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Nelson Mandela Metropolitan University (2000)

The effects of goat browsing on ecosystem patterns and processes in succulent thicket, South Africa

Lechmere-Oertel RG

Titre : The effects of goat browsing on ecosystem patterns and processes in succulent thicket, South Africa

Auteur : Lechmere-Oertel RG

Université de soutenance : Nelson Mandela Metropolitan University

Grade : Doctor of Philosophy (PhD) 2000

Résumé partiel
Transformation in the arid succulent thicket of the Eastern Cape of South Africa in response to unsustainable livestock production has been widespread, with less than 10% remaining intact. Transformation in succulent thicket has resulted in large areas of dense thicket (comprising a two-phase mosaic of perennial-vegetated patches separated by animal paths and bare patches) being replaced with a ’pseudo-savanna’ of remnant canopy trees with a structurally simple field layer of ephemeral and short-lived perennial grasses and forbs. There is an extensive literature describing the transformation of succulent thicket, with many speculative statements about the underlying mechanisms of transformation. The central focus of this study was to improve our mechanistic understanding of transformation in succulent thicket using field experiments. Hopefully these results will set another foundation upon which future management of succulent thicket can be improved and large-scale restoration initiated.

This study comprises four themes that are linked to the concept of landscape function. The central premise of landscape function is that functional landscapes have mechanisms that capture and retain scarce resources. Conversely, as landscapes become increasingly dysfunctional, so these mechanisms become disrupted. In succulent thicket, dysfunction appears to be linked to the reduced ability to harvest water, cycle carbon and a loss of organic carbon. In this thesis the author examined some of the key processes that influence water and organic carbon fluxes : perennial vegetation cover, soil fertility, litter fall and decomposition, and runoff and soil erosion. The experimental design that was used for all this work was a factorial ANOVA based on replicated fenceline contrasts that reflect differences in long-term management history. The main objectives of this thesis were to : (1) quantify the patterns of transformation in an arid form of succulent thicket, including changes in the biomass, cover and structure of the dominant vegetation guilds ; (2) test the stability of the transformed succulent thicket ecosystem to show whether it is a new stable state or an intermediate stage in a trajectory towards a highly desert�f�ed state where only the ephemeral grasses and forbs persist ; (3) describe and compare soil fertility across transformation contrasts, concentrating on changes in the spatial patterns of soil resources and the ability of the soil to harvest precipitation ; (4) to compare litter fall and decomposition of leaf material from the dominant plants in intact and transformed succulent thicket ; and (5) to quantify and compare run-off and erosion from run-off plots in intact and transformed succulent thicket.

Transformation and stability : The author quantified the changes in plant diversity, physiognomy and biomass that occur across transformation contrasts. Thicket transformation results in a significant loss of plant diversity and functional types. There is also a significant reduction in the biomass (e. 80 tha-1) and structural complexity of the vegetation, both vertically and horizontally. These results were interpreted in terms of their implications for ecosystem functioning and stability. To test the stability of the transformed succulent thicket the author used aerial photographs and ground-truthing to track the survivorship of canopy trees over 60 years in pseudo-savanna landscapes. The author also measured seedling establishment in different habitats. I show that the pseudo-savanna is not a stable state owing to ongoing adult mortality and no recruitment of canopy trees.

Soil fertility and water status : The author hypothesised that the above-ground changes in vegetation would be accompanied by similar trends in the pattern and levels of soil nutrient resources and the ability of the landscape to harvest precipitation. The author compared soil fertility (organic carbon, available nitrogen and phosphorus), texture, matric potential, and surface micro-topography in the two main micro-habitats on either side of the replicated fenceline contrasts. The results show that intact Spekboom thicket has a distinct spatial pattern of soil fertility where nutrients and organic carbon are concentrated under the patches of perennial shrubs, compared to under canopy trees and open spaces. Transformation results in a significant homogenisation out of this pattern and an overall reduction in the fertility of the landscape. The proportion of the landscape surface that would promote infiltration of water decreases from 60 - 0.6 %. Soil moisture retention (matric potential) also decreases with transformation. The author interpreted these patterns in terms of the ability of the landscape to harvest and release water after rainfall events.

Présentation (NRF)

Page publiée le 10 juillet 2017