Vegetation diversity and change in relation to land use, soil and rainfall — a case study from North-East District, Botswana

Journal of Arid Environments (2000) 44: 19–40 Article No. jare.1999.0566 Available online at http://www.idealibrary.com on

Vegetation diversity and change in relation to land use, soil and rainfall 0 a case study from North-East District, Botswana

Annika C. Dahlberg Environment and Development Studies Unit, Department of Physical Geography, Stockholm University (Received 10 October 1998, accepted 1 July 1999) The debate about the sustainability and productivity of communal lands, especially in comparison with privately managed land, is far from settled. Emerging theories emphasize the spatial and temporal diversity of the environment, and are often in agreement with local opportunistic land management strategies. This study explores differences in variables such as plant species richness, composition, and abundance of the field-layer and woody vegetation, between sites with different soils and different histories of land use (communal, private ranch, and rested from grazing), for 2 years. The results indicate that, for the study area chosen, differences in land use have not caused any major differences in the vegetation. However, there were clear differences depending on soil type, and plant production increased strongly with a slight increase in rainfall. Although people and livestock have had a strong impact on the vegetation, most indicators of degradation were absent, implying that the land has not lost its productive potential. ( 2000 Academic Press Keywords: vegetation change; vegetation diversity; semi-arid; Southern Africa; agropastoralism; land use; overgrazing; bush encroachment

Introduction In Botswana, as in large parts of Southern Africa, communal land is the most common form of land tenure. In eastern Botswana, the majority of the population derive their livelihood from rainfed subsistence farming, combined with livestock keeping and the utilization of a variety of natural produce (Silitshena & McLeod, 1989, pp. 79}86). For almost a century, the official view has been that this land use has resulted in increased land degradation, and a number of policies have been introduced to halt this process. As regards tenure, these have advocated the privatization of land use, such as the establishment of fenced commercial cattle ranches with exclusive grazing rights, to promote a more sustainable land management (Botswana, 1975; Sandford, 1980). In the last few decades it has been recognized that these privatization ventures have not had the envisaged positive effects (Botswana, 1991a; White, 1993). However, the official view is still that ‘the uncontrolled management of communal grazing is not only unproductive but has led to unprecedented range degradation’ (Botswana, 1991b, p. 10), and the debate about the rationale of communal land-use practices is as strong as 0140-1963/00/010019#22 $35.00/0

( 2000 Academic Press

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A. C. DAHLBERG

ever (Peters, 1987; Okoth-Ogendo, 1998). For many areas an opportunistic approach to land management, as practised by communal farmers, has been found to make ecological as well as economic sense (Abel & Blaikie, 1989; Behnke & Scoones, 1993; Dahlberg & Blaikie, 1999; Sullivan, 1999). In recent re-evaluations of ecosystem dynamics of semi-arid rangelands, including socio-environmental interactions, many established assumptions have been questioned. The earlier models describe potentially stable ecosystems, where range conditions vary linearly (or at least monotonically) with rainfall and grazing pressure respectively (Walker, 1993). Inherent here is the assumption that certain species or vegetation types indicate a specific stage of land degradation, often related to overgrazing. The emerging view places more emphasis on factors such as spatial heterogeneity, stochastic events, especially the dominant influence of fluctuating rainfall, and the persistence of ecosystems through time (Ellis & Swift, 1988; Behnke & Scoones, 1993). A number of studies have dealt with the interaction of rainfall and grazing on southern African rangelands (for example Milton & Hoffman, 1994; O’Connor & Roux, 1995; Jeltsch et al., 1997). Some have compared vegetation change on communal and private rangeland (O’Connor, 1995; Parsons et al., 1997), while others have looked more specifically at communal land (Shackleton et al., 1994; Harrison & Shackleton, 1999). Most previous studies have been conducted in pastoral areas, but the socio-environmental dynamics described apply also to agricultural ecosystems (Niemeijer, 1996). Also, grazing effects have mainly been related to the field-layer vegetation, while browse is seldom considered (BergstroK m, 1992). The present study was conducted in the North-East District, Botswana, where communal land use is characterized by alternating cultivation and grazing, in a spatial and temporal sense. Even before the turn of the century, a large part of the district was managed as private farms under freehold tenure. Under colonial rule the communal area decreased further, with an accompanying increase of human and livestock population densities. Freehold farms still cover more than half of the district (RAO, 1986). Since the 1930s, official reports have described a situation of ongoing and severe land degradation in the communal areas, although this has recently been questioned (Fortmann, 1989; Dahlberg, 1996; Kinlund, 1996). Aerial photographs and reconnaissance visits indicated that, in the study area, differences in management between communal and ranch land had not had an impact on environmental conditions. Thus, the hypothesis tested was that the heterogeneity of the landscape caused by soil distribution and rainfall fluctuation was more important for explaining the spatial and temporal diversity of the vegetation than the type of land use. Also, the long-term persistence of the ecosystem, as described in recent models of range ecology, and by local farmers, was tentatively explored by comparing the grazed land with an area rested from communal grazing for 4}6 years. The influence of land use, soil type, and rainfall on species richness, species composition, and abundance of the field-layer and woody vegetation, was compared. Characterization of the study area The study was carried out in and near the village of Kalakamate (Fig. 1). The environmental and social history of the area have been discussed in more detail elsewhere (Dahlberg, 1996; Dahlberg & Blaikie, 1999). In the colonial period the village was bordered in the east by a private cattle ranch, but until the late 1980s people and cattle were able to move freely in most directions, when a veterinary cordon fence curtailed movement to the north and north-east. Dense tree and bush savanna alternates with more open areas. Colophospermum mopane dominates in some areas, while the woody vegetation elsewhere is quite varied, other common genera being Acacia, Combretum and Commiphora. Grass cover varies between areas and years, and Aristida and Eragrostis are

VEGETATION DIVERSITY AND CHANGE

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Figure 1. Map of (a) Botswana, (b) North East District with Kalakamate village, freehold land and study area marked, and (c) detail of communal land, private ranch and veterinary cordon fence with sampling sites (1}6) indicated.

common genera. Sampling sites were located where communal land borders the fenced private ranch and the veterinary cordon fence. The latter constitutes a narrow corridor which has been ungrazed since 1987}88, thus representing a third land-use class. Two of the most common soil types occurred in each of the three land-use classes. The survey was conducted in March}April 1992, and was repeated 2 years later (March}April 1994) under different rainfall conditions. Land use Past and present land use was investigated by means of interviews and monotone aerial photographs from 1950, 1964, 1971, 1981, and 1988. On communal land the spatial pattern of land use has changed over the last few decades (Dahlberg & Blaikie, 1999). Farming activities had become more concentrated to certain areas, and at the study sites signs of previous cultivation had almost vanished by 1988. Grazing had also become less intense in this part of the village, although cattle were still encountered daily. Ideally the vegetation data should be related more exactly to grazing intensity. However, for the communal land, records of cattle were available only for 1976, 1981 and 1984}1992, and these data were not very reliable. During this time density numbers fluctuated

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A. C. DAHLBERG

between 5 and 25 cattle km!2 (11 cattle km!2 in 1992), with an average of 13 cattle km~2 (Greenhow, 1976; RAO R12D; Veterinary Office, 1992), and show no trend (Dahlberg & Blaikie, 1999). Goats were said to be increasing, but the few existing records were not enough to show any trend. They give an estimate of some 9}11 goats km~2 for the last few decades (Greenhow, 1976; Veterinary Office, 1992). On the ranch, which covers 36 km2 (RAO, 1986), small fields were cultivated near the study sites in the 1930s or 1940s, and the area was used by village livestock until the 1950s, when it was fenced. The present owner bought the land in 1971, and until the late 1980s rotational grazing was practised, but now the animals roam freely. Local information indicated that cattle numbers had been lower before 1971 than today. In 1971 cattle density was c. 6 cattle km~2, and since then numbers have fluctuated between 8 and 17 cattle km~2. Numbers of goats have varied between 3 and 6 goats km~2. In March 1992 there were approximately 11 cattle and 4 goats km~2. Thus, the main differences between communal and ranch land are mainly related to past management, and the data suggest a less intensive use of ranch land. The veterinary cordon fence consists of two wire fences running parallel 10 m apart, and at construction it was cleared of trees and shrubs. The corridor is patrolled to prevent cattle breaking through, and shrubs are continuously cleared. Soils Granite and gneiss are the common rocks in the area, and regosols the most common soil type. These are shallow, very sandy soils with a high content of gravel and stone and little development of structure. Dolerite intrusions, amphibolite and other basic rocks occur, with associated shallow to moderately deep sandy loams to clay loams (Litherland, 1975; Radcliffe, 1990). Patches of a particular soil are often small, i.e. 10,000 to 20,000 m2 or less, and the two common soil types both occurred in the study area. In March 1992, approximately 2 weeks after the latest rainfall occurrence, one soil pit was dug on each site, and samples taken from all horizons (see Dahlberg, 1996 for details). The first soil type is a reddish brown loamy sand to sandy loam, with a depth of 30}60 cm, occurring next to basic rock. The second soil type, derived from granite, is a greyish-white to greyish-brown coarse sand to loamy coarse sand, with clay content increasing with depth, and often less than 20 cm deep. Here, these soil types are termed ‘red’ and ‘white’ respectively. Organic carbon content was less than 4%, which is in accordance with previous surveys in the area (Radcliffe, 1990). On grazed land organic carbon content was highest for red soil, but on ungrazed land it was highest for white soil. Soil water content was low for all samples (1}2)6%), which is natural for sandy soils (Landon, 1984, pp. 84}85). Water content was similar for most sites on grazed land, but on ungrazed land slightly higher for white soil than for red. On average, samples from ‘ranch’ showed slightly lower values than from ‘communal’ and ‘ungrazed’. In 1992 and 1994 respectively, six samples per site were taken at the soil surface to measure bulk density (Landon, 1984), where high densities indicate soil compaction with an adverse effect on plant growth (Wambeke, 1992). Average values obtained for all sites and both years range from 1)3}1)6 g cm~3, which is normal for sands and sandy loams (Landon, 1984, pp. 79}80). The only significant difference between samples (p(0)001) was that values in general were 0)1 to 0)2 g cm~3 lower in 1994, the year with more rainfall. Rainfall Rainfall is extremely variable between and within years, and over short distances. As such the reliability of records and the proximity of the rain gauge are important. Sebina,

VEGETATION DIVERSITY AND CHANGE

23

Figure 2. Percentage deviation from long-term mean of annual rainfall (July}June) at Francistown (1922/23}1993/94). The two sampling years are indicated by black bars.

26 km from the study area, is the nearest gauge and records go back to 1958/59, but are not very reliable. Francistown, 66 km from the study area, has reliable weather records going back to 1922/23 (Department of Meteorological Services, Gaborone). These data are used to depict averages and variation (Figs 2 & 3). Annual rainfall data (1922/23}1993/94, hydrological year July}June) give the mean as 459 mm, the median as 426 mm, and the coefficient of variation as 39%. Rainfall data from Francistown and Sebina were correlated (Dahlberg & Blaikie, 1999), and for the years of sampling, records from Sebina show lower rainfall than those from Francistown. Due to the above considerations, differences in rainfall between sampling years should be treated qualitatively (‘more and less rainfall respectively’). Although both rainy seasons received below average rainfall, 1993/94 was slightly higher in total, and much higher in the period before sampling was conducted. For vegetation studies, rainfall in years preceding sampling is also important, and 1990/91 was close to ‘average’, while rainfall in the intervening year, 1992/93, was slightly above average in total, but below average towards the end.

Figure 3. Mean monthly potential evaporation, mean and median monthly rainfall (in mm), and coefficient of variation (CV) of monthly rainfall (in per cent) at Francistown. Rainfall values, including CV, are calculated on data from 1922}1994. Values of potential evaporation are based on available data from a U.S. Weather Bureau class A pan (Bhalotra, 1985), essentially covering the period 1958}1992: } } } ; mean potential evaporation; ——— ; mean rainfall; - - - - ; median rainfall; }}L}} ; CV% (right hand scale).

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A. C. DAHLBERG

Methods Sampling procedure Six sites were selected, one on each of the two soil types for each of the three land-use classes (Fig. 1). Three 50 m long transects were laid out randomly on each site, giving 18 transects in total. Along each transect, ten 1 m2 quadrats were evenly spaced to record variables of the field-layer vegetation, as well as three 100 m2 quadrats to measure the woody vegetation (Fig. 4). Measurements were conducted over a 4 week period in March to April 1992, the late rainy season, and repeated in the same quadrats in March to April 1994. In site 2 (white soil in ranch), markings for two transects had disappeared by 1994, so here measurements were repeated only along one transect. An ‘individual’ is here meant as a morphologically distinct part of a plant (e.g. a tiller or a tussock), and does not necessarily correspond to the genetic individual (Begon et al., 1986, pp. 125}128). Bare ground is defined as soil surface not covered by litter (the dead plant material) or by the projected cover of the field-layer vegetation. Most species could be identified on site, and were otherwise collected for later identification. However, several forbs could not be identified and are therefore classed as one group, and in 1992 the condition of some grasses was too poor for them to be identified with certainty.

Variables and analytical methods Most variables were measured in both years. Species richness, often equated with diversity, can be measured in many ways, and little consensus exists as to which measure is most appropriate (Kent & Coker, 1992, p. 95). The most commonly used method is a simple count of species (Magurran, 1988), which may be presented as the total number in a community, or as the mean of numbers of smaller units. Here, species richness was primarily calculated per transect. For each category (site, land-use class, soil type, and year, or specified combinations of these) values are given for the total number of species, and for the average of the number of species per transect. The woody vegetation, divided into the three height classes, 0}0)5 m (seedlings or small shrubs), 0)5}2 m (shrubs) and '2 m (tall shrubs or trees), was also analysed for density and species composition. For the field-layer vegetation, two estimates of abundance were made: cover and density. In 1992 and 1994, surface cover for the three classes, field layer, litter, and bare ground, was estimated for each 1 m2 quadrat using the Domin scale (Kent & Coker, 1992, p. 45). In 1992, cover per species (sometimes groups of species) was also

Figure 4. Sampling design along each transect.

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estimated (where the sum of individual estimates may differ from the estimated total). This provided estimates of the cover of annual and perennial grasses, and of grasses of different forage value. Annual grasses are generally considered to have less forage value than perennials, to provide less protection against erosion, and a dominance of annuals is often used as an indication of overgrazing (Rooyen et al., 1991). Forage value also differs between species, and has here been classed as poor, intermediate, or good, after information in Field (1976), Hendzel (1981), and Skerman & Riveros (1989). This classification, based on nutrient content and digestibility, is a simplification since these variables may vary over the year (Skarpe & BergstroK m, 1986), and between areas (Theunissen et al., 1992). A repeated estimation of relative cover of individual species after 2 years was considered too unreliable for comparisons. Instead densities (number of individuals per m2) of specific species, measured in both years, were compared. Here, comparisons of five common species, where the correlation between cover and density in 1992 was reasonably good (r-values vary between 0)72 and 0)97), are presented. For each separate year, the main interest was to compare the two soil types and the two land-use classes, ‘communal’ and ‘ranch’. When warranted, these land-use classes were grouped together as ‘grazed area’, and compared with the ‘ungrazed area’. These two classes also reflect differences other than grazing pressure. The absence of trees and shrubs in the ‘ungrazed area’ is likely to affect grass productivity, either positively or negatively depending on interactions with other processes (Belsky et al., 1989; Belsky & Amundson, 1998). Thus, only tentative conclusions can be drawn from comparisons with the ungrazed sites. Analyses for each separate year are based on all measured transects, while for comparisons between years the two transects missing in 1994 have also been excluded from the 1992 data. For most variables diagram comparisons are complemented by tests of difference. To limit the number of influencing factors, sites were chosen within a small area, and in Kalakamate a juxtaposition of the three land-use classes occurred only in one place (Fig. 1). This closeness means that the transects do not constitute independent representative samples of a specific soil type for the separate land-use classes. Rather, we have a natural approximation of a field experiment with six treatment combinations and three replicates (at the highest hierarchical level of replication) within each treatment. However, contrary to the design commonly applied in planned field experiments (e.g. randomized block design), the three transects for each treatment combination are, by necessity, spatially clustered (due to soil type variation over short distances). Despite the short distance between any two transects, it is probable that the transects within each site are more highly correlated to each other as regards non-studied factors which could influence the vegetation, than they are to transects in the other sites. Inherent here is a risk of spatial pseudoreplication, since the sample area is ‘smaller or more restricted than the inference space implicit in the hypothesis being tested’ (Hurlbert, 1984, p. 190). The risk of temporal pseudoreplication also exists, since several weeks were needed to collect all data, during which time variables may change. For studies at landscape level, controlled experiments are often very difficult and/or costly, and non-representativeness as discussed above can seldom be avoided. However, pseudoreplication should not be as related to sampling design and procedure, as to the application of inappropriate statistical analyses, or to the use of unallowable generalizations. Much ecological research, especially at the scale of the landscape or region, is inferential and inductive, but conclusions — with requisite qualifiers — need not be of less interest or value (Hargrove & Pickering, 1992). Due to the nature of the data (e.g. quadrat data were not normally distributed and transects were few), the non-parametric Mann-Whitney U test (for two independent samples) was used throughout (Siegel, 1956; Kent & Coker, 1992). The significance levels found were not compared to a priori requirements, but instead used as semiqualitative ‘relevance indicators’ for individual differences, giving structure to the

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data. In aggregate, the data enable us to judge, for instance, in what respect differences between soil types are more clearly evident than between land-use classes. Standard statistical symbols have been used (NS"p'0)05, *"0)01(p(0)05, **"0)001(p(0)01, and ***"p(0)001), and additional details are presented in Dahlberg (1996). Results Field-layer vegetation In total, 24 grass taxa (species or, in a few cases, groups of species) were identified, of which five were annuals. Ten species of forbs were identified (and several more were found), and these and the grasses are common for eastern Botswana (Field, 1976). In 1992, the species composition of grasses was similar for all sites on the ranch and communal land, but differed between these and the ungrazed sites. Two species were found only in the ranch, none were specific for communal land, and eight species occurred only in the ungrazed sites. In 1994, except for two species, the ranch and the ungrazed area had the same species composition. Overall species composition differed slightly between the two years, with three species from the first year not found in the second, when instead five new species were recorded. The number of annual grass species varied between three and four in all sites, except for ungrazed white soil in 1994, where all five were found. For all sites, the species richness of grasses (mean number of species per transect) was higher in the second year. Also, the total number of species per site (and category of land use and soil type) increased (except in site 5, and for ‘ungrazed areas’, where numbers were the same) (Table 1). More species were found on the ranch than on Table 1. Species richness for grasses — mean number of species per transect (in italics) and total number per site, land-use class, and soil type for each year Site Soil

1 (n"3) Red

2 (n"3) White

3 (n"3) Red

4 (n"3) White

5 (n"3) Red

6 (n"3) White

total (n"18)

1992

4)7}8

4)0}5 (3)

5)0}6

4)0}6

4)7}8

10)0}11

1994

9)7}14

8*

7)7}10

6)7}9

6)3}8

12)0}14

5)4 (5)5)} 18 (17) 8)4}19*

Land use 1992 1994 Land use 1992 1994

Soil 1992 1994

Ranch (n"6) 4)3 (4)3)}9 (8) 9)3}16*

Communal (n"6) 4)5}7 7)2}13

Grazed (n"12) 4)4 (4)4)}10 (9) 8)0}18* Only grazed sites (1}4) Red (n"6) White (n"6) 4)8}9 8)7}15

4)0 (3)8)}7 (6) 7)0}11*

Ungrazed (n"6) 7)3}15 9)2}15 All sites (1}6) Red (n"9) White (n"9) 4)8}11 7)9}15

6)0 (6)4)}15 (14) 9)1}17*

For 1994, * denotes that the number of transects equals the given n-value minus 2, because two transects on site 2 were not measured that year. For 1992, values in parentheses give means per transect or totals per category when the same transects are excluded.

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27

communal land, and for both these land-use classes the red soil was more species-rich than the white soil. In both years, the means per transect were higher in the ungrazed than in the grazed sites, but the increase from 1992 to 1994 was much higher in the grazed sites. In the ungrazed area, the white soil was more species-rich than the red soil, i.e. the opposite to the grazed sites. The cover of field-layer vegetation, litter, and the remaining area of bare ground, were estimated for each site (Fig. 5(a}c)), and tested for differences between soil types, land-use classes and years (Table 2). For grazed sites, field-layer cover was very low, especially in 1992. In neither year was there any significant difference between ranch and communal land, for this or the other two variables. However, in the grazed sites cover of living plants and litter were significantly higher on the red soil (although a few exceptional quadrat cover values causes the diagram presentation to show a reversed trend for the ranch in 1994). Although still low, field-layer cover on grazed sites was substantially higher in 1994, but the increase varied between sites. Field-layer cover was much higher in the ungrazed sites than on grazed sites, but this difference was less pronounced in the wetter year. For the ungrazed sites, no consistent differences in field-layer cover between soil types were found, while cover of litter was consistently higher on the red soil. Cover of litter was higher in the ungrazed sites in 1992, while in 1994 it was higher in the grazed sites. Cover of annuals (measured only in 1992) was completely dominated by one of the five species groups. Cover of annuals as a percentage of all grasses exceeded 33% for only 28 of the 180 quadrats, all on grazed land (five on red soil and 23 on white soil), and in most quadrats it was below 0)1%. No difference in proportional or actual cover of annuals was found between ranch and communal land (Fig. 6(a) and Table 3). For these sites, estimates of proportional cover indicate that annuals were more important on white than on red soil. However, actual cover of annuals did not differ between soil

Figure 5. Percentage area covered by (a) field-layer vegetation, (b) by litter and, (c) per cent area of bare ground for the six sites in 1992 ( ) and 1994 ( ). For 1992 all transects in site 2 have been included, also the two missing in 1994. The differences compared to values only for the transect remaining in 1994 were small (0)4 percentage units for field-layer cover, and about 3 percentage units for litter and bare ground). The intervals shown are 95% confidence intervals based on distribution of quadrat data. Note that scales differ between the diagrams.

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A. C. DAHLBERG

Table 2. Area of bare ground and cover of grasses and forbs, and of litter, in 1992 and 1994 — tests of difference

ONLY GRAZED SITES ranch vs. communal red vs. white soil

Grasses & forbs Litter Bare ground

1992

1994

1992

1994

lower vs. higher rainfall 1992 vs. 1994

NS NS NS

NS NS NS

**r ***r ***w

**r ***r ***w

***94 ***94 ***92

Sample sizes were as follows: ranch n"60 (40 in 1994), communal n"60, red soil n"60, white soil n"60 (40 in 1994), all grazed sites in 1992 n"120, in 1994 n"100.

grazed vs. ungrazed

Grasses & forbs Litter Bare ground

ALL SITES red vs. white soil

1992

1994

1992

1994

lower vs. higher rainfall 1992 vs. 1994

***un ***un ***gr

***un **gr NS

NS ***r *w

NS ***r ***w

***94 NS ***92

Sample sizes were as follows: grazed n"120 (100 in 1994), ungrazed n"60, red soil n"90, white soil n"90 (70 in 1994), all sites in 1992 n"180, in 1994 n"160. Mann-Whitney U test was used for all comparisons. Abbreviations show for which land-use class, soil type, or year, the values were highest: r"red soil, w"white soil, gr"grazed area, un"ungrazed area, 92"1992, 94"1994.

types; instead the actual cover of perennials was significantly higher on red soil. On ungrazed sites, average proportional cover of annuals was below 1% for both white and red soil. Of the 24 grass species (or groups) found, nine were classified as having ‘good’ forage value, nine as ‘intermediate’, and six as ‘poor’. On grazed sites, grass cover was dominated by species of poor forage value, especially on the red soil (Fig. 6(b) and Table 3). Proportional and actual cover of species of intermediate forage value were higher on white soil than on red. Comparing ranch and communal land, proportional

Figure 6. Cover (a) of annual ( ) and perennial ( ) grasses, and (b) of grasses of different forage value, poor ( ), intermediate ( ), and good ( ), as a percentage of total grass cover in 1992.

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Table 3. Cover of annual and perennial grasses, and of grasses of different fodder value in 1992 — tests of difference

ONLY GRAZED SITES ranch vs. red vs. communal white soil

ALL SITES grazed vs. red vs. ungrazed white soil

Annuals Perennials

NS NS

NS ***r

***gr ***un

NS **r

Good Intermediate Poor

NS *co NS

NS *w ***r

***un ***un NS

NS NS NS

Sample sizes were as follows. For only grazed sites: ranch, communal, red soil and white soil respectively n"60. For all sites: grazed n"120, ungrazed n"60, red and white soil respectively n"90. Mann-Whitney U test was used for all comparisons. Abbreviations show for which land-use class or soil type the values were highest: r"red soil, w"white soil, ra"ranch, co"communal, gr"grazed area, un"ungrazed area.

cover of grasses of good forage value was higher on the communal red soil site, but in actual cover no significant difference was found for any forage class. In the ungrazed area, proportional cover of species of good forage value was much higher than on grazed sites, and cover of ‘intermediates’ was more than double. For all sites together, no significant difference in actual cover for any forage class was found between soil types. Larger differences between communal and ranch land were found for cover of dominating species (measured in 1992). Proportional and actual cover (Fig. 7 and Table 4) of Aristida congesta was higher on ranch sites, while Oropetium capense was much more common on communal sites. Annual Eragrostis spp. showed no difference in proportional cover, but actual cover was higher on communal land. Forbs and Urochloa spp. were proportionally more important on communal land, while actual cover did not

Figure 7. Cover of forbs and of grass species as a percentage of total field-layer vegetation cover in 1992. The class ‘unspecified grasses’ mainly includes species that occurred at very low cover values, and also unidentified species. The group Eragrostis spp. were all annuals. The group Urochloa spp. mainly consists of the perennial U. mosambicensis, but also includes some individuals of the annual U. trichopus. : Forbs; Unspecified grasses; Eragrostis spp; Aristida congesta; Oropetium capense; Eragrostis rigidior; Heteropogon contortus; Urochloa spp.

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Table 4. Cover of forbs and dominating grass species in 1992 — tests of difference

ONLY GRAZED SITES ranch vs. red vs. communal white soil Forbs (mainly A}p) Eragrostis spp. (A}i) Aristida congesta (P}p) Oropetium capense (P}p) Eragrostis rigidior (P}i) Heteropogon contortus (P}i) Urochloa spp. (mainly P}g)

NS *co *ra ***co } } NS

*r *w ***r *r } } NS

ALL SITES grazed vs. red vs. ungrazed white soil ***un ***gr *un ***gr ***un ***un ***un

NS NS NS *r NS *w **r

Sample sizes were as follows. For only grazed sites: ranch, communal, red soil and white soil respectively n"60. For all sites: grazed n"120, ungrazed n"60, red and white soil respectively n"90. Mann-Whitney U test was used for all comparisons. Abbreviations describe the species as A"annual, P"perennial, and their fodder value as p"poor, i"intermediate, and g"good. Other abbreviations show for which land-use class or soil type the values were highest: r"red soil, w"white soil, ra"ranch, co"communal, gr"grazed area, un"ungrazed area, and — when no individuals were found.

differ significantly between communal and ranch sites. Differences between soil types also occurred. For grazed sites, actual cover of forbs, A. congesta and O. capense, was higher on red soil (not evident as proportional cover), while proportional and actual cover of Eragrostis spp. was higher on white soil. There was a significant difference in actual cover between grazed and ungrazed sites (Table 4). Species cover values were much higher on ungrazed sites, except for O. capense and annual Eragrostis spp., which had significantly higher values on grazed sites. For proportional cover, perennials of intermediate or good forage value (Eragrostis rigidior, Heteropogon contortus and Urochloa spp.) were dominant on ungrazed sites. Cover of H. contortus was higher on white soil, while cover of Urochloa spp. was higher on red (and cover of E. rigidior showed no difference). Individuals of each species were counted, and changes in abundance at species level between the two years were analysed using these data. For some species differences between soil types and/or land-use classes occurred only in one of the two years, or at different levels of significance (Table 5). A positive influence of the higher rainfall in the second year was clear, and expected, for total cover of grasses and forbs (Fig. 5(a) and Table 2), as well as for number of individuals of most species. However, for a few species, e.g. O. capense, average numbers (and cover) were less in the second year, and overall the degree of increase was found to vary substantially between sites (see examples in Fig. 8). The resulting large confidence intervals reflect the patchy occurrence of grasses. Tests of difference (based on distribution of quadrat data) confirmed that the significant changes represent a higher number of individuals in 1994 (see examples in Table 6). Woody vegetation For the two years together, 43 woody species were identified. Species richness (Table 7) was higher on red soil, while there was no difference between the two land-use classes ranch and communal. (No trees and only the odd shrub grew within the ungrazed area, so this land-use class is not included here.) The difference between soil types was even more pronounced than the corresponding pattern for the field-layer

VEGETATION DIVERSITY AND CHANGE

31

Table 5. Density (mean number of individuals m~2) of some common grass species — tests of difference between soil types and land-use classes, separately for each year

ONLY GRAZED SITES ranch vs. communal red vs. white soil 1992 1994 1992 1994 Chloris virgata (A}i) Tragus racemosus (A}p) Aristida congesta (P}p) Oropetium capense (P}p) Urochloa spp. (mainly P}g)

} *ra *ra ***co NS

NS NS NS ***co NS

} NS ***r *r NS

***w **w NS *r ***r

Sample sizes were as follows: ranch n"60 (40 in 1994), communal n"60, red soil n"60, white soil n"60 (40 in 1994).

ALL SITES grazed vs. ungrazed red vs. white soil 1992 1994 1992 1994 Chloris virgata (A}i) Tragus racemosus (A}p) Aristida congesta (P}p) Oropetium capense (P}p) Urochloa spp. (mainly P}g)

**un ***gr *un ***gr ***un

**un NS NS ***gr ***un

*w NS NS *r **r

***w NS NS *r ***r

Sample sizes were as follows: grazed n"120 (100 in 1994), ungrazed n"60, red soil n"90, white soil n"90 (70 in 1994). Mann-Whitney U test was used for all comparisons. Abbreviations describe the species as A"annual, P"perennial, and their fodder value as p"poor, i"intermediate, and g"good. Other abbreviations show for which land-use class or soil type the values were highest: r"red soil, w"white soil, ra"ranch, co"communal, gr"grazed area, un"ungrazed area, and — when no individuals were found. For the annuals C. virgata and T. racemosus numbers in 1992 were so low that the high correlation found may be spurious. However, individuals of these species did not differ much in size.

vegetation. All species not found in 1994 had in 1992 been represented by only 1}4 individuals, of which all except one were seedlings. (Two of the species not found in 1994 grew only on the missing transects.) Density (by height class) was tested for difference between soil types, between land-use classes and between years. Only two significant differences were indicated: (1) there were significantly (p(0)01) more shrubs (0)5}2 m) on communal land than in the ranch, and (2) there were significantly (p(0)05) more seedlings ((0)5 m) in 1992 than in 1994. Among the common species only one significant difference between the two years was found. Seedlings of Dichrostachys cinerea (often described as an indicator of overgrazing) occurred in significantly lower numbers in 1994 than in 1992. When combining species composition with density data, the most striking difference, for all height classes, was between the two soil types (Fig. 9(a}c) and Table 8). (Since there were so few differences between the two years, this analysis is given only for the 1992 data.) A number of the most common species were found in significantly higher densities on red soil, and in several cases this held true for all height classes, e.g. Acacia nigrescens, Combretum apiculatum, Grewia spp. and, for shrubs and trees, Commiphora mollis. The only species with a highly significant ‘preference’ for white soil, in all height classes, was Colophospermum mopane. At a lower level of significance, Ormocarpum trichocarpum, which mainly occurred as 0)5}1)5 m tall shrubs,

32

A. C. DAHLBERG

Figure 8. Difference in density (mean number of individuals m~2) between 1992 (s) and 1994 (d) for (a) Chloris virgata, (b) Tragus racemosus, (c) Aristida congesta, (d) Oropetium capense, and (e) Urochloa spp. For these species a reasonably strong correlation was found between number of individuals and cover. A"annual, P"perennial, p"poor forage value, i"intermediate forage value, g"good forage value. The intervals shown are 95% confidence intervals based on distribution of quadrat data. Note that scales differ between the diagrams.

was also more common on white soil. Density differences related to land use were less common, less consistent between height classes, and confirmed at a lower level of significance. The clearest difference was that Terminalia randii was more common on communal land. Discussion One of the main objectives of this study was to investigate whether, in this particular area, land management under different tenure conditions had resulted in differences in selected variables of the field-layer and of woody vegetation. The results indicate very few such differences, mostly with a low level of significance, and seldom consistent between the two years. For the field-layer vegetation, total species

VEGETATION DIVERSITY AND CHANGE

33

Table 6. Density (mean number of individuals m~2) of some common grass species — tests of difference between 1992 (low rainfall) and 1994 (higher rainfall)

LAND USE CLASSES

SOIL TYPES Only grazed All sites sites Ranch Communal Grazed Ungrazed Red White Red White Chloris virgata (A) Tragus racemosus (A) Aristida congesta (P) Oropetium capense (P) Urochloa spp. (mainly P)

NS NS ** NS

* ** *** NS

** * *** NS

** *** NS }

} NS *** NS

** *** *** NS

} NS *** NS

*** *** ** NS

***

***

***

**

***

NS

***

*

Sample sizes for each category and year were as follows. For land use classes: ranch n"60 (40 in 1994), communal n"60, grazed n"120 (100 in 1994), ungrazed n"60. Soil types for grazed sites only: red soil n"60, white soil n"60 (40 in 1994). Soil types for all sites: red soil n"90, white soil n"90 (70 in 1994). Mann-Whitney U test was used for all comparisons. An identified significant change in all cases meant a higher number of individuals in the year with higher rainfall (i.e. 1994 as compared to 1992). For these species a reasonably strong correlation was found between number of individuals and cover. Abbreviations describe the species as A"annual, P"perennial.

richness was slightly higher on the ranch than on communal land, while mean values per transect were less consistent, and in neither year did cover of the field-layer or litter differ between these land-use classes. For 1992, cover of annual and perennial grasses showed no difference between ranch and communal sites, while grasses of intermediate forage value (mainly annual Eragrostis spp. ) were slightly more abundant on communal sites, and a perennial of poor forage value, Oropetium capense, occurred almost solely on communal land. For other common species, differences were found only at low levels of significance, and were not consistent between the two years. For the woody vegetation, the difference between ranch and communal land was even less. There were very small, and inconsistent, differences in species richness, Table 7. Species richness of woody species. Mean number of species per transect (in italics) and total per site, land-use class, and soil type for each year

Site Soil

1 (n"3) Red

2 (n"3) White

3 (n"3) Red

4 (n"3) White

total (n"12)

1992 1994

23)0}33 22)3}33

17)7}24 (12) 13*

24)7}35 20)7}29

16)7}23 16)7}21

20)5 (20)5)}42 (40) 19)2}36*

Land use 1992 1994

Ranch (n"6) 20)3 (20)3)}35 (33) 20)0}33*

Communal (n"6) 20)7}37 18)7}31

Soil

Red (n"6)

White (n"6)

1992 1994

23)8}39 21)5}35

17)2 (15)5)}27 (24) 15)8}22*

For 1994, * denotes that the number of transects equals the given n-value minus 2, because two transects on site 2 were not measured that year. For 1992, values in parentheses give means per transect or totals per category when the same transects are excluded.

34

A. C. DAHLBERG

Figure 9. Density (mean number of individuals 100 m~2) of the woody vegetation, (a) seedlings, (b) shrubs, and (c) trees and tall shrubs, for each site, including values for the most common species for each height class. For 1992, data for site 2 are presented both for all three transects together and separately for the one that could be measured also in 1994. The intervals shown are 95% confidence intervals for total numbers within a height class based on distribution of quadrat data. Note that scales differ between the diagrams.

VEGETATION DIVERSITY AND CHANGE

35

Table 8. Density (mean number of individuals 100 m~2) of the most common woody species and genera in 1992, divided into height classes — tests of difference between soil types and land-use classes

Soil 0}0)5 m 0)5}2 m Acacia nigrescens Combretum apiculatum Grewia spp. Colophospermum mopane Commiphora mollis Terminalia randii Gardenia spp. Ormocarpum trichocarpum Securinega virosa Commiphora mossambicensis Euclea undulata Dichrostachys cinerea Commiphora pyracanthoides Kirkia acuminata

***r ***r **r ***w } NS NS NS NS NS NS NS } }

***r ***r ***r **w ***r NS *r *w *r } NS NS } }

'2 m *r ***r **r ***w **r NS NS } NS } } NS NS NS

Land use 0}0)5 m 0)5}2 m '2 m NS NS NS NS } *co NS NS NS *co NS NS } }

NS NS NS *co NS **co *co NS NS } NS NS } }

NS *ra *co NS NS **co NS } NS } } NS NS NS

Sample sizes were as follows: red soil n"18, white soil n"18 (9 in 1994), ranch n"18 (9 in 1994), communal n"18. Mann-Whitney U test was used for all comparisons. Abbreviations show for which landuse class or soil type the values were highest: r"red soil, w"white soil, ra"ranch, and co"communal.

while about half of the most common species differed somewhat in density, albeit at low levels of significance, and only for one height class. The exception, Terminalia randii, occurred in higher densities on communal land. This, and the corresponding higher abundance of O. capense, may be caused by the more recent cultivation and settlement on communal land. Until the 1970s, land use was less intensive on the ranch than on communal land, while recent stocking rates have been fairly similar. The early differences in management and stocking rate are not mirrored in vegetation composition and distribution which, except for O. capense and T. randii, are very similar. Parsons et al. (1997) found more pronounced differences for the herbaceous layer when comparing communal land and commercial cattle farms in South Africa. Here stocking rates were higher on communal land, but in spite of this variables such as plant density, basal cover and proportion of palatable species were all higher on communal land. Other studies (O’Connor, 1985, 1995) have indicated that stocking rate has a stronger influence on the vegetation than the land-use system, and that higher stocking densities have a detrimental impact on the vegetation. Since differences in stocking rate are often linked to differences in land management, it is difficult to separate the effect of one from the other (Parsons et al., 1997). In the present study recent land management as well as stocking rates were similar, as were the measured vegetation characteristics. This study shows, contrary to what is often assumed in policy documents, that a difference in tenure may not result in differences in management. Furthermore, the present findings indicate that past differences in management and stocking rate may not have a lasting impact on the vegetation, irrespective of any possible past differences in vegetation characteristics. During the present century the communal areas of the district have been described by scientists and policy-makers as having been subjected to severe and ongoing degradation, while villagers in Kalakamate hold a different opinion. Among scientists, land degradation implies that the productive capacity of the land has been depleted to the

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A. C. DAHLBERG

extent where its regenerative capacity has been seriously impaired. The ungrazed corridor is not an ideal example of what the land would look like without livestock, but a tentative comparison of grazed and ungrazed sites can be made. Overall, the condition of the field-layer vegetation was much better in the ungrazed sites, although for some variables the difference was small. Total cover was much higher in the ungrazed corridor in both years. In 1992, the proportional and actual cover of perennials, species of intermediate and good forage value, and many other species, were higher in the ungrazed sites. In 1994, cover of field-layer vegetation remained higher in the ungrazed sites, but the difference was not as large as in 1992. The protective cover of litter was often higher in the grazed (and wooded) sites, and for some common species there was no significant difference between grazed and ungrazed sites. These results indicate that although the field-layer vegetation in the grazed sites is strongly affected by grazing in the short-term, this impact has not destroyed the regenerative capacity of the soils. After only 4 years rest from grazing (with below average rainfall in 3 years), productivity substantially increased, and several species described as indicators of good grazing condition had become well established. These findings confirm those reported by Shackleton (1993) and Harrison & Shackleton (1999) for communal areas in South Africa. However, it should be noted that neither in these nor in the present study is it possible to ascertain whether the vegetation has reverted to a previously existing condition, or been replaced by a different one. The results also suggest that the difference between the grazed and ungrazed sites is higher in years with low rainfall, demonstrating the importance of rainfall irrespective of other factors (Behnke & Scoones, 1993). Results from the present study indicate an agreement with information given by villagers (Dahlberg, 1996), who state that grass cover fluctuates with rainfall, as do livestock numbers. A decline in vegetative cover is seen as temporary, and a good rainfall year now is claimed to produce as much grass as a good year earlier this century. A further objective of this study was to investigate whether differences in vegetation were related to soil type, and whether this was more important than the differences in management between ranch and communal land. For the study area, soil type had an overriding impact. Species richness of grasses and forbs differed between soil types for all land-use classes, while total cover of field-layer vegetation and litter differed between soil types only on grazed sites. For 1992, proportional cover of annual and perennial grasses and of grasses of different forage value were clearly different on the two soil types, while differences in actual cover were not as pronounced. For most of the common species differences in abundance between soil types were found, although the results differed between years, and for grazed and ungrazed sites. Parsons et al. (1997) reported less impact due to soil differences than to differences in land-use, but in their study differences in land management were more pronounced. In the present study differences between soil types were even more conspicuous for the woody vegetation. Species richness was higher on red soil, and several species were strongly associated with this soil type, while Colophospermum mopane was markedly associated with white soil. Species composition in combination with density was clearly different between soil types, but total density of seedlings, shrubs and trees did not differ between soil types. For the field-layer vegetation, the overall difference between years with different rainfall was, as expected, quite large. Species richness, total field-layer cover, and abundance of some species and groups of species, were higher in the high rainfall year of 1994. However, for many species this increase was not found for all land-use classes or all soil types. Rainfall fluctuations cause differences between years, but have a differential impact on different spatial units (e.g. delimited by soil type and land-use), making relationships in one year invalid or less significant in another. These local variations at different levels of heterogeneity were known and used by the villagers. The woody vegetation differed less between years. The overriding

VEGETATION DIVERSITY AND CHANGE

37

influence of rainfall is confirmed in many studies from arid and semi-arid areas (see for example Behnke et al., 1993; O’Connor, 1995; O’Connor & Roux, 1995). In light of the ongoing debate concerning our understanding of ecosystems characterized by disequilibria, the differential impact of rainfall must be stressed. The present findings confirm those of many previous studies, in demonstrating that rainfall fluctuations always affect the vegetation, but that this effect may differ in magnitude and type depending on factors such as stocking rate, land-use system, and soil type. A partial objective of the study was to relate the findings to the issue of bushencroachment, often used as an indication of overgrazing (Arntzen & Veenendaal, 1986; McLeod, 1992), and to the emerging interest in browse as an important forage resource. Of the nine species listed as the ‘main encroaching tree species in Botswana’ by Arntzen & Veenendaal (1986, pp. 49}50), six were found in the study area. Two of these (Acacia erubescens, Acacia mellifera) were represented by one individual each, and one (Acacia tortilis) by 16 scattered individuals. Acacia tortilis is common in the village area and together with Dichrostachys cinerea, these species are found as scattered individuals throughout the landscape. However, their occurrence is mainly concentrated in space and time to abandoned fields and homesteads that have been abandoned for a period of a few decades. There they grow in dense thickets which are gradually replaced by the mixed vegetation of the surrounding woodland (Dahlberg, 1995). In some areas abandoned fields are colonized by dense stands of C. mopane, which successively give way to open mopane woodland. Dichrostachys cinerea and C. mopane are both listed as ‘encroachers’, and both were very common on the study sites. Dichrostachys cinerea, although one of the 10 most common species, occurred as scattered individuals, not significantly related either to land-use class or soil type, while C. mopane was strongly linked to one soil type. The ‘encroachers’ Grewia spp. were quite common on the study sites, with their distribution mainly related to soil type. At a low level of significance, and only for the odd height class, Grewia spp. and C. mopane were found to be more common on communal land than on the ranch. Overall woody vegetation was quite dense, but species and height classes were mixed throughout, and in none of the sites did species composition or densities indicate bush encroachment. However, as pointed out by Jeltsch et al. (1997), bush encroachment, especially in more arid environments, is a slow process, and thus the short time-period of the present study precludes any definitive conclusions concerning possible long-term change. Browse is often an important component of livestock feed (Skarpe & BergstroK m, 1986; BergstroK m, 1992), and the economic value of woody species may be quite substantial (Campbell et al., 1997). This was confirmed by villagers in Kalakamate. The present results, in line with those from other studies (Dahlberg, 1996; Parsons et al., 1997), thus emphasize that communal land must be evaluated also for produce other than the grazing component. Concluding remarks Many studies have documented that spatial heterogeneity and temporal variation are inherent characteristics of semi-arid environments. The present study confirms observations that variation in soil type and, especially, fluctuations in rainfall, may strongly influence vegetation composition and cover, partially overriding the effect of land use. In the study area, livestock grazing had a strong impact on the vegetation, but no major difference between communal and private rangeland was found. The vegetative cover of grazed sites improved with higher rainfall, and sites rested from grazing recovered quite dramatically in just a few years, indicating that the regenerative capacity is relatively good. The study thus suggests, in accordance with other recent studies from the region, that generalized statements about the relatively bad condition of communal land, and especially the permanence of any specific condition, should be treated with

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A. C. DAHLBERG

caution. Finally, future studies should include more than the herbaceous cover in their investigations. Soil characteristics, overall productivity potential, and other resources used by local people, such as browse, wood and water, are also important. The Office of the President, Republic of Botswana, kindly authorized this research. Financial support was provided by the Swedish Agency for Research Cooperation with Developing Countries (SAREC), the Carl Mannerfelt, Hierta-Retzius, Axel Lagrelius, and Hans W:son Ahlmann foundations. Staff at the Department of Environmental Science, University of Botswana, provided advice and support. Assistance in the field was given by M. Sekei, W.S. Piti and D. Thosa, while W. Arnberg, B. Campbell, O. Eriksson, A. Nilsson, O. Inghe, W. Ellery and two anonymous reviewers gave valuable comments on earlier versions, and discussions with Erling Dahlberg made the work more challenging. K. Weilow and H. Drake drew the maps.

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