IOWA FALLS, Iowa — One by one, the old-fashioned barns that speckle this landscape are vanishing. Some are demolished to make way for new cornfields. Others, weak with years, simply crumple.

“We’re trying to ring that alarm bell,” said Mr. Scott, whom people here have come to call the Barn Guy for his insistence on trying to save some of Iowa’s 50,000 remaining barns, icons that turned up again and again in a guidebook to the state’s landmarks that was produced during the Great Depression and has recently been published online.

But the tale of the disappearing barn, a building whose purpose shifted, then faded away, tells a bigger story too, of how farming itself, a staple in this state then and now, has changed markedly since those writers drove through.