Shrub expansion into coastal grasslands: Seed dispersal and environmental filters determine patterns of invasion

Background/Question/Methods

Expansion of woody plants into grasslands has been observed throughout North America including barrier islands of the Atlantic coast where species of Morella, a bird dispersed shrub, are dominant. Invasion, colonization, and establishment patterns are mediated by a sequence of environmental filters that act upon the template of seed dispersal. To better understand invasion mechanisms, an ‘encroachment zone’ (the landscape where active Morella encroachment occurs) was identified on Hog Island, VA. Extent of seed dispersal throughout the encroachment zone and the proportion of seeds dispersed under different cover conditions were investigated by seeds traps (n = 82) of screen mesh and wood frame, each 0.21 m² in area. Seeds were collected monthly. Non-linear Least Squares Estimates (nl-LSEs) of the log₁₀-transformed density (# seeds/ m²) were used to determine the dispersal curve of Morella propagules throughout the encroachment zone with respect to distance from thicket edge. A one-way ANOVA with Tukey HSD contrasts was used to compare dispersal differences among cover types. Morella establishment was assessed throughout the encroachment zone by percent cover estimates. Soil chloride content is considered a major filter in the establishment process; spatial relationships of soil chlorides were compared with shrub establishment survey data.

Results/Conclusions

A total of 4667 seeds were collected. Non-Linear LSEs found that a negative exponential function best fit the distribution of seeds dispersed throughout the encroachment zone. The negative exponential function indicates a leptokurtic (fat-tailed) dispersal distribution which is characteristic of spreading populations. The ANOVA and Tukey HSD tests revealed differences in proportion of seed dispersal among cover types. As expected, the majority of seeds were dispersed closest to fruiting Morella. The proportion of seeds dispersed under co-occurring shrub species suggests a facilitative role in functioning as perches for frugivorous birds that disperse seeds away from parent plants and into uncolonized portions of a landscape. An association between established Morella and soil chlorides suggests that the effects of salinity on establishment processes may be a significant environmental filter that differs throughout the shrub life cycle. The interplay of biotic and abiotic processes such as extensive seed production, bird dispersal and habitat suitability contribute to invasion mechanisms and determine patterns of shrub expansion in coastal environments.