The ornamental design for a decorative christmastree, as shown and described.
FIG. 1 is a perspective view of a decorative Christmas tree showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is front view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a rear view thereof; and,
FIG. 7 is an enlarged, partial cut-away view of a joint of the horizontal ring and the inclined vertical support.
The broken line showings of the hanging ornaments are for the purpose of illustrating representative hanging ornaments and those specific embodiments depicted as such form no part of the claimed design.