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Woody Plant Encroachment in Arid and Mesic South African Savanna-Grasslands: Same Picture, Different Story?

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Woody Plant Encroachment in Arid and Mesic South African Savanna-Grasslands: Same Picture, Different Story? Woody plant encroachment in arid and mesic South African savanna-grasslands: same picture, different story? A thesis submitted in fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY of RHODES UNIVERSITY By ANDREW LUKE SKOWNO March 2018 Abstract Woody plant encroachment in South Africa’s savanna-grasslands has been considered a rangeland management problem since the early 1900s. This phenomenon, which has been observed globally, is particularly important in Africa given the extent of tropical grassy biomes on the continent and their importance for rural livelihoods. In this study, local and regional scale approaches were used to investigate woody cover change in South Africa across the important savanna-grassland rainfall threshold of 650 mm mean annual precipitation (MAP). The aim was to test this threshold using remote sensing and demographic surveys in order to better understand the patterns, mechanisms and drivers of encroachment. Rates of encroachment and population demographics of Vachelia karroo were compared at arid and mesic savanna sites in the Eastern Cape, using time-series analysis of historical aerial photographs in conjunction with field surveys. Changes in the extent of woodland vs. grassland were then quantified at a national scale (1990–2013) by combining optical and synthetic aperture radar remote sensing data. This produced the first map of woodland- grassland shifts for South Africa and provided the basis for a spatially explicit investigation of the key drivers of change. The local studies revealed higher rates of encroachment at mesic sites than at arid sites, with a correlation between drought and rate of encroachment at the arid site. Vachelia karroo seedlings and stunted saplings were more prevalent at mesic sites than at arid sites and the growth form of adult trees differed significantly between sites. The national remote sensing investigation showed that woodland replaced grassland in over 5% of South Africa’s savanna- grasslands between 1990 and 2014, at rates consistent with other global and regional studies. Spatially explicit models showed a pattern of incremental expansion of woodland along a ‘tree front’ and complex relationships between woodland increase and fire, rainfall, terrain ruggedness and temperature. Overall, the local and regional scale findings of this work highlight the importance of the savanna rainfall threshold (~650 mm MAP) and the presence / absence of fire in understanding savanna dynamics and woody cover change in the context of global drivers such as elevated atmospheric CO2. i Skowno PhD. (March 2018) Table of Contents Abstract ..................................................................................................................................... i List of figures .............................................................................................................................. iii List of tables ................................................................................................................................. v Acknowledgements .................................................................................................................... vii Chapter 1: General Introduction ............................................................................................ 1 Woody plant increase in savanna-grasslands............................................................................ 1 South African context and terminology .................................................................................... 4 Potential consequences of woody plant increase ...................................................................... 8 Methods and approaches ......................................................................................................... 10 Summary of chapters and key questions................................................................................. 13 Chapter 2: Six decades of woody canopy cover change across a savanna rainfall gradient in the Eastern Cape, South Africa ............................................................................................ 15 Abstract ...................................................................................................................................... 15 Introduction ................................................................................................................................ 16 Methods ...................................................................................................................................... 17 Study site................................................................................................................................. 17 Historical aerial photograph analysis ...................................................................................... 18 Rainfall trends ......................................................................................................................... 22 Fire records ............................................................................................................................. 22 Analyses .................................................................................................................................. 23 Results ........................................................................................................................................ 26 Validation of method .............................................................................................................. 26 General trends at each site ...................................................................................................... 26 Type analysis .......................................................................................................................... 27 Rate analysis ........................................................................................................................... 28 Discussion .................................................................................................................................. 34 Overall trends .......................................................................................................................... 34 Rates of encroachment and rainfall/drought ........................................................................... 34 Alignment with previous studies ............................................................................................ 35 Historical changes ................................................................................................................... 36 Chapter 3: Demography of Vachelia karroo at an arid and mesic savanna site with histories of bush encroachment ............................................................................................................ 38 Abstract ...................................................................................................................................... 38 Introduction ................................................................................................................................ 39 Demography............................................................................................................................ 40 Plant architecture .................................................................................................................... 41 Key questions .......................................................................................................................... 41 Methods ...................................................................................................................................... 42 Study site................................................................................................................................. 42 Transect data ........................................................................................................................... 42 Analyses .................................................................................................................................. 43 Results ........................................................................................................................................ 45 Discussion .................................................................................................................................. 52 Demography............................................................................................................................ 52 Architecture ............................................................................................................................ 53 Implications for managers ...................................................................................................... 53 ii Skowno PhD. (March 2018) Chapter 4: Woodland expansion in South African grassy biomes based on satellite observations (1990-2013): general patterns and potential drivers .................................... 55 Abstract ...................................................................................................................................... 55 Introduction ................................................................................................................................ 56 Methods ...................................................................................................................................... 57 Study area ..............................................................................................................................
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