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.