JON ANTHONY STALLINS Barrier Island Morphology and Dune Vegetation Pattern and Process in the Georgia Bight (Under the Direction ALBERT J. PARKER) Vegetation patterns reflect the interplay of two processes: the imposition of site factors along gradients and disturbance exposure and recovery. I investigated how these processes interact to generate patterns in dune plant species composition, diversity, and soil-vegetation relationships on two barrier islands in the Georgia Bight: South Core Banks, North Carolina, a frequently storm- overwashed barrier island, and Sapelo Island, Georgia, an infrequently overwashed barrier. A single transverse compositional gradient characterized South Core versus two intersecting gradients on Sapelo. Dune ridge-and-swale topography on Sapelo restricts the spread of overwash, thus disturbance-structured vegetation adjoins the prominent transverse vegetation zonation expressed along gradients. On South Core, topography reinforces the spread of disturbance across the landscape, thus folding disturbance and transverse vegetation zonation into a single overlapping gradient. These contrasting gradient structures contributed to Sapelo’s higher longitudinal and transverse compositional variability. Biogeomorphic feedback and threshold processes that mold compositional patterns decouple disturbance exposure and effect on Sapelo, while this coupling is reinforced on South Core. The Intermediate Disturbance Hypothesis (IDH) presupposes a geographically-consistent direct coupling between disturbance exposure and effect, and thus was applicable at the landscape-scale only on South Core. Here, disturbance is frequent, and biogeomorphic processes reinforced an inhibition-structuring of diversity patterns. On Sapelo, diversity patterns did not conform to IDH, largely due to a decrease in the spatial scale of the processes regulating species patterns. The Dynamic Equilibrium Model (DEM) was a more robust framework for examining the geographic patterns of species diversity in this study given its ability to incorporate direct and indirect structuring effects of disturbance, and changes in the scale expression of diversity patterns. Infrequent overwash enhanced edaphic organization on Sapelo, while frequent overwash dampened edaphic organization on South Core. Relatively stronger associations between soil and total vegetation cover were expressed on Sapelo, however, this may not necessarily be due to edaphic organization. Individual species on both islands were responsive to variability in edaphic properties. This association between the dominant dune-building and sediment-stabilizing taxa and edaphic variability on both islands reinforces their importance as regulators of topography and the spread of disturbance. INDEX WORDS: Barrier islands, Biogeomorphology, Dune Vegetation, Disturbance, Gradient zonation, Feedbacks, Thresholds, Intermediate Disturbance Hypothesis, Dynamic Equilibrium Model. BARRIER ISLAND MORPHOLOGY AND DUNE VEGETATION PATTERN AND PROCESS IN THE GEORGIA BIGHT by JON ANTHONY STALLINS B.S., Florida State University, Tallahassee, 1985 M.S., Georgia State University, Atlanta, 1995 A Dissertation Submitted to the Graduate Faculty of The University of Georgia in Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY ATHENS, GEORGIA 2000 © 2000 Jon Anthony Stallins All Rights Reserved BARRIER ISLAND MORPHOLOGY AND DUNE VEGETATION PATTERN AND PROCESS IN THE GEORGIA BIGHT by JON ANTHONY STALLINS Approved: Major Professor: Albert J. Parker Committee: David S. Leigh Vernon Meentemeyer Thomas L. Mote Kathy C. Parker Electronic Version Approved: Gordhan L. Patel Dean of the Graduate School The University of Georgia June 2000 ACKNOWLEDGMENTS This research was funded by a grant from the National Science Foundation Doctoral Dissertation Improvement Program (Division of Geography and Regional Science) to Albert J. Parker and Jon A. Stallins (10-21-RR-175-1250). I am foremost grateful to my major advisor, Al Parker, for his insightful contributions to this dissertation. His patience and demeanor are exemplary as a mentor. My other committee members, Kathy Parker, David Leigh, Tom Mote, and Vernon Meentemeyer, have also contributed to the successful completion of this dissertation, and their professionalism sets a high standard for me to follow. At the University of Georgia Marine Institute, Steve Pennings and Jon Garbisch provided invaluable logistical support, as did Michael Rikard and staff at Cape Lookout National Seashore, NC. I am indebted to the help in the field provided by Scott Kissman, Amanda Wrona, John Rodgers, and Jenny Cruse. Thanks also go to the Geography Department staff members Kim, Loretta, Audrey, and Emily who provided invaluable office support. iv TABLE OF CONTENTS Page ACKNOWLEDGMENTS ..........................................................................................................iv CHAPTER 1 INTRODUCTION....................................................................................................1 2 DUNE PLANT COMPOSITIONAL PATTERNS IN TWO BARRIER ISLAND GEOMORPHIC ENVIRONMENTS.......................................................................14 Overview.................................................................................................................14 Introduction ............................................................................................................15 Background.............................................................................................................17 Study Area..............................................................................................................26 Methods..................................................................................................................29 Results....................................................................................................................36 Discussion ..............................................................................................................67 Conclusion..............................................................................................................76 Literature Cited.......................................................................................................78 3 DUNE PLANT SPECIES DIVERSITY IN TWO BARRIER ISLAND GEOMORPHIC ENVIRONMENTS ......................................................................88 Overview ................................................................................................................88 Introduction ............................................................................................................89 Background.............................................................................................................91 Study Area ............................................................................................................95 Methods ................................................................................................................100 Results..................................................................................................................103 Discussion......................................................................................... ........................................................ ...................116 Conclusion........................................................................ ...................................123 v vi Literature Cited.....................................................................................................125 4 DUNE SOIL AND VEGETATION PATTERNS IN TWO BARRIER ISLAND GEOMORPHIC ENVIRONMENTS.....................................................................134 .................................... Overview ..............................................................................................................134 Introduction...........................................................................................................135 ....................................................... Background...........................................................................................................136 Study Area............................................................................................................137 Methods ................................................................................................................139 Results..................................................................................................................143 Discussion.............................................................................................................155 ........................................................ Conclusion............................................................................................................159 Literature Cited.....................................................................................................160 5 CONCLUSIONS ..................................................................................................165 LITERATURE CITED............................................................................................................174 APPENDICES.........................................................................................................................187 A TRANSECT TOPOGRAPHIC DESCRIPTORS..............................................187 B QUADRAT SAMPLING INTENSITY ...........................................................189