species accumulation curve and multivariate analyses. The grass and forb diversity was quantified based on the species × quadrat abundance matrix, the estimated species richness using the *accuncomp* function of the *Biodiversity R* package (Kindt and Coe 2005). A sample-based rarefaction procedure was used to estimate the 95% confidence intervals and compare the patterns of plant richness among grazing intensities. To determine the impact of grazing on the species similarity, we performed an Analysis of Similarities (ANOSIM) test using the “anosim” function in the *vegan* package (Oksanen et al. 2022). ANOSIM is a non-parametric test of significant difference between two or more groups, based on any distance measure (Clarke and Ainsworth 1993), and used for taxa-in-sample data, where groups of samples are to be compared. Then, a non-metric multidimensional scaling (NMDS) was used to group plots with similar species into separate classes using the Bray–Curtis dissimilarity matrix. The NMDS analysis was performed using the *metaMDS* function in the *vegan* package (Oksanen et al. 2022). Stress value was used as a criterion of efficiency, where stress is the departure from monotonicity in the plot of distance in the original p-dimensional space (dissimilarity) vs. distance in the ordination space (k-dimensional space) (Fasham 1977). A rule of thumb is that stress < 0.05 provides an excellent representation in reduced dimensions, < 0.1 is great, < 0.2 is good/ok, and stress > 0.3 provides a poor representation (Clarke 1993). Moreover, species composition collected from the quadrats was averaged for each of the four grazing areas to determine the relative dominance of each species of the four grazing sites.

We used General Linear Models (GLM) to test for significant ($p < 0.05$) effects of grazing intensities on forb-, grass- and total percentage cover, Shannon diversity and standing aboveground biomass. Specifically, GLM with family binomial was used for forb-, grass- and total percentage cover modeled as percentage data (Zuur et al. 2009), while the Gaussian family was used for Shannon diversity and total biomass due to the normality of their distribution. Mean values and standard errors were represented graphically for better visibility. In addition, we tested for the significance of grazing intensity-dependent effects of Shannon diversity on biomass. Specifically, we assessed both the main and interaction effects of the grazing intensity with Shannon diversity, using linear models (biomass ~ grazing intensity × Shannon diversity).

Finally, we used piecewise Structural Equation Modelling (pSEM) to investigate how grazing intensity
<figure><img src="image_4.png" /><figcaption>**Fig. 2** Species accumulation curves showing for each grazing intensity the expected number of species as a function of sampled m²</figcaption></figure>

influenced biomass directly and indirectly through species diversity and vegetation ground cover. We used pSEM because it can accommodate a variety of model structures and assumptions on the response variables (Lefcheck 2016). Specifically, we first assessed how grazing intensity influenced vegetation ground cover, species diversity and biomass. Next, we evaluated the direct influence of ground cover and species diversity on biomass. In the pSEM grazing intensity was analysed as an ordinal categorical variable recoded as 0 for no grazing, 1 for light grazing, 2 for moderate grazing and 3 for high grazing intensity. The pSEM was fitted using pSEM function in the piecewiseSEM package (Lefcheck

2016). The overall fit of the pSEM was assessed based on the Fisher's C statistic and associated *p* value (Lefcheck 2016).

## Results

### Species richness and composition

A total of 48 herbaceous species from 17 families were identified in the study area. The higher numbers of species were observed in the no and light grazing sites than in the moderate and high grazing sites (Fig. 2).

The herbaceous plant community composition varied significantly (ANOSIM: *R*=0.46, permutations=999, *p*<0.001) among sites with different grazing intensity (Fig. 3). The NMDS ordination revealed that plant community composition changed markedly with decreasing grazing intensity (half change = 1.16, stress value = 0.18, and *R*²=0.97; Fig. 3b).

Notably, about 85% of the sites in the moderate and high grazing and 80% of the site in the light grazing were dominated by the forb species of *Diodella sarmentosa*. However, in the no grazing site, *D. sarmentosa* was not found, while grass species such as *Cenchrus biflorus* and *Chloris prieurii* were dominant at 28% and 33%, respectively (Additional file 1: Table S1).

### Herbaceous ground cover, diversity and biomass

Percent of vegetation ground cover was significantly (Chi-square = 80.6, *p* < 0.001; Table 1) different between the different grazing intensities. The vegetation total cover and grass cover decreased with increasing grazing intensity (Fig. 4). On the contrary, forb cover was

<figure><img src="image_5.png" /><figcaption>**Fig. 3** Ordination plot **a** of the plant community composition obtained from the non-metric multidimensional scaling analysis (NMDS) based on Bray–Curtis dissimilarities, and **b** stress plot of the NMDS</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2** Species accumulation curves showing for each grazing intensity the expected number of species as a function of sampled m²</figcaption></figure>

influenced biomass directly and indirectly through species diversity and vegetation ground cover. We used pSEM because it can accommodate a variety of model structures and assumptions on the response variables (Lefcheck 2016). Specifically, we first assessed how grazing intensity influenced vegetation ground cover, species diversity and biomass. Next, we evaluated the direct influence of ground cover and species diversity on biomass. In the pSEM grazing intensity was analysed as an ordinal categorical variable recoded as 0 for no grazing, 1 for light grazing, 2 for moderate grazing and 3 for high grazing intensity. The pSEM was fitted using pSEM function in the piecewiseSEM package (Lefcheck

2016). The overall fit of the pSEM was assessed based on the Fisher's C statistic and associated *p* value (Lefcheck 2016).

## Results

### Species richness and composition

A total of 48 herbaceous species from 17 families were identified in the study area. The higher numbers of species were observed in the no and light grazing sites than in the moderate and high grazing sites (Fig. 2).

The herbaceous plant community composition varied significantly (ANOSIM: *R*=0.46, permutations=999, *p*<0.001) among sites with different grazing intensity (Fig. 3). The NMDS ordination revealed that plant community composition changed markedly with decreasing grazing intensity (half change = 1.16, stress value = 0.18, and *R*²=0.97; Fig. 3b).

Notably, about 85% of the sites in the moderate and high grazing and 80% of the site in the light grazing were dominated by the forb species of *Diodella sarmentosa*. However, in the no grazing site, *D. sarmentosa* was not found, while grass species such as *Cenchrus biflorus* and *Chloris prieurii* were dominant at 28% and 33%, respectively (Additional file 1: Table S1).

### Herbaceous ground cover, diversity and biomass

Percent of vegetation ground cover was significantly (Chi-square = 80.6, *p* < 0.001; Table 1) different between the different grazing intensities. The vegetation total cover and grass cover decreased with increasing grazing intensity (Fig. 4). On the contrary, forb cover was

<figure><img src="image_5.png" /><figcaption>**Fig. 3** Ordination plot **a** of the plant community composition obtained from the non-metric multidimensional scaling analysis (NMDS) based on Bray–Curtis dissimilarities, and **b** stress plot of the NMDS</figcaption></figure>
<table><caption>**Table 1** Results of general linear models testing the effects of grazing intensities on forb, grass and total percentage cover, Shannon diversity and total biomass</caption><thead><tr><th></th><th>Df</th><th>Deviance/Sum Sq</th><th>Mean Sq</th><th>LR Chisq/F value</th><th>p value</th></tr></thead><tbody><tr><td>Percentage of forb cover</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>50.11</td><td></td><td>420.3</td><td>&lt;0.001</td></tr><tr><td>Residuals</td><td></td><td>9.58</td><td></td><td></td><td></td></tr><tr><td>Percentage of grass cover</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>50.21</td><td></td><td>437.1</td><td>&lt;0.001</td></tr><tr><td>Residuals</td><td></td><td>9.26</td><td></td><td></td><td></td></tr><tr><td>Total percentage cover</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>17.23</td><td></td><td>80.6</td><td>&lt;0.001</td></tr><tr><td>Residuals</td><td></td><td>9.50</td><td></td><td></td><td></td></tr><tr><td>Shannon diversity</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>7.43</td><td>2.48</td><td>20.79</td><td>&lt;0.001</td></tr><tr><td>Residuals</td><td>95</td><td>11.31</td><td>0.12</td><td></td><td></td></tr><tr><td>Shapiro-Wilk test (W=0.98; p=0.525)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Total biomass</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>9.59</td><td>3.19</td><td>23.61</td><td>&lt;0.001</td></tr><tr><td>Residuals</td><td>95</td><td>12.88</td><td>0.14</td><td></td><td></td></tr><tr><td>Shapiro-Wilk test (W=0.97; p=0.105)</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Df: degrees of freedom; sum Sq: sum of squares; Mean Sq: mean square; LR Chisq: likelihood ratio Chi-square

significantly lower in the no grazing sites than in the grazing sites (Table 1; Fig. 4).

Species diversity (Shannon diversity) was also significantly ($F=20.79, df=3, p<0.001$) different among grazing intensities. Significantly higher species diversity was found in the low compared to the high grazing intensity plots (Fig. 4), also corroborating the results of species accumulation curves. Total biomass varied significantly among grazing intensities, with higher values on no grazing and light grazing sites and lowest values on moderate and high grazing sites (Table 1; Fig. 4).

### Main and interaction effects of grazing and diversity on herbaceous biomass

Herbaceous total biomass was significantly related to diversity ($p<0.001$) and grazing intensity ($p<0.001$). However, the interaction effects of diversity and grazing intensity were not significant (Table 2). Results further showed a positive relationship of species diversity with total herbaceous biomass across all grazing sites (Fig. 5).

### Direct and indirect influence of grazing intensity on total herbaceous biomass

The piecewiseSEM explained 37% of the variance in total biomass and showed a good fit to the data ($p>0.05$; Fig. 6). In terms of the direct effects, increasing grazing intensity significantly decreased species diversity, vegetation cover and total biomass (Table 3; Fig. 6). However,

unlike vegetation cover, increasing species diversity was associated with higher total herbaceous biomass (Fig. 6). Grazing intensity also influenced total herbaceous biomass indirectly through its negative effect on species diversity ($\beta=-0.56 \times 0.21 = -0.12$; Table 3; Fig. 6).

### Discussion

We found that the species richness accumulations for no, light, moderate, and high grazing intensities were 31, 30, 22, and 13, respectively (Fig. 2). The sites considered as high and moderate grazing in this study severely impede the regenerative ability of grass species and result in a decline in species richness possibly due to the local extinction of native species. Higher number of grass species were found in the no-grazing and light-grazing sites, but abundance was low in the light-grazing sites (Additional file 1: Table S1). The low abundance of grass species in the light grazing site might be related to the deficiency of viable soil seed banks. It has been suggested that the recovery of vegetation after the removal of grazing depends on several factors, including the availability of soil seed banks (Gebregergs et al. 2019).

Moreover, the species composition showed a clear relationship with grazing intensity (Additional file 1: Table S1; Fig. 3). The ability of a species to persist in a grazing environment is a result of both grazing resistance through tolerance and avoidance (Zainelabdeen
<table><caption>**Table 1** Results of general linear models testing the effects of grazing intensities on forb, grass and total percentage cover, Shannon diversity and total biomass</caption><thead><tr><th></th><th>Df</th><th>Deviance/Sum Sq</th><th>Mean Sq</th><th>LR Chisq/F value</th><th>p value</th></tr></thead><tbody><tr><td>Percentage of forb cover</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>50.11</td><td></td><td>420.3</td><td>&lt; 0.001</td></tr><tr><td>Residuals</td><td></td><td>9.58</td><td></td><td></td><td></td></tr><tr><td>Percentage of grass cover</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>50.21</td><td></td><td>437.1</td><td>&lt; 0.001</td></tr><tr><td>Residuals</td><td></td><td>9.26</td><td></td><td></td><td></td></tr><tr><td>Total percentage cover</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>17.23</td><td></td><td>80.6</td><td>&lt; 0.001</td></tr><tr><td>Residuals</td><td></td><td>9.50</td><td></td><td></td><td></td></tr><tr><td>Shannon diversity</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>7.43</td><td>2.48</td><td>20.79</td><td>&lt; 0.001</td></tr><tr><td>Residuals</td><td>95</td><td>11.31</td><td>0.12</td><td></td><td></td></tr><tr><td>Shapiro-Wilk test (W = 0.98; p = 0.525)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Total biomass</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Grazing intensity</td><td>3</td><td>9.59</td><td>3.19</td><td>23.61</td><td>&lt; 0.001</td></tr><tr><td>Residuals</td><td>95</td><td>12.88</td><td>0.14</td><td></td><td></td></tr><tr><td>Shapiro-Wilk test (W = 0.97; p = 0.105)</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Df: degrees of freedom; sum Sq: sum of squares; Mean Sq: mean square; LR Chisq: likelihood ratio Chi-square

significantly lower in the no grazing sites than in the grazing sites (Table 1; Fig. 4).

Species diversity (Shannon diversity) was also significantly ($F=20.79, df=3, p<0.001$) different among grazing intensities. Significantly higher species diversity was found in the low compared to the high grazing intensity plots (Fig. 4), also corroborating the results of species accumulation curves. Total biomass varied significantly among grazing intensities, with higher values on no grazing and light grazing sites and lowest values on moderate and high grazing sites (Table 1; Fig. 4).

### Main and interaction effects of grazing and diversity on herbaceous biomass

Herbaceous total biomass was significantly related to diversity ($p<0.001$) and grazing intensity ($p<0.001$). However, the interaction effects of diversity and grazing intensity were not significant (Table 2). Results further showed a positive relationship of species diversity with total herbaceous biomass across all grazing sites (Fig. 5).

### Direct and indirect influence of grazing intensity on total herbaceous biomass

The piecewiseSEM explained 37% of the variance in total biomass and showed a good fit to the data ($p>0.05$; Fig. 6). In terms of the direct effects, increasing grazing intensity significantly decreased species diversity, vegetation cover and total biomass (Table 3; Fig. 6). However,

unlike vegetation cover, increasing species diversity was associated with higher total herbaceous biomass (Fig. 6). Grazing intensity also influenced total herbaceous biomass indirectly through its negative effect on species diversity ($\beta=-0.56 \times 0.21 = -0.12$; Table 3; Fig. 6).

### Discussion

We found that the species richness accumulations for no, light, moderate, and high grazing intensities were 31, 30, 22, and 13, respectively (Fig. 2). The sites considered as high and moderate grazing in this study severely impede the regenerative ability of grass species and result in a decline in species richness possibly due to the local extinction of native species. Higher number of grass species were found in the no-grazing and light-grazing sites, but abundance was low in the light-grazing sites (Additional file 1: Table S1). The low abundance of grass species in the light grazing site might be related to the deficiency of viable soil seed banks. It has been suggested that the recovery of vegetation after the removal of grazing depends on several factors, including the availability of soil seed banks (Gebregergs et al. 2019).

Moreover, the species composition showed a clear relationship with grazing intensity (Additional file 1: Table S1; Fig. 3). The ability of a species to persist in a grazing environment is a result of both grazing resistance through tolerance and avoidance (Zainelabdeen
<figure><img src="image_6.png" /><figcaption>**Fig. 4** Barplots showing means ± standard error of forb, grass and total vegetation cover, total biomass and Shannon diversity. The letters denote comparison between grazing intensity levels</figcaption></figure>

et al. 2020), as can also be seen in the finding that about 85% of the high and moderate grazing and 80% of the light grazing areas were dominated by *Diodella sarmen-tosa* (Additional file 1: Table S1). This species was not identified during the period 1964 to 2011 (Ndiaye et al. 2015b), indicating that it aggressively invaded the study area in recent years suppressing the formerly dominant native species of the area. *D. sarmentosa* produces a large number of viable seeds, hairy leaf surface, and delayed elevation of growing points as mechanisms for grazing tolerance and avoidance.

<table><caption>**Table 2** Analysis of variance resulting from the linear model testing for effects of grazing intensity and Shannon diversity on total herbaceous biomass</caption><thead><tr><th>Total herbaceous biomass</th><th>Df</th><th>F value</th><th>Pr (&gt; F)</th></tr></thead><tbody><tr><td>Shannon diversity</td><td>1</td><td>37.14</td><td>&lt; 0.001</td></tr><tr><td>Grazing intensity</td><td>3</td><td>11.74</td><td>&lt; 0.001</td></tr><tr><td>Grazing intensity × Shannon diversity</td><td>3</td><td>0.87</td><td>0.462</td></tr><tr><td>Residuals</td><td>91</td><td></td><td></td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Fig. 5** Scatterplot showing the effect of species diversity (Shannon diversity) on herbaceous biomass across all grazing sites</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4** Barplots showing means ± standard error of forb, grass and total vegetation cover, total biomass and Shannon diversity. The letters denote comparison between grazing intensity levels</figcaption></figure>

et al. 2020), as can also be seen in the finding that about 85% of the high and moderate grazing and 80% of the light grazing areas were dominated by *Diodella sarmen-tosa* (Additional file 1: Table S1). This species was not identified during the period 1964 to 2011 (Ndiaye et al. 2015b), indicating that it aggressively invaded the study area in recent years suppressing the formerly dominant native species of the area. *D. sarmentosa* produces a large number of viable seeds, hairy leaf surface, and delayed elevation of growing points as mechanisms for grazing tolerance and avoidance.

<table><caption>**Table 2** Analysis of variance resulting from the linear model testing for effects of grazing intensity and Shannon diversity on total herbaceous biomass</caption><thead><tr><th>Total herbaceous biomass</th><th>Df</th><th>F value</th><th>Pr (&gt; F)</th></tr></thead><tbody><tr><td>Shannon diversity</td><td>1</td><td>37.14</td><td>&lt; 0.001</td></tr><tr><td>Grazing intensity</td><td>3</td><td>11.74</td><td>&lt; 0.001</td></tr><tr><td>Grazing intensity × Shannon diversity</td><td>3</td><td>0.87</td><td>0.462</td></tr><tr><td>Residuals</td><td>91</td><td></td><td></td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Fig. 5** Scatterplot showing the effect of species diversity (Shannon diversity) on herbaceous biomass across all grazing sites</figcaption></figure>
<figure><img src="image_8.png" /></figure>

DF=2; Fisher's C = 0.714 with P-value = 0.700

**Fig. 6** Graphical representation of the piecewise structural equation model showing the inter-related pathways between grazing intensity, species diversity (Shannon diversity), vegetation cover and total herbaceous biomass. Arrows are the hypothesized causal paths. The values next to the arrows are the standardized path coefficients and their significance is given in Table 3. Paths with dark-orange colour stand for negative effects, whereas paths in turquoise colour indicate positive effects. Non-significant effects are shown with dashed arrows. *df* degree of freedom; ns: *p* > 0.05; \* *p* < 0.05; \*\*\* *p* < 0.001

However, plants that invest heavily in defence may not be as competitive as plants that invest little in such mechanisms under low grazing (Trlica and Rittenhouse 1993). In line with this, *D. sarmentosa* was not observed in the no-grazing site; instead, the grass species such as *Cenchrus biflorus* and *Chloris prieurii* were the most dominant. These results collectively imply that dominance of plant species depends on a trade-off between grazing avoidance and competitive ability. Moreover, *Senna obtusifolia* is an invasive forb species recently introduced to the study area, but its distribution is still low (Additional file 1: Table S1). Studies demonstrated that *S. obtusifolia* could completely dominate grass species, reducing pasture growth and excluding stock (Dunlop et al. 2006; Gebrekiros and Tessema 2018). Hence, proper management that restricts the expansion of this species is required before the natural ecosystems in the Sahel are under threat of invasion.

Studies have shown that herbaceous plants with prostrate growth form might have a competitive advantage, as they can better escape frequent grazing and shading by the tall grass species (Noy-Meir et al. 1989; Hirata et al. 2010). In the current study, *Zornia glochidiata* with prostrate growth form was the second most

<table><caption>**Table 3** Significance of the piecewiseSEM paths testing the inter-relations between grazing intensity, species diversity (Shannon diversity), vegetation cover and total biomass</caption><thead><tr><th>Response variable</th><th>Predictor</th><th>Standardized estimate</th><th>p value</th></tr></thead><tbody><tr><td>Shannon diversity</td><td>Grazing intensity</td><td>-0.560</td><td>0.000</td></tr><tr><td>Percentage cover</td><td>Grazing intensity</td><td>-0.301</td><td>0.024</td></tr><tr><td>Total biomass</td><td>Shannon diversity</td><td>0.213</td><td>0.033</td></tr><tr><td>Total biomass</td><td>Percentage cover</td><td>0.002</td><td>0.982</td></tr><tr><td>Total biomass</td><td>Grazing intensity</td><td>-0.464</td><td>0.000</td></tr></tbody></table>

dominant species at the sites with high and moderate grazing pressure. Moreover, previous studies showed that the species with prostrate growth form, such as *Dactiloterium aegyptium* and *Z. glochidiata*, were the most dominant in the current grazing sites (Ndiaye et al. 2015b; Tagesson et al. 2015). This indicated that in the absence of the invasive species of *D. sarmentosa*, the native species with prostrate growth form could have a better competitive ability for grazing pressure and the harsh environmental conditions in the Sahelian savannah ecosystem.

Overall, grazing had a significant impact on the segregation of vegetation cover of the rangeland. All the grazing sites were dominated by forb species, whereas the site without grazing had a dominant grass cover (Fig. 4). This suggests that certain forb species benefit from reduced competition with grass and were able to colonize the grazed areas. The decline in the cover of highly palatable grass species observed is also consistent with findings from other semi-arid ecosystems (Sanaei et al. 2021). As indicated by Gemedo-Dalle et al. (2006) the decrease in grass species with increasing grazing pressure might be an indicator of the deteriorating conditions of the rangelands. The total vegetation ground cover decreased with the increase in grazing intensity. This finding aligns with the results reported by Sternberg et al. (2000), who observed a decrease in vegetation ground cover with higher grazing intensity in the Mediterranean herbaceous community.

Species diversity was significantly higher in the no-grazing site than in the light-grazing site (Fig. 4). Moreover, higher species diversity was recorded in the light-grazing site than in the moderate and high-grazing sites. This result also complements previous findings that areas protected from grazing yield higher species diversity compared with continuously grazed sites (Miehe et al. 2010). The intermediate disturbance hypothesis suggests that species diversity is expected to be highest at intermediate levels of disturbance and decline at low or high levels of disturbance (Connell
<figure><img src="image_8.png" /></figure>

DF=2; Fisher's C = 0.714 with P-value = 0.700

**Fig. 6** Graphical representation of the piecewise structural equation model showing the inter-related pathways between grazing intensity, species diversity (Shannon diversity), vegetation cover and total herbaceous biomass. Arrows are the hypothesized causal paths. The values next to the arrows are the standardized path coefficients and their significance is given in Table 3. Paths with dark-orange colour stand for negative effects, whereas paths in turquoise colour indicate positive effects. Non-significant effects are shown with dashed arrows. *df* degree of freedom; ns: *p* > 0.05; \* *p* < 0.05; \*\*\* *p* < 0.001

However, plants that invest heavily in defence may not be as competitive as plants that invest little in such mechanisms under low grazing (Trlica and Rittenhouse 1993). In line with this, *D. sarmentosa* was not observed in the no-grazing site; instead, the grass species such as *Cenchrus biflorus* and *Chloris prieurii* were the most dominant. These results collectively imply that dominance of plant species depends on a trade-off between grazing avoidance and competitive ability. Moreover, *Senna obtusifolia* is an invasive forb species recently introduced to the study area, but its distribution is still low (Additional file 1: Table S1). Studies demonstrated that *S. obtusifolia* could completely dominate grass species, reducing pasture growth and excluding stock (Dunlop et al. 2006; Gebrekiros and Tessema 2018). Hence, proper management that restricts the expansion of this species is required before the natural ecosystems in the Sahel are under threat of invasion.

Studies have shown that herbaceous plants with prostrate growth form might have a competitive advantage, as they can better escape frequent grazing and shading by the tall grass species (Noy-Meir et al. 1989; Hirata et al. 2010). In the current study, *Zornia glochidiata* with prostrate growth form was the second most

<table><caption>**Table 3** Significance of the piecewiseSEM paths testing the inter-relations between grazing intensity, species diversity (Shannon diversity), vegetation cover and total biomass</caption><thead><tr><th>Response variable</th><th>Predictor</th><th>Standardized estimate</th><th>p value</th></tr></thead><tbody><tr><td>Shannon diversity</td><td>Grazing intensity</td><td>-0.560</td><td>0.000</td></tr><tr><td>Percentage cover</td><td>Grazing intensity</td><td>-0.301</td><td>0.024</td></tr><tr><td>Total biomass</td><td>Shannon diversity</td><td>0.213</td><td>0.033</td></tr><tr><td>Total biomass</td><td>Percentage cover</td><td>0.002</td><td>0.982</td></tr><tr><td>Total biomass</td><td>Grazing intensity</td><td>-0.464</td><td>0.000</td></tr></tbody></table>

dominant species at the sites with high and moderate grazing pressure. Moreover, previous studies showed that the species with prostrate growth form, such as *Dactiloterium aegyptium* and *Z. glochidiata*, were the most dominant in the current grazing sites (Ndiaye et al. 2015b; Tagesson et al. 2015). This indicated that in the absence of the invasive species of *D. sarmentosa*, the native species with prostrate growth form could have a better competitive ability for grazing pressure and the harsh environmental conditions in the Sahelian savannah ecosystem.

Overall, grazing had a significant impact on the segregation of vegetation cover of the rangeland. All the grazing sites were dominated by forb species, whereas the site without grazing had a dominant grass cover (Fig. 4). This suggests that certain forb species benefit from reduced competition with grass and were able to colonize the grazed areas. The decline in the cover of highly palatable grass species observed is also consistent with findings from other semi-arid ecosystems (Sanaei et al. 2021). As indicated by Gemedo-Dalle et al. (2006) the decrease in grass species with increasing grazing pressure might be an indicator of the deteriorating conditions of the rangelands. The total vegetation ground cover decreased with the increase in grazing intensity. This finding aligns with the results reported by Sternberg et al. (2000), who observed a decrease in vegetation ground cover with higher grazing intensity in the Mediterranean herbaceous community.

Species diversity was significantly higher in the no-grazing site than in the light-grazing site (Fig. 4). Moreover, higher species diversity was recorded in the light-grazing site than in the moderate and high-grazing sites. This result also complements previous findings that areas protected from grazing yield higher species diversity compared with continuously grazed sites (Miehe et al. 2010). The intermediate disturbance hypothesis suggests that species diversity is expected to be highest at intermediate levels of disturbance and decline at low or high levels of disturbance (Connell
1978). However, in this study, high species diversity was observed in the low disturbance, no-grazing site and increasing species diversity was associated with higher total biomass. This finding deviates from the expectations of the intermediate disturbance hypothesis. A possible explanation could be that the medium grazing intensity area is most likely also relatively highly grazed, even though being less than the highly grazing site.

The herbaceous biomass in the high-grazing sites was twice lower than the no-grazing sites. In line with this study, Biondini et al. (1998) found that grazing pressures lead to a removal of 50% of vegetation productivity. In contrast, Tagesson et al. (2016) reported that years with high grazing had higher vegetation productivity than years with low grazing. The increase in herbaceous biomass in light and no grazing sites could be linked to the reduction in grazing pressure and subsequent accumulation of soil organic matter during the resting period (Noulèkoun et al. 2021a; Gebremedhn et al. 2022). In line with this study, there exists a possibility for increased biomass under wet-season resting periods (Ash et al. 2011) and less or no grazing (Mekuria et al. 2018).

While both diversity and biomass declined with increasing grazing intensity, there was little evidence for significant interaction effects of grazing intensity and species diversity on biomass, which suggests that diversity effect on herbaceous biomass did not depend on grazing intensity. A plausible explanation for this finding is the proportionate response of diversity and biomass to grazing intensity (Fig. 4), but the idiosyncratic patterns of forb and grass response to increasing grazing could also partly play an active role. In particular, we argued that the dominance of grasses contributed to the diversity effects on biomass in the no grazing sites while dominance of forbs seemed to contribute to the diversity effects on biomass in higher grazing sites.

The bivariate analysis of species diversity and herbaceous biomass indicated that, across all grazing sites, diversity had a positive effect on biomass. The piece-wiseSEM revealed that increasing grazing intensity had direct effects on species diversity, vegetation ground cover, and total herbaceous biomass. In addition, grazing intensity had an indirect negative influence on total herbaceous biomass through its negative impact on species diversity. These results corroborate the insights that grazing intensity plays a crucial role in shaping these ecological variables. The positive effect of diversity on herbaceous biomass is reminiscent of recent findings, and has repeatedly been explained by mechanisms, such as complementarity and dominance effects as diversity increases (Noulèkoun et al. 2021b). Specifically, increase in biomass production with increasing species diversity is attributed to the fact that species-rich ecosystems

experience high-level resource-use complementarity and facilitation by co-occurring species. This is possibly the case in the no-grazing sites which had high number of grass species (> 60% cover) but also > 30% coverage in forb species. On the contrary, high occurrence of forb species (e.g., *Diodella sarmentosa*) in the moderately and heavily grazed sites reflect the dominance effects facilitated by grazing activities, but also some reduced competition with grass, enabling these forbs to colonize grazed areas and increase biomass production.

## Conclusions

The results of the study indicate that there were significant differences in herbaceous species richness and composition among the plots with varying grazing pressure. The plant species composition shifted from a dominance of grasses in no grazing sites to a dominance of forbs in higher grazing sites. Plots with low grazing pressure had higher species diversity, herbaceous biomass, and ground cover compared to plots with moderately and highly grazing pressure. Species diversity was found to be significantly related to total herbaceous biomass across all sites. However, diversity effect on biomass did not vary between grazing intensities, possibly due the dominance of grass species in no-grazing site, and of forb species on sites with higher grazing intensity. Grazing intensity also had a negative indirect influence on total biomass through its negative influence on species diversity. Based on these findings, it can be concluded that managing grazing intensity can result in higher plant productivity and promote the establishment of mixed forage in the dryland savannah ecosystem of the Sahel. This information is valuable for guiding land management strategies and promoting sustainable grazing practices that consider both livestock needs and the conservation of the ecosystem health and biodiversity.

## Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s13717-023-00468-3.

Additional file 1: Contains Figure S1 and Table S1. Figure S1. Pictures of field sites in dry and rainy season at Dahra savannah ecosystem of Senegal. Table S1. Herbaceous species origin, growth form and their composition (%) under the four different grazing intensities in savannah ecosystem of Sahel (NG = No grazing; LG = light grazing, MG = moderate grazing; HG = High grazing). Intermediate falls between prostrate and erect growth forms; Climbing means twining growth form or using surrounding structures for support.

## Acknowledgements

HHG and SM are grateful to the UK Research and Innovation (UKRI) through the Global Challenges Research Fund (GCRF) program, Grant Ref: ES/P011306, implemented by the Regional Universities Forum for Capacity Building in Agriculture (RUFORUM), which enabled further international collaborations. The authors would also like to acknowledge the reviewers and editor for their comments, which helped improve the manuscript greatly.
1978). However, in this study, high species diversity was observed in the low disturbance, no-grazing site and increasing species diversity was associated with higher total biomass. This finding deviates from the expectations of the intermediate disturbance hypothesis. A possible explanation could be that the medium grazing intensity area is most likely also relatively highly grazed, even though being less than the highly grazing site.

The herbaceous biomass in the high-grazing sites was twice lower than the no-grazing sites. In line with this study, Biondini et al. (1998) found that grazing pressures lead to a removal of 50% of vegetation productivity. In contrast, Tagesson et al. (2016) reported that years with high grazing had higher vegetation productivity than years with low grazing. The increase in herbaceous biomass in light and no grazing sites could be linked to the reduction in grazing pressure and subsequent accumulation of soil organic matter during the resting period (Noulèkoun et al. 2021a; Gebremedhn et al. 2022). In line with this study, there exists a possibility for increased biomass under wet-season resting periods (Ash et al. 2011) and less or no grazing (Mekuria et al. 2018).

While both diversity and biomass declined with increasing grazing intensity, there was little evidence for significant interaction effects of grazing intensity and species diversity on biomass, which suggests that diversity effect on herbaceous biomass did not depend on grazing intensity. A plausible explanation for this finding is the proportionate response of diversity and biomass to grazing intensity (Fig. 4), but the idiosyncratic patterns of forb and grass response to increasing grazing could also partly play an active role. In particular, we argued that the dominance of grasses contributed to the diversity effects on biomass in the no grazing sites while dominance of forbs seemed to contribute to the diversity effects on biomass in higher grazing sites.

The bivariate analysis of species diversity and herbaceous biomass indicated that, across all grazing sites, diversity had a positive effect on biomass. The piece-wiseSEM revealed that increasing grazing intensity had direct effects on species diversity, vegetation ground cover, and total herbaceous biomass. In addition, grazing intensity had an indirect negative influence on total herbaceous biomass through its negative impact on species diversity. These results corroborate the insights that grazing intensity plays a crucial role in shaping these ecological variables. The positive effect of diversity on herbaceous biomass is reminiscent of recent findings, and has repeatedly been explained by mechanisms, such as complementarity and dominance effects as diversity increases (Noulèkoun et al. 2021b). Specifically, increase in biomass production with increasing species diversity is attributed to the fact that species-rich ecosystems

experience high-level resource-use complementarity and facilitation by co-occurring species. This is possibly the case in the no-grazing sites which had high number of grass species (> 60% cover) but also > 30% coverage in forb species. On the contrary, high occurrence of forb species (e.g., *Diodella sarmentosa*) in the moderately and heavily grazed sites reflect the dominance effects facilitated by grazing activities, but also some reduced competition with grass, enabling these forbs to colonize grazed areas and increase biomass production.

## Conclusions

The results of the study indicate that there were significant differences in herbaceous species richness and composition among the plots with varying grazing pressure. The plant species composition shifted from a dominance of grasses in no grazing sites to a dominance of forbs in higher grazing sites. Plots with low grazing pressure had higher species diversity, herbaceous biomass, and ground cover compared to plots with moderately and highly grazing pressure. Species diversity was found to be significantly related to total herbaceous biomass across all sites. However, diversity effect on biomass did not vary between grazing intensities, possibly due the dominance of grass species in no-grazing site, and of forb species on sites with higher grazing intensity. Grazing intensity also had a negative indirect influence on total biomass through its negative influence on species diversity. Based on these findings, it can be concluded that managing grazing intensity can result in higher plant productivity and promote the establishment of mixed forage in the dryland savannah ecosystem of the Sahel. This information is valuable for guiding land management strategies and promoting sustainable grazing practices that consider both livestock needs and the conservation of the ecosystem health and biodiversity.

## Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s13717-023-00468-3.

**Additional file 1: Contains Figure S1 and Table S1. Figure S1.** Pictures of field sites in dry and rainy season at Dahra savannah ecosystem of Senegal. **Table S1.** Herbaceous species origin, growth form and their composition (%) under the four different grazing intensities in savannah ecosystem of Sahel (NG = No grazing; LG = light grazing, MG = moderate grazing; HG = High grazing). Intermediate falls between prostrate and erect growth forms; Climbing means twining growth form or using surrounding structures for support.

## Acknowledgements

HHG and SM are grateful to the UK Research and Innovation (UKRI) through the Global Challenges Research Fund (GCRF) program, Grant Ref: ES/P011306, implemented by the Regional Universities Forum for Capacity Building in Agriculture (RUFORUM), which enabled further international collaborations. The authors would also like to acknowledge the reviewers and editor for their comments, which helped improve the manuscript greatly.
**Author contributions**

Conceptualization, HHG; methodology, HHG; software, HHG; validation, HHG, SM and PS; formal analysis, HHG and SM; investigation, HHG, ON, CF, ST, TT and PS; data curation, HHG; writing—original draft preparation, HHG; writing—review and editing, HHG, ON, SM; CF, ST, TT and PS; project administration, HHG and PS; funding acquisition, HHG, ST and PS. All authors read and approved the final manuscript.

**Funding**

This work was funded by the New Zealand Government to support the objectives of the Global Research Alliance on Agricultural Greenhouse Gases; and the CASSECS project (Carbon Sequestration and Green-house Gas Emissions in (Agro) Sylvopastoral Ecosystems in the Sahelian CILSS States) [FOOD/2019/410-169]. Tagesson was additionally funded by the Swedish National Space Agency (SNSA 2021-00144; 2021-00111) and FORMAS (Dnr. 2021-00644).

**Availability of data and materials**

The data sets used and/or analysed during the current study are available from the corresponding author on reasonable request.

**Declarations**

**Ethics approval and consent to participate**

Not applicable.

**Consent for publication**

Not applicable.

**Competing interests**

The authors declare that they have no competing interests.

**Author details**

¹African Center of Excellence for Climate-Smart Agriculture and Biodiversity Conservation, Haramaya University, P.O. Box 138, Dire Dawa, Ethiopia. ²Dispositif en Partenariat « Pôle Pastoralisme Et Zones Sèches », Route du Front de Terre BP 2057, Pôle de Recherche de Hann, ISRA, Dakar, Senegal. ³Centre de Recherches Zootechniques de Dahra, Institut Sénégalais de Recherches Agricoles, Dakar, Senegal. ⁴Laboratoire de Biomathématiques et d'Estimations Forestières, Faculté des Sciences Agronomiques, Université d'Abomey-Calavi, Cotonou, Benin. ⁵Chair of Forest Growth and Dendroecology, Albert-Ludwigs-Universität Freiburg, Freiburg Im Breisgau, Germany. ⁶Faculté des Sciences et Techniques, Département BV, Université Cheikh-Anta-Diop, Dakar, Senegal. ⁷UMR SELMET, CIRAD, INRAE, Institut Agro Univ Montpellier, Montpellier, France. ⁸Department of Physical Geography and Ecosystem Science, Lund University, Sölvegatan 12, 223 62 Lund, Sweden. ⁹Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark.

Received: 15 June 2023 Accepted: 6 November 2023

Published online: 17 November 2023

**References**

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Amole T, Augustine A, Balehegn M, Adesogoan AT (2022) Livestock feed resources in the West African Sahel. *Agron J* 114:26–45. https://doi.org/10.1002/agj2.20955

Archer S, Pyke DA (1991) Plant-animal interactions affecting plant establishment and persistence on revegetated rangeland. *J Range Manag* 4:558–565. https://doi.org/10.2307/4003036

Ash AJ, Corfield JP, McIvor JG, Ksiksi TS (2011) Grazing management in tropical savannas: utilization and rest strategies to manipulate rangeland condition. *Rangel Ecol Manag* 64:223–239. https://doi.org/10.2111/REM-D-09-00111.1

Assouma MH, Lecomte P, Hiernaux P et al (2018) How to better account for livestock diversity and fodder seasonality in assessing the fodder intake of livestock grazing semi-arid sub-Saharan Africa rangelands. *Livest Sci* 216:16–23. https://doi.org/10.1016/j.livsci.2018.07.002

Backéus I, Rulangaranga ZK, Skoglund J (1994) Vegetation changes on formerly overgrazed hill slopes in semi-arid central Tanzania. *J Veg Sci* 5:327–336. https://doi.org/10.2307/3235856

Behnke RH (2000) Equilibrium and non-equilibrium models of livestock population dynamics in pastoral Africa: their relevance to Arctic grazing systems. *Rangifer* 20:141–152. https://doi.org/10.7557/2.20.2-3.1509

Biondini ME, Patton BD, Nyren PE (1998) Grazing intensity and ecosystem processes in a northern mixed-grass prairie, USA. *Ecol Appl* 8:469–479. https://doi.org/10.1890/1051-0761(1998)008[0469:GIAEPI]2.0.CO;2

Booker K, Huntsinger L, Bartolome JW et al (2013) What can ecological science tell us about opportunities for carbon sequestration on arid rangelands in the United States? *Glob Environ Chang* 23:240–251. https://doi.org/10.1016/j.gloenvcha.2012.10.001

Cingolani AM, Noy-Meir I, Díaz S (2005) Grazing effects on rangeland diversity: a synthesis of contemporary models. *Ecol Appl* 15:757–773. https://doi.org/10.1890/03-5272

Clarke KR (1993) Non-parametric multivariate analyses of changes in community structure. *Aust J Ecol* 18:117–143. https://doi.org/10.1111/j.1442-9993.1993.tb00438.x

Clarke KR, Ainsworth M (1993) A method of linking multivariate community structure to environmental variables. *Mar Ecol Prog Ser* 92:205–219. https://doi.org/10.3354/meps092205

Connell JH (1978) Diversity in tropical rain forests and coral reefs: high diversity of trees and corals is maintained only in a nonequilibrium state. *Science* 199:1302–1310

Derry JF, Boone RB (2010) Grazing systems are a result of equilibrium and non-equilibrium dynamics. *J Arid Environ* 74:307–309. https://doi.org/10.1016/j.jaridenv.2009.07.010

Drawe DL, Box TW (1968) Forage ratings for deer and cattle on the welder wildlife refuge. *J Range Manag* 21:225. https://doi.org/10.2307/3895819

Dunlop EA, Wilson JC, Mackey AP (2006) The potential geographic distribution of the invasive weed *Senna obtusifolia* in Australia. *Weed Res* 46:404–413. https://doi.org/10.1111/j.1365-3180.2006.00524.x

Egeru A, Magaya JP, Kuule DA et al (2020) Savannah phenological dynamics reveal spatio-temporal landscape heterogeneity in Karamoja Sub-region, Uganda. *Front Sustain Food Syst* 4:541170. https://doi.org/10.3389/fsufs.2020.541170

Fasham MJR (1977) A comparison of nonmetric multidimensional scaling, principal components and reciprocal averaging for the ordination of simulated coenoclines. *Ecology* 58:551–561

Fleischner TL (1994) Ecological costs of livestock grazing in western North America. *Conserv Biol* 8:629–644. https://doi.org/10.1046/j.1523-1739.1994.08030629.x

Fulbright TE, Drabek DJ, Ortega-S JA et al (2021) Forb standing crop response to grazing and precipitation. *Rangel Ecol Manag* 79:175–185. https://doi.org/10.1016/j.rama.2021.08.007

Gebreikros MG, Tessema ZK (2018) Effect of *Senna obtusifolia* (L.) invasion on herbaceous vegetation and soil properties of rangelands in the western Tigray, northern Ethiopia. *Ecol Process* 7:9. https://doi.org/10.1186/s13717-018-0121-0

Gebregrergs T, Tessema ZK, Solomon N, Birschke E (2019) Carbon sequestration and soil restoration potential of grazing lands under exclosure management in a semi-arid environment of northern Ethiopia. *Ecol Evol* 9:6468–6479. https://doi.org/10.1002/ece3.5223

Gebremedhn HH, Kelkay TZ, Tesfay Y et al (2022) Carbon stock and change rate under different grazing management practices in semiarid pastoral ecosystem of Eastern Ethiopia. *Land* 11:639. https://doi.org/10.3390/land11050639

Gemedo-Dalle MBL, Issestein J (2006) Rangeland condition and trend in the semi-arid Borana lowlands, southern Oromia, Ethiopia. *Afr J Range Forage Sci* 23:49–58. https://doi.org/10.2989/10220110609485886

Grillot M, Vayssières J, Guerrin F, Lecomte P (2018) Modélisation conceptuelle de la gestion adaptive de la biomasse face à l'aléa climatique en systèmes agro-sylvopastoraux. *Cah Agric* 27:55004. https://doi.org/10.1051/cagri/2018034

Haftay H, Yayneshet T, Animut G, Treydte AC (2013) Rangeland vegetation responses to traditional enclosure management in eastern Ethiopia. *Rangel J* 35:29–36
## Author contributions

Conceptualization, HHG; methodology, HHG; software, HHG; validation, HHG, SM and PS; formal analysis, HHG and SM; investigation, HHG, ON, CF, ST, TT and PS; data curation, HHG; writing—original draft preparation, HHG; writing—review and editing, HHG, ON, SM; CF, ST, TT and PS; project administration, HHG and PS; funding acquisition, HHG, ST and PS. All authors read and approved the final manuscript.

## Funding

This work was funded by the New Zealand Government to support the objectives of the Global Research Alliance on Agricultural Greenhouse Gases; and the CASSECS project (Carbon Sequestration and Green-house Gas Emissions in (Agro) Sylvopastoral Ecosystems in the Sahelian CILSS States) [FOOD/2019/410-169]. Tagesson was additionally funded by the Swedish National Space Agency (SNSA 2021-00144; 2021-00111) and FORMAS (Dnr. 2021-00644).

## Availability of data and materials

The data sets used and/or analysed during the current study are available from the corresponding author on reasonable request.

## Declarations

### Ethics approval and consent to participate

Not applicable.

### Consent for publication

Not applicable.

### Competing interests

The authors declare that they have no competing interests.

## Author details

¹African Center of Excellence for Climate-Smart Agriculture and Biodiversity Conservation, Haramaya University, P.O. Box 138, Dire Dawa, Ethiopia. ²Dispositif en Partenariat « Pôle Pastoralisme Et Zones Sèches », Route du Front de Terre BP 2057, Pôle de Recherche de Hann, ISRA, Dakar, Senegal. ³Centre de Recherches Zootechniques de Dahra, Institut Sénégalais de Recherches Agricoles, Dakar, Senegal. ⁴Laboratoire de Biomathématiques et d'Estimations Forestières, Faculté des Sciences Agronomiques, Université d'Abomey-Calavi, Cotonou, Benin. ⁵Chair of Forest Growth and Dendroecology, Albert-Ludwigs-Universität Freiburg, Freiburg Im Breisgau, Germany. ⁶Faculté des Sciences et Techniques, Département BV, Université Cheikh-Anta-Diop, Dakar, Senegal. ⁷UMR SELMET, CIRAD, INRAE, Institut Agro Univ Montpellier, Montpellier, France. ⁸Department of Physical Geography and Ecosystem Science, Lund University, Sölvegatan 12, 223 62 Lund, Sweden. ⁹Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark.

Received: 15 June 2023 Accepted: 6 November 2023

Published online: 17 November 2023

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Amole T, Augustine A, Balehegn M, Adesogoan AT (2022) Livestock feed resources in the West African Sahel. *Agron J* 114:26–45. https://doi.org/10.1002/agj2.20955

Archer S, Pyke DA (1991) Plant-animal interactions affecting plant establishment and persistence on revegetated rangeland. *J Range Manag* 4:558–565. https://doi.org/10.2307/4003036

Ash AJ, Corfield JP, McIvor JG, Ksiksi TS (2011) Grazing management in tropical savannas: utilization and rest strategies to manipulate rangeland condition. *Rangel Ecol Manag* 64:223–239. https://doi.org/10.2111/REM-D-09-00111.1

Assouma MH, Lecomte P, Hiernaux P et al (2018) How to better account for livestock diversity and fodder seasonality in assessing the fodder intake of livestock grazing semi-arid sub-Saharan Africa rangelands. *Livest Sci* 216:16–23. https://doi.org/10.1016/j.livsci.2018.07.002

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Zuur AF, Ieno EN, Walker et al (2009) Mixed effects models and extensions in ecology with R. Springer, New York, p 574. https://doi.org/10.1007/978-0-387-87458-6

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Submit your manuscript to a SpringerOpen journal and benefit from:

Convenient online submission

Rigorous peer review

Open access: articles freely available online

High visibility within the field

Retaining the copyright to your article

Submit your next manuscript at ► springeropen.com