Ecophysiology of barrier island beach plants: Responses in form and function to daily, seasonal and episodic stresses

Barrier islands (BIs) are transient, highly dynamic geological structures of fairly recent origin found along most continental shorelines. 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. Unfortunately, research on the physiological ecology of BI beach plants is essentially nonexistent. 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. In general, leaves of these plants fit model predictions for a stressful environment with some exceptions. 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. It was determined that water availability may be greater than previously thought. Each of the species demonstrated physiological and life-history attributes that reduced water stress and likely maximized photosynthetic carbon gain over a given year. The importance of photosynthetic carbon gain (PCG) as a common currency was addressed in a third study. 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. Photosynthetic carbon gain was found to be the most efficient single parameter to assess plant fitness/success in the BI beach environment. Total yearly PCG for each species varied and there appeared to be a threshold value that must be assimilated. This carbon threshold could be related to functional group traits. Conceptual models were presented that relate PCG reductions due to abiotic stresses, plant response to these stresses and overall impacts on plant fitness.