The invasion of Pteronia incana (Blue bush) along a range of gradients in the Eastern Cape Province: It’s spectral characteristics and implications for soil moisture flux JOHN ODHIAMBO ODINDI Submitted in fulfilment of the requirement for the degree of PHILOSOPHIAE DOCTOR in the Faculty of Science at the Nelson Mandela Metropolitan University January 2009 Promoter: Professor Vincent Kakembo i Abstract Extensive areas of the Eastern Cape Province have been invaded by Pteronia incana (Blue bush), a non-palatable patchy invader shrub that is associated with soil degradation. This study sought to establish the relationship between the invasion and a range of eco-physical and land use gradients. The impact of the invader on soil moisture flux was investigated by comparing soil moisture variations under grass, bare and P. incana invaded surfaces. Field based and laboratory spectroscopy was used to validate P. incana spectral characteristics identified from multi-temporal High Resolution Imagery (HRI). A belt transect was surveyed to gain an understanding of the occurrence of the invasion across land use, isohyetic, geologic, vegetation, pedologic and altitudinal gradients. Soil moisture sensors were calibrated and installed under the respective surfaces in order to determine soil moisture trends over a period of six months. To classify the surfaces using HRI, the pixel and sub-pixel based Perpendicular Vegetation Index (PVI) and Spectral Mixture Analysis (SMA) respectively were used. There was no clear trend established between the underlying geology and P. incana invasion. Land disturbance in general was strongly associated with the invasion, as the endemic zone for the invasion mainly comprised abandoned cultivated and overgrazed land. Isohyetic gradients emerged as the major limiting factor of the invasion; a distinct zone below 619mm of mean annual rainfall was identified as the apparent boundary for the invasion. Low organic matter content identified under invaded areas was attributed to the patchy nature of the invader, leading to loss of the top soil in the bare inter-patch areas. The area covered by grass had consistently higher moisture values than P. incana and bare surfaces. The difference in post-rainfall moisture retention between grass and P. incana surfaces was significant up to about six days, after which a near parallel trend was noticed towards the ensuing rainfall episode. Whereas a higher amount of moisture was recorded on grass, the surface experienced moisture loss faster than the invaded and bare surfaces after each rainfall episode. i There was consistency in multi-temporal Digital Number (DN) values for the surfaces investigated. The typically low P. incana reflectance in the Near Infrared band, identified from the multi-temporal HRI was validated by field and laboratory spectroscopy. The PVI showed clear spectral separability between all the land surfaces in the respective multi-temporal HRI. The consistence of the PVI with the unmixed surface image fractions from the SMA illustrates that using HRI, the effectiveness of the PVI is not impeded by the mixed pixel problem. Results of the laboratory spectroscopy that validated HRI analyses showed that P. incana’s typically low reflectance is a function of its leaf canopy, as higher proportions of leaves gave a higher reflectance. Future research directions could focus on comparisons between P. incana and typical green vegetation internal leaf structures as potential causes of spectral differences. Collection of spectra for P incana and other invader vegetation types, some of which have similar characteristics, with a view to assembling a spectral library for delineating invaded environments using imagery, is another research direction. ii ACKNOWLEDGEMENTS I would like to thank the many people whose support in different ways made this thesis possible, Prof. Vincent Kakembo for his enthusiasm, inspiration, guidance and support throughout my doctoral studies. This study could also not have been possible without the NRF grant holders funding he secured. Dr. Jenipher Gush and the Amakhala Game Reserve Conservation Centre team (Shahid Razzaq, Dr. Nathalie Razzaq, Lauren Le Roux and Giles Gush) for the support during field work. Dr. Jaques Petersen for statistical support Mr. Peter Bradshow and Ms Phozisa Mamfengu of SANParks – Park Planning and Development for providing GIS data and advice. Staff in the Geosciences department for always being there for me. Special thanks goes to Willy Deysel and Paul Baldwin for making sure that everything I needed for my laboratory work was available. My Colleagues and friends (Mhangara, Nyamugama, Manjoro, Dhliwayo, Mengwe, Nohoyeka, Mamfengu, Onyancha, Kleyi and Gitonga) for providing a stimulating environment to learn and grow. My brother Dr. Mak’Ochieng and his family for all the sacrifices. My immediate and extended family, particularly my parents and my first cousin Peter Were for making me who I am. Generous funding from NRF grant holder bursary and the NMMU postgraduate funding from the Research Office is hereby appreciated. To God for the strength and determination iii TABLE OF CONTENTS Page ABSTRACT i ACKNOWLEDGEMENTS iii TABLE OF CONTENTS iv APPENDICES viii LIST OF FIGURES ix LIST OF TABLES xii LIST OF ACRONYMS xiii Chapter 1: General introduction 1.1 Introduction 1 1.2 The research problem 2 1.3 Aim of the study 3 1.4 Specific objectives 3 1.5 Chapter outline 5 Chapter 2: Plant invasions across gradients, hydrological response and spectral characteristics: A theoretical background 2.1 Introduction 7 2.2 Plant invasions across ecological and physical gradients 7 2.3 P. incana : Origin, floristic structure and invasion implications 9 2.4 Relationship between soil moisture and vegetation patchiness 10 2.4.1 Moisture retention: Implications for invasion control and restoration of invaded areas 12 2.4.2 Techniques for monitoring soil moisture flux 12 2.4.2.1 Capacitance moisture probes 13 2.5 Classification of P. incana invaded surfaces using pixel and sub-pixel based techniques 14 2.5.1 Separation of P. incana using ratio based indices 14 2.5.2 Perpendicular Vegetation Indices 15 2.5.3 Pixel and sub-pixel based techniques 16 iv 2.5.4 Endmember selection, validation and applicable resolutions 18 2.6. The use of spectroscopy for validation of surface reflectance 20 2.6.1 Role of spectroscopy in remote sensing 20 2.6.2 The spectroscopy process 21 2.6.3 In-situ versus laboratory spectroscopy 22 2.6.4 Spectral reflectance at different wavelengths 23 2.6.5 Importance of spectral derivatives 26 2.7 Summary 26 Chapter 3: P. incana occurrence across a range of gradients 3.1 Introduction 28 3.2 Major gradients within the transects 29 3.2.1 Geological formations 30 3.2.2 Land use types 30 3.2.3 Vegetation types 30 3.2.4 Rainfall 31 3.3 Methods 33 3.4 Results 35 3.5 Discussion 39 3.5.1 P. incana invasion and the underlying geology 39 3.5.2 Land use and P. incana invasion 39 3.5.3 Disturbance as a cause of invasion 40 3.5.4 Isohyet gradient and P. incana invasion 41 3.5.5 P. incana invasion and soil characteristics 42 3.6 Conclusion 43 Chapter 4: Hydrological response of P. incana invaded areas: implications for landscape functionality 4.1 Introduction 44 4.2 The study area 45 4.3 Materials and methods 47 4.3.1 Capacitance sensor: Theory and instrumentation 47 4.3.2 Sensor calibration 48 v 4.3.3 Field installation 49 4.3.4 Data presentation and analysis 50 4.4 Results and discussion 50 4.4.1 Moisture variations 50 4.4.2 Episodic moisture flux 51 4.4.3 Soil moisture trends 54 4.4.4 Day/night moisture oscillations 58 4. 4.5 Implications of P. incana invasion for landscape function 62 4.5 Conclusion 63 Chapter 5: A comparison of pixel and sub-pixel based techniques to separate P. incana invaded areas using multi-temporal High Resolution Imagery 5.1 Introduction 64 5.2 The study area 66 5.3 Methods 68 5.3.1 High Resolution Imagery acquisition 68 5.3.2 Image rectification 69 5.3.3 Image enhancement 70 5.3.4 Multi-temporal image analyses 70 5.3.5 Surfaces sample spectroscopy 76 5.4 Results 77 5.5 Discussion and conclusion 85 Chapter 6: The use of laboratory spectroscopy to establish Pteronia incana spectral trends and its separability from bare surfaces and green vegetation 6.1 Introduction 87 6.2 The study area 89 6.3 Materials and methods 91 6.4 Results and discussion 94 6.5 Conclusion 101 vi Chapter 7: Synthesis 7.1 Introduction 102 7.2 P. incana invasion correlation with macro-scale gradients 102 7.3 P. incana invasion and soil moisture flux 103 7.4 P. incana spectral characteristics 104 7.5 Application of pixel and sub-pixel based classifications in P. incana invaded areas 104 References 107 vii APPENDICES Appendix A: P. incana canopy mixtures with respective leaves to branch ratios 141 Appendix B: Green vegetation, bare soil and P. incana monthly sample reflectance spectra 142 Appendix C: First order derivatives of the monthly reflectance spectra 145 Appendix D: Portion of calibrated sensor moisture logs for the three episodes at 1hr interval 148 viii LIST OF FIGURES Figure 2.1: Pteronia incana invader shrub 10 Figure 2.2: The influence of terrain on moisture infiltration 11 Figure 2.3: Perpendicular Vegetation Index (PVI) 15 Figure 2.4: Vegetation spectra and portions that react to different plant components 22 Figure 2.5: Spectral response of soils at oven dried, 0.03, 0.12, 0.20, 0.30 and 0.42 gravimetric water contents (g/g) 25 Figure 3.1 Transects and GPS invasion nodes 34 Figure 3.2: P. incana invaded nodes on underlying geology 35 Figure 3.3: P.