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Ecophysiology of Barrier Island Beach Plants: Responses in Form and Function to Daily, Seasonal and Episodic Stresses

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Ecophysiology of Barrier Island Beach Plants: Responses in Form and Function to Daily, Seasonal and Episodic Stresses
Hancock, Thomas Earl
Barrier islands (BIs) are transient, highly dynamic geological structures of fairly recent origin found along most continental shorelines. In particular, eighty-five percent of the East and Gulf Coasts of the United States are fronted by BIs. Within this environment, the BI beach is considered physically controlled due to the harsh abiotic factors that predominate. The small suite of plant species that survive in this habitat have been the subject of numerous ecological studies for greater than one-hundred years. This research has provided a general understanding of how BI beach plants and their environment interact and affect one another. Unfortunately, research on the physiological ecology of BI beach plants is essentially nonexistent, with the exception of several West Coast (USA) studies, and therefore a detailed understanding of the physiological processes involved in plant response to stress is lacking. The most general objective of this dissertation is to incorporate the tools of physiological ecology into an ecological study that investigates the response of four plant species representing different functional groups to daily, seasonal and episodic stresses present on Topsail Island, North Carolina (USA). Leaf stomatal frequency measurements, microscopic examination of internal leaf anatomy and photosynthetic light response measurements of Amaranthus pumilus, Cakile edentula, Hydrocotyle bonariensis and Iva imbricata were made in an effort to test predictions of a leaf form and function model proposed by Smith et al. (1997). Amaranthus pumilus, a C4 summer annual, had more stomata on the adaxial than abaxial leaf surface, typical Kranz anatomy, and higher adaxial photosynthetic light response. Cakile edentula, a C3 cool weather annual, had slightly more stomata on the adaxial leaf surface, unifacial leaf anatomy and equal adaxial and abaxial photosynthetic light response. Hydrocotyle bonariensis, a C3 perennial that grows by asexual rhizomes, had equal numbers of stomata on both leaf surfaces, bifacial leaf anatomy and equal adaxial and abaxial photosynthetic light response. Iva imbricata, a C3 woody perennial, had equal numbers of stomata on both leaf surfaces, unifacial leaf anatomy and equal adaxial and abaxial photosynthetic light response. These results generally fit model predictions for a stressful environment. In a second study, micrometeorological measurements were taken and soil water content determined in an effort to assess water availability in the BI beach environment. Additionally, plant strategies for mediating water stress were addressed via life-history trait determination, xylem water potential and gas exchange measurements. Moderate to high levels of rainfall occurred February through September, with forty percent of the yearly total associated with three storm systems (March, May and August). Soil water content was approximately three percent which is similar to values reported for several previous BI studies. Amaranthus pumilus had the shortest growing season (seven months), some of the highest assimilation rates and appeared to mediate transpiration and xylem water potential via stomatal control. Cakile edentula was absent during the hottest months of the year when vapor pressure deficits would have been greatest (late July through late October). Cakile edentula had moderate to high assimilation rates, some of the highest stomatal conductance rates and lowest xylem water potential measurements. Hydrocotyle bonariensis exhibited the longest growing season (ten months), lowest assimilation rates and maintained an almost constant, high xylem water potential due to rhizomes that accessed water from slacks located inland of the foredunes. Iva imbricata had a nine month growing season, moderate assimilation and transpiration rates as well as moderate xylem water potential values. Based on these findings, A. pumilus, H. bonariensis and I. imbricata could be considered to employ tolerance strategies with C. edentula employing an avoidance strategy. The importance of photosynthetic carbon gain (PCG) as a common currency was addressed in a third study by modeling the response of A. pumilus, C. edentula, H. bonariensis and I. imbricata to the present day BI beach environment. Micrometeorological, gas exchange and photosynthetic light response measurements as well as a transplantation experiment were conducted to estimate potential and realized PCG for each species. The realized yearly PCG was 63.14 mol CO2 m-2, 69.91 mol CO2 m-2, 59.10 mol CO2 m-2 and 80.16 mol CO2 m-2 for A. pumilus, C. edentula, H. bonariensis and I. imbricata, respectively. A PCG model based upon PAR measurements and the photosynthetic light response of each species suggested that there were several environmental factors responsible for the difference in realized and potential PCG. The response of each species to environmental stresses (including the genetic potential to respond to these stresses) can account for species differences in both potential and realized PCG. A yearly PCG threshold may exist, as well as numerous life-stage PCG thresholds, that must be met in order for plants to survive and produce viable offspring. Each species in this study can acquire these thresholds via a unique combination of life-history, anatomical and physiological attributes. The final chapter of this dissertation explored predictions for the future of A. pumilus, C. edentula, H. bonariensis and I. imbricata in the face of climate change. Many studies have indicated that in addition to a continuing (and possibly accelerated) rise in sea level, the intensity and frequency of extreme episodic storm events are likely to increase. These pressures when added to current and future habitat loss due to development and hardening of the shoreline will certainly result in a very different BI beach landscape than is seen today. Shorelines are predicted to retreat a few kilometers to tens of kilometers inland depending upon geographic location. How humans respond to these changes will determine the future of many BI beach plant species. Of the four species addressed in this study, A. pumilus is likely to be most negatively affected. Amaranthus pumilus was listed as federally threatened in 1993 due to habitat loss. Studies have shown that A. pumilus requires areas of suitable habitat that include accreting ends of BIs that do not contain hardened structures. These habitats are likely to become less common except in areas that are preserved or properly managed as dynamic landscapes. The Coastal Barrier Island Network is a recently funded (National Science Foundation) diverse group of biologists, geologists, engineers, economists and sociologists that have begun to work toward a national policy that promotes sustainable preservation, conservation and development of BI ecosystems within the natural limits imposed by this highly dynamic environment.
barrier islands
plant physiology
plant ecology
leaf form
water stress
climate change
Baxley, John V. (committee chair)
Smith, William K. (committee member)
Kron, Kathleen A. (committee member)
Dimock, Ronald V. (committee member)
Silman, Miles R. (committee member)
2009-09-04T12:40:53Z (accessioned)
2010-06-18T18:59:09Z (accessioned)
2009-09-04T12:40:53Z (available)
2010-06-18T18:59:09Z (available)
2009-09-04T12:40:53Z (issued)
Biology (discipline)
http://hdl.handle.net/10339/14831 (uri)
en_US (iso)
Wake Forest University
Release the entire work immediately for access worldwide. (accessRights)

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