Before Arbor Day was a date on calendars across the U.S., it was an idea taking shape inside this sprawling Nebraska City estate. Arbor Lodge began as a modest home in 1855, but its owner, J. Sterling Morton, expanded it over time until it ended up a 52-room mansion.

Morton, a newspaper editor and later U.S. Secretary of Agriculture, believed trees could make the plains of Nebraska feel more hospitable and humane. In 1872, he proposed a day dedicated to planting them. That first Arbor Day reportedly inspired the planting of more than a million trees across the state, and over the next decade, it spread into a national tradition.

Today, Arbor Lodge State Historical Park preserves both the house and the philosophy behind it.…