Cynodon dactylon meadows occupying seasonally inundated areas of the Pongolo river floodplain, South Africa, decompose during periods of submergence . The loss of dry matter and nutrients from both fresh and dried material enclosed in litter bags was studied . The pattern of loss of dry matter was diphasic . Loss was more rapid from dried than from fresh material, half the mass being lost in 18 and 28 days respectively . Nutrient loss followed an exponential pattern . Nitrogen, phosphorus and magnesium were lost more rapidly from dried than from fresh material whilst rates of loss of sodium, potassium and calcium were similar . The significance of Cynodon decomposition in the functioning of the floodplain is assessed from estimates of the above-and below-ground standing crop around one small lake before and after submergence . It is concluded that decomposition of plants growing in seasonally inundated areas of the floodplain may contribute significantly to the productivity of the system, but that the extent of the contribution can be greatly influenced by the flooding regime . The completion of the Pongolapoort dam upstream of the floodplain will alter the natural flooding pattern, effecting a reduction in floodplain productivity, unless a programme for flood release can be initiated .
The Braun-Blanquet method of phytosociological analysis has been used to identify the plant communities of the seasonally flooded areas of the Pongolo River Floodplain. Six communities and two sub-communities, whose distribution is closely related to relative periods of exposure and inundation, were recognized. The struc ture and interrelationships o f the communities are considered and the role of human activities in determining their extent is commented upon. Comparison is made with similar situations elsewhere in Africa.
RÉSUMÉ VÉGÉTATION DES Z O N E S PÉRIODIQUEM ENTINONDÉES DANS LE LIT MAJEUR DE LA RIVIERE PONGOLOOn a utilisé la méthode d 'analyse phytosociologique de Braun-Blanquet pour identifier les communautés végétales des zones périodiquement inondées dans le lit majeur de la riviere Pongolo. On y a reconnu six communautés, dont la distribution est en corrélation étroite avec les périodes relatives d'exposition et d 'inondation. On considêre la structure et les interrelations des communautés et io n commente le róle joué par les activités humaines dans la détermination de leur amplitude. La situation est comparée á d 'autres du même genre qui se présentent ailleurs en Afrique.
Cynodon dactylon (L.) Pers. forms extensive meadows on seasonally inundated areas of the Pongolo river floodplain. As floodwaters receded during winter, the shoots of C. dactylon which had not decomposed during submergence sprouted. Crop growth rate was high (up to 23 kg/ ha/ d dry mass) and the meadows were heavily grazed by domestic stock which removed up to 15 kg/ha/d. The high crop growth rate was not maintained because as exposure period increased, soils became dry and water stress conditions developed. This caused the C. dactylon to become less palatable and brought about a shift in grazing which allowed a build-up of the grazeable and ungrazeable fraction standing crop. Maximum standing crop occurred at the onset of the next flood. Grazing is shown to have a net negative effect on production, although at different times during exposure defoliation had either a depressing or stimulating effect on growth. It is postulated that the duration of the inundation period is more significant than the duration of the exposure period in maintaining the C. dactylon meadows.
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