The yellow dunes begin to show a greater diversity of plants as conditions become more favourable. As plants die and decay, a humus layer builds up and this traps both water and nutrients. The pH is now only slightly alkaline (about 7.5), there is more shelter and less salt spray. Marram usually still dominates the vegetation but as can be seen in the picture on the right, other plants are taking hold on the stabilised surface. Plants may include creeping fescue, sand sedge, mosses, lichens, sea holly and sea spurge.The dunes by this stage may well have reached 5-10 metres in height. Up to 80% of the sand surface may now be vegetated. Rabbits and othe mammals may add their droppings to help enrich the developing soil.