Sentence:  [http://books.google.com.au/books?id=eSKTYgEACAAJ&dq=%22mountain+ash%22%7C%22mountain+ashes%22&hl=en&sa=X&ei=jv-bUu_SD8ydlQW844CYBA&redir_esc=y <WSD>Mountain Ash</WSD>: Formally Rowan Tree] 
Meaning of the word marked with <WSD> tags:  The European species "Sorbus aucuparia", commonly known as rowan or more specifically European rowan.