Yellowstone National Park has suspended improvements on a small ski area in the park. Last month, workers began installing a used poma lift at Undine Falls ski area, five miles east of Mammoth Hot Springs, Wyo., to replace a rope tow that was considered unsafe. But after receiving phone calls and letters from environmental groups and local residents, the Park Service halted construction to further review the issue, says Yellowstone public affairs officer Marsha Karle. People began complaining about the lift last December, when workers removed two dozen trees, including some 200-year-old Douglas-firs, and poured concrete pads for lift towers. If the project continues, eight brown towers, ranging in height from 18 to 23 feet, will be erected on the slope. The ski run is used primarily by a ski club, composed mostly of people living in Mammoth Hot Springs and Gardiner, Mont., says Karle. The Yellowstone elementary school in Mammoth, which uses the area for downhill ski classes, is paying for the $60,000 project with discretionary funds. Michael Scott, Northern Rockies director for The Wilderness Society, says the ski area is inappropriate in the park and that children could downhill ski at nearby Big Sky and Bridger Bowl resorts.