The vegetation ecology of the lower Mkuze river floodplain, Northern KwaZulu-Natal : a landscape ecology perspective.
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Date
2001
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Abstract
The overall aim of this study was to develop an understanding of the vegetation ecology of
the lower Mkuze River floodplain from a landscape ecology perspective. The lower Mkuze
River floodplain and its associated wetlands are located east of the Lebombo Mountains
and north of Lake St. Lucia on the Maputaland Coastal Plain in northern KwaZulu-Natal.
This system is defined as a storage floodplain wetland and comprises a mosaic of different
wetland types. In addition it has a complex history of resource use and management.
Landscape ecology proved to be an ideal theoretical framework for this study because it
enables the examination of complex ecological processes and phenomena in an integrated
and holistic manner. It achieves this by explicitly recognizing the spatial heterogeneity,
dynamics and hierarchical organization of the landscape; concepts that proved useful in
developing an understanding of the ecological patterns and processes operating within the
lower Mkuze River floodplain.
The vegetation of the study area was classified, using multivariate techniques, into six
plant communities. The distribution of these plant communities was correlated with
underlying environmental gradients that summarized the interactions between hydrology,
substrate properties and topography within the floodplain system. Within the study area the
Phragmites mauritianus reed swamp community was found where there was slow moving
water, in semi- to permanently saturated soil. This was usually around the edges of pans or
in extensive stands in low-lying areas in the distal reaches of the floodplain. The Imperata
cylindrica hygrophilous grassland community was uncommon and was found in isolated
stands towards the edge of the region of seasonal flooding. The Echinochloa pyramidalis
backswamp community was the most extensive of all the plant communities identified.
This community was tolerant of flooding and was found in damp places such as seasonal
pans, backswamps and riverbanks as well as in standing water. The distribution of the
Ficus sycomorus riparian forest community was restricted to elevated levees adjacent to
the river channel that experienced inundation when floods were large enough to overtop
channel banks. The Cynodon dactylon floodplain community was generally found towards
the floodplain-terrestrial upland boundary in elevated areas with sandy well-drained soils.
The Acacia xanthophloea woodland community was distributed on the floodplain margin
in elevated areas on sandy soils, primarily fringing the linear pans draining towards the
Mkuze River from the north. The description of the plant community types and the
underlying environmental determinants of their distribution provided a useful foundation
for the examination of ecological processes and phenomena operating at spatially coarser
levels within the landscape hierarchy.
Plant communities were aggregated into functional types based on criteria such as
exposure to similar flooding and sedimentation regimes. The identification and mapping of
these functional types, using a Geographical Information System (GIS), enabled one to
identify a hydrogeomorphic continuum that described the interaction between floodplain
processes and vegetation distribution. Within the study area the proximal-seasonally
inundated functional type comprised plant community types found on channel levees and
within backswamp areas. These areas were functionally connected to the Mkuze River in
that they were exposed to seasonal flood events and associated sedimentation. The distal-permanently inundated functional type was typically found in the lower reaches of the
floodplain that were rarely exposed to hydrological and sedimentological inputs from the
Mkuze River. This functional type was permanently inundated and characterized by
standing water and/or permanently saturated soils that were generally associated with the
large floodplain pans. The distal-infrequently inundated functional type was typically
located in sandy areas along the southern distal reaches of the floodplain. These areas were
infrequently inundated by overbank floodwaters from the Mkuze River and were not
characterized by substantial clastic sedimentation. The distribution and interaction between
these functional types made it possible to develop process-based understanding of the
ecosystem patterns and processes operating within the lower Mkuze River floodplain.
Landscape ecology theory emphasises the importance of a temporal analysis of spatial
heterogeneity and the role of disturbance in ecosystem patterns and processes. Therefore a
temporal analysis of the landscape mosaic from 1937 to 1996 was undertaken, using a GIS,
in order to quantify landscape change over time. The landscape characteristics utilised to
examine this change were total category area, percentage contribution to the total
landscape area, number of patches, mean patch size, median patch size, patch size standard deviation and the mean perimeter-area ratio. These spatial statistics were calculated for
each year using PATCH ANALYST, an ArcView GIS extension and they were used to
illustrate the role of anthropogenic disturbance on the landscape mosaic at a variety of
levels within the landscape hierarchy. Anthropogenic disturbance was found to affect
landscape content and configuration and therefore had the potential to undermine the
underlying environmental determinants of landscape patterns and processes. Once the
underlying functional processes are undermined, irreversible ecosystem degradation is a
possible outcome.
The examination of the different levels within the landscape hierarchy and the dynamics of
ecosystem patterns and processes operating within the Mkuze River floodplain made it
possible to develop deeper insights into ecosystem patterns and processes than a
conventional vegetation ecology study that typically focuses primarily on plant community
classification. The use of landscape ecology as an overarching theory that guided the
research process and aided the interpretation of findings by explicitly recognising the
importance of examining spatial heterogeneity, hierarchical organisation and dynamics,
proved invaluable in developing process-based understanding of the lower Mkuze River
floodplain.
Description
Thesis (M.Sc.)-University of Natal, Durban, 2001.
Keywords
Plant ecology--KwaZulu-Natal., Landscape ecology--KwaZulu-Natal., Plant communities--KwaZulu-Natal--Greater Mkuze Wetland System., Theses--Marine and coastal management.