The Zonation of Coastal Dune Plants in Relation to Sand Burial, Resource Availability and Physiological Adaptation
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THE ZONATION OF COASTAL DUNE PLANTS IN RELATION TO SAND BURIAL, RESOURCE AVAILABILITY AND PHYSIOLOGICAL ADAPTATION A thesis submitted in fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY (Science) of RHODES UNIVERSITY by MATTHEW EDMUND GILBERT December 2007 ii ABSTRACT When considering the large amount of work done on dune ecology, and that a number of the classical ecological theories originate from work on dunes, it is apparent that there remains a need for physiological and mechanistic explanations of dune plant phenomena. This thesis demonstrated that in the extreme coastal environment dune plants must survive both high rates of burial (disturbance), and low nutrient availability (stress). The ability of four species to respond to these two factors corresponded with their position in a vegetation gradient on the dunes. A low stem tissue density was shown to enhance the potential stem elongation rate of buried plants, but reduced the maximum height to which a plant could grow. Such a tradeoff implies that tall light-competitive plants are able to survive only in stable areas, while burial responsive mobile-dune plants are limited to areas of low vegetation height. This stem tissue density tradeoff was suggested as the mechanism determining the zonation that species show within the dune vegetation gradient present at various sites in South Africa. Finally, detailed investigations of dune plant ecophysiology found that: 1) The resources used in the response to burial derive from external sources of carbon and nitrogen, as well as simple physiological and physical mechanisms of resource allocation. 2) The leaves of dune plants were found to be operating at one extreme of the photosynthetic continuum; viz efficient use of leaf nitrogen at the expense of water loss. 3) Contrary to other ecosystems, the environmental characteristics of dunes may iii allow plants to occupy a high disturbance, high stress niche, through the maintenance of lowered competition. 4) At least two mobile-dune species form steep dunes, and are able to optimise growth, on steeper dunes, such that they have to grow less in response to burial than plants that form more shallow dunes. In this thesis, it was shown that the link between the carbon and nitrogen economies of dune plants was pivotal in determining species distributions and survival under extreme environmental conditions. As vast areas of the world’s surface are covered by sand dunes these observations are not just of passing interest. iv ACKNOWLEDGEMENTS • Kristen Frole, Douglas Ibrahim and Gareth Coombs for their help in the setup and maintenance of these experiments • Candice Thesen and Emily McGregor whose third year projects allowed more hands to make lighter work. • Dugmore Gaba and the Fish River Sun management. • Garth Sampson and Coleen de Villiers of the South African Weather Service, for so promptly providing so much information. • The South African National Research Foundation and Rhodes University Joint Research Council from whom financial support was obtained. In particular, NRF Grant Holder Bursaries from Norman Pammenter, and subsequently a NRF Prestigious Doctoral Scholarship. • Adele Cutten for allowing us to use her house as accommodation. • Doug, Dad, Norman and Brad for so thoroughly reviewing the thesis. • David Orpen, for helping develop analytical mathematical solutions for many of the problems in this thesis: nitrogen mean retention time estimation, the interception of CO 2 response curves with supply functions, and the vector mathematics necessary for determining the shortest route of plant growth on three dimensional dune surfaces. • Kristen, for never once complaining about the time I’ve spent completing this thesis. • Brad and Norman for pointing me in the right directions and supporting whatever research this cat might have dragged in. • Brad for encouraging me to go out there and just DO rather than pontificate. • My parents for their unlimited encouragement. v TABLE OF CONTENTS ABSTRACT ........................................................................................... ii ACKNOWLEDGEMENTS ................................................................. iv LIST OF TABLES ............................................................................... xv LIST OF FIGURES ............................................................................ xvi LIST OF PLATES .............................................................................. xix SECTION 1: Introduction .................................................................... 1 Chapter 1 ............................................................................................... 1 What determines plant growth? .......................................................... 1 What is known about dune ecosystems? ............................................. 1 THESIS OUTLINE: AIMS AND QUESTIONS ............................................ 5 Growth, fitness and survival ............................................................... 8 STUDY SITES .............................................................................................. 9 Old Woman’s River ............................................................................ 9 Dune field southwest of Cintsa West .................................................. 9 Glengariff ........................................................................................... 9 THE ZONAL DISTRIBUTION OF DUNE SPECIES .................................. 11 Similarity of the species’ zonation at these sites and the dune succession at Mtunzini ...................................................................... 14 SPECIES DESCRIPTION ........................................................................... 14 SECTION 2: ABIOTIC FACTORS IN DUNE ECOLOGY ............. 17 Chapter 2 ............................................................................................. 17 A detailed description of the abiotic environment of the coastal dunes of the Eastern Cape ............................................................................. 17 2.1. WIND AND SAND BURIAL ....................................................... 17 INTRODUCTION ....................................................................................... 17 Mechanisms of sand transport in relation to wind ............................. 19 Other effects of wind ........................................................................ 20 AIMS ........................................................................................................... 21 METHODS .................................................................................................. 21 Measurement of sand burial .............................................................. 21 Burial regime of a dominant dune building species ........................... 21 Burial regime of four species from different zones on the dune gradient ......................................................................................................... 22 Burial gradient with distance from the sea ........................................ 22 Spatially explicit measurements of burial .......................................... 22 Weather data .................................................................................... 23 Weather model of sand transport ...................................................... 23 RESULTS .................................................................................................... 26 The characteristics of burial .............................................................. 26 vi The effect of wind on burial .............................................................. 29 Wind model of sand transport: directional movement of sand ........... 31 Wind model of sand transport: Periodicity and stochasticity ............. 31 Wind model of sand transport: Seasonality ....................................... 34 DISCUSSION .............................................................................................. 35 Is there a zonation of burial? ............................................................. 35 Characteristics of burial: burial measurements .................................. 35 Modelling burial from wind data ...................................................... 36 Limitations of the Fryberger technique ............................................. 37 Characteristics of burial: wind data ................................................... 38 Implications of burial for dune plants ............................................... 39 Comparison with other sites .............................................................. 40 Special cases .................................................................................... 43 2.2. NUTRIENT DYNAMICS............................................................. 44 INTRODUCTION ....................................................................................... 44 Nutrient limitation: Method of measurement .................................... 45 Nutrient limitation ............................................................................ 45 Nutrient limitation and salt spray ...................................................... 46 Nitrogen budgets .............................................................................. 47 AIMS ........................................................................................................... 48 METHODS .................................................................................................