American Forests History: American ReLeaf celebrates its 30th anniversary Millions of monarchs overwinter in the high-elevation forests of Michoacán, Mexico, where they cluster in oyamel firs for warmth and protection. Credit: Gami Photo Agency / Shutterstock. EVERY YEAR, millions of monarchs overwinter in the high-elevation forests of Michoacán, Mexico, where they cluster in oyamel firs for warmth and protection. Here, a Goldilocks combination of temperature and humidity keeps the butterflies healthy until spring. But for decades, legal and illegal loggers have been felling the monarchs’...