In this wealthy old community, a bastion of gentility and reserve, the leading citizens would hardly be expected to greet Mohawk hair styles.

But in the case of Mr. T, the gruff, burly, television actor, it was not the way he trimmed his hair that sent this Chicago suburb into an uproar, but rather the way he trimmed his property.

In recent weeks, Mr. T, reportedly suffering from allergies, has already cut down more than a hundred of the oak trees on his English Tudor estate here, violating an unwritten code of esthetics in this picturesque town along the shore of Lake Michigan.

Angry residents here are calling it ''The Lake Forest Chain Saw Massacre.''

The telephones at City Hall have been ringing ceaselessly with complaints from neighbors. The local newspaper, The Lake Forest News/Voice, condemned Mr. T in an editorial last week for what it described as his ''arrogant, insensitive action.'' And an alderwoman, Mary Barb Johnson, has promised to draft an ordinance to prohibit any further ''outrageous destruction.'' Pride in 'Tree City, U.S.A.'