The Ammophila decline: A field experiment on the effects of mineral nutrition

American beachgrass (Ammophila breviligulata) is a pioneer plant on coastal dunes of northeastern North America, where it forms dense, monospecific st ands on the foredune. Although both plant and population growth are active on the front of the foredune, stand decline sometimes occurs on the back. S everal hypotheses have been proposed to explain this phenomenon. The aims o f the present study were to assess the effects of mineral nutrition on the performance of adult A. breviligulata plants. In a field experiment, the re sponse of A. breviligulata to fertilizer addition was determined on the fro nt (in a vigorous stand) and on the back (in a decline zone) of the foredun e at the Dune du Sud, Iles de la Madeleine, Quebec. There were differences in substrate nutrient concentration between positions on the foredune at th e beginning of the growth season: potassium was somewhat higher on the fron t, and nitrogen on the back of the foredune in the decline zone. However, t hese differences were slight and not biologically significant. On the contr ol plots, root mass density was higher on the back than on the front of the foredune. Fertilizer addition increased the density and the mass of A. bre viligulata shoots both on the front and the back of the foredune. Plants in the decline zone responded strongly to nutrient addition and performed bet ter than those in the control plots on the front of the foredune. Thus, it seems that fertilizer addition largely overtook the effect of the factor(s) responsible for the decline. Because there were no biologically significan t differences in substrate nutrient concentration between the front and the back of the foredune, and because root mass density was higher on the back (in the decline zone) than on the front of the foredune, we suggest that t he decline might be associated with a lower efficiency of the roots to abso rb nutrients. This might be related to differences in mycorrhiza infections between the back and the front of the foredune.