CELEBRATING A MILESTONE, or remembering a late loved one? Plant a commemorative tree. Your Yard: Look for a tree that reminds you of the traits you admire about the honoree—a flowering dogwood for kindness, a maple for generosity, an oak for strength. Affix a marker. Nature Preserve: Check for commemorative programs in your area. University: Adopt a tree in a campus garden or arboretum. —Monica Cardoza