Sex and shrub expansion: The interplay of sex ratio, seed production, and environmental filtering in shrub expansion patterns on an Atlantic coast barrier island

Background/Question/Methods

On Atlantic coast barrier islands, recent shrub expansion has been related to changes in island topography and climatic variables. Expansion is dependent on seed production and distance to open suitable habitat. In addition, the relationship between environment and sex determination for the dominant shrub Myrica cerifera (a dioecious evergreen) can affect the rate and patterns of expansion relative to salinity and freshwater availability. Studies were conducted on Hog Island, a Virginia barrier island that has experienced rapid sand accretion on the northern end over the past 30 years while the southern shoreline has remained stable. Despite 285% increase in shrub cover, successional processes alone do not explain the patterns of invasion, thus we sought to determine how variable driving forces influence new shrub establishment and thicket expansion.

The spatial associations of salinity variations and distribution of sexes was assessed. In addition, seed rain density, dispersal distance, and germination, were measured to quantify the influence of viable females on local expansion. Quantifying the importance of spatial variations in sex ratio, seed rain, and seedling establishment relative to patterns of shrub expansion will facilitate more accurate predictions about the future community composition.

Results/Conclusions

Differences in seed production and seed rain were closely related to island position as indicated by elevation above sea-level and distance from the ocean shoreline. Dispersal density measures were greatest within a half meter of viable females but declined rapidly with distance. Furthermore, seed germination and seedling survival was most affected by elevation but much less so by distance from the shoreline. There was also a relationship between salinity and shrub sex ratio. Shrub expansion patterns on Hog Island are more complex than previously understood. Low elevation swales allow access to freshwater and serve as protection for both female plants and their progeny. Thus, LiDAR based spatial analysis of island topography will serve as an indicator for shrub expansion potential.