West African Savanna ecosystems are undergoing severe changes in their vegetation composition due to the impact of human land use and changes in climatic conditions. This study aims to examine the effect of climate, land use, and their interaction on species richness and composition of West African herbaceous vegetation. Plot based vegetation sampling was done in Burkina Faso. Specific richness and diversity indices were used to determine the effect of land use, climate, and their interaction. An importance value was computed to determine herbaceous species dominating the communities. Frequency of species is used to examine their distribution pattern. The results showed that climate significantly influenced herbaceous specific richness more than land use. However, land use had a significant effect on herbaceous vegetation composition. Herbaceous species diversity changed with environmental conditions. The floristic composition of dominant species is driven by both climate and land use. The frequency of distribution demonstrated that herbaceous species occurrences were more influenced by the mixed effect of climate and land use than their separate effects. Occasional and rare species are the most important part of herbaceous vegetation. Thus heterogeneity of Savanna ecosystem and vulnerability of herbaceous species are high.
West African Savanna ecosystems are undergoing severe changes in their vegetation composition and species cover due to the impact of human land use and changes in climatic conditions. Indeed, African Savannas represent a fundamental environment for people practicing stock breeding and agriculture [
Several studies predicted dramatic changes for West Africa vegetation for the next 50 years: for example, Savannas are predicted to expand in the next few centuries at the expense of tropical forests, mainly as a result of deforestation and human fires [
Many authors studied the effect of land use and climate on West African Savannas vegetation separately. Most of these studies showed that anthropogenic activities affect woody vegetation composition [
The scope of this study is to determine the effect of climate, land use, and their interaction on herbaceous species richness and composition in Savanna vegetation. We assume that investigating these factors and their interaction may clarify which change may take place in West African Savanna herbaceous diversity. With this study we will test the following hypotheses: The specific richness or taxonomic richness of herbaceous vegetation is more driven by climate than by land use. The composition of herbaceous vegetation varies due to climate and land use conditions. The frequency of herbaceous species depends more on the interaction between climate and land use than on their individual effects.
The study was conducted along a climatic gradient in three protected areas and their surroundings used by local people for multiple purposes (fallows, grazing, collecting of nonforest product, etc.). The three study sites are representative for the phytogeographical zoning of Burkina Faso described by Guinko [ The Sylvopatoral Reserve and Partial Fauna Reserve of Sahel: it is a protected area of IUCN category VI [ The Northern part of National W Biosphere Reserve: W Biosphere Reserve is a protected area of IUCN category II [ The Southern part of Classified Forest and Partial Fauna Reserve of Pama: Pama Reserve is a protected area of IUCN category IV [
We distinguished between two types of land use (communal area and protected area); in “protected areas” all human activities are strictly prohibited, while in “communal areas” all kinds of human land use activities are allowed (like grazing, agriculture, and harvest of nontimber forest products).
Study sites.
Per study site, two contrasting land use types were compared: protected and communal areas. Per land use type and study site, herbaceous vegetation composition was sampled during the rainy season (September to November) on plots of 10 m × 10 m. Additionally, we distinguished between four different habitats (dry, bowal, fresh, and wet). For each habitat 10 plots were sampled. Thus, vegetation sampling resulted in 240 plots (2 land use types × 4 habitats × 3 study areas × 10 repetitions). Per plot the percentage of cover occupied by each herbaceous species was visually estimated using the method of Braun-Blanquet [
Specific richness or taxonomic richness of an ecosystem is defined as the number of species or taxa that can be found there regardless of the number of individuals or the biomass of each taxon [
There are several numbers of diversity which differ greatly and make comparisons difficult and confusing, which hampers also a sound interpretation [ Evenness
Diversity indices were analyzed with the linear model (
To evaluate
The importance value of each species was calculated per climate and land use combination to identify dominant herbaceous species. The importance value index was considered as the sum of Relative Frequency and Relative Dominance according to the following parameters: Frequency = number of plots in which the species occurs/total plot number × 100. Relative Frequency = frequency of a species/sum of all frequencies × 100. Relative Dominance = total cover of a species/total cover of all species × 100. Importance value = Relative Frequency + Relative Dominance. Frequency ≥ 50%: frequent species. 25 ≥ Frequency < 50%: fairly frequent species. 5 ≥ Frequency < 25%: occasional species. Frequency ≤ 5%: rare species.
In order to determine the effect of climate, land use, and their interactions on the occurrence of herbaceous species, the frequency of each species was computed and compared [
All statistical analyses were done using R.3.0.2 [
The inventoried flora of the three sites included 374 species of which 307 are herbaceous species and 67 juveniles of woody species. The herbaceous species belong to 152 genera and 39 families. Overall, herbaceous vegetation is dominated by the families of Poaceae (31.8%), Fabaceae (13.2%), Cyperaceae (9.5%), and Malvaceae (6.4%).
Specific richness increased along the climatic gradient from the Sahel to the South Sudanian vegetation zone (Table
Climate effect on specific richness of herbaceous vegetation (GLMs analysis).
Sahel | North Sudanian | South Sudanian |
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| ||
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Families | Overall | 24 | 33 | 34 | 2.0800 | 0.3535 |
Protected area | 15 | 30 | 28 | 5.9071 | 0.0522 | |
Communal area | 25 | 27 | 33 | 1.2019 | 0.5483 | |
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Genus | Overall | 74 | 117 | 125 | 15.1200 | 0.0005 |
Protected area | 31 | 100 | 102 | 49.2051 | <0.0001 |
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Communal area | 74 | 95 | 105 | 7.6152 | 0.0403 |
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Species richness | Overall | 130 | 198 | 208 | 21.2500 | <0.0001 |
Protected area | 41 | 147 | 161 | 87.9035 | <0.0001 |
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Communal area | 128 | 150 | 168 | 7.4399 | 0.0459 |
Analyzing the influence of land use, species, genera, and family richness did not vary significantly between protected and communal areas (
Land use effect on specific richness of herbaceous vegetation (GLMs analysis).
Protected area | Communal area |
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| ||
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Families | Overall | 33 | 37 | 0.2287 | 0.6325 |
Sahel | 12 | 25 | 4.6665 | 0.0308 |
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North Sudanian | 30 | 27 | 0.1580 | 0.6910 | |
South Sudanian | 28 | 33 | 0.4103 | 0.5218 | |
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Genus | Overall | 125 | 145 | 1.4828 | 0.2233 |
Sahel | 31 | 74 | 18.1381 | <0.0001 |
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North Sudanian | 100 | 95 | 0.1282 | 0.7203 | |
South Sudanian | 102 | 105 | 0.0435 | 0.8348 | |
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Species richness | Overall | 223 | 263 | 3.2959 | 0.0695 |
Sahel | 41 | 128 | 47.0106 | <0.0001 |
|
North Sudanian | 147 | 150 | 0.0303 | 0.8618 | |
South Sudanian | 161 | 168 | 0.1489 | 0.6995 |
The analysis of diversity of herbaceous vegetation by Hill numbers allowed underlining the variations of mean species numbers
Herbaceous vegetation diversity indices along climate-land use conditions.
Climatic zone | Land use | Mean richness |
|
|
Evenness ( |
---|---|---|---|---|---|
Sahel | Communal area | 14.34 ± 4.71 |
8.23 ± 3.15 |
1.27 ± 0.23 |
0.77 ± 0.09 |
Protected area | 19.50 ± 4.74 |
10.05 ± 3.12 |
1.20 ± 0.06 |
0.77 ± 0.05 |
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North Sudanian | Communal area | 21.67 ± 7.90 |
13.31 ± 5.43 |
1.15 ± 0.08 |
0.83 ± 0.06 |
Protected area | 15.86 ± 5.90 |
10.58 ± 4.14 |
1.19 ± 0.12 |
0.84 ± 0.06 |
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South Sudanian | Communal area | 25.58 ± 6.34 |
18.00 ± 4.60 |
1.09 ± 0.04 |
0.89 ± 0.03 |
Protected area | 19.51 ± 6.35 |
14.63 ± 4.96 |
1.11 ± 0.04 |
0.90 ± 0.03 |
Superscript letters indicate significant differences according to Student's
Mean species numbers by plot
The analysis of herbaceous vegetation by
By considering
The evenness pattern increased significantly (d.f. = 5,
The composition of herbaceous vegetation varied according to the climate and land use conditions. Sorensen coefficient of similarity reveals that similarity is low between all climate and land use conditions (Table
Sorensen similarities of climate and land use types.
Climatic zone | Land use | Sahel | North Sudanian | South Sudanian | |||
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Communal | Protected | Communal | Protected | Communal | Protected | ||
Sahel | Communal | 1 | |||||
Protected | 0.25 | 1 | |||||
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North Sudanian | Communal | 0.23 | 0.17 | 1 | |||
Protected | 0.16 | 0.07 |
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1 | |||
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South Sudanian | Communal | 0.20 | 0.07 |
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0.45 | 1 | |
Protected | 0.16 | 0.07 | 0.31 | 0.48 |
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1 |
In the Sahelian zone, four herbaceous species were found as new for the flora of Burkina Faso. They are
We identified the dominant herbaceous species per climatic zone and land use type on the basis of their importance value index (IVI ≥ 5) (Table
Herbaceous species with an importance value ≥ 5 according to climate and land use conditions.
Climatic zone | Sahel | North Sudanian | South Sudanian | |||
---|---|---|---|---|---|---|
Land use | Communal area | Protected area | Communal area | Protected area | Communal area | Protected area |
|
|
|
4.64 | 2.82 | 6.16 | 2.99 |
|
3.72 | — | — |
|
3.37 |
|
|
— | — | — |
|
|
|
|
5.44 | — | — | — | — | — |
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4.38 | — | — | 3.24 | — |
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3.19 |
|
— | — | — | — |
|
— | 7.22 | — | — | — | — |
|
— | 5.87 | — | — | — | — |
|
2.79 | 6.51 | 3.28 | 2.86 | 4.55 |
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4.03 | 6.87 | 3.55 | — | — | — |
|
— | 5.18 | — | — | — | — |
|
— |
|
— | — | — | — |
|
5.41 |
|
— | — | — | — |
|
— | — | — |
|
— |
|
|
— | — | — | — | 3.77 |
|
|
— | 7.37 | — | — | — | — |
|
— | — |
|
|
— | 3.11 |
|
— | — |
|
|
|
— |
|
— | — | 5.08 | — | — | — |
|
|
— | — | — | — | — |
|
— | — |
|
5.17 |
|
4.42 |
|
|
— | — | — | — | — |
|
— | — | 5.93 | — | — | — |
|
— | — | — | — | 3.40 | 5.43 |
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5.32 | 7.03 | 3.70 | — | 4.12 | — |
|
— | — | 3.90 | 3.03 |
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5.40 |
|
— | — | 5.44 | — | 3.29 | — |
|
— | 6.82 | — | — | — | — |
|
— | — | — | 2.63 | — | 5.11 |
|
— | 4.79 | 5.78 | 3.06 |
|
— |
|
— | — |
|
5.00 | 4.10 | — |
|
|
|
|
— | — | — |
The occurrence of 23.6% of all species was affected by land use type and 59.8% by climate (Figure
Percentage of herbaceous species according to their responses to land use, climate gradient, and their interaction (
In all climate zone and land use combinations, herbaceous vegetation is dominated by occasionally occurring and rare species excepted in the protected area of the Sahel (Figure
Abundance of herbaceous species according to their frequency classes.
The rank-frequency diagram allows distinguishing frequent species, fairly frequent species, occasional species, and rare species. Figure
Rank-frequency diagram.
In the Sahel communal area,
In the North Sudanian communal area,
In the South Sudanian communal area,
In Savanna ecosystems, herbaceous species richness increases with increasing annual precipitation. Species richness in the Sudanian zone is higher than in the Sahelian zone. These results are in accordance with Bognounou et al. [
The absence of a significant difference in species richness between the protected and the communal area in the Sudanian zone provides evidence that land use affects species richness of herbaceous vegetation less than climate. Devineau et al. [
The difference observed for the Sahel between species richness in communal and protected areas might be explained by the fact that this protected area was realized by fencing. Compared to the Sudanian zone of Burkina Faso, in the Sahel zone, the protected area (the Sylvopatoral Reserve and Partial Fauna Reserve of Sahel) was most affected by human pressure. To reduce the impact of human pressure in this protected area in Sahel zone the work was done in fenced area. Consequently, these protection issues probably might explain the observed difference in species richness between protected and communal areas.
The dominance of families of Poaceae, Fabaceae, Cyperaceae, and Malvaceae in all study areas reflects that these families are the most frequent in Savanna herbaceous vegetation in West Africa [
Hill’s diversity numbers reveal that herbaceous vegetation was more diverse in the Sudanian zone than the Sahelian zone. The significant difference of mean species and abundant species numbers along the climate gradient shows that precipitation increase favors an increase of herbaceous species richness in Savanna ecosystem. The low values found for very abundant species show that the different climate and land use conditions were dominated by a group of species and the relative high evenness value underlines that dominant species have a regular distribution of their individuals [
The similarity in herbaceous species composition and abundance between climate and land use conditions is low. The low similarity in herbaceous species composition and abundance between climate-land use conditions reflects differences in climate conditions and land use regimes. In addition, the low to moderate level of similarity between climate and land use combinations indicates high beta-diversity and accentuates the importance of climate and land use in explaining herbaceous species diversity at larger spatial scale.
The difference in herbaceous vegetation composition is related to the dual effect of climate and land use conditions. Climate conditions explain the difference in species composition between different climate zones [
The frequency of herbaceous species is more influenced by the interaction between climatic conditions and land use regimes than their separate effects. In fact, the distribution of herbaceous species is principally affected by climate, which determines geographical affinity [
Overall, most of the listed species are classified as occasional or rare. The low percentage of frequent species and the high rate of occasional and rare species implies on the one hand a great heterogeneity among herbaceous plant communities of each climate and land use condition and on the other hand the vulnerability of these rare herbaceous species. These results are in concordance with the studies of Sambaré [
This study shows that specific richness of herbaceous vegetation increases significantly with precipitation, and is, thus, mainly determined by climate, whereas the effect of land use is less important. Herbaceous vegetation diversity described by mean richness and the numbers of abundant species reveals that the Sudanian zone and the communal areas were, respectively, more diverse than the Sahelian zone and the protected areas. The number of very important species demonstrated that only few species are dominant and the evenness pattern showed that these dominant herbaceous species exhibit a regular distribution of their individuals within herbaceous communities. The composition of Savanna herbaceous species is dependent on climate and land use conditions. Land use affects more the composition of herbaceous vegetation than its richness. The occurrence of herbaceous species in Savanna ecosystem is more controlled by the interaction between climate and land use than by their separate effects. Occasional and rare species constitute the most important part of Savanna herbaceous vegetation diversity, characteristic of heterogeneity of ecosystems and vulnerability of herbaceous species.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank the DFG (German Research Foundation, BE 4143/2-1, and HA 6789/1-1) and the Ministry of Secondary and Higher Education of Burkina Faso for financial support.