The Monarch Butterfly Biosphere Reserve is a World Heritage Site containing most of the over-wintering sites of the eastern population of the monarch butterfly. The reserve is located in the Trans-Mexican Volcanic Belt pine-oak forests ecoregion on the border of Michoacán and Mexico State, 100 km (62 milles), northwest of Mexico City. Millions of butterflies arrive in the preserve annually. Butterflies only inhabit a fraction of the 56,000 hectares of the reserve from October–March.