The ornamental design for a decorativecornstalktree, as shown and described.
FIG. 1 is a front view of a decorative cornstalk tree, showing my new design in a used condition with cornstalks shown in broken lines;
FIG. 2 is a back view thereof;
FIG. 3 is a side view thereof;
FIG. 4 is an opposite side view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
In FIGS. 1-6, the broken lines show environmental structure for purposes of illustrating the decorative cornstalk tree in a used condition. The broken line structure shown in FIGS. 1-6 form no part of the claimed design.