 The amount of forested land in the state is declining, from 16.7 million acres (68,000 km²) in 1990 to in 2004; however, the average forest is maturing.From 1999 to 2004 the average annual growth within the state was 550 million board-feet (1,300,000 m³) of timber, while the average amount harvested was only 330 million board-feet (780,000 m³) per year.Minnesota was famous for its soft-ore iron mines which produced a significant portion of the world's iron ore for over a century.