Has there been any serious studies into the relationship between the Asherah tree and the Christmas trees, with the possibility of the former being the source of the latter?
There is no historical connection.
The cultures that worshiped Asherah were mostly destroyed when the Babylonians invaded the kingdom of Judah and took a large portion of the populace as captives. The last reference to Asherah worship in the Bible is 2 Kings 23, during the reign of Josiah (r. 640 to 609 BCE).
Christmas trees became popular in Europe starting in Germany in the 1500s. There is no historical or cultural connection to the practices within Judah.