Psammosere succession (Primary succesion on sand dunes)
Stage 1) Occurs on yellow dunes, 2nd smallest 2nd nearest the sea front. (unstable)
Marram and couch grass are pioneer species that colonise on yellow dunes.
These grasses are salt intolerant so can't survive on embryo dunes.
Stage 2) Grey dunes are 50m further back, larger and more stable than yellow dunes.
More humus, meaning soil is more fertile, providing a base for range of species to colonise (Lichens and heather)
More stable due to the roots of these plants securing the dune in place.
No Marram/Couch grass as it is outcompeted by hardier perennials that take the space, nutrients…