Seasonal modifications in leaf form and physiology for two barrier island plant species in response to a dynamic abiotic environment

Background/Question/Methods

Abiotic factors of the barrier island sand dune environment undergo extreme variations spatially and temporally, and include edaphic and microclimate parameters such as a shifting sand substrate, periodic saltwater and freshwater immersion, and corresponding alterations in sunlight exposure and temperature.  Although several studies have associated spatial effects of certain abiotic factors (e.g. salt spray and sand movement) with distinct vegetation patterns and zonation, few studies have investigated specific ecophysiological responses of species to such a dynamic environment.  During the summers of 2007-8, leaf morphological and physiological characteristics were assessed on Topsail Island, North Carolina for two dominant sand dune species: Iva imbricata, a perennial succulent shrub, and Hydrocotyle bonariensis, a clonal broadleaf perennial.  To determine seasonal responses, changes in microclimate, leaf form, and gas exchange data were monitored for each month from May to September.  A transect was established from the first embryo dune on the upper beach 200 meters, extending through the primary dunes, swale, secondary dunes, and dune meadow.  Five leaves of I. imbricata and ten leaves of H. bonariensis were randomly chosen at ten meter intervals (if present) along the transect (N = 5 x 75 = 375 and N = 5 x 50 = 250, respectively). 

Results/Conclusions

There were significant differences according to month for both I. imbricata and H. bonariensis leaves for all variables measured except abaxial/adaxial stomata density.  For both species, mean leaf temperatures closely mirrored mean air temperatures in each month measured, suggesting both species had mechanisms for regulating leaf temperature via leaf form alterations (e.g. structure and orientation) that functioned to maximize photosynthetic carbon gain by avoiding overheating, high transpiration rates, and potential photoinhibitory effects.  However, the two species presented distinct morphological and physiological responses, suggesting the utilization of the avoidance strategy by H. bonariensis and tolerance by I. imbricata.  H. bonariensis leaves showed differences in seasonal and daily leaf inclination and azimuth, resulting in relatively constant sunlight incidence at levels generating stable leaf temperatures, and leaves experienced increased transpiration rates, stomata conductance, leaf area, and leaf weight throughout season.  I. imbricata leaves, however, showed variation in leaf form (e.g. modifications in thickness, area, and weight) that prevented high leaf temperatures and decreased transpiration rates and stomata conductance.  The results of this study suggest a variety of adaptive strategies used by sand dune vegetation on barrier islands to acclimate to extreme seasonal and spatial changes in the abiotic environment.