Woody vegetation spatial patterns in a semi-arid savanna of Burkina ...

211

Plant Ecology 132: 211–227, 1997. c 1997 Kluwer Academic Publishers. Printed in Belgium.

Woody vegetation spatial patterns in a semi-arid savanna of Burkina Faso, West Africa Pierre Couteron1 & Kouami Kokou2 1

Ecole Nationale du Génie Rural des Eaux et des Forêts, B.P. 5093, 34033 Montpellier, France; 2 Département de Botanique et Biologie Végétale, Université du Bénin, B.P. 1515, Lomé, Togo Received 8 October 1996; accepted in revised form 10 June 1997

Key words: Density regulation, Recruitment, Sahel, Second order spatial analysis, Spatial point processes

Abstract Spatial patterns of woody individuals were studied in a semi-arid savanna of West Africa located in Burkina Faso at and around 14 120 N and 2 270 W. The study was based upon a 10.24 ha plot within which individuals were mapped. Spatial pattern analysis was carried out using second order characteristics of point processes as K functions and pair correlations. The overall density amounted to 298 individuals ha ,1 . The most abundant species were Combretum micranthum G. Don., Grewia bicolor Juss. and Pterocarpus lucens Lepr. Anogeissus leiocarpus (D.C.) G. et Perr. was also an important constituant of this vegetation type, owing to its taller stature. Clumped spatial distributions were identified for all species except for two, for which complete spatial randomness (CSR) was found (including P. lucens, a dominant woody plant). No regular pattern was found even when tall individuals were considered alone. Aggregation dominates interspecific relationships, resulting in multispecific clumps and patches. The overall aggregation pattern was constituted by two different structures. A coarse-grain pattern of ca. 30–40 m was based on edaphic features, and expresses the contrast between sparse stands on petroferric outcrops and denser patches on less shallow soils. A finer-grain pattern made of clumps ca. 5–10 m wide, with no obvious relation to pre-existing soil heterogeneity. There was no overall pattern for saplings (between 0.5 m and 1.5 m in height) irrespective of species, and thus no obvious common facilitation factor. For species with a high recruitment level there was no significant relationship between mature adult and saplings. The only case of clumped saplings with randomly distributed adults was found in P. lucens. However, this cannot be unequivocally interpreted as density dependent regulation since the existence of such a process was not consistent with the spatial distribution of dead P. lucens individuals (victims of the last drought). The mean density around dead P. lucens was lower than around surviving ones, indicating that the last drought tended to reinforce clumping rather than promote a regular pattern of trees. Spatial pattern analysis yielded no evidence supporting a hypothesis of stand density regulation through competition between individuals. Other processes, as surface sealing of bare soils or insufficient recruitment, may play a more important role in preventing a savanna-like vegetation from turning into denser woodlands or thickets. Introduction Tropical savannas are usually defined as vegetation types featuring a more or less continuous herbaceous layer and an open woody upper-storey (Menaut 1983; Huntley 1982). This widespread definition based upon physiognomical criteria has often been associated with presumed regulatory mechanisms preventing (or delaying) complete takeover by one component on another. However, the mechanisms invoked have been highly

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variable according to authors and geographical contexts. Although most authors agree in recognising fire as tremendously important for influencing woody populations of humid savannas (Sandford et al. 1982; Menaut et al. 1990), several conflicting hypotheses have been proposed regarding more arid savanna-like vegetation types (see Belsky 1990; Skarpe 1992, for a review). Among them was competition for scarce water resources between grasses and trees (Walker et al. 1981; Eagleson & Segarra 1985) and/or between

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212 trees themselves. However, the second kind of competition has been very scarcely investigated and documented in tropical semi-arid ecosystems. Theoretically it should result in density-dependent mortality among woody individuals, at least during certain critical phases of stand development, or in relation to drought periods (Fowler 1986). Such dynamic processes can be expected to shape spatial distributions of woody individuals, and pattern analysis has been therefore considered as an indirect approach of stand dynamics in very little known vegetation types (GreigSmith & Chadwick 1965; Phillips & MacMahon 1981; Prentice & Werger 1985; Skarpe 1991). This is the scope of the present paper dealing with a semi-arid savanna-like vegetation, that is extensively encountered across the borderline between Burkina Faso and Mali (from 13 300 up to 14 300 north latitude). Such a vegetation type is very scarcely documented and little is known about its current dynamics, although recurrent droughts between the early seventies and the mid-eighties can be thought to have had a significant impact on woody stands. The main questions addressed here are to which extent spatial patterns of individual trees and shrubs may have been shaped by density-dependent regulation, and how patterns of mortality and recruitment may have been influenced by two decades of unfavourable rainfall. As diachronic data are not available, an indirect approach through the study of present spatial patterns is used. However, the analysis has to deal with a complex multispecific vegetation type, influenced by several factors as heterogeneity of the substract, contrasted ecologies of constituting species and both intra and interspecific interactions between individuals. As a consequence, there is a need of a more integrating and flexible statistical framework than usual analyses (e.g., based on nearest-neighbours distances). Such a framework can be provided by the theory of ‘marked point processes’ (Goulard et al. 1995), which has inspired recent applications regarding ecology of temperate forest stands (Pentinen et al. 1992; Goulard et al. 1995), and is applied, hereafter, to multispecific savanna stands.

Material and methods Study area Climate is semi-arid tropical with hot temperatures (mean annual values are 29–30 C) and a potential evapotranspiration (Penman) of about

2000 mm year,1 . There is a long dry season from October to May, with a short wet season from June to September. The highest monthly rainfall is usually observed in August. Average rainfall recorded at Banh (14 050 N, 2 270 W) was 486 mm (SD = 92 mm) between 1986 and 1994. However, a broader perspective on rainfall variability can been obtained from Ouahigouya (50 km south) which has a record from 1922. The average annual rainfall was 745 mm between 1950 and 1967 and only 550 mm between 1968 and 1985. The drought experienced during this second period was probably the worst that has occurred during this century, for West Tropical Africa as a whole (Lawesson 1990; Morel 1992). The early eighties were particularly dry (with less than 200 mm at Banh in 1984). A slight increase in rainfall began in 1986 and has continued since 1991. The study area encompasses a major geological divide between the old Precambrian ‘Mossi shelf’ (granites and schists), and the ‘Gondo’ plain, constituted of continental deposits (sandstones, dolomites), and of coarse material eroded from the shelf. This study focused on savannas which are extensively encountered in the plain, whilst slopes from the Mossi area, carry a more or less striped vegetation (i.e., brousse tigrée or tiger bush; White 1970; Couteron et al. 1996). In the plain there is no consistent slope and micro-relief is barely perceptible to the eye. Soils are shallow and poorly developed on a more or less continuous petroferric contact, consisting of ironstone gravels and stones. Around the study site, this material originates from the destruction of former ironpan layers in the Mossi shelf and has been locally re-consolidated into a discontinuous cuirass. Soil depth ranges from 0 to 80 cm, depending on microrelief which provides mosaic complexity. Topsoil texture varies from sandy-clay on the humps to clay silt in the bottoms. Ironstone gravels are abundant in this topsoil from place to place, especially in the former situations, locally denoting very shallow soils. Vegetation belongs to the ‘Sahel regional transition zone’, with most woody species related to the ‘Sudanian regional centre of endemism’ (White 1983). Woody vegetation is dominated by shrubs and small trees which are often multi-stemmed. The herbaceous strata is constituted mainly of annual grasses and forbs whose occurrence is highly dependent on rainfall distribution (Grouzis 1992; Seghieri et al. 1994). Perennial grasses are very scarce. As a consequence, herders do not use fire to promote grass regrowth at the end of the dry season as is often done in more southern Sudanian

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213 Table 1. Overall stand characteristics for individuals taller than 1.5 m. Data for sixteen species are presented (15 species with more than 2 individuals ha,1 plus Adansonia digitata). Densities are expressed in terms of live individuals and not of stems. Species

Acacia ataxacantha DC. Acacia macrostachya Reich. Acacia senegal (L.) Willd Adansonia digitata L. Anogeissus leiocarpus (DC.) G. et Perr Boscia salicifolia Oliv. Boscia senegalensis (Pers.) Lam Combretum glutinosum Perr Combretum micranthum G. Don. Combretum nigricans Lepr. Commiphora africana (A. Rich.) Engl Dalbergia melanoxylon G. et Perr Grewia bicolor Juss. Grewia flavescens Juss. Guiera senegalensis J.F. Gmel. Pterocarpus lucens Lepr All species

Density (N ha,1 )

Biological volume (B.V.) (m3 ha,1 )

Proportion of total B.V. (%)

Average height (m)

10.2 3.0 2.8 1.1 10.9 6.9 6.5 4.3 77.4 10.2 30.7 14.8 47.2 19.5 8.8 35.0

531 149 188 567 3220 51 52 168 1664 428 775 926 1843 259 259 4863

3.3 0.9 1.2 3.5 19.7 0.3 0.3 1.0 10.2 2.6 4.8 5.7 11.3 1.6 1.6 29.8

3.4 3.4 3.7 9.4 7.5 2.9 2.0 3.9 2.8 4.2 3.7 4.3 3.4 2.7 2.6 5.3

298.2

16321

100.0

3.6

Including scarcer species not presented in the table. savannas (Granier & Cabanis 1976). Fires are thus very unlikely and woody vegetation is indeed dominated by fire-sensitive species. In case of severe drought (as in 1983–1985), absence of perennial grasses also resulted in an immediate collapse of herbaceous vegetation, which suggests an absence of competition with woody plants for water resource during particularly dry years. Historically the area has been ruled by Fulani pastoralists since the end of the eighteenth century (Martinelli 1995). It is still mainly devoted to pastoral activities with a rather low human population (less than 10 inhabitants km,2 ). Savannas in the Gondo plains are used by cattle and goats during the rainy season and by goats to the end of the dry season. The stocking rate was estimated in 1990 (Couteron et al. 1992) as 10.8 Tropical Livestock Units km,2 over six months (one T.L.U. is the equivalent of 250 kg live weight; Lamprey 1983, p. 652). This value is considered as moderate under current African range management standards (Lamprey 1983, p. 654). There is no significant wood gathering or cutting except in the immediate vicinity of human settlements.

Field data A square plot of 10.24 ha (320 m by 320 m) was delimited in the savanna of the Gondo plain at about 14 120 2700 N and 2 270 2300 W (GPS coordinates). Field work was carried out in 1993. The plot was divided into 10 m by 10 m quadrats inside which all woody individuals with a height over 1.5 m were identified. Plant nomenclature follows Hutchinson & Dalziel (1954–1973). Individuals were mapped by their XY coordinates in the plot. Total height and crown diameters (two perpendicular diameters were considered, N–S and E–W) were measured to the nearest decimeter. Crown area was appraised from the two diameters by assuming an elliptical shape (S = d1 d2 =4). An index of bio-volume was also computed as crown area times total height. ‘Saplings’ were defined as individuals with a height between 0.5 and 1.5 m. This threshold of 1.5 m is relevant for species with a tree life form (e.g., Anogeissus leiocarpus, Commiphora africana, Dalbergia melanoxylon, Pterocarpus lucens; see Table 1 for authority), for which an individual under 1.5 m cannot be mature. This is not the case for the smal-

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214 lest shrubby species (e.g., Boscia senegalensis, Grewia flavescens) which were not considered in the analyses for recruitment. Small individuals belonging to the first group of species were mapped. For the remaining species, indviduals less than 1.5 m were only identified and counted in the 10 m by 10 m quadrats. For each quadrat, the herbaceous cover and the abundance of ironstone gravels were appraised by visual assessment using an ordinal scale of 20 classes, corresponding each to a 5% cover interval. Data analysis Principles Spatial pattern analysis is considered within the framework of point process theory (Diggle 1983). A pointprocess is a stochastic model ruling the locations of points in some topological set (e.g., points correspond to individuals within a study area). A point process is usually studied through its ‘intensity’ (i.e., the expected number of points per unit area), and second order characteristics dealing with spatial dependence between regions of the study site. An important technique for second order analysis is the ‘K function’ (or reduced second moment function) introduced by Ripley (1977). The K function K(r) is defined from the expected number of extra points within distance r of an arbitrary point of the process, which is K (r) . This definition can be applied to a pattern of a single kind of points or to a pattern of two (or more) kinds of points (say, i and j ). In this case j Kij (r) is the expected number of points of the j kind within distance r of an arbitrary point of the i kind (Diggle, 1983). Under independence between the two kinds of points Kij (r) = Kji (r) = r2 . Regarding vegetation studies, different kinds of points can be distinguished on the basis of species, size class or lifestage. To date, analysis of spatial patterns involving several kinds of points has had limited use in plant ecology (see nevertheless Kenkel 1988 or Pentinen et al. 1992), while simple K functions have often been utilised (Diggle 1983; Prentice & Werger 1985; Getis & Franklin 1987; Skarpe 1991). Under complete spatial randomness (CSR) between points of a given kind (Diggle 1983, p. 4), K (r) = r2 . Under regularity (‘inhibition between points’), K(r) tends to be less than r2 , whereas under clustering (‘attraction’) K(r) tends to be greater than r2 . A common practice is to use a transformed version,

( )=

L r

r

( ):

K r

(1)

Under CSR, L(r)-r has an expectation of zero for all values of r (Besag in Ripley, 1977). Another useful tool for second-order analysis of spatial point patterns is the pair correlation function gij (r) which can be defined from the K function as dKij (r) 1 : gij (r) = (2) dr 2r (Provided Kij (r) is differentiable; Stoyan 1988.) Although closely related, pair correlations and L functions provide complementary insights into spatial point patterns. The pair correlations are often easier to interpret whereas cumulative L are useful for hypotheses testing (Pentinen et al. 1992). Pair correlation is above 1 with clustering and under 1 in the case of ‘inhibition’; it remains close to 1 in case of CSR (for a single kind of points) or of independence between two kinds of points. Estimations In practice, i is estimated as,

^ = Ni jE j

i

(3)

(where jE j notes the area of the study site E , and Ni is the number of points of the i kind within it). Kij (r) is estimated as, skl ^ ij (r) = 1 K (4) ;

X

^ 020 ha,1) such as Commiphora africana, Grewia flavescens and Combretum micranthum. For all these species there is also a rapid decline of pair correlations in relation to distance. Significant departures from complete spatial randomness are observed up to 60 m. However, since L functions are cumulative characteristics, significant departures from CSR usually persist beyond the range of dependence between individuals. According to Hanisch & Stoyan (1983), the range of dependence should often correspond to a distance for which the L function shows a sharp decrease in slope (‘salient point method’). With respect to this criterion, C. nigricans, G. senegalensis and Commiphora africana have a range of less than 10 m indicating fine-grained aggregation. Another kind of pattern can be distinguished for species with lower correlations at close distance (e.g., Acacia ataxacantha, Anogeissus leiocarpus, Dalbergia melanoxylon, Grewia bicolor and to a lesser extent C. micranthum). For these species there is little aggregation under 10 m while CSR is to be rejected for greater distances (at least to 60 m). Independence is reached at 20–30 m for A. ataxacantha and G. bicolor and at 50 m for A. leiocarpus. There is even no indication of independence for D. melanoxylon under 60 m. These underlying spatial patterns suggest coarse-grained variations in density with little fine-grained aggregation between individuals. For some species (e.g., Combretum nigricans, Guiera senegalensis), complex nested patterns

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216

Figure 1. Spatial distributions of the most frequent species, analysed from pair correlations (on the left) and L functions (on the right). Circled points indicate distances with significant departures from CSR, and the arrow denotes the range of dependence between individuals (determined according to the ‘salient point method’).

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217

Figure 1. Continued.

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218 can nevertheless be observed with both fine-grained aggregativity and coarse-grained variations of density. Finally, Boscia senegalensis and Pterocarpus lucens are two species which do not display any departure from CSR whatever the distance. Interspecific relationships Interspecific relationships have been analysed from the results of the test on Lij functions, that is, random shifts of one pattern while the other is kept fixed. Distance intervals for which the null hypothesis (no spatial relationship between species) has been rejected are summarised in Table 2, where species have been sorted to emphasise similar patterns of relationship. For several species (upper-right part of the matrix), there is a strong tendency toward interspecific association within distances up to 30–40 m (and sometimes a little more). Among these species are found (1) species of medium abundance (less than 15 individuals ha ,1 ) such as Acacia ataxacantha, Anogeissus leiocarpus and Dalbergia melanoxylon and (2) species of high abundance (more than 30 individuals ha,1 ) such as Combretum micranthum, Commiphora africana and Grewia bicolor. Most of these species were previously recognised as occurring within large patches, which are revealed by the present analysis as multispecific ones. Another group of species can be identified (middle part of Table 2) with no systematic pattern of interspecific dependence. The null hypothesis is often not rejected and significant departures from independence are encountered only at close distance (in most cases less than 10 m). Among this group is the abundant Pterocarpus lucens, the less abundant Grewia flavescens and the rather scarce Boscia salicifolia. Guiera senegalensis and Boscia senegalensis display some similarities with this group. However, they are also characterised by significant ‘inhibitions’ with some other species for distances of 10–20 m. In other words, they are scarce within this range of distance from individuals of other species. Hence two pairwise interspecific patterns seem to arise: first, one made of large ‘patches’ with a grain of about 30–40 m, and a second of ‘small clumps’ with a grain of less than 10 m. Most species distributions are involved in one of these patterns or both, with the exception of B. senegalensis, for which an important proportion of individuals are outside the clumps or patches.

Taking size into account Size (in terms of height) has been taken into account to investigate further the above clumping patterns. Species have been subdivided into height classes, based either on the quartiles (three classes, the two intermediate quartiles being pooled) for abundant species or on the median (two classes) for scarcer ones. The results are discussed for the two dominant species, Anogeissus leiocarpus and Pterocarpus lucens. For this latter species, neither medium-sized nor small individuals depart from CSR (test on L functions), in spite of a slight aggregation of the latter category (Figure 2). However, tall individuals, had a slight tendency towards regularity at close distance, although this is not significant. There is also a slight, though insignificant (test on Lij functions) inhibition between large and medium-sized individuals. No significant relationships between height classes of P. lucens were found. In Anogeissus leiocarpus, the pattern of small trees (under the median, i.e., 7.1 m) was slightly aggregated for distances of more than 20 m. Large individuals (above the median) are randomly distributed under 10 m, and strongly aggregated beyond. The structure of large patches is thus particularly significant for this height class. Furthermore, small individuals are clustered around large ones within close distance. Relationships between the two dominant species are characterised by an association between most height classes (Table 3), for distances under 10 m; this is also true regarding large individuals for which there is thus no obvious interspecific inhibition. The same holds when all ‘tall’ individuals are considered (i.e., the upper quartile of the height distribution; (H > 6:4 m), regardless of species, with the difference that aggregation is also significant beyond 10 m (Figure 3). Thus, spatial distribution of tall individuals clearly expresses two levels of aggregativity (between 2–5 m and from 20 m to ca. 50 m). Spatial distributions of all woody species have been considered in relation to the pattern of tall individuals (H > 6:4 m), in order to check for a potential influence of the latter. Most species have individuals (height < 6:5 m) significantly clustered around large trees, but the range of dependence is variable. Combretum micranthum and Grewia bicolor are dependent from 2 to 60 m, whereas Grewia flavescens is dependent from 5 to 40 m and Commiphora africana or Guiera senegalensis from 2 to 20 m. Other significant dependencies found were Acacia ataxacantha, at 20 and 40 m, Combretum nigricans at 10 m and Dalbergia melan-

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219 Table 2. Ranges of significant spatial relationships between species (distances are expressed in meters). The indicates significant spatial segregation between two species (i.e., ‘inhibition’, where one species tends to be scarce at a given distance from the other), otherwise distances refer to clustering. NS denotes no significant relationship at any distance for the species pair. Distances less than 2 m have not been tested. 1 1 2 3 4 11 12 5 6 7 8 9 10

Acacia ataxacantha Anogeissus leiocarpus Dalbergia melanoxylon Combretum nigricans Combretum micranthum Grewia bicolor Commiphora africana Grewia flavescens Boscia salicifolia Pterocarpus lucens Guiera senegalensis Boscia senegalensis

2 2–10 2–30 2–30 2–30 5-20 2–20 NS NS NS 20 NS

3 2–40 30 2–40 2–30 2–10 5 NS 2–5 NS 10

4 10–20 2–30 2–20 20–50 2–10 NS NS NS NS

11 2–20 20 30 NS NS 2–20 5 NS

12 2–50 2–50 2–5 2–5 2–5 NS NS

5 2–30 2–10 2–5 NS NS 5

6 2–10 2–5 2 5 2

7 NS 2 NS 2–5

8 2–5 NS 10

9 NS NS

NS

Figure 2. Spatial distributions of size classes of Pterocarpus lucens (analysed from pair correlations): (a) Size classes considered alone and (b) Spatial relationships between size classes. Size classes are determined from the quartiles of the height distribution. The two intermediate quartiles are pooled to form the ‘medium’ class (from 3.6 m to 6.4 m, bounds included).

oxylon at 2 m and at 30–40 m. All these distances are far higher than the range of influence of the crown of a large tree, since the mean crown radius is only 3.8 m (S:D: = 2:0 m). However, some species display little dependence on large trees, such as Boscia senegalensis and P. lucens. It should be remembered that these two species have a CSR distribution, and do not show the overall pattern of

denser clumps and patches, since a significant number of their individuals have been able to develop outside the patches. Influence of edaphic variability In the study plot, the main cause of edaphic variability is linked to outcrops of petroferric material which

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220

Figure 3. Spatial distribution of tall individuals (H > 6:4 m) analysed from the L function. Circled points denote significant departures from CSR. Table 3. Ranges of significant spatial relationships between size classes of Anogeissus leiocarpus and Pterocarpus lucens. Distances of less than 2 m have not been tested. P. lucens Small

Medium

Large

Small A. leiocarpus

2–10 m

10–20 m

NS

Large A. leiocarpus

2–5 m

2–5 m

2–5 m

locally determine very shallow soils. These situations have been mapped using the cover of ironstone gravels on the soil surface (Figure 4). Notable outcrops of petroferric material account for 16% of the total area, but carry only 8% of woody individuals (and only 6.3% of the large trees). Most species have lower densities on these outcrops (Table 4). The less sensitive species (Boscia salicifolia, Guiera senegalensis and Boscia senegalensis) have a low overall density. Species that are the most negatively influenced by petroferric outcrops are Combretum micranthum, Acacia ataxacantha, Anogeissus leiocarpus and Grewia bicolor. All these species have been previously identified as having spatial distributions forming large multispecific patches. From the present analysis it appears that these patches probably stand out in contrast with sparse stands on shallow soils. To investigate this hypothesis further, quadrats on shallow gravelly soils of outcrops (Figure 4) have been set aside, and corresponding trees/shrubs ignored in the computations. This resulted in a higher density for most species or size classes. With such an approach the study area E is considered as including ‘holes’ (corresponding to dark areas). The correction for edge-effects, defined by Equation (5), easily allow the analysis of patchy study sites. Pair correlation functions are presented for some species/size classes whose spatial distribution has been

Figure 4. Map of petroferric outcrops (dark areas) within the study plot.

previously identified as more or less aggregative within large patches (Figure 5). In most cases, the difference due to the omission of shallow soils is particularly notable for distances larger than 10–20 m, with lower values of the pair correlation, and little, if any, aggregation beyond this range. This confirms that large patches are principally determined by edaphic features associated with outcroops. At close distances there were greater differences between categories of woody individuals. Thus, for A. leiocarpus and Dalbergia melanoxylon there were only minor changes in pair correlations under 10 m. This was not the case regarding large Anogeissus for which there was a decrease in pair correlations, whatever the distance. As most large Anogeissus (95%) were not found on shallow soils, the overall density was underestimated when the whole area of the plot was considered. Not including shallow soils results in a higher, more realistic, density, and consequently in lower pair correlations. In some other cases (e.g., small Pterocarpus lucens), a decrease in pair correlation is particularly notable at close distances; this can be explained by a higher aggregation between individuals on shallow soils, as compared to the rest of the population. For categories with no departure from CSR (e.g., large P. lucens), omission of shallow soils situations results only in minor changes in pair correlations.

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221 Table 4. Species sensitivity to shallow soils on petroferric outcrops. Proportion of individuals on shallow soils (%) Species with moderate sensitivity

Boscia salicifolia Guiera senegalensis Boscia senegalensis Pterocarpus lucens Commiphora africana Grewia flavescens Combretum nigricans

15.6 13.3 10.5 9.5 9.3 9.0 8.7

Species with high sensitivity

Dalbergia melanoxylon Grewia bicolor Anogeissus leiocarpus Acacia ataxacantha Combretum micranthum

7.9 6.6 6.3 5.7 5.4

Spatial patterns of recruitment Abundance of saplings varies greatly according to species. Grewia bicolor, Pterocarpus lucens and Commiphora africana are the most abundant whilst Anogeissus leiocarpus, Dalbergia melanoxylon and all other species are rather scarce. The first three species all have highly clumped spatial distributions. For G. bicolor, there was no mapping and aggregation can only be deduced from 10 m by 10 m quadrat counts, which depart strongly from a Poisson distribution (chi-square test, p = 0:001). P. lucens saplings had a very aggregated spatial distribution, but the range of dependence is reached at ca. 10 m (determined from L function). Saplings C. africana displayed a slight aggregation but only at closer distance (2–5 m). Furthermore, there was little relationship between saplings of these two species, indicating that there was no obvious common facilitation in seedling establishment and growth into saplings. For P. lucens, the spatial pattern of young individuals displayed no significant relationship with that of mature individuals (Figure 6). Consequently, saplings aggregation ought not be viewed as a spatial dependence on parent trees. In fact such a dependence is unlikely owing to P. lucens propagule (a winged achene, easily transported by wind during the dry season). Saplings of Commiphora were also independent of adults. The spatial distribution of Pterocarpus seedlings was compared with the distribution of mature individuals of several species, but few significant relationships were obtained. The only exception was an association with Combretum micranthum, within a

distance range of ca. 10–20 m, and a segregation with Boscia senegalensis (Figure 6). The first relationship seems to denote a favourable environment for seedling survival at the fringes of existing clumps of C. micranthum (the most dense species in this vegetation type with patches of about 20 m of radius; Figure 1); the second expresses conversely the inhospitability of small open areas (up to ca. 20 m of radius), where B. senegalensis was often the only species encountered (Table 2). Spatial patterns of mortality Dead woody individuals are frequently encountered in the study area. According to local people, most of them died during the last drought at the beginning of the eighties. Dead individuals of Anogeissus leiocarpus, Dalbergia melanoxylon and Pterocarpus lucens, were mapped within the plot. These three species have hard wood and can reach tree size. A dead individual usually takes more than 10 years to fall and fragment and there was no significant gathering of dead wood in and around the study plot. Ratios of dead individuals still standing unbroken with respect to live ones have been calculated. The similarities in life form and wood durability make ratios comparable between species. For D. melanoxylon and P. lucens there was an important proportion of dead individuals on shallow soils (Table 5). It is not the case for A. leiocarpus, whose low mortality ratio might be a consequence of having few individuals established on shallow soils before the drought. Dead A. leiocarpus were too scarce to allow a

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222

Figure 5. Influence of edaphic variability on some pair correlations. Unfilled symbols correspond to the pair correlation based on all individuals and filled symbols are used when shallow soils are not considered.

Table 5. Impact of mortality on three species. Species

Density of dead individuals (N ha,1 )

Ratio of mortality (a )

Proportion of dead individuals located on shallow soils (%)

Anogeissus leiocarpus Dalbergia melanoxylon Pterocarpus lucens

0.6 3.3 7.5

0.05 0.22 0.22

0 30 39

a Ratio

of standing dead individuals to live ones.

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Figure 6. Spatial relationships, analysed from L functions, between Pterocarpus lucens saplings and mature individuals: (a) P. lucens, (b) Boscia senegalensis and (c) Combretum micranthum. Circled points denote significant departure from independence based on L functions.

systematic analysis of their spatial distribution, which was nevertheless carried out for the two other species. The pair correlation function for dead P. lucens demonstrates that dead individuals constitute large patches of more than 40–50 m (Figure 7), which were strongly influenced by edaphic condition. When gravelly situations were included, pair correlations were notably higher. When the corresponding data were not included there was no significant aggregation except between 5–10 m. Dead P. lucens were rather scarce within 10 m of live ones. The same remained true when considering all individuals taller than 4 m (i.e., the upper tercile of the height distribution), whatever the species. Regarding D. melanoxylon, dead individuals were clumped only for small distances, which means that edaphic influence is not as strong as for P. lucens. Dead individuals were also scarce near live ones. There is little relationship between the spatial distributions of dead individuals of these two species. Potential density-dependent mortality was investigated by comparing mean densities (live+dead) around both dead and live individuals. Computation of mean densities

Figure 7. Spatial distributions of dead P. lucens (analysed from pair correlations). Unfilled symbols correspond to the pair correlation with data from shallow soils included and filled symbols are used when shallow soils are not included.

MCd(r) and MCv(r) (for dead and live individuals, respectively) is straightforward from bivariate K functions, since

( ) = dKdd(r) + vKdv(r); M Cv (r) = vKvv (r) + dKvd(r); M Cd r

(8)

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224 where Kij is the bivariate K function considered for either dead (i; j = d) or live individuals (i; j = v ). When P. lucens was considered alone, crowding around dead individuals was lower than around live ones for close distances (under 5 m; Figure 8a). This difference resulted mainly from lower densities of P. lucens stands on gravelly shallow soils, where the mortality ratio was high (0.86); it vanished when dead individuals on gravelly soils were not taken into account (Figure 8b). This is not the case when crowding was computed from all live large individuals (defined as belonging to the upper tercile of the height distribution), which could have been supposed as having strong competitive influence. Crowding from large individuals was far higher around live P. lucens than around dead ones, even when situations with shallow soils were ignored (Figure 8c). Small multispecific clumps appear to have been favourable to the survival of P. lucens during the last drought. Also D. melanoxylon densities of large trees were far lower around a dead individual than around a live one (Figure 8d). For both species, isolated individuals appear more exposed to drought-related mortality.

Discussion In semi-arid savannas of the Gondo plain, there was a notable diversity of spatial patterns depending on species and size classes. Aggregation dominated in most monospecific distributions, with the exception of Boscia senegalensis and mature Pterocarpus lucens for which complete spatial randomness was retained. No regular pattern was found as statistically significant, however, regularity was not tested for any distances under 2 m. Spatial clustering was also a dominant scheme with respect to interspecific relationships, the only exception being B. senegalensis, and to some extent P. lucens. The overall aggregation patterns appear to have resulted from the superposition of two distinct structures. The first expressed the contrast between sparse stands on petroferric outcrops and denser ones on less shallow soils. The dense multispecific patches contained most large trees, around which individuals of lesser stature were found clustered (for most species except B. Senegalensis and P. lucens). As this relationship extented far beyond the range of influence of a canopy, large patches cannot be seen as created around pre-existing large trees. Indeed, they reflect more edaphic variability than effective interactions between woody individuals. The second spa-

tial pattern identified consisted in small multi-specific clumps, whose relationship to edaphic heterogeneity was less obvious. Perhaps this could be investigated by a more complete spatialisation of soil characteristics such as depth and texture. However, that such a clumping pattern was determined by soil features was not demonstrated. The overall conclusion of aggregation diverges from several studies concluding that woody populations densities could be regulated by intraspecific competition in arid or semi-arid ecosystems, a process that was inferred from a decay of aggregation to randomness as larger individuals interact (e.g., Greig-Smith & Chadwick 1965; Phillips & MacMahon 1981; Prentice & Werger 1985; Skarpe 1991). Most of these studies constated such a result over distances larger than 2 m (the shorter distance we investigated here). However, that the observed patterns of large trees did not tend toward regularity or randomness does not mean that competition did not occur, but only that it had no visible effect on densities of mature individuals. Competition may nevertheless have had significant consequences on aspects which were not investigated here, as individual growth or reproductive outputs. Futhermore, as already pointed out by Smith & Goodman (1986), most of the studies quoted above dealt with pre-desertic scrubs instead of semi-arid savannas. There have been few studies dealing with spatial distributions in semi-arid savannas (e.g., Skarpe 1991). Moreover, only savannas in Austral Africa are documented and no results are available regarding any West African savanna unaffected by burning. Existing studies from Austral Africa considered ecological conditions which may present major differences with the situation analysed here. For example, Smith & Goodman (1986) conducted their study on deep welldeveloped soils, and Skarpe (1991) dealt with deep sandy soils under more arid conditions. Some other differences can be underlined regarding local flora, especially with respect to the herbaceous layer, which is dominated by perennials in Austral Africa but by annuals on our site. For instance, annuals cannot be supposed to interact with woody vegetation for soil moisture during very dry years since they are absent or little developed, contrary to what can be supposed with perennials (Walker et al. 1981). However, P. lucens, one of the co-dominant species in the savannas of the Gondo plain, had spatial distributions which might be interpreted in terms of intraspecific interferences. There was indeed a general trend from clumping to randomness with increas-

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225

Figure 8. Local mean densities around dead and live individuals. Filled symbols are mean densities around live individuals, whereas unfilled symbols are densities around dead ones. (a) Mean densities of Pterocarpus lucens alone (all data considered), (b) Mean densities of P. lucens alone (shallow soils not included), (c) Mean densities of large trees around P. lucens (shallow soils not included) and (d) Mean densities of large trees around Dalbergia melanoxylon (shallow soils not included).

ing size, and even a slight inhibition between large trees. Similar observations were considered by several authors as supporting the hypothesis of intraspecific regulation (Greig-Smith & Chadwick 1965; Phillips & MacMahon 1981). However, such an assumption was not corroborated by mortality data. Thus, dead individuals were scarce within 10 m of live ones, a fact that leads to the conclusion that little mortality resulted from a one-sided competition between a dominant individual and a dominated one (sensu Kenkel 1988). Otherwise, dead ‘losers’ should have been frequently observed very close to surviving ‘winners’. Furthermore, no increasing relationship was found between mortality and density. In fact, the reverse was observed, where local mean crowding was higher around surviving individuals than around dead ones. The observed trend was hence toward slightly more clumped distributions, though no significant departures from CSR were detected. Any change toward regular distributions was unlikely since regular distributions can be thought

as being shaped from clumped or random distributions during droughts which are severe. (e.g., the early eighties were the driest years recorded since 1922). If intraspecific density regulation was to occur, it should hence have been observed in dense clumps of saplings instead of scattered mature trees. However, other processes may be thought of as explaining the coexistence of clumped saplings and randomly distributed trees. For instance, present patterns of mature trees and shrubs might have developed from a less clumped distribution of saplings which was established several decades ago under different climatic conditions. In these semi-arid savannas, recruitment is a difficult process, which is highly variable in relation to rainfall fluctuations. Additional field evidence, such as soil water budgets around trees, are thus needed to further confront dynamic hypotheses. Above all, it should be born in mind that any mortality pattern may be notably influenced by fine-grained environmental heterogen-

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226 eity, which was sufficiently high to interact with the intraspecific dynamics of P. lucens. Moreover, soil features are not the only cause of environmental heterogeneity, and biotic interaction ought also to be considered when dealing with availability of scarce resources. If water is the limiting resource, an hypothesis that was not directly addressed by this paper, its availability would not depend only on local density of woody vegetation. Water budget is known to result from complex interactions between climate, soil and vegetation (Skarpe 1992), and one very important aspect is the amount of water infiltrating from rainfall. It is fairly well established that soil surface features are of overriding influence on infiltration under semi-arid climates characterised by torrential rainfall events. Soil surface displays a high level of permeability within dense vegetation patches (Joffre & Rambal 1993), owing to biopores formed by root systems and macrofauna activities. In the absence of significant vegetation cover, topsoil is often capped by surface crusts which prevent infiltration (Valentin & Bresson 1992). In the presence of a consistent slope, water is reallocated from bare areas to vegetated ones through run-off, and such a process has been fairly well documented regarding tiger bush or related patterned vegetations (Cornet et al. 1992; Ludwig & Tongway 1995). In savanna-like vegetation types with no consistent slope, such a process cannot be systematically invoked, and the pattern of alternated dense thickets and bare soils is indeed not encountered. However, there is a juxtaposition of small woody clumps and sparsely covered areas which are often circular. Such a pattern may be influenced – if not determined – by complex biotic interactions between the vegetation itself and soil macrofauna. Active large termite mounds (Macrotermes spp.) were frequent inside the clumps while unactive eroded ones are often observed in sparsely covered areas between them. Erosion of termite mounds is known to determine a sealing of soil surface (White 1970). Surface sealing reduces water infiltration within a radius of several meters around the former mound. This results in a major hindrance to vegetation extension from clumps to nearby areas. During severe droughts sealing may also cause an increased mortality of isolated woody individuals in the immediate vicinity of eroded termitic mounds. In these situations only the highly drought-resistant Boscia senegalensis, which has Saharian affinities (Lawesson 1990), is able to

survive during particularly dry years. Bare soils on presently degrading mounds can be thought as highly persistent since their colonisation by vegetation is hindered by an extremely compact surface which prevents seeds from lodging (White 1970). This persistence is likely to play an important role in maintaining a fine-grained heterogeneity of vegetation. An overall shift toward denser stands might be also simply prevented or delayed by reduced numbers of seedlings reaching sapling size during periods of unfavorable rainfall.

Acknowledgements This study is part of the IGBP’s Savannas on the Long Term (SALT) project. We are very grateful to M. Goulard (INRA, Avignon) for his advice on point process analysis and for comments on a first draft. We are also thankful to two anonymous reviewers who have greatly helped in improving the paper.

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