FIG. 1 is a perspective view of the tree grate embodying the new design; FIG. 2 is a top elevational view thereof; FIG. 3 is a right side elevational view thereof; FIG. 4 is a left side elevational view thereof; FIG. 5 is a front elevational view thereof; FIG. 6 is a rear elevational view thereof; FIG. 7 is a top elevational view thereof showing segments of one type of assembled tree grate in broken lines; and, FIG. 8 is a perspective view thereof showing segments of one type of assembled tree grate in broken lines. FIGS. 1-8 show portions of a tree grate in broken lines. The broken lines are shown for environmental purposes and form no part of the claimed design.