Historical clue formulations

• Certain upper-growth limit — 2016-07-09
• Fringe of the green? — 2005-12-16
• Mountain demarcation — 2004-11-20
• Tundra borderer — 2003-12-06
• The edge of the green? — 2000-03-25
• Where the top begins to get bare — 1999-09-11
• It's about 2,500 feet up on Mt. McKinley — 1999-04-30