The use of multiple discriminant analysis in reconstructing recent vegetation changes on the Nuwelveldverg, South Africa

Review of Palaeobotany and Palynology, 60 (1989): 131-147 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

131

THE USE OF MULTIPLE DISCRIMINANT ANALYSIS IN RECONSTRUCTING RECENT VEGETATION CHANGES ON THE NUWEVELDBERG, SOUTH AFRICA J E A N M. S U G D E N a n d M I C H A E L E. M E A D O W S Department of Environmental and Geographical Science, University of Cape Town, 7700 Rondebosch (South Africa)

Abstract Sugden, J.M. and Meadows, M.E., 1989. The use of multiple discriminant analysis in reconstructing recent vegetation changes on the Nuweveldberg, South Africa. Rev. Palaeobot. Palynol., 60:131 147. Pollen analysis of vlei or swamp sediments from the Nuweveldberg Mountains in the Central Karoo yields a vegetation history spanning the last 760 years. It sheds light on the local vegetation shifts in response to fluctuations in climate and the possible effects of changing land-use, particularly when Khoi-Khoi herders began occupying the area which was previously inhabited by San hunter-foragers. Multiple discriminant analysis, which compares Holocene fossil pollen assemblages with modern pollen spectra, is used as a tool in palaeovegetational reconstruction. Visual, subjective zoning of the pollen diagram is substantiated by multiple discriminant analysis. This technique is shown to be useful in determining whether modern analogues exist for the fossil pollen assemblages and for identifying misclassified vegetation zones and zones not identified during the initial subjective zoning. Used in this way, multiple discriminant analysis can considerably enhance the interpretation of fossil pollen spectra.

Introduction The K a r o o N a m i b r e g i o n (Werger, 1978; White, 1983) in s o u t h e r n A f r i c a is subdivided on the basis of the S u m m e r A r i d i t y Index, w h i c h is the p e r c e n t a g e w i n t e r half-year rainfall and life-form mix, into t h r e e biomes, viz. the N a m a - K a r o o , S u c c u l e n t K a r o o a n d Desert Biomes ( R u t h e r f o r d and Westfall, 1986). Lifeform mix is defined as the m o r p h o l o g i c a l expression of the a d a p t a t i o n of o r g a n i s m s to t h e i r e n v i r o n m e n t (Taylor, 1984). Q u a t e r n a r y p a l a e o e n v i r o n m e n t a l evidence is s c a r c e in the i n t e r i o r of S o u t h Africa, p a r t i c u l a r l y for the K a r o o region. This is, in part, due to the lack of sites suitable for p a l y n o l o g i c a l i n v e s t i g a t i o n , as few o r g a n i c deposits c o n t a i n i n g fossil pollen or m a t e r i a l for r a d i o c a r b o n d a t i n g h a v e accum u l a t e d in the semi-arid e n v i r o n m e n t (Scott, 1982; M e a d o w s and Meadows, 1988). At h i g h e r 0034-6667/89/$03.50

altitudes, w h e r e t e m p e r a t u r e s are cooler and p r e c i p i t a t i o n s o m e w h a t greater, o r g a n i c sedim e n t s m a y a c c u m u l a t e , p a r t i c u l a r l y in vleis. M/ickel (1974) describes d a m b o s (they are k n o w n as vleis in s o u t h e r n Africa: Meadows, 1988) or swamps, as s h a l l o w l i n e a r depressions in the h e a d w a t e r zones of rivers, w i t h o u t m a r k e d s t r e a m channels. P a l y n o l o g i c a l analysis of s u c h deposits sheds light on the vegetation h i s t o r y of the K a r o o , w h i c h m u s t h a v e been affected by late Q u a t e r n a r y e n v i r o n m e n tal c h a n g e s r e c o r d e d elsewhere in s o u t h e r n A f r i c a (Klein, 1984). This s t u d y of the vlei sediments from the N u w e v e l d b e r g in the Central K a r o o , is p a r t of a b r o a d e r study, in w h i c h u p l a n d a r e a s of the K a r o o (Fig.l) are being examined. The b r o a d e r s t u d y a t t e m p t s to o b t a i n a c l e a r e r p i c t u r e of w h a t a p p e a r s to be a s p a t i a l l y d y n a m i c K a r r o i d v e g e t a t i o n , w h i c h is p u r p o r t e d l y e x p a n d i n g in an e a s t e r l y d i r e c t i o n

© 1989 Elsevier Science Publishers B.V.

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(Acocks, 1953; 1975). This expansion i s d u e to either human-induced disturbance or a longer term natural environmental trend, or both. The observed palaeoecological changes which have occurred within the Karoo Biome have been subtle and therefore statistical techniques, which have been useful elsewhere (e.g. Birks and Birks, 1980; Birks and Peglar, 1980; Birks and Gordon, 1985; Liu and Lam, 1985; MacDonald and Ritchie, 1986), may be valuable in improving interpretation of these changes. The primary aim of this paper is to

reconstruct past vegetational changes from pollen-stratigraphic evidence obtained from vlei sediments on the Nuweveldberg, Central Karoo. This is done by investigating the pollen diagram in a conventional way, followed by multiple discriminant analysis, which originally proved successful in studies by Liu and Lam (1985) and MacDonald and Ritchie (1986). Discriminant analysis is used to compare modern pollen spectra with the Holocene fossil pollen assemblages from Bokkraal Vlei. The results are used to reconstruct patterns and

133

rates of vegetation development and to determine whether modern analogues exist for the fossil pollen assemblages. A variety of statistical analyses are available, the relative merits of which are discussed below. Discriminant analysis was selected as most appropriate for this study.

S t u d y area Location

Bokkraal Vlei, at an altitude of 1820 m, is situated in the Nuweveldberg, Central Karoo (Fig.2). The geology comprises sedimentary

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134 rocks of the Karoo sequence, intruded by dolerite, to form the upstanding mountain ranges. The dolerite capping has been eroded, resulting in a terraced escarpment, which rises sharply from the Karoo plains (840 m) to the Middle plateau (1200 m) and finally to the Upper plateau (1800 m) (Visser, 1986). Although many vleis exist on the upland plateau, most have been disturbed or ploughed for agricultural purposes and only a few remain suitable for palaeoecological investigation. The semi-arid climate, characterised by the typical extremes in temperature and rainfall (Booysen and Rosswell, 1983), is an important mitigating factor on peat formation and pollen preservation. Summer temperatures may rise to above 40°C, whereas winter nights may be extremely cold, dropping to below - 10°C (Schulze, 1979). In this summer rainfall area, precipitation varies from 250 mm on the plains, increasing to 750 mm per annum on the Upper plateau (Schulze, 1979), favouring organic sediment accumulation in topographical depressions and water courses, especially at increased altitudes.

Vegetation The vegetation of the Nuweveldberg is heterogeneous, but has been divided into several vegetation assemblages by Acocks (1953, 1975); these correspond broadly with the geomorphological zones i.e. Karoo plains, Middle plateau and Upper plateau (Fig.2). The vegetation assemblages or veld types are defined by Acocks (1953, 1975) as ~'... units of vegetation whose range of variation is small enough to permit the whole of it to have the same farming potentialities (p.1)." Karroid Brokenveld (Acocks' veld type 26) on the Karoo Plains. The vegetation cover is low, consisting of a variety of small, droughtresistant shrubs, particularly of the Asteraceous type. Small hardy shrubs such as Pentzia incana, Lycium oxalicladum, Psilocaulon absimile and Rhigozum obovata dominate, while ephemeral grasses, particularly Stipagrostis spp. are conspicuous on the flats during a wet season (Acocks, 1953).

Central Upper Karoo (veld type 27) on the Middle Plateau. Grasses, which are more abundant than on the plains, are represented by Eragrostis lehmannia, Aristida congestus, Merxmuellera disticha a n d M. stricta. Shrubs include Grewia robusta, Chrysocoma tenuifolia and Elytropappus rhinocerotis (Acocks, 1953). Merxmuellera Mountainveld replaced by Karoo (veld type 42) on the Upper plateau. Sourveld grasslands dominate, particularly Merxmuellera disticha and Themeda triandra. Since the proclamation of this former stock farming area as a National Park almost ten years ago, there has been a decrease in Karroid shrubs (H. Braak, pers. commun., 1987) and this area may now be classified as Merxmuellera Mountainveld (type 60). Trees are absent, apart from a few Cliffortia arborea and Kiggelaria africana on the sheltered leeward slopes of the upper plateau. In fertile, channelised vlei areas, the Karroid Merxmuellera grasslands are invaded by Karroid shrubs forming an aromatic, resinous, almost Macchia-like vegetation. The vlei environment itself is covered by a hydrophytic vegetation, dominated by species of Cyperaceae, Liliaceae and Mentha aquatica and surrounded by Karroid Merxmuellera Mountainveld.

Initial t e c h n i q u e s o f data collection and analysis

Contemporary pollen analysis Contemporary pollen data are essential for establishing the relationship between the present-day pollen spectra and the vegetation which produces it. In order to obtain a picture of the pollen rain in the different vegetation zones in the Nuweveldberg area, 17 pollen traps of a design used by Meadows (1984), were set up in a range of vegetation communities (Fig.2). Due to the dwarf, Karroid vegetation, pollen traps placed both 1 meter from the surface and at surface level were used. After one year, they were replaced and their pollen content examined (Fig.3). Surface soil samples

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Fossil pollen analysis Polleniferous material, from core samples, was extracted from the vlei using a Dutch manufactured gouge auger. The cores were subsampled at 5 cm intervals and the pollen acetolysed, treated with hydrofluoric acid and concentrated, using an adaptation (Davis, 1966) of the chemical procedure proposed by Faegri and Iversen (1975) and Moore and Webb (1975). Pollen concentrations in number of grains per cm 3, obtained from the pollen suspended in silicone oil, were tabulated and computed from counts at 630 times magnification on a standard Zeiss photomicroscope.

Bokkraal vlei pollen diagram Reconstruction of past vegetation was limited by the depth to which coring was possible and the age of the sediments. Vlei sediments have a mean organic content of 13.07%, as determined by the Walkley-Black technique (Smith and Atkinson, 1975), which is sufficient for radiocarbon dating. The sediment stratigraphy is shown on Fig.4. A radiocarbon-dated basal sample has yielded an age of 760 + 50 years B.P. (Pta-4351). To facilitate interpretation of the pollen diagram, the local vlei vegetation is distinguished from the regional and extralocal vegetation. Taxa representative of the local environment are extrapolated from the surface sample spectra and pollen trap spectra (Fig.3). Taxa occurring in the present vlei or swamp environment and ecologically known to be vlei taxa, are taken as being representative of the local environment. These taxa are usually over-represented in the contemporary pollen spectra from the vlei. Taxa not found in the contemporary vlei samples, but found in pollen traps and surface samples from immediately adjacent upland areas (extralocal vegetation) and the regional areas (Janssen, 1967), are therefore representative of the regional environment. Throughout the discussion, the

term regional vegetation is used to represent both the regional and extralocal vegetation. Zonation of the pollen diagram is initially based on a visual classification and then substantiated using multiple discriminant analysis. Three zones are distinguishable, although no sharply defined boundaries are evident. Zone Na (118-105 cm) The base of Bokkraal core is characterised by higher pollen percentages than zone Nb of Asteraceae, Caryophyllaceae, Rosaceae, particularly Cliffortia and herbaceous taxa of Fabaceae. The frequencies of Poaceae, Aizoaceae and Tiliaceae pollen are, however, lower than zone Nb. Zone Nb (105 25 cm) For the major portion of this zone, the percentages of Asteraceae, Aizoaceae, Bignoniaceae and Thymelaeaceae pollen are high, whereas the percentages of Poaceae, Caryophyllaceae and Cliffortia pollen have decreased in relation to zone Na. Zone Nc (25-0 cm) In the uppermost zone, changes in frequency include increases in Poaceae, Caryophyllaceae, Geraniaceae, Fabaceae, particularly Acacia pollen and an increasing proportion of Elytropappus pollen within the Asteraceae. There is an associated decrease in Bignoniaceae, Aizoaceae and Thymelaeaceae pollen. The overall percentages of the major taxa within the local environment, i.e. Cyperaceae, Juncaceae, Labiatae and Ranunculaceae, do not fluctuate greatly throughout the time span of the pollen diagram (Fig.4), indicating an established moist vlei environment, which has not changed markedly over the past 760 years. Within the regional environment, the occurrence of Caryophyllaceae and Cliffortia pollen, both indicator-species of moist environments, the associated increase in Poaceae pollen percentages and the onset of organic sedimentation at the base of the core (Zone Na), indicate a relatively moist climate prevailing during the time period of zone Na. A gradual change to a somewhat drier environment, indicated by higher Asteraceae, Bignoniaceae and Chenopodiaceae pollen percentages and an associated decrease in the frequencies of

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grass, Caryophyllaceae and other pollen indicators of moist environments, is then apparent. The increasing frequencies of Poaceae and decreasing frequencies of Asteraceae pollen in zone Nc are suggestive of a return to a moister Nuweveldberg environment, where moisture availability probably increased and conditions were again suitable for Merxmuellera grasslands. This idea is at variance with the generally accepted ideas of Acocks (1953), t h a t the recent past has been somewhat drier and resulted in the eastward extension of the karroid and even desert communities in southern Africa. The simultaneous, but gradual increase in percentages of Stoebe, Elytropappus, Chenopodiaceae, Acanthaceae and Tiliaceae pollen from zone Nb to the surface may, however, indicate deterioration of the vegetation rather than decreased moisture availability. The gradual increase in the above pollen percentages may be associated with the change in land-use, due to the occupation of the plateau by the Khoi-Khoi pastoralists, as opposed to the San hunter-foragers, who had previously occupied the area (Sampson, 1986). The arrival of the Dutch Trekboers and stockherders in the early 17O0's (Sampson, 1986) seems to have had little noticeable impact on the vegetation. The simultaneous increase in percentages of Poaceae, Elytropappus, Acacia and Populus pollen in zone Nc could be interpreted in the landscape as being indicative of sedentary agricultural practices, as opposed to nomadic hunter-gatherer and pastoral management. Charcoal numbers, estimated from the absolute number of charcoal particles per slide, yield some information about the possible frequency of veld fires. The occurrence of natural lightning fires in the Nuweveldberg in the present day is unusual and therefore these changes in fire frequencies may be associated with the human-induced vegetation changes. The fires seem to increase during the more xeric period (zone Nb), when the Khoi-Khoi herders occupied this mountainous area.

Multiple discriminant analysis Statistical analyses In addition to the standard zonation, description and interpretation of pollen assemblages in the core (Fig.4), discriminant analysis has been used to aid reconstruction of the vegetation history. Techniques of multivariate data analysis are routinely used to display patterns among pollen spectra. Principal components analysis, dissimilarity coefficients and canonical variate analysis have been used to compare and display the affinities of fossil samples (Birks, 1976; Prentice, 1980; Overpeck et al., 1985; Clark et al., 1986). These techniques are extremely useful, but have certain limitations. A problem with these multivariate approaches is that they do not provide a direct measurement of the "degree of analogy" (Overpeck et al., 1985) and fossil samples with no close modern analogue may be placed spuriously close to some modern samples in the plots of the first few principal components or canonical variates. Canonical variate analysis assumes that each of the fossil pollen samples originates from one of the vegetation types included in the modern sample. If fossil spectra from other vegetation types are present, they will be positioned in the parts of the canonical variate space in which no modern samples are present. This can aid in the recognition of fossil samples for which no modern analogue is represented (Birks, 1976). An extension of canonical variate analysis may be used, which provides the possibility of a discriminant test based on Mahalanobis D 2 between a fossil sample and a set of modern samples. Multiple discriminant analysis was chosen in preference to principal components analysis, because discrimination and allocation may be more explicitly performed by the former. Although principal components analysis permits comparison between fossil and modern pollen spectra in multidimensional space, as shown by Birks and Birks (1980), Birks and Gordon (1985) and Thackeray and Scott (in press), it does not provide a quantitative

139 criterion value to discriminate between groups of samples or intermediate samples. The results may then be influenced by subjective decisions and prevent assigning group membership to new samples of unknown identity (Liu and Lam, 1985). An advantage of principal components analysis, however, is that it allows for continuous vegetation and palynological change and can allow for samples that fall within ecotones, whereas multiple discriminant analysis works with a priori assemblages. The advantage of multiple discriminant analysis is that it can be used in both multivariate discrimination of modern surface samples and subsequent comparison of fossil and modern assemblages. Overpeck et al. (1985) reviewed the problems of comparing modern and fossil pollen spectra and assessed the usefulness of several dissimilarity coefficients for this purpose. They concluded that all of the dissimilarity coefficients produced congruent results. They suggested that Mahalanobis D 2 distances, measured from the canonical variate analysis, such as those provided by discriminant analysis would be a useful method of determining the degree of analogy between fossil and modern pollen samples (Prentice, 1980; MacDonald and Ritchie, 1986). Direct, fossil sample to modern sample measures of distance, such as those examined by Overpeck et al. (1985) and Clark et al. (1986), are preferable in situations with high heterogeneity among the modern pollen samples from the same vegetation type. Discriminant analysis has been selected in this case, however, as modern pollen samples do not have a high heterogeneity. Multiple discriminant analysis compares fossil pollen assemblages from vleis with modern pollen assemblages collected from known vegetation regions (Liu and Lam, 1985). Modern pollen assemblages (i.e. modern analogues), which are representative of those shown in the fossil record are then identified. The application of discriminant analysis to palaeovegetational reconstruction for modern and fossil pollen data is constrained by a number of ecological and statistical assumptions. The

ecological assumption that the contemporary pollen samples selected for the analysis adequately represent the palynological signatures of their parent regions, depends on the number and spatial distribution of modern pollen spectra used in the analysis in relation to spatial and statistical variability of' the pollen rain in these vegetation regions. Given the limited availability of pollen spectra in some of the vegetation regions, the spatial distribution of our modern pollen data points are uneven, but representative of the area and should not affect the results. Although only 22 traps were used to obtain contemporary data, analyses and interpretation of the data indicate that they are representative of their surrounding vegetation. The pollen traps and surface samples are analysed annually and show little variation from year to year. The assumption, that the same pollen-vegetation relationship exists today as in the past is necessary for the concept of a modern analogue to hold (Birks and Birks, 1980). Although difficult to test, this assumption is probably valid for the time scale being considered (i.e. the Holocene). The term "modern analogue" is used to define a modern pollen spectrum, which matches a fossil pollen sample and is used to interpret the past vegetation (MacDonald and Ritchie, 1986). The statistical assumptions for discriminant analysis are: (1) samples are randomly chosen; (2) probabilities of group membership are equal for all groups; (3) samples are correctly classified; (4) the variance-covariance matrices of groups are statistically equal; and (5) the variables are normally distributed within each group. In the present data set, the first assumption is justified. The second assumption may not be completely justified, because there is an unequal number of samples from each of the different zones in the study area. Assumption three is justified once the additional "disturbed Mountainveld" assemblage is included to account for ecotonal areas. The fourth assumption was tested statistically using Box's M statistic. Multivariate skewness and kurtosis measures are used to test assumption five (Nie et al., 1975). Unfortunately, measures

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of skewness and kurtosis could not be calculated for the Upper Karoo group, due to the paucity of contemporary pollen samples. Multivariate normality holds for Merxmuellera Mountainveld, Disturbed Mountainveld and Karroid Brokenveld. Although the present data set does not meet all the assumptions of discriminant analysis, this is not unusual for ecological data and the technique appears to be relatively robust (Liu and Lain, 1985; MacDonald and Ritchie, 1986). The BMDP subprogram, DISCRIMINANT, was used to perform the analysis (Jenrich and Sampson, 1983). Discriminant analysis reveals: (1) whether and to what extent a fossil pollen assemblage has modern analogues based on an index called the probability of modern analogue; and (2) if this modern analogue exists, it relates the fossil pollen assemblage quantitatively to the group of modern pollen spectra representing its modern analogue, based on the probability of group membership. In discriminant analysis, the samples or in this case taxa, are divided into a priori groups and the analysis finds discriminant functions or linear combinations which best characterise the differences between the groups, so that samples of unknown group identity may be assigned to one of the groups. The discriminant functions are also useful for classifying new samples (Hair et al., 1979). The division of samples into a priori steps is a critical, yet potentially restricting step, as it does not allow for samples that fall within ecotonal areas. The analysis may be explained by plotting each sample as a point in multidimensional space, where each variable represents a dimension. The points are projected onto a plane, which is selected so that the separation between groups is maximised. This plot of variables (function 1 vs. function 2) forms part of the canonical variates analysis, which finds linear combinations of the dominant sets of variables in the classification function and permits comparison between modern and fossil spectra. The second set of variables is used to indicate group membership (Jenrich and Sampson, 1983). Discriminant functions are therefore derived to

ensure maximum separation between a priori groups and thus to distinguish between pollen assemblages from known vegetation types. Mathematical details of the application of discriminant analysis to palynological data are presented in Appendix I. In all the analyses, pollen percentages were calculated based on the sum of 33 taxa. These taxa include all those in Fig.3, including the generic subdivisions, but omitting spores, fungal spores, unknowns, unidentifiables and Salicaceae. Salicaceae pollen comes exclusively from recent plantations and would have little significance in characterising fossil pollen zones. In this analysis, Acocks' (1953; 1975) vegetation veld types are used as the a priori groups, but these vegetation boundaries are not clearly defined in the Nuweveldberg area and therefore ecotonal vegetational has to be closely examined. An additional "disturbed" assemblage is used to account for the diffuse ecotone and disturbed vegetation assemblage between Merxmuellera Mountainveld and the Upper Karroid vegetation. The third statistical assumption which has to be made when using discriminant analysis is therefore sufficiently justified by making the disturbed area a discrete entity and avoiding misplacement.

Results and interpretation Discriminant analysis : contemporary pollen samples Discriminant functions are derived which classify all contemporary pollen samples from known vegetation regions in the Nuweveldberg into one of the five a priori groups or veld types. These groups are as statistically distinct from each other as possible. A comparison of the objectively predicted group membership with the a priori group membership shows that 100% of the samples were correctly classified. The high classification rate of these samples indicates that the pollen trap spectra are reliable contemporary data bases, which may then be used to determine analogue palaeovegetation assemblages. 97% of the between-

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variables, i.e. functions 1 and 2, are calculated for each contemporary pollen sample. In all cases, the second most probable group is the vegetation assemblage that is geographically adjacent to the predicted assemblage. These probabilities reflect the zonal pattern of the vegetation along a gradient. Therefore, it is possible to convert the probabilities of these first two groups (functions 1 and 2, Fig.5) into a single "vegetation zonal index" for each sample. Contemporary pollen spectra typical of a vegetation region (100% probability of group membership) are assigned specific zonal indices, corresponding with the vegetation assemblages along the gradient, i.e. Vlei in Mountainveld, Merxmuellera Mountainveld, Disturbed Mountainveld, Upper Karoo and Karroid Brokenveld are assigned indices of 1.0,2.0,3.0,4.0 and 5.0 respectively. Samples

group to within-group variance of modern pollen data is accounted for by two discriminant functions. The discriminant scores for the eleven pollen trap samples are plotted along the first two discriminant functions. The scores show that the four group centroids are clearly separated from each other and the samples have little variation about their group centroid. This plot indicates that the vegetation communities may be separated into distinct assemblages on the basis of their pollen rain characteristics (Fig.5). The surface samples also proved useful for classifying modern analogues and show similar trends to those of the pollen trap spectra (Fig.3).

Vegetation zonal index The probabilities of group membership in the predicted and second most probable groups of

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classified as transitional between two vegetation assemblages are then assigned intermediate zonal indices relative to the two probabilities of group membership. For example, a sample classified as 90% Upper Karoo (i.e. the predicted group) and 10% Karroid Brokenveld, would have a vegetation zonal index of 4.1. The vegetation zonal indices for surface and pollen trap data all approximate 100°, as the samples are from known vegetation assemblages and ecotonal areas are accounted for by the "Disturbed Mountainveld" assemblage.

are found in disturbed areas, as the component taxa vary greatly from one disturbed area to another. The low probabilities for Karroid Brokenveld may be attributed to the diversity of plants occurring in this assemblage and the age since disturbance which would influence the successional stage and species diversity.

Fossil pollen samples The probabilities of modern analogue and vegetation zonal index are calculated for each fossil pollen spectrum in the same way as contemporary data. These values are then plotted stratigraphically for the pollen sequence of Bokkraal vlei. Vegetation zonal indices are plotted for fossil subsamples (Fig.7). The zonal index is relatively high for Karroid Brokenveld and the Upper Karoo, approximating 3.1 (90%) and 2.0 (100%) respectively. The index is lower for Merxmuellera Mountainveld, at; 1.5 (50%), which indicates a transition or intermediate value, between MerxmueUera Mountainveld and the Upper Karroid zone and corresponds with Disturbed Merxmuellera Mountainveld in

Probability of a modern analogue This index provides the probability that samples from the modern pollen group, to which a fossil sample is assigned, will be positioned in the vicinity of the fossil sample on the canonical variate axes. In palynological terms, it compares a pollen assemblage with the "palynological signature" of its assigned vegetation region, as represented by the group centroid. For three-quarters of the contemporary samples (Fig.6), the probability of modern analogues is high, i.e. 1.0, equivalent to low beta diversity, whereas low probabilities of 0.0,

SAMPLE NO

VLEI IN I MERXMLIELLERA I DISTURBED ~1UPPER KAROO MOUNTAINVELD I MOUNTAINVELD I MOUNTAINVELD I i

I

~-.

I

KARROID BROKENVELD VLEI IN MERX MOUNTAINVELD

i

MERXMUEL[ ERA I

MOUNTAINb ELD

MOUNTAINyELD 9 11

p.. (~ 0!5 1!0 1'0 PROBABILITY OF MODERN ANALOGUE

BROKENVEI-D

2'0

'

3'0

'

VEGETATION ZONAL INDEX

4'0

B'0 VEGETATION

Fig.6. Results of the d i s c r i m i n a n t a n a l y s i s for the 11 pollen traps located in different vegetation zones.

143

I DEPTH (cm) POLLEN ZONE

I

MERXMUELLERA I MOUNTAINVELD

Nc i

] I I

I UPPER KAROO

I I I

KARROID BROKENVELD

MERXMUELLEI

J

MOUNTAINVE[

/

Nb4

KARROID BROKENVELD

Nb3 I

UPPER KAROC

]

Nb 2 i

KARROID BROKENVELD

____pB Nb~ UPPER KAROC

i

760 yr:

:-14 DATE

Na

i

PROBABILITY OF

c__

MERXMUELLEI MOUNTAINVE[ UPPER KAROf"

110 ' 21.0 VEGETATION ZONAL INDEX

'

3'.0 PALAEOVEGETATION

MODERN ANALOGUE

Fig.7. Results of discriminant analysis for the pollen stratigraphy from Bokkraal Vlei.

zone Nc. Generally, the sharpness of resolution of zones may be due to good discriminators, characteristic of specific communities, and the absence of the "disturbed" Merxmuellera Mountainveld category in the lower part of the core (zones Na and Nb). Probability of a modern analogue Palaeovegetation types, inferred from fossil pollen assemblages, without modern analogues are detected by computing the probability of a modern analogue, thus helping to elucidate vegetational assemblages (Fig.7). Vegetation zonal indices indicate that the initial vegetation (palaeovegetation) prevailing about 760 years B.P., was Upper Karoo followed by Merxmuellera Mountainveld, returning to Upper Karoo and then Karroid Brokenveld until zone Nc. The change to Upper Karoo (zone Nb2) during a period when Karroid Brokenveld

prevailed, may be due to a fluctuating moisture availability and the sensitivity of this vegetation to disturbance by Khoi pastoralists (T. Hart, pers. commun., 1987). Irregularities are found in zone Nc, where the a priori palaeovegetation is Merxmuellera Mountainveld (Zonal index of 1.0) and the predicted value is 1.5, indicative of an intermediate, ecotonal vegetation type (Fig.7). The probability of a modern analogue is low (0.5) for samples of this zone, indicating that although Merxmuellera Mountainveld is predicted to have prevailed during this period, it does not have a modern analogue. When further investigated, using "Disturbed Merxmuellera Mountainveld" as an a priori assemblage, the modern analogue was found to correspond with this vegetation assemblage. Although Merxmuellera Mountainveld appears to be well-defined in Fig.8, the vegetation

144

KEY

40-

V 30-

I 2 3 4

20-

f3~ c~ 10-

5 z

Group Centroid

• Samplesn=22 SURFACE SPECTRA KarroidBrokenveld UpperKaroo MerxmuelleraMountainveld Disturbed Mountainveld

\_.) KARRO2

/~

BROKENVELD

2

/

I

0"

LL

MERXMUELLERA MOUNTAINVELD (--'~-~,

10-

~.

4

20-

30-

40 -40

-~o

-~o

t'o

~

;o

2'o

3'o

,~o

FUNCTION 1

Fig.8. Ordination plot of the 22 fossil pollen samples from Bokkraal Vlei along discriminant functions 1 and 2. Dotted lines indicate the range of contemporary pollen spectra from 4 vegetation regions, which are superimposed for comparison.

zonal indices in Fig.7 indicate an intermediate or disturbed Merxmuellera Mountainveld (index of 1.5) in zone Nc. This verifies Acocks' (1953) description of this pollen zone, who describes the vegetation of this area during the past century as being Merxmuellera Mountainveld replaced by Karoo. In order to substantiate the interpretation t h a t the pollen assemblage in the top 25 cm (zone Nc) represents a vegetation assemblage without a modern analogue, the discriminant scores for the fossil pollen spectra were plotted along the first two discriminant functions (Fig.8) and compared to those of the surface samples. The pollen zone Nc, representing MerxmueUera Mountainveld, lies outside the range of the modern samples and has a greater affinity to the "Disturbed Mountainveld" than to pure Merxmuellera Mountainveld. The palaeovegetation categories are found

to correspond broadly with the three contemporary vegetation assemblages. Using this statistical analysis, the pollen diagram can be divided into five zones, representing small, though significant, changes in vegetation and hence fluctuations in environmental conditions. These zones, not apparent through visual zoning techniques, become obvious when statistical procedures based on fossil data, are implemented. This analysis therefore classifies the upper section of the pollen diagram as "disturbed" MerxmueUera Mountainveld, and shows an invasion of the Mountainveld by Upper Karroid vegetation, thus shedding light on the irregularities found in the pollen frequencies of this zone. The unexpected increase in Poaceae pollen in the top 20cm may be explained by the dominance of Merxmuellera disticha on the upper plateau of the Nuweveld-

145 berg. M e r x m u e l l e r a is a hardy grass and may have replaced T h e m e d a t r i a n d r a , which is sensitive to harsh climates and disturbance. M e r x m u e U e r a is unpalatable to domestic sheep and cattle, but would have been eaten by buffalo and blue-buck, which are today extinct (K. McCabe, pers. commun., 1987). M e r x m u e l lera has become established since these extinctions in the 1930's and now dominates the area. The higher grass pollen frequencies towards the surface may therefore be explained by a greater abundance of this type of grass, which is an annual and reproduces sexually, not vegetatively as does T h e m e d a t r i a n d r a .

Conclusion The overall implication is that small fluctuations in climate have occurred over the past 760 years, causing associated shifts in vegetation assemblages. These shifts are possibly associated with climatic oscillations, with the human-induced oscillations being of secondary importance, except in the zone Nc, where human disturbance, possibly by Dutch trekboers (Sampson, 1986), is more marked and highlighted by discriminant analysis. The onset of sedimentation at 760 years B.P. coincides with other dates of peat initiation and acceleration of peat deposition within southern Africa (Meadows, 1988). The onset of sedimentation could be associated with wetter conditions than the present, followed by a drier period and possible increases in environmental disturbance, due to the arrival of Khoi pastoralists in the area. Discriminant analysis proves to be an effective aid in elucidating small fluctuations in the palaeovegetation, which were not apparent by visual investigation. Bearing in mind that assumptions are made and the contemporary pollen data set is limited, it is probably wise to consider the results of discriminant analysis as exploratory and suggestive, rather t h a n confirmatory in any rigid statistical sense. Although discriminant analysis is usually more effective for larger scale vegetation changes, it has proved to be a valuable tool in

this palynological research, particularly for identifying relatively minor shifts in vegetation assemblages over the past 760 years.

Acknowledgements This research was carried out with the aid of a CSIR/FRD Karoo Biome Project research grant. The radiocarbon dating was done by Dr. J.C. Vogel of the CSIR in Pretoria.

Appendix I Notation and description follow Nie et al. (1975), Liu and Lam (1985) and MacDonald and Ritchie (1986). Discriminant analysis tries to statistically distinguish between two or more groups or samples. Discriminant functions are formed to achieve this. These functions are linear combinations of variables (taxa) and are formed to maximise separation between groups relative to variation within groups. They are of the form: D1 = d l l z l + d12z2 + .... dlpzp

(1)

where D 1 is the score of discriminant function i, the d's are weighting coefficients and the z's are the standardised values of the p discriminating variables. The discriminant scores (D) from the cases in one group should be similar (see Figs.5 and 8). The maximum number of discriminant functions derived is either one less t h a n the number of groups or equal to the number of discriminating variables. A stepwise procedure is employed in this analysis to test the significance of the discriminating variables. The probability of group membership is compared on the basis of the distance between the case (pollen sample) and the centroid of each a priori group. The probability is represented by p ( H J X i ) , the probability of case i belonging to group k, given the values in i of the m variables (Xli,X2i ..... Xmi). This is calculated by: gik -~ log Pk -- '/2(l°g/Dk/ + Xik 2)

(2)

where Pk is the prior probability for membership in group k. ~Ok~ represents the determi-

146 n a n t of t h e w i t h i n - g r o u p v a r i a n c e - c o v a r i a n c e m a t r i x of g r o u p k a n d Xik 2 a n d is c o m p u t e d by: Xik 2 =

dikD - l dik

(3)

w h e r e D - 1 is t h e i n v e r s e of t h e p o o l e d w i t h i n g r o u p v a r i a n c e c o v a r i a n c e m a t r i x , a n d dik is the vector containing the difference between s a m p l e i a n d t h e c e n t r o i d of g r o u p k. T h e p r o b a b i l i t y of g r o u p m e m b e r s h i p is t h e n computed: exp

P(Hk/X,)

=

exp k-1

g~k

m a xk g~kt

(maxgik) glk

(4)

k

T h e s u m of t h e p r o b a b i l i t i e s for t h e g r o u p m e m b e r s h i p o f e a c h s a m p l e i is e q u a l to u n i t y . S P S S ( N i e et al., 1975) a s s i g n s t h e s a m p l e to g r o u p k to w h i c h its p r o b a b i l i t y of m e m b e r s h i p is h i g h e s t . T h e r e s u l t s i n e v e r y s a m p l e a r e a s s i g n e d to a g r o u p . T h e p r o b a b i l i t y of g r o u p m e m b e r s h i p is a s s e s s e d b y c a l c u l a t i n g t h e p r o b a b i l i t y of m o d e r n a n a l o g u e i n d e x , d e n o t e d as P(XJH~). T h i s p r o b a b i l i t y r e p r e s e n t s t h e p r o p o r t i o n of s a m p l e s a l o n g m e m b e r s of g r o u p k w h i c h a r e o r d i n a t e d i n t h e v i c i n i t y of s a m p l e i. T h i s is d e t e r m i n e d b y first c a l c u l a t i n g t h e c h i - s q u a r e d i s t a n c e b e t w e e n sample i a n d c e n t r o i d of g r o u p k i n e q u a t i o n 3. Xik 2 h a s a c h i - s q u a r e d i s t r i b u t i o n w i t h m d e g r e e s of f r e e d o m a n d P(Xi/H ~ is t h e s i g n i f i c a n c e l e v e l o f s u c h a Xik 2. P(XJHk) m e a s u r e s t h e s i m i l a r i t y of s a m p l e i i n group k with other samples in the same group.

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Booysen, J. and Rosswell, B.I., 1983. The drought problem in Karoo areas. Proc. Grassl. Soc. S. Afr., 18:40 45. Clark, J.S., Overpeck, J.T., Webb III, T. and Patterson III, W.A., 1986. Pollen stratigraphic correlation and dating of barrier-beach peat sections. Rev. Palaeobot. Palynol., 47:145 168. Davis, M.B., 1966. Determination of absolute pollen frequency. Ecology, 47:310 312. Faegri, K. and Iversen, J., 1975. Textbook of Pollen Analysis. 3rd ed. Blackwell, Oxford, 295 pp. Hair, J.F., Anderson, R.E., Tatham, R.L. and Grablowsky, B.J., 1979. Multivariate Data Analysis. Petroleum Publishing Company, Oklahoma, pp.81 111. Janssen, C.R., 1967. Stevens Pond: a postglacial pollen diagram from a small Typha swamp in northwestern Minnesota interpreted from pollen indicators and surface samples. Ecol. Monogr., 37(2): 145 172. Jenrich, R. and Sampson, P., 1983. P7M Discriminant Analysis Programme. In: W.J. Dixon (Editor), BMDP Statistical Software. University of California Press, London, pp.519 537. Klein, R.G., 1984. The large mammals of southern Africa: Late Pliocene to Recent. In: R.G. Klein (Editor), Southern African Prehistory and Palaeoenvironments. A.A. Balkema, Rotterdam, pp.107-180. Liu, K. and Lam, N.S, 1985. Palaeoecological reconstruction based on modern and fossil pollen data: an application of discriminant analysis. Ann. Ass. Am. Geogr., 75(1): 115 130. MacDonald, G.M. and Ritchie, J.C., 1986. Modern pollen spectra from the western interior of Canada and the interpretation of Late Quaternary vegetation development. New Phytol., 103:245 268. M/~ckel, R., 1974. Dambos: a study in morphodynamic activity on the plateau regions of Zambia. Catena, 1: 327-365. Meadows, M.E., 1984. Contemporary polhm spectra and vegetation of the Nyika Plateau, Malawi. J. Biogeogr., 11: 223-233. Meadows, M.E., 1988. Late Quaternary peat accumulation in southern Africa. Catena, 15: 459- 472. Meadows, M.E. and Meadows, K.F., 1988. Late Quaternary vegetation history of the Winterberg, Eastern Cape. S. Afr. J. Sci., 84(4): 253 259. Moore, P.D. and Webb, J.A., 1978. An Illustrated Guide to Pollen Analysis. Hodder and Stoughton, London, 133 pp. Nie, N.H., Hull, C.A., Jenkins, J.G., Steinbrenner, K. and Brent, D.H., 1975. SPSS: Statistical Package for the Social Sciences. McGraw-Hill, New York, 806 pp. Overpeck, J.T., Webb III, T. and Prentice, I.C., 1985. Quantitative interpretation of fossil pollen spectra: dissimilarity coefficients and the method of modern analogs. Quat. Res., 23:87 108. Prentice, I.C., 1980. Multidimensional scaling as a research tool in Quaternary palynology: a review of theory and methods. Rev. Palaeobot. Palynol., 31:71 104. Rutherford, M.C. and Westfall, R., 1986. Southern African biomes. Mem. Bot. Surv. S. Afr., 54. Government Press, Pretoria, 98 pp. Sampson, C.G., 1986. Changes in the stylistic boundaries of

147 Mobile Hunter-Foragers: Investigations of the Band/ Territory Level. Nat. Sci. Found. Proj. Summ., U.S.A., 26 pp (unpubl.). Schulze, B.R., 1979. Climate of South Africa. P a r t 8, 3rd ed. General Survey. S.A. Weather Bureau, 28, 330 pp. Scott, L., 1982. A late Quaternary pollen record from the Transvaal Bushveld, South Africa. Quat. Res., 17: 339 370. Smith, R.T. and Atkinson, K., 1975. Techniques in Pedology. Elek Science, London, 212 pp. Taylor, J.A., 1984. Themes in Biogeography. Croom Helm, Kent, 404 pp.

Thackeray, J.F. and Scott, L., in press. Quantification of climatic change during the late Quaternary in southern Africa. Palaeoecol. Afr., 19. Visser, J.N.J., 1986. Geology. In: R.M. Cowling, P.W. Roux and A.J.H. Pieterse (Editors), The Karoo Biome: a preliminary synthesis. S. Afr. Nat. Sci. Progr. Rep., 124: 1 16. Werger, M.J.A., 1978. The Karoo-Namib Region. In: Werger, M.J.A. (Editor) Biogeography and Ecology of S o u t h e r n Africa. J u n k , The Hague, 1439 pp. White, F., 1983. The Vegetation of Africa. Unesco, Paris, 356 pp.