From seedling establishment to thicketization 

Background/Question/Methods

Development of shrub thickets on barrier islands along the Atlantic coast may be the most rapid encroachment of woody species into grassland.  This is partially due to a mesic environment and low diversity relative to other ecosystems. The dominant shrub, Morella cerifera (formerly Myrica, Myricaceae), is an evergreen nitrogen fixer that produces a prolific number of bird dispersed seeds.  A secondary woody species, Baccharis halimifolia (Asteraceae), is deciduous and produces large quantities of wind dispersed seeds.  The objective of this study was to compare the functional mechanisms associated with seedling establishment and thicket formation for these two species.  A thorough understanding of mechanisms involved in woody encroachment will provide insight towards predicting further expansion and will aid in development of management plans

Field studies over the past fifteen years have focused on identifying patterns and processes associated with island shrub thicket expansion.  Established plots and cross-island transects, in conjunction with hyperspectral and LiDAR imagery, were used to quantify shrub seedling establishment, changes in shrub abundance and growth characteristics, and identification of factors that promote thicket formation. 

Results/Conclusions

Despite differences in seed dispersal, there was nearly a 50/50 ratio of seedling abundance for the two species and seedlings approximate a random distribution in plots.  At least 99% of all shrub seedlings establish within grass-dominated swales where microclimate is moderated to facilitate seedling growth.  In addition, the N-fixing endosymbiont, Frankia must be present in the soil for M. cerifera.  As shrubs emerge above the grass canopy, densities decrease for both species and aboveground growth accelerates.  Small patches of shrubs begin to merge and form thickets, B. halmifolia is reduced is to a few individuals along the thicket edge.  Experimental plots with variable leaf litter depth and environmental measurement at the thicket grassland interface demonstrated that thicket formation and exclusion of the former graminoid dominated community and, perhaps, B. halimifolia, is due to the inhibitory effects of a thick layer of leaf litter and a reduced light environment.  These are the result of rapid growth and an exceptionally high LAI of M. cerifera thickets.  Upon complete canopy closure, M. cerifera shrub density is reduced and approximates a regular distribution leading to a dense, nearly uniform, impenetrable canopy, ie. a thicket.  Considering life history and physiologically based functional mechanisms, and the relatively few competitive woody species, M. cerifera often forms dense, monospecific thickets that continue to rapidly expand into barrier island swales.