While some nordic trails and popular backcountry routes are in areas reserved for self-propelled travel (see map, or posted signs), in general Lake Alpine is shared between skiers and snow machine users.  If you see a snow machine, or evidence thereof, in the ski-only area, please report the infraction to the US Forest Service (Hathaway Pines Ranger Station) at (209) 795-1381.  But as multiple recreation groups contend for a scarce wilderness, relations between skiers and snow machiners can be strained; in interactions please be polite and strive to be a considerate representative of the skiing community.