Possible Clues:

• Quakers in a forest
• Rocky Mountain National Park sights
• Trees associated with Colorado
• White-barked trees
• Trees that 'at the cross-roads talk together,' in an Edward Thomas poem
• Deciduous trees famously photographed in New Mexico by Ansel Adams
• Trees with trembling foliage
• Snake regularly meanest in trees
• Trees with trembly leaves
• Ones trembling not out of fear