The Nature Conservancy says it will keep 240 acres of rare old-growth forest in northeastern Minnesota protected under a purchase from polar explorer and St. Thomas alum Will Steger.

The land near George H. Crosby Manitou State Park has towering trees believed to be older than the state of Minnesota.

The conservancy said Wednesday that it’s adding the property — which contains old-growth white spruce, white cedar, sugar maple and yellow birch — to its Upper Manitou Forest Preserve, expanding it to 2,450 acres that are open to the public.

Minnesota has less than 4 percent of its original old-growth forest, defined as natural forests at least 120 years old that haven’t been burned or logged.

Steger, who refused offers from loggers and developers, says he’s pleased the land will remain protected.