Woody plant encroachment into coastal grasslands: consequences for soil properties and plant diversity

Northeastern U.S. coastal grasslands are biologically, culturally, and historically significant but are being lost through the invasion of woody plants, including native species Gaylussacia baccata Wangenh. and Smilax rotundifolia L. Soil changes induced by these woody plants have important implications for herbaceous species diversity and restoration potential. We determined the effects of young and old G. baccata and S. rotundifolia patches on soil properties and plant diversity and identified relationships among soil properties and plant diversity. On a coastal grassland on Naushon Island (MA, USA) in 2015 and 2016, we used a time sequence of satellite imagery to identify young (< 5 years) and old (> 20 years) patches of G. baccata and S. rotundifolia. In these areas and in remaining grassland patches, we sampled soil properties, leaf 15N, and herbaceous plant diversity. All soils had low pH and higher extractable NH4+ than NO3−. G. baccata had relatively low extractable soil NO3−, soil potential nitrification, and leaf δ15N, while S. rotundifolia had relatively high extractable soil NH4+, low soil pH, and high leaf %N. Herbaceous plant diversity was low under S. rotundifolia and nearly absent under G. baccata, and most effects were more pronounced in older woody plant patches. Species diversity correlated with different soil variables between the two woody plants, indicating different mechanisms may drive diversity loss. Especially in older woody plant patches, restoration efforts should take into account soil changes, such as by making use of soil amendments. This could help offset soil effects after woody plant clearance and better enable grassland species recovery.