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University of Venda (2022)

Vegetation dynamics in a semi-arid freshwater wetland : a measure for restoration success

Murungweni, Florence Mazwi

Titre : Vegetation dynamics in a semi-arid freshwater wetland : a measure for restoration success

Auteur : Murungweni, Florence Mazwi

Université de soutenance : University of Venda

Grade : Doctor of Philosophy (PhD) in Environmental Sciences 2022

Résumé partiel
The functioning of wetland ecosystems is impacted by human anthropogenic activities, which affect wetlands’ vegetation development and their ability to offer ecosystem services. The need to understand vegetations’ natural regeneration, in previously degraded wetlands of semi-arid regions is essential to measure restoration success. Nylsvley freshwater wetland is the largest inland flood plain found in the semi-arid region of South Africa and a Ramsar site of worldwide conservation importance. It provides a habitat to some endangered species ; however, it is facing degradation problems leading to a declining of its biodiversity. The importance of Nylsvley wetland prompted an initiation into its ecological restoration, to try and reverse the ecosystem degradation and monitor vegetation development of this Ramsar site. This study aimed to evaluate restoration efforts, through an analysis of vegetation dynamics, in a restored semi-arid freshwater wetland, so as to understand the success story of restoration efforts. The first objective of the study examined historical rainfall trends in the Nyl River system as rainfall affects vegetation productivity. Daily rainfall data was measured from eight rainfall stations for the period, 1950 to 2016, to generate seasonal and annual rainfall data. The Mann-Kendall and quantile regression were applied to assess trends in this rainfall data. Normalized Difference Vegetation Index derived from satellite images from between 1984 and 2003, utilising zonal statistics was correlated with rainfall of the same period to assess vegetation dynamics. The Mann-Kendall and Sen’s slope estimator showed that only one station had significant increased rainfall trend, annually and seasonally, at p < 0.05, whereas all the other stations showed insignificant trends during these rainfall seasons. Quantile regression showed that 50% and 62.5% of the stations had increasing annual and seasonal rainfall, respectively. Three of the eight stations had data which were statistically significant at p < 0.05, indicating increasing and decreasing rainfall trends. These rainfall trends show that the rainfall at Nylsvley decreased Vegetation dynamics in a semi-arid freshwater wetland : a measure for restoration success between 1995 and 2003. The R2 between rainfall and Normalized Difference Vegetation Index of Nylsvley is 55% indicating the influence of rainfall variability on vegetation productivity. The second objective assessed spatio-temporal changes of natural vegetation development within Vogelfontein, part of Nylsvley, utilising 1984, 2005 and 2016 satellite imagery which provided at least 10 years’ evaluation of conditions, before and after restoration. Land cover maps were generated from classified Landsat images using maximum likelihood classification algorithm with five land uses - old fields, water, bare ground, sparse and dense vegetation. The results showed a worsening of natural vegetation regeneration from 1984 until 2005, as bare ground and old field increased by over 19%. This decline was attributed to cultivation, which ended in 1996 in Vogelfontein. The period 2005 to 2016 saw the classes gaining in natural recovery of vegetation (7%). This indicates the necessity for wetland restoration, as there was evidence of vegetation recovery in a landscape facing a degradation trajectory.

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Page publiée le 21 janvier 2023