Influence of selective tree cutting, livestock and prescribed fire on herbaceous biomass in the savannah woodlands of Burkina Faso, West Africa

Agriculture, Ecosystems and Environment 105 (2005) 335–345

Influence of selective tree cutting, livestock and prescribed fire on herbaceous biomass in the savannah woodlands of Burkina Faso, West Africa Louis Sawadogo a , Daniel Tiveau b , Robert Nygård b,∗ b

a Département Production Forestière, CNRST, INERA, BP 10 Koudougou, Burkina Faso Department of Silviculture, Swedish University of Agricultural Sciences SLU, 901 83 Umeå, Sweden

Received 12 June 2003; received in revised form 23 January 2004; accepted 4 February 2004

Abstract In West Africa policies for grazing, selective tree cutting and prescribed early fire in the savannah woodlands are rarely based on long-term experimental studies. The purpose of this study was to provide scientific evidence based on field data from two case studies for an informed discussion on the effects of various management options. The main findings were by and large specific for the species, the growth form and the location. This supports the argument for devolution of management responsibility to the local level where there is such site-specific ecological knowledge. Effects of selective tree cutting (50% of basal area), livestock (1–1.4 tropical livestock unit ha−1 ) and prescribed early annual fire on herbaceous biomass were studied in the state forests of Laba (shallow sandy soils) and Tiogo (deep clayey soils) in the savannah woodlands in the Sudanian Zone of Burkina Faso, from 1993 to 2001. The herbaceous biomass was analysed on three levels: total herbaceous biomass, per growth form (annual and perennial grasses and forbs) and for each of the four main species in the study areas (Andropogon pseudapricus, Loudetia togoensis, Andropogon ascinodis and Andropogon gayanus). At both sites, mean total biomass during the study period was reduced by the presence of livestock while it was not significantly affected by early prescribed fire or by selective cutting. There was interaction between the three treatments at both sites. Statistically significant treatment effects were found at each site when analysing each growth form and species individually. For instance, at the growth form level, grazing reduced the biomass of annual grasses in Tiogo, perennial grasses in Laba and forbs at both sites. Site and species-specific response to grazing was found for A. ascinodis with increased biomass in Tiogo and reduced biomass of A. gayanus in Laba. Although the effect of prescribed early fire was not statistically significant the trend was the same at both sites with increased biomass of annual grasses and decreased biomass of perennial grasses. This homogenising effect of fire was statistically significant at the species level with increased biomass of the annual grass L. togoensis and decreased biomass of the perennial grass A. gayanus at both sites. Selective cutting significantly increased mean biomass of annual grasses in Tiogo whereas there was no difference in Laba. © 2004 Elsevier B.V. All rights reserved. Keywords: Browsing; Pastoralism; Rangeland; Protected area; Selective logging; Disturbance

1. Introduction ∗ Corresponding author. Tel.: +46-90-7868365; fax: +46-90-7867669. E-mail address: [email protected] (R. Nygård).

The herbaceous vegetation makes up 75–90% of the total biomass in tropical savannah ecosystems (Garnier

0167-8809/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2004.02.004

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and Dajoz, 2001b). On the ecological scale, it is fuel for fires that are necessary to maintain the two components constituting the savannah biome, ligneous and herbaceous vegetation. On the socio-economical scale, the herbaceous component in West Africa is essential as forage for domestic as well as wild animals, for construction material, for arts and crafts, as medication and as food for humans during difficult times. Unfortunately in West Africa, models and experimental studies investigating the roles of biotic and abiotic factors influencing the coexistence of these two components have tended to focus on tree population dynamics (Menaut, 1977; Gignoux et al., 1997; Gambiza et al., 2000; Sawadogo et al., 2002). Both fire and grazing are two important management tools that can readily be manipulated (Frost et al., 1986; Meurer, 1994; Liedloff et al., 2001). Meurer (1994) observes that fire suppression leads to a decrease of the grass production in savannah grasslands and woodlands since large quantities of dead herbaceous matter reduce the tillering ability. Menaut and Cesar (1979) and Garnier and Dajoz (2001b) come to the same conclusion that fire maintains tussocks and increases their cover by favouring tillering. This could in turn reduce runoff and have repercussions on the percolation of water. Fires burning early in the dry season tend to be of low intensity as the fuel dominated by the herbaceous layer still holds moisture from the wet season. These fires often burn in a patchy manner and while removing litter and much of the dead grass layer, they are less harmful for the ligneous vegetation compared to late fires. Cesar (1990) stresses that fire should not be seen as a transformation factor but rather as a conservation factor that maintains the savannah, its species composition and especially its herbaceous vegetation component. Livestock is often associated with land degradation, especially in the Sahelian zone. Too high stocking rates may lead to land degradation or even desertification (Keya, 1998; Guevara et al., 1999) and loss of species richness, while too little grazing may lead to succession from grassland to woodland (Watkinson and Ormerod, 2001). Not only the level of grazing is important but also the timing and the animal species involved. This has been shown in other geographic areas (Grant et al., 1996; Hulme et al., 1999; Humphrey and Patterson, 2000). Balancing stocking rates with herbaceous production is particularly difficult in arid and semi-arid areas where there is

considerable variability in rainfall (Watkinson and Ormerod, 2001). Many authors are in favour of a non-equilibrium view of savannah ecology and argue that annual rainfall has more impact on herbaceous production than the stocking rate has (Ellis and Swift, 1988; Westoby et al., 1989; Behnke et al., 1993). Others argue that biotic factors are more important than the abiotic ones (Hutchinson, 1996; Keya, 1998; Fynn and O’Connor, 2000). Livestock interacts with fire in both time and space since much of the livestock is attracted to recently burnt ground to feed on the post-fire grass regrowth. Livestock reduces grass biomass and therefore lowers the short-term possibility of the area sustaining another burn. Patchy grazing consequently causes patchy fire and vice versa (Frost et al., 1986; Coughenour, 1991). The effects of fire and livestock depend on the growth form (annual-, perennial grass, etc.), intensity, frequency and season of use, growth stage of the plant, grass moisture content, air temperature, humidity, wind speed, soil type and soil moisture conditions (Trollope, 1982; Frost et al., 1986; Coughenour, 1991; Cheney et al., 1993). When tree density increases, the grass production decreases (Grunow et al., 1980; Mordelet and Menaut, 1995) both due to thermal interference (Ball et al., 2002), shading and due to increased competition for water and for nutrients, although the opposite has sometimes been found (Belsky, 1994; Grouzis and Akpo, 1997) especially for isolated trees (Belsky et al., 1989; Weltzin and Coughenour, 1990; Belsky et al., 1993). Tree removal results in increased grass production but the total aboveground plant production usually decreases (Scholes and Walker, 1993). In Burkina Faso, state forests are important for supplying both fuelwood to urban areas and feed for livestock in particular during the rainy season when agricultural fields are being cultivated. Currently in Burkina Faso, local authorities prohibit cattle in the state forests and recommend both selective tree cutting by harvesting 50% of the basal area on a 20-year rotation period and applying annual early prescribed fire (Bellefontaine et al., 2000), but these prescriptions are not based on scientific evidence. Thus an experiment was conducted on permanent sample plots during a 9-year period to study the effects of livestock, fire and tree removal on the herbaceous biomass. More specifically, the hypothesis tested was that different herbaceous species respond differently to fire, grazing

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and selective tree cutting. The specific aim of the study was to evaluate if fire and grazing in combination with selective tree cutting could be used as tools for sustainable management of savannah woodlands.

2. Material and methods 2.1. Study area The experimental sites were located on flat areas in Laba State Forest (11◦ 40 N and 2◦ 50 W) and in Tiogo State Forest (12◦ 13 N, 2◦ 42 W) both at an altitude of 300 m a.s.l. in Burkina Faso, West Africa. The Laba and Tiogo State Forests were delimited by the colonial French administration in 1936 and cover 17,000 and 30,000 ha, respectively. Both forests were located along the only permanent river (Mouhoun formerly known as Black Volta) in the country. Phyto-geographically they were situated in the Sudanian regional centre of endemism (White, 1983) in the transition from the north to south Sudanian Zone (Guinko, 1984). The unimodal rainy season lasted about 6 months, from May to October. The mean annual rainfall for the study period (1993–2001) was 910 ± 138 mm for Laba and 836 ± 219 mm for Tiogo with large inter-annual variability. The number of rainy days per annum during the study period was 76 ± 13 and 70 ± 9 for the two sites, respectively (Table 1). Mean daily minimum and maximum temperatures were 16 and 32 ◦ C in January (the coldest month) and 26 and 40 ◦ C in April (the hottest month),

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Table 2 Main soil characteristics at two sites (Tiogo and Laba) Laba Clay (%) Fine silt (%) Coarse silt (%) Fine sand (%) Coarse sand (%) Total organic matter (%) Total N (%) C/N (%) Available P (ppm) pH H2 O

17.5 8.7 16.4 16.7 40.0 2.1 0.1 15.9 1.3 6.2

Tiogo ± ± ± ± ± ± ± ± ± ±

8.8 2.4 6.2 4.3 11.6 0.6 0.0 4.9 1.0 0.7

24.8 15.0 25.4 21.7 13.1 1.8 0.1 11.4 1.4 6.2

± ± ± ± ± ± ± ± ± ±

7.7 4.3 3.0 6.7 4.2 0.7 0.0 4.6 0.7 0.5

Values are mean ± S.D.

yielding an aridity index (Brown and Lugo, 1982) of 3.2 and 3.5 for Laba and Tiogo, respectively. Most frequently encountered were Lixisols (LX) according to the FAO soil classification system (Driessen et al., 2001). The soils were shallow (<45 cm depth) silty-sand at the Laba site and mainly deep (>75 cm) silty-clay at the Tiogo site. Some of their physical and chemical characteristics are summarised in Table 2. These soils were representative of large tracts of the Sudanian Zone in Burkina Faso (Pallo, 1998). At both sites, the vegetation was a tree and bush savannah with a grass layer dominated by the annual grasses Andropogon pseudapricus and Loudetia togoensis as well as and the perennial grasses Andropogon gayanus (dominant in Tiogo) and Andropogon ascinodis (dominant in Laba). In the study area, these two perennial grasses were the most important species because of their fodder value and also because they were used for

Table 1 Annual rainfall for 1993–2001 at two sites (Tiogo and Laba) in the Sudanian Zone of Burkina Faso, West Africa Year

1993 1994 1995 1996 1997 1998 1999 2000 2001 Mean ± S.D.

Laba

Tiogo

Rainfall (mm)

Number of rainy days

Rainfall (mm)

Number of rainy days

1021 987 742 1094 987 822 1017 734 785

72 88 81 79 79 83 89 53 56

686 1131 703 676 839 1195 986 581 723

59 79 77 66 67 73 87 68 58

910 ± 138

76 ± 13

836 ± 219

70 ± 9

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construction (roofs and fences) and handicraft. The main forb species were Cochlospermum planchoni, Borreria spp. and Wissadula amplissima. Identification of species and families of plants were made according to Hutchinson et al. (1954). Mimosaceae and Combretaceae dominated the woody vegetation component at both sites. In terms of basal area, the main species were Detarium microcarpum, Combretum nigricans, Acacia macrostachya, Entada africana, Lannea acida, Anogeissus leiocarpus and Vitellaria paradoxa. At Laba experimental site, at the beginning of the study period mean basal area at stump level (20 cm) was 10.7 and 6.3 m2 ha−1 at breast height (130 cm) and stand density was 582 woody individuals ha−1 having at least one stem ≥10 cm GBH (girth at breast height). At Tiogo the equivalent figures were a basal area of 10.9 m2 ha−1 at stump level, 6.1 m2 ha−1 at breast height and a stand density of 542 woody individuals ha−1 . Before the establishment of the research sites, the area was frequented by livestock and wild animals. Bush fires occurred almost every year, often late in the dry season (November–May). Livestock was dominated by cattle although sheep and goats also grazed on the sites. The sites were also, occasionally, visited by elephants. The presence of livestock in the two state forests varied spatially and temporally, mainly occurring during the rainy season (June–October) when the grass was green and the surrounding areas were cultivated. During the dry season, when the crops had been harvested, the cattle mostly frequented the agricultural fields and at this time of the year the animals came to the forest mostly in search of water along the river. They then grazed on straws in the bushclumps that had escaped the fire as well as the young shoots of perennial grass species and young woody foliage induced by the fire. The livestock carrying capacity in Laba State Forest was 1.0 tropical livestock unit ha−1 (T.L.U. ha−1 ) compared to 1.4 T.L.U. ha−1 in Tiogo State Forest (Sawadogo, 1996) and the grazing pressure at both experimental sites was about half of this capacity. Trees were mainly cut for commercial fuelwood and poles by local populations that were organised in co-operatives in both forests. The wood was transported to towns 50 and 145 km away. Non-timber forest products such as fruits, leaves, tubers, perennial grass straw and hay were also harvested in these state forests.

2.2. Experimental design A split-plot experiment with four replicates of 4.5 ha was established in each of the two state forests (Table 3). Each experimental site (18 ha) was split into two contiguous main plots of which one was fenced off at the beginning of the dry season in December 1992 as to exclude livestock. Each main plot was further divided into four blocks of 2.25 ha, each containing nine subplots of 0.25 ha (50 m × 50 m). The subplots were separated from each other by 20–30 m fire-breaks. To the nine subplots within each block, three treatments were randomly assigned as no cutting, selective cutting of 50% of the basal area at stump level and selective cutting of 50% of the basal area followed by direct seeding of tree species. Cutting took place in December 1993 in Tiogo and 1 month later in January 1994 in Laba. To each subplot that had received the same cutting treatment, one of three fire treatments was applied: fire protection, 2-year fire protection followed by early annual fire and early annual fire since the establishment of the trials. The prescribed early fire was applied at the end of the rainy season (October–November) each year beginning 1992, when the grass layer humidity was approximately 40%. Table 3 Experimental design Livestock

Fire

Cutting

Number of plots

Grazing

Fire

No cutting Cutting Cutting + seeding

4 4 4

No fire

No cutting Cutting Cutting + seeding

4 4 4

2-Year fire protection

No cutting Cutting Cutting + seeding

4 4 4

Fire

No cutting Cutting Cutting + seeding

4 4 4

No fire

No cutting Cutting Cutting + seeding

4 4 4

2-Year fire protection

No cutting Cutting Cutting + seeding

4 4 4

No grazing

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This study is part of a larger experiment and in this paper we did not assess “2-year fire protection” or “cutting + seeding”. Analyses in this paper are based on mean values from 9 years and “2-year fire protection” would require analyses of the temporal variation during the study period. The seeding did not affect the herbaceous cover so there are in fact two subplots (0.25 ha) of the selective cutting treatment for each block having received the cutting treatment. The herbaceous biomass assessment was done by handclipping at peak biomass each year (mid-October) of six 1 m2 quadrats in each 50 m × 50 m subplot. The location of these quadrates was chosen at random without picking the same location in consecutive years. The samples were sorted according to species, bagged, dried and weighed. For each site and each treatment, the mean herbaceous standing biomass during the study period (1993–2001) was analysed by considering the total herbaceous biomass, biomass per growth form (annual grass, perennial grass and forbs) and the biomass of the main grass species (A. gayanus, A. ascinodis, A. pseudapricus and L. togoensis). 2.3. Statistical methods The analysis of variance was performed with the following general linear model (GLM): Yijkl = µ + βi + Gj + Fk + Cl + βGij + GFjk + GCjl + FCkl + GFCjkl + eijkl where Yijkl was the response variable for the herbaceous biomass parameter, µ the overall mean, βi the block effect (replication) i, Gj the effect of livestock

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(main plot) j, Fk the effect of fire k and Cl effect of selective cutting l. The parameters Gj , Fk , Cl and their interactions were regarded as fixed effects and the parameter βi as random. Multiple comparisons were made with Tukey’s test to detect differences between treatments (Zar, 1984) at 5% level of significance. 3. Results 3.1. Herbaceous biomass at the two sites Overall from 1993 to 2001, the Laba site was less productive, with a mean production of 3.47 (±1.37) t DM ha−1 (tonnes of dry matter ha−1 ) compared to 4.01 (±1.51) t DM ha−1 for the Tiogo site (Table 4). At both sites, the mean production of perennial grasses was highest (1.65 (±0.79) and 2.40 (±0.69) t DM ha−1 ), followed by annual grasses (1.49 (±0.78) and 1.39 (±0.79) t DM ha−1 ). The mean forb biomass was only 0.32 (±0.27) and 0.22 (±0.14) t DM ha−1 for the Laba and Tiogo sites respectively, during the same period. The main grass species (A. gayanus, A. ascinodis, A. pseudapricus and L. togoensis) make up 52 and 68% of the total biomass at Laba and Tiogo, respectively. There was a large inter-annual variation of the herbaceous biomass during the study period at both sites (Table 4). For example, in Laba the herbaceous biomass varied from 5.1 t DM ha−1 in 1997 to 1.6 t DM ha−1 in 2001, a more than three-fold variation. Similarly, in Tiogo the herbaceous biomass varied from 6.5 t DM ha−1 in 1993 to 1.7 t DM ha−1 in 1999, an almost four-fold variation.

Table 4 Annual biomass per growth form in tonnes of dry matter per hectare for 1993–2001 at two sites (Laba and Tiogo) Year

Laba

Tiogo

1993

1994

1995

1996

1997

1998

1999

2000

2001

Mean ± S.D.

Annual grasses Perennial grasses Forbs

1.25 1.00 0.43

2.22 1.82 0.75

2.35 1.49 0.73

2.61 1.94 0.40

1.81 3.22 0.06

0.90 1.88 0.08

0.97 1.55 0.07

0.85 1.08 0.14

0.44 0.89 0.24

1.49 ± 0.78 1.65 ± 0.71 0.32 ± 0.27

Total Laba

2.68

4.79

4.57

4.95

5.09

2.86

2.60

2.08

1.57

3.47 ± 1.37

Annual grasses Perennial grasses Forbs

2.91 2.28 0.27

3.15 2.96 0.41

1.27 3.02 0.16

1.03 2.57 0.25

0.95 3.14 0.15

1.20 2.98 0.20

0.30 1.39 0.00

0.46 1.55 0.07

1.28 1.69 0.41

1.39 ± 0.99 2.40 ± 0.69 0.22 ± 0.14

Total Tiogo

5.46

6.52

4.46

3.85

4.24

4.38

1.69

2.08

3.38

4.01 ± 1.51

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Table 5 Impact of livestock on the mean herbaceous biomass during 1993–2001 in tonnes of dry matter per hectare for total herbaceous biomass, each growth form and for the four most common species at the two sites (Laba and Tiogo), two replicates

Table 6 Impact of fire on the mean herbaceous biomass during 1993–2001 in tonnes of dry matter per hectare for each growth form and for the four most common species at the two sites (Laba and Tiogo), four replicates

Livestock

Fire

Mean total biomass Annual grasses A. pseudapricus L. togoensis Perennial grasses A. ascinodis A. gayanus Forbs ∗

Laba

Tiog

Grazing

No grazing

Grazing

No grazing

2.99∗ 1.55 0.20 0.19∗ 1.19∗ 0.74∗ 0.34∗ 0.25∗

3.94 1.45 0.18 0.08 2.08 1.14 0.75 0.41

3.68 1.12∗ 0.29 0.27 2.37 0.49∗ 1.51 0.19∗

4.33 1.69 0.43 0.49 2.41 0.33 1.57 0.24

P < 0.05.

3.2. Effects of livestock Mean total biomass during the study period was significantly reduced by grazing in Laba and close to significantly reduced in Tiogo (P = 0.045 and P = 0.054, respectively) (Table 5). At the growth form level, there was a significant biomass reduction of annual grasses in Tiogo (P = 0.045) and perennial grasses in Laba (P = 0.030). The forb biomass decreased significantly at both sites (P < 0.034). The main species reacted differently according to the site. Grazing reduced the biomass of A. ascinodis in Laba while it was increased significantly in Tiogo (P < 0.036). The biomass of A. gayanus decreased significantly in Laba (P = 0.043) while in Tiogo there was only a tendency towards decreased biomass. Grazing had a tendency to increase the biomass of A. pseudapricus in Laba while the opposite was the case in Tiogo. L. togoensis showed a significantly increased biomass in Laba (P = 0.045) while there was a tendency to decreased biomass in Tiogo. 3.3. Effect of prescribed early fire The effect of prescribed fire was not significant for the mean total biomass at either site (Table 6). At the growth form level, the treatment had a tendency to increase the biomass of annual grasses and to decrease the biomass of perennial grasses at both sites. The forb biomass increased in Tiogo (P = 0.008) and was not

Laba

Mean total biomass Annual grasses A. pseudapricus L. togoensis Perennial grasses A. ascinodis A. gayanus Forbs ∗

Tiogo

Early fire

No fire

Early fire

No fire

3.40 1.62 0.19 0.21∗ 1.48 1.05 0.27∗ 0.30

3.36 1.32 0.17 0.03 1.75 0.75 0.83 0.30

3.99 1.58 0.42 0.59∗ 2.20 0.46 1.21∗ 0.20∗

4.04 1.32 0.40 0.21 2.57 0.41 1.80 0.15

P < 0.05.

significantly influenced in Laba. At the species level, the biomass of A. gayanus was significantly decreased with prescribed early fire at both sites (P < 0.001). The opposite effect was found for L. togoensis at both sites (P < 0.032). Prescribed fire had a tendency to increase A. ascinodis and A. pseudapricus biomass at both sites. 3.4. Effects of selective tree cutting Mean total biomass during the study period was not significantly influenced by selective cutting at neither Laba nor Tiogo (Table 7). The different growth Table 7 Impact of selective tree cutting on the mean herbaceous biomass during 1993–2001 in tonnes of dry matter per hectare for each growth form and for the four most common species at the two sites Laba and Tiogo, four replicates Cutting

Mean Total biomass Annual grasses A. pseudapricus L. togoensis Perennial grasses A. ascinodis A. gayanus Forbs ∗

P < 0.05.

Laba

Tiogo

Cutting

No cutting

Cutting

No cutting

3.44 1.49 0.15∗ 0.14∗ 1.61 0.92 0.54 0.34

3.51 1.50 0.26 0.11 1.70 0.98 0.56 0.31

4.01 1.54∗ 0.41∗ 0.48∗ 2.28 0.42 1.42∗ 0.20∗

4.00 1.14 0.28 0.20 2.60 0.40 1.82 0.25

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forms (annual grasses, perennial grasses and forbs) reacted however, differently to the treatment. The mean biomass of annual grasses increased (P = 0.000) following cutting compared to the no cutting treatment in Tiogo, while there was no difference in Laba. Cutting had a tendency to reduce the perennial grass biomass at both sites. The forb biomass following cutting decreased in Tiogo (P = 0.000) but increased slightly in Laba. At the species level, the mean biomass of A. gayanus decreased significantly (P < 0.000) in Tiogo. A. ascinodis had a tendency for decreased biomass in Laba and increased in Tiogo. L. togoensis increased significantly at both sites following cutting (P < 0.000) compared to the no cutting reference. A. pseudapricus biomass decreased significantly in Laba (P < 0.000) whereas it increased significantly in Tiogo (P < 0.000). 3.5. Interaction effects A selective cutting×livestock ×fire interaction was observed in Laba for perennial grasses (P = 0.034) and A. ascinodis (P = 0.042) while in Tiogo, a similar interaction was found for perennial grasses (P = 0.003) and forbs (P = 0.024). A significant selective cutting × livestock interaction was significant at both sites for mean total biomass (P < 0.022) during the study period and in Tiogo at the species level for A. ascinodis and A. gayanus (P < 0.026). Another set of significant interactions between selective cutting and fire was found in Laba for the mean total biomass (P = 0.009), perennial grasses (P = 0.044) and A. ascinodis (P = 0.038). In Tiogo, similar interactions were found for forbs (P = 0.046) and L. togoensis (P = 0.034). A fire × livestock interaction was significant for the forbs in Laba while in Tiogo, this interaction was significant for the mean total biomass (P = 0.009). 4. Discussion 4.1. Herbaceous biomass at the two sites Four species dominated the herbaceous biomass at the two sites. The higher biomass at Tiogo compared to Laba could might be attributed to different soil characteristics. The deeper soils at Tiogo were more

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favourable for the biomass of perennial grasses such as A. gayanus and Diheteropogon amplectens which account for a large part of the biomass in the savannahs of the Sudanian Zone (personal observation). The texture of the soils is also an important determinant for the herbaceous biomass. Sandy soils are more productive than clayey soils during years with below average rainfall while the opposite is the case during better years (Frost et al., 1986; Fournier, 1991; Seghieri et al., 1995). Deep clayey soils accentuates the seasonal contrasts that cause more marked aridity in dry season or intra-seasonal dry periods and greater quantities of water are available over a longer period during above average rainfall years (Seghieri et al., 1994). The risk of high inter-annual variation in herbaceous cover is greater on soils with high clay content than on sandy soils (Dye and Spear, 1982). This could explain the fact that the silty-sandy soils of Laba produced more than the silty-clayey soils of Tiogo for some years during the study period. The large inter-annual variability of the biomass was partly due to the rainfall patterns. In the present study, a correlation coefficient of only 38% was found between herbaceous biomass and annual rainfall. This coefficient increased slightly when taking the number of rainy days per year into account. Primary productivity in natural vegetation is closely related to rainfall and therefore varies greatly, both spatially and temporally (Barnes and McNeill, 1978). Herbaceous biomass is particularly variable and differences of over 500% may occur between successive seasons. Other factors such as temporal rainfall distribution are also important (Beatley, 1974; Deshmukh, 1984; Lehouerou et al., 1988), not only for the peak biomass but also for the species composition (Seghieri et al., 1994). Inter-annual variation in fire intensity and grazing pressure are other factors that influence the annual herbaceous biomass. 4.2. Effects of livestock At the two sites grazing had the same impact on the total herbaceous biomass but there were differences at the growth form as well as at the species levels and the response to grazing was by and large specific for each site. In Laba there was a significantly decreased biomass of perennial grasses i.e. A. gayanus whereas in Tiogo there was a decrease of annual grasses with grazing versus the no-grazing treatment. This suggests

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that in shallow soils, grazing favoured the establishment of annual grass species and hindered the perennials, whereas the opposite was the case for deeper soils. Our findings for A. gayanus in Laba were similar to those of Fournier and Nignan (1997) who under similar climatic conditions found that cattle grazing inhibited its establishment. Competition between different herbaceous species could be the origin of the different grazing responses. North of the area studied, in the Sahelian zone, L. togoensis has been found to be the most competitive species (Seghieri et al., 1995). The establishment of this species is favoured by the fact that it was not grazed after its flowering stage (Sawadogo, 1996). Type of livestock (cattle, sheep, goats, etc.), grazing intensity and the time of the year that the grazing takes place are all factors that vary from one year to another. During the dry season, the non-burned plots were most frequently visited and the straws were grazed. The reduction of litter would favour higher biomass production during the rainy season. The burned plots were frequented to graze the young perennial shoots that had been induced by the fire. This could lead to an exhaustion of the reserves of these perennial grasses especially on shallow soils. For the Sahel, Hiernaux et al. (1988) confirm that severe defoliation might limit next-year production through the depletion of seed stocks. At the beginning of the rainy season, the livestock mostly visited the plots that had received the early fire treatment, that at this time held more biomass since in the unburnt plots dry matter from previous years hinders not only germination and tillering but also grazing. Hiernaux and Turner (1996) conclude that repetitive defoliation during the rainy season could lower same-season production by 50% or more. In the semi-arid savannahs of South Africa, high rainfall and light grazing promote tufted perennial grasses (Fynn and O’Connor, 2000). These researchers also found annuals and weakly tufted perennials favoured by low rainfall but some annuals were also favoured by heavy grazing and high rainfall. Moreover, trampling by cattle and fertilisation by animal dung may have enhanced decomposition by compacting necromass and increasing its contact with the soil, thus exposing it to a potentially more humid and microbial-rich environment. A reduction in herbaceous biomass by grazing and trampling led to reduced

fire intensity, which in turn was favourable for the vegetative propagation of ligneous vegetation (Sawadogo et al., 2002). Pastoralism could therefore not only be compatible with silviculture but may in fact be an essential management option to maintain the ligneous production. In West Africa there is a need to recognise that the monetary value as well as the ecological value of the savannah woodlands can be increased due to the presence of livestock (Gambiza et al., 2000). 4.3. Prescribed early fire effects In this study, effects of prescribed early fire did not significantly influence the total herbaceous biomass at either site. This was most likely due to the opposite reactions from the fire treatment with increased annual and decreased perennial grass biomass which balanced the overall effect. This effect was the same at both sites and in this respect the burning had a homogenising effect on the sites. The organic matter accumulated in the unburnt plots may have hindered both germination of the annual grasses and hampered the tillering of perennial grasses. In general it is believed that the release of minerals from the ashes following burning favour higher herbaceous production. Smoke has also been found to improve seed germination (Adkins and Peters, 2001). The biomass of A. gayanus was significantly reduced by the prescribed early fire at both sites. This could partly be explained by the combined effects of fire and grazing leading to exhaustion of the reserve (Rietkerk et al., 1998). The decrease in production of this species is favourable for higher ligneous production. A. gayanus is the most productive and tallest (over 3 m) grass species and induces very intense fire with high flames. Higher tree mortality after fire where A. gayanus species dominate the herbaceous cover has been observed (personal field observation). In general, it is believed that pastoral groups use fire to improve pasture, however in our study burning had a tendency to lower the pastoral value by a shift to annual grass species at the expense of perennials. When comparing the Sudanian with the Sahelian zone it should be noted however that the fires in latter area are lighter. Perennial grasses are more resistant to fire due to their underground reserves and the fact that they are commonly dormant during the dry season when fires occur (Liedloff et al., 2001). Adult individuals

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of perennial grasses are also more resistant due to their meristems being located at the base of the plant where they are protected from heat by densely packed stems and leaves (Garnier and Dajoz, 2001b; Jensen et al., 2001). Variable seed awn length is another fire adaptation that was found to be correlated to seed burial depth for Hyparrhenia diplandra (Garnier and Dajoz, 2001a). In Ivory Coast, Garnier and Dajoz (2001b) found that seedling growth and survival rate of the perennial grass H. diplandra were significantly higher in unburned plots and that there is a correlation between size of the seedling and its ability to sustain a fire. They also suggested that less frequent but still regular fires may allow establishment while preventing the accumulation of litter. Apart from its role as combustibles, herbaceous litter is thought to interfere with emergence and growth of seedlings (Bergelson, 1990; Facelli, 1994). 4.4. Effects of selective cutting The effects of selective tree cutting differed according to site, growth form and species. Some species were favoured by cutting while for other species we believe the treatment may have had the opposite effect. This could explain the fact that selective cutting of ligneous vegetation had no significant effect on the average total herbaceous biomass during the study period at neither of the two sites. Tree cutting was assumed to increase herbaceous production by reducing the competition for water and nutrients and by increasing the availability of light (Frost et al., 1986). In the absence of grazing, removing harvestable trees increased grass production (Gambiza et al., 2000). However, this may not be valid in arid and semi-arid areas where regeneration of trees was usually achieved by vegetative propagation. The compensation response of the trees might increase the competition for water and nutrients to the detriment of the herbaceous layer, as the harvested trees need more resources to regenerate. Indeed, all the tree species that were cut regenerate by vegetative propagation (Sawadogo et al., 2002). Some tree species that are very common at the study sites such as D. microcarpum and E. africana responded to cutting not only by vigorous sprouting but also by producing numerous root suckers. Moreover, since the selective cutting treatment only reduced the basal area of the ligneous vegetation by about 50%, there

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was no major reduction in the shading effect. In this respect, clear-cutting of the ligneous vegetation component would certainly be more favourable for the herbaceous vegetation than selective cutting.

5. Conclusions Different species respond differently to disturbance by fire, grazing and selective tree cutting in accordance with the aim of this study. These treatments could therefore be used as tools for sustainable management of savannah woodlands but it should be noted that the response is very much site-specific. In the Sudanian Zone, the presence of livestock in state forests can be a valuable option for sustainable multiple-use management. Indeed, by reducing herbaceous biomass, the presence of livestock may favour ligneous production by reducing the severity of fire. Nevertheless, further studies should be undertaken mainly to determine the level of grazing and cutting for sustainable management. Furthermore, data from long-term experiments are needed to be able to take the highly irregular rainfall as well as how different species react to the treatments into account.

Acknowledgements Funding for this study was provided by Swedish International Development Cooperation Agency (Sida). We are grateful for statistical advice from Sören Holm at Department of Forest Resource Management and Geomatics at Swedish University of Agricultural Sciences.

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