The facilitative role of trees in tree-grass interactions savannas The facilitative role of trees in tree-grass interactions in savannas K. V. R. Priyadarshini V. K. K. V. R. Priyadarshini The facilitative role of trees in tree-grass interactions in savannas K. V. R. Priyadarshini Thesis committee Promotors Prof. Dr H.H.T. Prins Professor of Resource Ecology Wageningen University Prof. Dr S. de Bie Professor of Sustainable Use of Living Resources Wageningen University Co-promotor Dr I.M.A. Heitkönig Assistant Professor, Resource Ecology Group Wageningen University Other members Prof. Dr E. Hoffland, Wageningen University Prof. Dr L. Mommer, Wageningen University Dr K.T. Rebel, Utrecht University Dr E.S. Bakker, Netherlands Institute of Ecology, Wageningen This research was conducted under the auspices of the C.T. de Wit Graduate School for Production Ecology & Resource Conservation (PE&RC). The facilitative role of trees in tree-grass interactions in savannas K. V. R. Priyadarshini Thesis submitted in fulfilment of the requirements for the degree of doctor at Wageningen University by the authority of the Rector Magnificus Prof. Dr A.P.J. Mol, in the presence of the Thesis Committee appointed by the Academic Board to be defended in public on Tuesday 24 May 2016 at 1.30 p.m. in the Aula. K. V. R. Priyadarshini The facilitative role of trees in tree-grass interactions in savannas, 134 pages. PhD Thesis, Wageningen University, Wageningen, NL (2016) With references, with summaries in English and Dutch ISBN 978-94-6257-700-8 Propositions 1. Understanding the role of soil organisms in mediating and sustaining nutrient transfer among plants, and between soil and plants is crucial for explaining tree-grass coexistence in savannas. (this thesis) 2. A comprehensive understanding of tree-grass interactions in savannas demands a broad, multitrophic approach rather than a narrow one based on Gaussian principles of competitive exclusion. (this thesis) 3. The fundamental tenet of natural resource economics and environmental economics is that all natural resources are substitutable or recyclable (Farley, J & R. Constanza, 2010, Ecological Economics 69, 2060–2068) is flawed. 4. The notion of ecosystems being ‘Natural Capital’ and natural processes being ‘Ecosystem Services’(Adams, W. M., 2014, Science 346, 549–551) is fundamentally erroneous since such commodification of ecosystem and natural processes suggests that the natural world is otherwise worthless. 5. Arresting the unsustainable increase in human population is possible through reducing human fertility with modern contraception techniques and communication without religious and socio-political interference. 6. The distinction between freedom of action and freedom of will is that freedom of action is real and freedom of will is imaginary. Propositions belonging to the Ph.D. thesis entitled: ‘The facilitative role of trees in tree-grass interactions in savannas’ K. V. R. Priyadarshini Wageningen, 24 May 2016 TABLE OF CONTENTS CHAPTER 1 .............................................................................................................................. 1 GENERAL INTRODUCTION .................................................................................................. 1 Tree-grass interactions in savannas ........................................................................................... 1 Resource-use and tree-grass interactions ................................................................................... 3 Outline of the thesis ................................................................................................................... 5 CHAPTER 2 .............................................................................................................................. 7 SEASONALITY OF HYDRAULIC-REDISTRIBUTION BY TREES TO GRASSES AND CHANGES IN THEIR WATER-SOURCE USE THAT CHANGE TREE-GRASS INTERACTIONS ...................................................................................................................... 7 Abstract ...................................................................................................................................... 7 Introduction ................................................................................................................................ 8 Materials and methods ............................................................................................................. 10 Results ...................................................................................................................................... 15 Discussion ................................................................................................................................ 20 Appendix 2.1 ............................................................................................................................ 25 Appendix 2.2 ............................................................................................................................ 26 Appendix 2.3 ............................................................................................................................ 27 CHAPTER 3 ............................................................................................................................ 28 OVERLAP IN NITROGEN SOURCES AND REDISTRIBUTION OF NITROGEN BETWEEN TREES AND GRASSES IN A SEMI-ARID SAVANNA .................................. 28 Abstract .................................................................................................................................... 28 Introduction .............................................................................................................................. 29 Methods.................................................................................................................................... 30 Results ...................................................................................................................................... 34 Discussion ................................................................................................................................ 38 Appendix 3.1 ............................................................................................................................ 45 Appendix 3.2 ............................................................................................................................ 46 CHAPTER 4 ............................................................................................................................ 49 COMPETITION WITH TREES DOES NOT INFLUENCE ROOT CHARACTERISTICS OF PERENNIAL GRASSES IN SEMI-ARID AND ARID SAVANNAS IN SOUTH AFRICA. .................................................................................................................................. 49 Abstract .................................................................................................................................... 49 Introduction .............................................................................................................................. 50 Methods.................................................................................................................................... 51 Results ...................................................................................................................................... 57 Discussion ................................................................................................................................ 58 Appendix 4.1 ............................................................................................................................ 66 CHAPTER 5 ............................................................................................................................ 67 THE IMPORTANCE OF TREES IN HUMAN LAND-USE SYSTEMS IN AFRICAN DRYLANDS: AN ECOLOGICAL AND BIOGEOCHEMICAL PERSPECTIVE ................ 67 Abstract .................................................................................................................................... 67 Introduction .............................................................................................................................. 68 Tree impacts on ecosystem functions: water and nutrient redistribution ................................ 69 Trees in human-use landscapes ................................................................................................ 74 Conclusion ............................................................................................................................... 86 CHAPTER 6 ............................................................................................................................ 88 SYNTHESIS: THE IMPORTANCE OF FACILITATIVE INTERACTIONS FOR TREE- GRASS COEXISTENCE IN SAVANNAS ............................................................................ 88 Introduction .............................................................................................................................. 88 Facilitative effects of trees on grasses as a mechanism of tree-grass co-existence in arid and semi-arid savannas ................................................................................................................... 89 Biotic influence on water and nitrogen dynamics on plant interactions in savannas .............. 98 Conclusion ............................................................................................................................