The ornamental design for a modified christmastree, as shown and described.
FIG. 1 is a front perspective view of a modified Christmas tree, the broken line showing therein of a person, support wall and Christmas tree ornaments is for illustrative purposes only and forms no part of the claimed design;
FIG. 2 is a rear elevational view; and,
FIG. 3 is a right side elevational view, the left side elevational view being a mirror image thereof.
All surfaces not shown form no part of the claimed design.