The plant ecology of seasonally flooded areas of the Pongolo River Floodplain, with particular reference to Cynodon dactylon (L.) Pers.
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Date
1982
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Abstract
The impounding of the waters of the Pongolo river, upstream of its
floodplain on the Mocambique coastal plain, may adversely affect the
functioning of the floodplain system. A multidisciplinary study of the
functioning of the floodplain was initiated to provide a basis for the
development of a management strategy for the floodplain.
The study reported in this dissertation considered the flood dependence
and functioning of the vegetation of the seasonally inundated
area. The vegetation was mapped and the communities ordinated, according
to the Braun-Blanquet technique, in relation to their positions relative
to high flood level (HFL) and the level of the water after flood subsidence
(i.e. maximum retention level, MRL). Community distribution was
shown to be strictly determined by both the height of the floods and by
the MRL. It was concluded that periodic floods are essential for the
maintenance of the communities.
The Cynodon dactylon (L.) Pers. Community, which forms extensive
meadows in the zone of periodic inundation, was studied in detail. As
the water level receded during winter, productivity was high (up to
23 kg ha¯¹ d¯¹1 dry mass) and a palatable sward was produced. This is
heavily grazed by domestic stock, but as the soil dries out and water
stress becomes significant, production decreases, C. dactylon becomes
less palatable, and grazing shifts to newly exposed areas. The shift in
grazing allows the build up of a large standing crop of both grazeable
and ungrazeable (below ground and stolons) material. At the time of
inundation by the next floods c. 910 kg ha¯¹ of dry mass, c. 17 kg ha¯¹
nitrogen and c. 2 kg ha¯¹ phosphorus have been removed by grazers. It
is concluded that this production, which is flood dependent, forms an
important supplement to stock grazing during winter. It is suggested
that this source of grazing could be stimulated by irrigation during
winter. Cynodon dactylon is shown to decompose rapidly during inundation,
losing half of its mass and nutrients in c . 28 days. It therefore
represents a major energy and nutrient input during the aquatic phase.
The extent to which it is grazed during submergence is unknown. The
nutrient input is derived ultimately from the soils of the inundated
areas and, since nutrients are being removed by both terrestrial grazers
and flushing, continued production is reliant upon the annual sediment
load reaching the floodplain. Most of the sediment load will now be
deposited in the impoundment, and fertilization may be necessary to
maintain productivity.
The response of C. dactylon to the seasonal fluctuations in water
level are used to formulate proposals for water release from the dam.
These include proposals for the short-term, i.e. until the demand for
irrigation water conflicts with the requirements of the floodplain, and
for the long-term, when less water will be available for the floodplain
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1981.
Keywords
Plant ecology--KwaZulu-Natal--Pongolo River Floodplain., Floodplains--KwaZulu-Natal--Pongolo River Floodplain., Grasslands--KwaZulu-Natal--Pongolo River Floodplain., Theses--Botany.