One of Big Green's biggest outfits, the Nature Conservancy (2011 revenue $997 million; assets $6 billion), is once again under fire. The new accusation is that it used improper influence over an elderly landowner to get her to donate her family property.

This time it's the 16,500-acre Roberts Ranch in Larimer County, Colo., owned by a 92-year-old woman. TNC allegedly influenced her to donate the land and cattle operation to the county, where it would be run as open space at taxpayer expense -- or, if the county declines the donation, the ranch would go to "a conservation group," according to county officials.

The real kicker is TNC's questionable part in cutting off inheritance rights of three living descendants of 1874 homesteader Robert O. Roberts, Derek Roberts and his two sons, Burke and Benjamin. That's brewing up a storm of protest from hundreds of locals who know the heirs.

In 1993, something similar happened with the Gibbs Farm near Valparaiso, Ind. There was no cheering section to help the Gibbs brothers, Frederic and Erich -- both medical researchers -- get their property back from TNC when their elderly father gave it to TNC after many blandishments.

The father, Frederic A. Gibbs, M.D., had been a world-renowned doctor and medical scientist; he invented of the electroencephalograph. But he was in decline and of questionable mental ability when he signed documents giving the family farm to TNC, forcing the brothers to file a lawsuit they could not afford in order to get it back.

As plaintiffs, the brothers alleged that their father was not mentally competent to sign the documents and accused defendant TNC of fraud and undue influence.

Court documents show that on Oct. 26, 1993, after a lengthy trial, federal Judge Rudy Lozano asked the jury foreman, "Have the plaintiffs proven by a preponderance of the evidence that the legal documents executed by Frederic Gibbs on Sept. 26, 1991, are the product of undue influence?" Answer: yes.

The plaintiffs could not prove their father's mental incompetence or that TNC committed fraud, but the undue-influence verdict was enough to get the farm back. TNC appealed and lost. The farm went to the brothers, who told me they had to sell it to pay their legal bills -- a heartbreaking Pyrrhic victory.

The Roberts Ranch case looks eerily like the Gibbs Farm case. In a hearing last week, local news sources reported, nearly 100 people "in cowboy hats and boots and work jeans" -- including the three heirs -- heatedly told the Larimer County Open Lands Advisory Board to not accept TNC's gift of the Roberts Ranch, said the report.

The sole owner of the ranch is Catherine Roberts, the 92-year-old second wife of the late Evan Roberts. She is the stepmother and stepgrandmother of the three descendants. Derek Roberts is Evan's only child. He and his sons are fighting the donation -- and the ownership of the ranch.

I contacted Derek Roberts by phone and asked about the ownership fight. "I feel like our heritage was stolen," he told me. He then went through a list of allegations that sounded all too familiar: forged signatures on key documents, a will that's being ignored, misrepresentations and broken promises, and undue influence by TNC.

Catherine Roberts has made herself unavailable for interviews. TNC only comments that "Catherine Roberts is the legal owner."

At last week's meeting, Derek Roberts and his two sons asked for an investigation of their concerns. The Open Lands Advisory Board voted to table the issue to gather more information.

Examiner Columnist Ron Arnold is executive vice president of the Center for the Defense of Free Enterprise.