A Christmas tree was typically any conifer tree that was decorated with lights and ornaments during the Christmas season.
Remote-controlled Christmas trees were used by the Roboforms during the alien invasions of Earth that occurred on Christmas of 2006 and 2007. They were dealt with by the Tenth Doctor on both occasions. (TV: The Christmas Invasion, The Runaway Bride)
Princess Cilia used Ulian shrinking technology to make herself six inches tall and hid on top of a Christmas tree, pretending to be an angel. The Ulian robots which had been sent to protect her tracked her down with the Tenth Doctor's help. The Doctor convinced her to return home with the robots. (PROSE: The Doctor on My Shoulder)
When the Zentra came to Earth, they disguised Zentrabots as the Christmas "toy craze". When the Zentrabots were underneath children's Christmas trees, the Zentra sent an activation signal to kidnap a hundred thousand children, and demanded to one of Earth's governments that they would hand over all its weapons or the children would die. The children were successfully rescued by the Eleventh Doctor. (COMIC: The Zentrabot Invasion)
Christmas trees were a common sight in the town of Christmas on Trenzalore. (TV: The Time of the Doctor)
A Christmas tree was on display in the waiting room of the hospital Rory Williams worked at during the Year of the Slow Invasion in the early 21st century. (TV: The Power of Three)