Is the Christmas Tree a Christian tradition?
Well, yes - and no!
First the "No"The use of greens, especially evergreens, to celebrate the Winter solstice, dates back to the ancient Egyptians. At the solstice, the days would again start to become longer - and since they worshipped the Sun god, Ra, they would decorate their homes with green palm branches as a symbol of life being victorious over death - as the Sun was growing "weaker and weaker" culminating on the shortest day of daylight (December 21st or 22nd), after the solstice the days were getting longer again and soon would come Spring and Summer when nature came back to life again.
The Romans also celebrated the solstice for similar reasons as the Egyptians before them. They knew that soon would come the Spring and farms and orchards would come back to life again. The Romans would decorate their homes and temples with evergreen boughs to celebrate the lengthening of days.
Similarly, the ancient Druids/Celts decorated their temples with evergreen boughs, again symbolizing not just new life, but everlasting life. The Vikings also believed evergreens were a special gift of the Sun god, Balder.
And now the "Yes"Christmas Trees, as we have come to know them now, started in Germany in the 16th century. Most credit Martin Luther with being the first to tie candles to the branches with wire and lighting the tree. It is said that Luther was on a walk one Winter evening and was inspired by the stars shining among the evergreens. He used an evergreen tree in his home to show his family what he experienced on his walk.The evergreen being a symbol of everlasting life, the eternal life in heaven; the candles represent Jesus Christ, the Light of the World. He also put red decorations on the tree to represent the blood of Christ - keeping in mind that while they were celebrating Christmas, Jesus came to earth to be sacrificed on the Cross.
Christmas Trees in the United States were basically unheard of until the 19th century when German settlers to America brought with them the tradition of the Christmas Tree.