The ornamental design for a christmas tree, as shown and described.
FIG. 1 is a side view of christmas tree showing my new design; all other side views being substantially identical thereto; and,
FIG. 2 is a side view of a second embodiment of a christmas tree showing my new design; all other side views being substantially identical thereto.
The tones forming the alternating contrasting horizontal stripes of the claimed design represent contrasting color tones and are not limited to any specific colors.