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. The mansion is brimming with unique architectural additions that reflect Morton’s changing tastes and ambitions, while the surrounding arboretum is filled with plant species Morton championed—a wide variety of native and non-native trees.…