In Lake County, about 4,000 trees, many mature and healthy, have been felled from four preserves in the last three year in an effort to let in sunlight to grow more oak trees. Though pests have also been a major reason for tree removal — the Cook County forest district had to clear 9,000 trees because of the emerald ash borer, for example — the large-scale removal of trees to restore and preserve habitat marks a continuing evolution in conservation.