Where have all the flowers gone? Are our woodland flowers disappearing?

The owner of a wood in Norfolk once told me that he thought that the lily of the valley has declined over the last 20 years. In Wytham Woods near Oxford the dense bramble patches I studied in the 1970s didn't seem so thick ten years later. Were these slips of the memory or did Pete Seeger have a point with his 1961 song - Where have all the flowers gone?