The Vegetation Ecology of the Lower Mkuze River Floodplain, Northern

The vegetation ecology ofthe lower Mkuze River floodplain, northern KwaZulu-Natal: A landscape ecology perspective. Submitted in fulfillment ofthe requirements for the degree ofMaster ofScience in the School ofLife and Environmental Sciences, University ofNatal-Durban. 2001 Marian J. Neal PREFACE The work described was carried out between January 1999 and December 2001, in the School of Life and Environmental Sciences (previously the Department of Geographical and Environmental Sciences) at the University of Natal-Durban, under the supervision of Prof. W.N. Ellery. The study represents original work by the author and has not been submitted in any other form to another university. Where use was made of the work of others, it has been duly acknowledged in the text. 11 ACKNOWLEDGEMENTS This study fonns part of an ongoing research programme conducted in the Greater Mkuze Wetland System, northern KwaZulu-Natal and was made possible through the funding from Wildlife & Environment Society of southern Africa, the University ofNatal Masters special fund and the National Research Foundation. I would like to acknowledge the following people without whom this thesis would not have been possible: • Prof. Fred Ellery, for the enthusiasm he has for wetlands, for the discussions, time and encouragement and for instilling in me a work ethic I will carry with me throughout my career. • Dr Annika Dahlberg, for advice, support and assistance, especially in the field. • Prof. Gerry Garland, for the assistance and advice during the soil analysis phase. • Ezemvelo KwaZulu-Natal Wildlife, for granting pennission to access the study area and for the use of the Manzibomvu camp. Special mention must go to Drikus Gissing for his hospitality and logistical support. • The Mnqobokazi community, for their permission to access land under their jurisdiction and their continuous support for the project. • Ashleigh Schoutlz, Kirsten Bames, Andrew Green, Blessing Mhlanga, Steven Dunlevey, for their assistance in the field during the data collection phase. • Frank Sokolic, for his unfailing support and assistance with GIS dramas, general computer troubleshooting and the production ofsome ofthe figures. • Issac Abboy, for organising maps, aerial photos and field equipment. • Gill Woods, for her continuous support and encouragement. • Hem Hurrypursad, for the production ofsome ofthe figures. • The staff of department, for giving me the unique opportunity to work in an environment that encourages multidisciplinary co-operation, generosity and support. • Vasna Ramasar and Jennifer Houghton for their unflagging support and friendship. • My Family, for their support, encouragement and love. 111 ABSTRACT The overall aim ofthis study was to develop an understanding ofthe 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 ofdifferent wetland types. In addition it has a complex history ofresource 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 ofpans or in extensive stands in low-lying areas in the distal reaches ofthe 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. IV 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 ofthe 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 ofdisturbance in ecosystem patterns and processes. Therefore a temporal analysis ofthe 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 ofpatches, mean patch size, median patch size, patch size standard v 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 ofthe 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. VI TABLE OF CONTENTS Title Page ·..··.······ 1 Preface 11 Acknowledgements 111 Abstract 1V Table ofContents Vll List ofFigures Xl List ofTables X111 1. INTRODUCTION 1 2. THEORETICAL FRAMEWORK 3 2.1 Introduction 3 2.2 The Emergence ofthe Hierarchical Patch Dynamics Paradigm 3 2.2.1 The Equilibrium Paradigm3 .. 2.2.2 The Alternative Paradigm 4 2.2.3 The Hierarchical Patch Dynamics Paradigm 5 2.3 Landscape Ecolog)' 6 2.3.1 Hierarchy 7 2.3.2 Heterogeneity 8 2.4 Wetland Ecolo~..........•............................•.••..••.•..............•.................•..............•.•....... 12 2.4.1 Wetland Patterns and Processes

Load more