Water requirements and distribution of Ammophila arenaria and Scaevola plumieri on South African coastal dunes Thesis Submitted in fulfilment of the requirements for the degree of Master of Science of Rhodes University by CRAIG INGRAM PETER Department of Botany Rhodes University February 2000 Map 1 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 -25 Maputo -26 Cabo de Santa Maria -27 The South African Coastline Kosi Bay The South African coastline is approximately 3000 kilometers long and includes the dry west coast, the warm temperate south-west -28 and south coast, and the sub-tropical east coast. Important coastal Cape Vidal towns and cities are identified, as are important landmarks. The Oranjemund Richards Bay Cape St Lucia -29 distribution limits of Ammophila arenaria are marked in blue, while Mtunzini the western distribution limit of Scaevola plumieri is marked in red. Tugela River Durban -30 Port Shepstone -31 Southbroom Port St Johns -32 Lambert’s Bay St Helena Bay Cape Columbine East London Kei River -33 Saldanha Langebaan Lagoon Grahamstown Port Alfred Fish River / Old Womans River Cape Town Knysna Port Elizabeth Cape Padrone -34 Mossel Bay Algoa Bay Cape Recief False Cape Seal Cape of Good Hope Bay Cape Vacca Cape St Francis Cape Infanta Danger Point Arniston Cape Agulhas -35 0 100km -36 Map 2 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 -26 -27 -28 The South African Coastal Vegetation KwaZulu Natal coast: Dune thicket and Cape Vidal Classification of the South African coastline as suggested by Lubke coastal forest et al (1997, red) and as proposed in the current work (blue). Boxes Cape St Lucia -29 give general vegetation types in the different regions as defined by Lubke et al (1997) and correspond to these authors classification. Tugela River Durban -30 West coast: Transkei coast: East coast Desert or strandveld Natal coast Coastal grassland, West coast (subdesert) -31 West coast dune thicket and coastal forest. KwaZulu -32 South east coast: South coast: Dune Fynbos (west), dune thicket with coastal Cape Columbine South west coast: Dune fynbos with mosaics of -33 Fynbos with patches grassland and forest (east). thicket (east) and afromontane East London of dune thicket forest patches (central) Transkei & southern KZN Port Elizabeth -34 Cape Town Algoa Bay Cape Padrone False Transkei coast South west coast Cape Seal Cape Recief Bay Cape Vacca South west coast Cape Infanta East coast -35 Cape Agulhas South coast -36 South east coast 0 100km South coast -37 “There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact” Mark Twain Abstract Phenomenological models are presented which predicts transpiration rates (E) of individual leaves of Scaevola plumieri, an indigenous dune pioneer, and Ammophila arenaria, an exotic grass species introduced to stabilise mobile sand. In both cases E is predictably related to atmospheric vapour pressure deficit (VPD). VPD is calculated from measurements of ambient temperature and humidity, hence, where these two environmental variables are known, E can be calculated. Possible physiological reasons for the relationships of E to VPD in both species are discussed. Scaling from measurements of E at the leaf level to the canopy level is achieved by summing the leaf area of the canopy in question. E is predicted for the entire canopy leaf area by extrapolation to this larger leaf area. Predicted transpiration rates of individual shoot within the canopy were tested gravimetrically and shown to be accurate in the case of S. plumieri, but less so in the case of A. arenaria. Using this model, the amount of water used by a known area of sand dune is shown to be less than the rainfall input in the case of S. plumieri in wet and dry years. The water use of A. arenaria exceeds rainfall in the low-rainfall year of 1995, while in 1998 rainfall input is slightly higher than water extraction by the plants. Using a geographic information system (GIS), regional maps (surfaces) of transpiration were calculated from surfaces of mean monthly temperature and mean monthly relative humidity. Monthly surfaces of transpiration were subtracted from the monthly median rainfall to produce a surface of mean monthly water deficit. Areas of water surpluses along the coast correspond with the recorded distribution of both species in the seasons that the plants are most actively growing and reproducing. This suggests that unfavourable water availability during these two species growth periods limit their distributions along the coast. In addition to unfavourable water deficits, additional climatic variables that may be important in limiting the distribution of these two species were investigated using a discriminant function analysis. III Table of Contents ABSTRACT...............................................................................................................................................................III TABLE OF CONTENTS..........................................................................................................................................IV LIST OF FIGURES ...................................................................................................................................................VI LIST OF TABLES .....................................................................................................................................................X ACKNOWLEDGEMENTS ......................................................................................................................................XI CHAPTER 1: INTRODUCTION........................................................................................................................... 12 RATIONALE ..........................................................................................................................................................12 THE INVASS PROGRAMME................................................................................................................................15 AIMS AND QUESTIONS........................................................................................................................................17 AN INTRODUCTION TO THE SPECIES: SCAEVOLA PLUMIERI AND AMMOPHILA ARENARIA .....................18 Scaevola plumieri......................................................................................................................................... 19 Nomenclature .............................................................................................................................................19 Ecology and distribution..........................................................................................................................20 Physiology .................................................................................................................................................20 Ammophila arenaria .................................................................................................................................... 22 Nomenclature .............................................................................................................................................22 Ecology and distribution..........................................................................................................................22 Physiology .................................................................................................................................................24 SAND DUNES AND CLIMATE OF THE SOUTH AFRICAN COAST ...................................................................25 An overview of South African coastal climate......................................................................................... 26 The distribution of sandy beaches along the South African coastline............................................... 28 Dune vegetation in South Africa................................................................................................................ 28 The coastal sand dunes environment........................................................................................................ 31 CHAPTER 2: ESTIMATING TRANSPIRATION RATES OF SCAEVOLA PLUMIERI AND AMMOPHILA ARENARIA LEAVES................................................................................................................. 33 INTRODUCTION....................................................................................................................................................35 Transpiration................................................................................................................................................. 35 The control of transpiration by stomata................................................................................................... 39 Modelling transpiration.............................................................................................................................. 41 STUDY SITES........................................................................................................................................................44 MATERIALS AND METHODS..............................................................................................................................45 Calibration and configuration of the IRGA............................................................................................. 45 Field measurements.....................................................................................................................................