Possible Clues:

• Ecological Seuss character, with "The"
• Environmentalist in a Dr. Seuss story
• Seuss character, with "The"
• Kid's character who ''speaks for the trees''
• Truffula Tree defender, with 'the'
• "I speak for the trees" speaker
• Critic of the Once-ler, with 'the'
• Fictional protector of Truffula trees
• Kid-lit ecologist
• Orange environmentalist of kid lit