I first noticed the upside-down Xmas trees last year. There's a mention of them from 2005 Here, which gives a 12th century origin for the European X-mas tree - though it wasn't until 1841 that they became popular in England, taking its place alongside other anciently traditional evergreen decorations - the holly, ivy & mistletoe - but just how anciently traditional is difficult to say, much less to ascertain any actual significance (in terms of symbolic meaning), pagan or otherwise. Frazer has a lot to answer for!