Phytodiversity dynamics in pastured and protected West African savannas

PHYTODIVERSITY DYNAMICS IN PASTURED ANDPROTECTED WEST AFRICAN SAVANNAS

KAREN HAHN-HADJALI1, MARCO SCHMIDT2 & ADJIMA THIOMBIANO2

1J.W. Goethe-University, Botanical Institute, Siesmayerstr. 70, 60054 Frankfurt/Main,Germany

2Department of Botany and Molecular Evolution, Research Institute Senckenberg,Senckenberganlage 25, 60325 Frankfurt/Main, Germany

[email protected]; [email protected]

AbstractIn the last decades vegetation changes in West African savannas due to changing

climatic conditions and human impact have become more and more obvious. Whereasthe dynamics of vegetation cover were intensively observed, the knowledge aboutchanging species composition and phytodiversity dynamics remains scarce. For a betterunderstanding of current dynamics, especially with regard to the impact of grazing,permanent plots were installed in protected and pastured areas in the Sudanian zoneof Burkina Faso. By standardized floristic inventories and phytosociological relevésphytodiversity and species composition were analyzed. The investigations are partof a continent wide monitoring of the BMBF-funded BIOTA Africa project(http://www.biota-africa.de/). Results from the first inventories reveal a significantlyhigher species richness on the pastured plots, and also big differences in speciescomposition. The pastured plots show a higher proportion of forbs and annual grassesthe majority being typical elements of the segetal and ruderal flora brought by cattleinto the savanna. The species richness of the woody layer is so far not affected bygrazing. However, regarding the species composition of the different woody strata, forseveral woody species the regeneration seems to be disturbed in the pastured plots,which at long term might lead to their decline. Further monitoring is needed tounderstand these population dynamics in the long run.

RésuméPhytodiversity dynamics in pastured and protected West African savannas. Pendant

les dernières décennies les variations de végétation des savanes Ouest Africaines, duesau changement des conditions climatiques et á l’impact de l’homme sont devenues deplus en plus visibles. La dynamique du couvert végétal a été intensivement étudiée maisles observations sur le changement de composition des espèces et la dynamique de laphytodiversité demeurent plus rares. Pour une meilleure compréhension desdynamiques actuelles, principalement par rapport a l’impact du pâturage, des sitespermanents furent installés dans des zones protégées et d’autres dans des zonesconsacrées au bétail dans la zone soudanienne au Burkina-Faso. La phytodiversité et lacomposition en espèces furent analysées par des inventaires standardisés de la flore etdes relevés phytosociologiques. Ces investigations font part d’un projet de suivis ál’échelle du continent du BMBF financée par BIOTA projet Afrique (http://www.biota-

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Hahn-Hadjali, K., Schmidt, M. & Thiombiano, A. (2006). Phytodiversity dynamics in pastured andprotected West African savannas. In: S.A. Ghazanfar & H.J. Beentje (eds), Taxonomy and ecology ofAfrican plants, their conservation and sustainable use, pp. 351–359. Royal Botanic Gardens, Kew.

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Taxonomy and ecology of African plants, their conservation and sustainable use

africa.de). Les résultats des premiers inventaires révèlent une significative plus granderichesse en espèces dans les sites avec pâturage ainsi qu’une plus grande différence dansla composition des espèces. Les sites pâturés montrent une plus grande proportion enherbes messicoles et annuelles transportées par le bétail dans la savane. Jusqu’ici larichesse végétale en espèces de la strate arborée n’est pas affectée par le pâturage.Cependant concernant la composition en espèces des différentes strates arborées pourplusieurs espèces arbustives, la régénération semble modifiée dans les sites pâturés, cequi a long terme pourrai conduire a leur déclin. Plus de suivis seront nécessaires pourcomprendre la dynamique de ces populations á long terme.

Key words: Burkino Faso, pasture pressure, phytodiversity dynamics, savannas, West Africa

1 Introduction

Changes in vegetation cover and habitat degradation are increasing andaccelerating in many West African savanna regions due to changing climatic conditionsand human impact. This general development is still contrasted by a lack of detailedknowledge concerning the current state of plant diversity, species composition andtheir dynamics. In this context the impact of land use practices on vegetationdevelopment is of particular interest, to distinguish the effects of human activities andclimate. This is stressed by the currently revived discussion on degradation andrecovery of the Sahelian vegetation, based on results of satellite time series indicatingan increase in seasonal greenness for large areas of the Sahel (UNEP/FAO/UNCCD,2003). It is pointed out, that a current “greenness” value similar to a former state mightresult from a different species composition.

Livestock grazing is one of the important forms of land use, being a major influenceon the composition and diversity of species not only in the Sahel (Le Houérou, 1998,1996; Hiernaux, 1998; Toutain, 1999) but also in the Sudanian zone. Generally lightgrazing is considered as resulting in an increase in species richness (Naveh &Whittaker, 1979; Noy-Meir et al., 1989) whereas heavy grazing leads to a decrease inspecies diversity (Olsvig-Whittaker et al., 1993). Frequently observed changes associatedwith increasing grazing pressure are shifts from perennial to annual vegetation(Taddese et al., 2002; Milton et al., 1994), and heavy grazing can result in the loss ofpalatable plants and select for weedy, generalist species (Milton et al., 1994; Huenneke& Noble, 1996; McIntyre & Lavorel, 2001). However, most of the studies were carriedout in semi-arid areas and the consequences of livestock grazing can be diverse, anddiffer depending on the region. Species fostered by grazing in the Sahel of Niger(Hiernaux, 1998) include, for example, some of the best forage species. For theSudanian zone detailed knowledge about the species composition and distribution inresponse to grazing pressure is still lacking. Partly this may be due to the fact thatpastures are often at the same time fallows as part of shifting cultivation cycles, whichcomplicates the interpretation of different species composition in response to pastureor fallow succession.

For a better understanding how grazing affects plant species composition andspecies richness in the Sudanian zone permanent plots were installed in protected andpastured areas in Burkina Faso. The investigations are part of a continent wide longterm monitoring of the BMBF-funded BIOTA Africa project (www.biota-africa.de) toinvestigate biodiversity dynamics throughout a continental climatic gradient. Thepresent results refer to the first inventories carried out in 2001.

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Phytodiversity dynamics in pastured and protected West African savannas

FIG. 1. The study area.

2 Materials and methods

The study area is located in the eastern part of Burkina Faso (Fig. 1) in the northernSudanian zone. The vegetation cover is composed of a mosaic of various types ofwoodland, grass, shrub and tree savannas, fallows and gallery forests. Nearly the wholearea is pastured mainly by cattle and to lesser degree by small ruminants, and annualbush fires pass through. The region receives 825 mm of rain per annum; the rainyseason lasting from April to October.

A typical Sudanian woodland savanna with a tree cover of about 60% was selectedfor the analysis of the grazing impact on plant species richness and composition. Twosites, distinguished by their degree of grazing, were chosen to establish the permanentplots. One is situated in a protected area in which only sporadic influence of wildlifebecomes effective the other one is located in an area with heavy cattle grazing pressure.Stocking rates are unknown, as sedentary as well as transhuman herders use thecommunal rangeland. The grazing regime can be characterized as being heavily andcontinuously grazed.

At both sites five permanent plots each of a size of 1 ha were made. In every hectareplot smaller plots were installed: a 20 × 50 m2 plot for making an inventory of the woodylayer, and a 20 × 20 m2 and 10 × 10 m2 for the investigations of the herb layer. Thedifferent plot sizes for the woody and herb layer correspond to results of minimumarea analyses for the adequate assessment of the different strata. The inventories arecarried out according to standardized methods in the frame of the BIOTA researchnetwork. The following parameters were recorded:

• 100 × 100 m2: list of all plant species

• 20 × 50 m2 relevés of the woody layer: estimation of the percentage covercontributed by each species (Londo-scale: 10% intervals), divided into differentstrata (0–0.5, 0.5–1, 1–2, 2–5, 5–10, >10 m)

• 10 × 10 m2 relevés of the herb layer: estimation of the percentage covercontributed by each species (Londo-scale: 10% intervals)

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3 Results

3.1 Species richnessThe first inventories in 2001 revealed for the hectare plot size a mean species

richness of 81.4 in the protected site and 96.6 for the pastured site. Student’s t-testshowed that the species richness of all species is significantly higher in the pasturedsites for both the 20 × 50 m2 and 10 × 10 m2 plots (p = 0.066) and the hectare plots (p= 0.053) (Fig. 2). Contrary to the differences in species richness of all species the plotsof the woody layer (20 × 50 m2 plots) contain in protected and pastured sites an equalnumber of species, which means that the woody layer’s species richness is at the currentstate not affected by pasture. But the woody layer’s evenness (Fig. 3) is significantlyhigher in the protected 20 × 50 m2 plots (p = 0.02). This indicates that the woodyspecies composition is more even in the protected area whereas some species dominatethe woody stratum in the pastured plots (see also Table 1). For the herb layer, oppositetendencies can be detected. The pastured plots show a higher evenness while thedominance of some few tall grass species like Andropogon gayanus and Hyparrheniainvolucrata leads to a lower value in the protected plots.

3.2 Life formsThe spectra of life forms for the plots clearly show a difference between the

compared areas (Fig. 4). Not only the species richness is increased in pastured plots, butalso the occurrence of therophytes and chamaephytes is significantly higher (p < 0.001)under grazing, whereas the number of hemicryptophytes is slightly increased (p = 0.03)in the protected plots.

Within the pastured plots the majority of the therophytes are typical elements of thesegetal and ruderal flora of the Sudanian Zone, some of them also widely distributedin the Sahel (e.g. Cassia tora, Zornia glochidiata, Microchloa indica). These species arepartly palatable species, some of them even good quality fodder (Zornia glochidiata),partly unpalatable herbs like Cassia tora. The higher share of chamaephytes in the

TABLE 1. Woody cover per strata for certain species in the 20 × 50 m2 plots.

Species/strata 0–0.5 m 0–5–1 m 1–2 m 2–5 m 5–10 m 0–0.5 m 0.5–1 m 1–2 m 2–5 m 5–10 m

0–1 2 5 10% (mean cover of 5 plots)

Vitellaria paradoxaAnnona senegalensis

Piliostigma reticulatum

Pteleopsis suberosa

Crossopteryx febrifuga

Maytenus senegalensis

Pterocarpus erinaceus

Feretia apodanthera

Dichrostachys cinerea

Anogeissus leiocarpus

Lannea acida

Acacia dudgeoni

Lonchocarpus laxiflorus

Tamarindus indica

FIG. 2. Species richness in pastured and protected plots.95% confidence intervals.

protectedpastured

10 m × 10 m herb layer 20 m × 50 m woody layer 100 m × 100 m

200

150

100

50

0

Species richness

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FIG. 3. Evenness in pastured and protected plots. 95% confidence intervals.

protectedpastured

10 m × 10 m herb layer 20 m × 50 m woody layer

100

90

80

70

60

50

40

30

20

10

0

Evenness

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FIG. 4. Life forms in pastured and protected plots.

FIG. 5. Species richness in 20 × 50 m2 plots.

protectedpastured

not a

ssign

ed

Species

Herb

Semi-p

arasi

te

Thero

phyte

Geoph

yte

Hemicr

ypto

phyte

Chamae

phyte Lia

na

Phan

erop

hyte

protectedpastured

5–10 m

2–5 m

1–2 m

0,5–1 m

0–0,5 m

0 5 10 15 20 25 30

50

40

30

20

10

0

Life forms in protected and pastured plots

Species richness in 20 ×× 50 m plots

pastured plots is due to the increase of unpalatable species (Tephrosia pedicellata,Waltheria indica), covering much of the herb layer. The protected plots however have ahigher percentage of perennial grasses (hemicryptophytes) in terms of species richnessand abundance.

3.3 Woody plants per stratumSpecies richness as well as the mean cover of the woody layer was analyzed according

to the different strata to detect differences in woody species regeneration in responseto grazing. While the total species richness shows little difference, species richness issignificantly higher for the 0.5–1 m stratum (p = 0.05) in the protected plots. All otherstrata have a similar species richness. The woody layer’s mean cover is highly variablefor both pastured and protected plots, being higher in the protected plots in the 0.5–1m stratum and in the pastured plots in the strata 0–0.5 m and 2–5 m.

3.4 Woody species regenerationWith regard to the species composition of the woody layer the comparison of the

pastured and protected sites reveals a different abundance per strata for several treesand shrubs (Table 1). Some of the woody species (Crossopteryx febrifuga to Tamarindusindica) are regularly distributed in all strata in the protected sites and can be found inthe pastured plots only in the lowest stratum or in the highest without any juveniles.This indicates a disturbed regeneration for these species in response to heavy grazing.

Other species show a more regular occurrence and higher cover in all strata of thepastured plots (e.g. Vitellaria paradoxa, Piliostigma reticulata). This is partly due to theformer exploitation of the site (fallow-field-cycles with protection of Vitellaria paradoxamore than 50 years ago) but in the case of Piliostigma reticulata and also Annonasenegalensis it is an effect of the grazing influence.

357


FIG. 6. Woody cover in 20 × 50 m2 plots.

protectedpastured

5–10 m

2–5 m

1–2 m

0,5–1 m

0–0,5 m

0 5 10 15 20 25 %

Woody cover 20 ×× 50 m-plots

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4 Discussion

The comparison of protected and heavily pastured sites clearly shows the effect ofgrazing by livestock on species richness, which is significantly increased under grazing.This result is in contrast with other studies, which have found either a decrease ofspecies richness under grazing impact (Olsvig-Whittaker et al., 1993) or no impact ofheavy grazing at all on the number of species (Naveh & Whittaker, 1979; Todd &Hoffmann, 1999). This might be due to the fact that most of the studies were carriedout in semi-arid regions, where savannas have a different vegetation structure. In theSudanian Zone the vegetation of the protected sites is dominated by a two to threemeters tall and very dense grass layer, composed mainly of perennial species, leavinglittle space for species underneath. When this dense grass layer is opened by grazingmany annuals, pioneers and forbs can establish. The results of the evenness alsoindicate these differences between the pastured and protected sites. However, it is alsopossible that the increase of species richness under long-term impact of heavy pastureturns into a decrease. The grazing intensity on the investigated plots has increased onlyin the last few years according to the local population. Modeling experiments suggest,that vegetation changes due to overgrazing might take many years to become evident(Wiegand & Milton, 1996). Further monitoring will provide information on this. With regard to the impact of grazing on the life forms and species composition theresults show the typical shift from perennial herbaceous species to therophytes andchamaephytes. This compares favorably with several other studies that have found analteration of the dominance of perennials in favor of annuals (Naveh & Whittaker,1979; Todd & Hoffmann, 1999). The increase of geophytes observed by Todd &Hoffmann (1999) did not take place at our sites. This may be due to its formeragricultural use.

The therophytes and chamaephytes which increase on the pastured sites are not justthe unpalatable species. They consist of a number of unpalatable as well as somepalatable species and even good fodder. These results confirm well with those ofHiernaux (1998) for Sahelian rangelands, were no relationship was found betweenspecies response to grazing and palatability. Others (Milchunas & Lauenroth, 1993;Todd & Hoffmann, 1999; McIntyre & Lavorel, 2001) have observed a selection for lowgrowing prostrate growth forms in response to grazing.

While the herb layer shows distinct changes in response to grazing, the speciesrichness of the woody layer is not affected. The comparison between pastured andprotected sites broken down into strata, which might show changes in regeneration ofthe woody species, differs only in the species richness of the 0.5–1m stratum. Thelower values in the pastured plots suggest the disturbance of woody regeneration bycattle at this height. Lower cover values in the same stratum stress this interpretation.This agrees partly with Higgins et al. (1999), who observed a lower level of regenerationof woody plants under higher pasture pressure in semi-arid savannas. However, in thisregard the individual species response has to be considered. Regarding the coverdistribution of the species per stratum, different species groups can be distinguished.Some of them (e.g. Piliostigma reticulata) are apparently increasing, whereas for others,like Pterocarpus erinaceus and Anogeissus leiocarpus the irregular occurrence in thedifferent strata indicates a disturbed regeneration in the pastured plots, which in thelong term is supposed to lead to their severe decline. These first results highlight theneed for further studies on the impact of grazing pressure especially that on thewoody vegetation.

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