Long-term effects of nitrogen fertilization on plant community structure on a coastal barrier island dune chronosequence

Nitrogen limitation typically has considerable influence on plant community composition and structure on coastal barrier island dune ecosystems. The purpose of this study was to quantify plant community responses of different age dunes on a coastal barrier island in Virginia, USA to nitrogen addition. We hypothesized that nitrogen addition would increase density and cover, but with differential effects among species as a result of increased interspecific competition, and decrease species diversity. Density, cover and species diversity were monitored along a dune chronosequence on Hog Island (Virginia Coast Reserve Long-Term Ecological Research Site) over a seven year period. There were few significant changes in species dominance in control plots between 1992 and 1998; however, in fertilized plots, Ammophila breviligulata cover increased (except on the oldest dune) and Spartina patens cover decreased (except on the youngest dune). After seven growing seasons, results suggested more intense negative effects of competition in nitrogen-fertilized plots. Greater cover of Ammophila in fertilized plots suggests Ammophila is in a better position to compete for light with enhanced aboveground dominance. Diversity was lower in fertilized plots on all but the dune formed in 1967 and diversity decreased most dramatically in fertilized plots on the oldest dune. The increase in total density with fertilization as diversity decreased, coupled with the shifting composition of Ammophila and other dominants, appears to support the interspecific competitive exclusion hypothesis. Changes in the positions of free surfaces (groundwater level in particular) appear to influence plant community composition. The decline in Spartina density may be at least partially attributed to development of drier conditions on the older dunes.