The influence of rainfall on vegetation composition in different conditional states

JournalofAridEnvironments(1995) 30:185-190

The influence of rainfall on vegetation composition different conditional states

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S. J. Steenekamp & O. J. H. Bosch Department of Plant Sciences, Potchefstroom University for Christian Higher Education, 2520 Potchefstroom, South Africa (Received 20 October 1993, accepted I0 November 1993) The impact of ecological factors such as changes in rainfall could lead to large compositional changes in the diverse indigenous flora of southern Africa. A study was therefore initiated to determine the influence of rainfall conditions on species composition changes, which could serve as a basis for determining the long term influence of rainfall increase (e.g. cloud stimulation) and rainfall decrease (e.g. long term droughts). Certain species were found to occur in all the rainfall zones while others were limited to either the high or the low rainfall area. The reaction of species on the rainfall gradient also differs in vegetation representing different degradation stadia. The predictions of the reactions of key species on rainfall decrease or increase are of special importance for the eventual determination of grazing capacity.

Keywords: vegetation; rainfall increase; rainfall decrease; southern Africa

In~oducfion Large species diversity is an inherent characteristic o f the indigenous flora of southern Africa. T h e impact of ecological factors such as changes in grazing pressure or rainfall could therefore lead to large compositional changes (Bosch, 1989). Seasonal and long t e r m droughts is a natural p h e n o m e n a in southern Africa, which can have a p r o f o u n d influence on the composition of the rangelands. O n the other h a n d cloud stimulation is used to increase the long t e r m rainfall in certain localities (G6rgens & R o o s e b o o m , 1990). It could be expected, as in the case of crops, that short t e r m rainfall changes will have a large effect on the p r o d u c t i o n of natural vegetation. However, species composition of different rainfall zones differ greatly f r o m each other and could it be expected that an increase or decrease in rainfall will have a definite effect on species composition changes in the long term. T h e purpose of this study was therefore to study the influence o f rainfall conditions on species composition changes, which could serve as a basis for determining the long t e r m influence of rainfall increase (e.g. cloud stimulation) and rainfall decrease (e.g. long t e r m droughts).

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© 1995 Academic Press Limited

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S.J. STEENEKAMP & O. J. H. BOSCH

Materials and methods Study (lFea

The study has been conducted in the semi-arid climatic climax grasslands of southern Africa (Mentis & Huntley, 1982) in five consecutive rainfall zones, namely zone 1( < 600 m m p.a.), zone 2 (600-650 m m p.a.), zone 3 (650-700 m m p.a.), zone 4 (700-750 m m p.a.) and zone 5 ( > 750 m m p.a.). T h e rainfall occurs mainly in the summer months (October to March) with the highest amounts in D e c e m b e r and January. Rainfall is erratic and seasonal droughts occur on a regular basis. Temperatures vary from a mean maximum of 27°C and a minimum of 15°C in December to a mean maximum of 16°C and m i n i m u m o f - 3 ° C in July.

Fieldwork Vegetation surveys were made on the pediments of localities in the five consecutive rainfall zones. T h e surveys were done by means of a computerized monitor apparatus (Booysen & Bosch, 1991). Only the nearest plant within a radius of 200 m m was recorded. T h e sample sites in each locality were selected in such a way that they represented different degradation stadia (Bosch & Kellner, 1991). For this vegetation surveys were done at different distances from watering points or other areas of animal concentration (e.g. shade), on both sides offence lines as well as in areas that were under-utilized in the long term (Bosch, 1989). This selection of sample sites provided compositions that were spread over the whole spectrum of the degradation gradient. During the field surveys habitat diversity was kept at a minimum to ensure that species composition changes could mainly be attributed to the grazing factor.

Data analysis Rainfall gradients were constructed for different degradation stadia. For t_his the data were first analysed by means of quantitative ordination techniques in order to construct a degradation gradient for each locality (Bosch & Gauch, 1991; Bosch & Booysen, 1992). T h e first axis of the ordination represents a degradation gradient while the second and higher axis were combined in a single value, namely the residual (Bosch & Gauch, 1991). Sample sites with a residual value higher than an arbitrary 50% of the Euclidian distance (distance between a sample site and the first ordination axis) were regarded as outliers and were removed from the data matrix (Bosch & Kellner, 1991). T h e first ordination axis of the degradation gradient of each locality was then subdivided into four utilization or degradation levels, i.e. under-utilized, optimum utilization, moderate over-utilization and severe over-utilization. T h e I S P D package (Bosch et al., 1992) was used to create data sets containing samples of the same utilization level of the different rainfall zones. Each data set was again ordinated by means of the ordination module of ISPD. As the different sample sites of each data set were representative of the same condition but from different rainfall zones, a rainfall gradient developed on the first ordination axis. T h e sample sites of the different rainfall zones were used to confirm this gradient on the first axis (Fig. 1). T h e reaction of individual species on the rainfall gradient has been determined graphically by means of various regression techniques for each conditional state (Booysen, 1990). Gaussian curves provided the best fittings and only those with a D-statistic value higher than 0.5 were regarded as significant. Species that did not

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show any reaction on the rainfall gradient (D-statistic value smaller than 0.5) were r e m o v e d f r o m the data matrix in order to obtain m o r e accurate ordinations.

Results and discussions

Species can be grouped into three different categories on the basis of their reaction to rainfall, namely species that showed a clear reaction on the gradient, species that only occur in high or in low rainfall zones and species that did not show any reaction on the rainfall gradient.

Species that showed a reaction on the rainfall gradient Examples of the reaction of species on the rainfall gradient at four different conditional states are given in Fig. 2. Themeda triandra Forssk. and Eragrosris curvula (Schrad.) N e e s occur over the total rainfall gradient and b o t h species react clearly to the influence of rainfall. Themeda triandra, which has b e e n classified by Bosch & T h e u n i s s e n (1992) as a decreaser species (i.e. those species which d o m i n a t e in veld which is in a good condition and which decline in a b u n d a n c e w h e n reid deteriorates in condition), is d o m i n a n t u n d e r optimal grazing conditions as well as to a lesser extend in under-utilized conditions in all the rainfall zones. In over-utilized conditions Themeda rriandra occurs only in the. high rainfall areas. Eragrostis curvula, a low ecological status species (Bosch & Theunissen, 1992), is m o r e i m p o r t a n t u n d e r moderately-utilized conditions in all the rainfall zones. In over-utilized conditions this species is only d o m i n a n t in the high rainfall zones. Setaria sphacelata (Schumach.) M o s s has a high a b u n d a n c e in all four of the conditional states in the low rainfall zone. Heteropogon contortus (L.) Roem. & Schult. occurs u n d e r moderately- and under-utilized conditions only in the high

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S. J. STEENEKAMP& O. J. H. BOSCH

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rainfall zones, while it occurs u n d e r optimally and under-utilized conditions over the whole rainfall gradient.

Species which occur only in low or high rainfall areas Examples of the reaction of species which occur only in low or high rainfall areas are given in Fig. 2. Aristida congesta Roem. & Schult., Cynodon dactylon (L.) Pet's., Eragrostis obtusa M u n r o ex Fical. & H i e m and Microchloa caffra Nees occur only in the low rainfall areas and are more dominant under moderately- and over-utilized conditions than the other conditional states. T h e s e species are also regarded by Gibbs Russel et al. (1990) and Janse van Rensburg & Bosch (1990) as low ecological status species. Cymbopogon plurinodis (Stapf) Stapf ex Burtt Davy and Elionurus muticus, (Spreng.) Kunth, which were classified by Janse van Rensburg & Bosch (1990) as increaser

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1-species (i.e. species which are not abundant in veld which is in good condition, but which increase when veld is under-utilized), are dominant under optimally and underutilized conditions, but are also present in low frequencies under moderately- utilized conditions. Both these species, but especially Cymbopogon plurinodus, are only found in the low rainfall zones. Hyparrhenia hirta (L.) Stapf occurs only in under-utilized conditions in the low rainfall area. Eragrostis plana Nees and Eragrostis capensis (Thunb.) Trin. occur only in the high rainfall zone and are dominant under moderately-and under-utilized conditions. Gibbs Russel et al. (1990) associated these two species with wet and disturbed habitat conditions. Eragrostis plana does occur in low frequencies under optimally and underutilized conditions. According to Gibbs Russel et al. (1990), the occurrence of Eragrostis plana in areas with a low rainfall is restricted to water courses and other wet habitats. As the surveys were only carried out on the pediments, Eragrostis plana was not recorded in poor condition vegetation in the low rainfall zones. Tristachya leucothrix Nees and Havpechloa falx (L.) Kuntze occur only in the high rainfall areas. They are dominant under optimally and under-utilized conditions, but also occur in low frequencies under moderately- and over-utilized conditions. Gibbs Russel et al. (1990) stated that Havpechloafalx occurs in wetter areas, while Tristachya leucothrix is found in sour veld (which is associated with high rainfall). Alloteropsis semialata (R.Br.) Hitchc. and Monocymbium ceresiiforme (Nees) Stapf occur only under optimal conditions in the high rainfall zones.

Species with no reaction w the rainfall gradient The following species showed no reaction to the influence of rainfall, namely Aristida canescem Henr., Brachiaria nigropedata (Fical. & Hiem) Stapf, Chloris virgata Swartz, Cymbopogon excavatus (Hochst.) Stapf ex Burtt Davy, Digitaria eriantha Steud., Diheteropogon amplectens (Nees) W.D. Clayton, Eragrostis racemosa (Thunb.) Steud., Melinis repens (Willd.) C.E. Hubb, Eragrostis gummiflua Nees, Sporobolus fimbriatus (Trin.) Nees, Trachypogon spicatus (L.f.) Kuntze and Tragus berteronianus Schult. The fact that the frequencies of the above species in the data are low could also be a reason why no significant fitting on the rainfall gradient was obtained for these species. Bosch & Theunissen (1992) indicated that ecotypic variation in Digitaria eriantha and Eragrostis racemosa is associated with specific habitat conditions and that no relationships with geographical distribution exist (including rainfall).

Conclusion Certain species occur in all the rainfall zones while others are limited to either the high or the low rainfall area. T he reaction of species on the rainfall gradient also differs in vegetation, representing different degradation stadia. This could probably also be attributed to ecotypic variations (Gibbs Russel, 1983; Bosch & Theunissen, 1992; Theunissen, 1992a, b). Species composition changes under different conditional states can be predicted by means of the reaction that the key species would have on an increase or decrease in rainfall. These predictions are of particular importance to the eventual determination of grazing capacity, as the calculation of effectively available plant production is based on species composition.

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Industrial Research. Report No. 62. Theunissen, J.D. (1992a). An ecosystematic investigation of two graminoids (Digitar~a er~antha and Setaria sphacelata vat. torta) in the semi-arid grasslands of southern Africa. Botanical Bulletin of Academia Sinica, 33:123-132. Theunissen, J.D. (1992b). An ecosystematic investigation of Themeda triandra (Poaceae: Andropogoneae) in the semi-arid grasslands of southern Africa. Journal of Arid Environments, 22:1-i0.