Why Do Leaves Change Color in Autumn?

Frame the driving question: every autumn, green disappears from leaves and is replaced by yellows, oranges, and reds. Ask the learner to notice that the green must go somewhere, or be hidden.

• Leaves are green in summer
• Leaves turn other colors in autumn
• The green does not simply vanish

Ask the learner to commit to their best guess about why leaves change color before any mechanism is revealed.

• Make one independent choice before seeing the evidence

Let the learner slide a 'chlorophyll mask' away from a leaf to reveal the yellow and orange pigments that were underneath all summer, simulating paper chromatography separation.

• Green chlorophyll covered other pigments
• Yellow and orange carotenoids exist in summer leaves
• Removing green reveals what was always there

Present the three main pigment families: chlorophylls (green), carotenoids (yellow and orange), and anthocyanins (red and purple), with their roles and seasonal behavior.

• Chlorophylls dominate summer
• Carotenoids persist year-round but are masked
• Anthocyanins appear only in autumn for some species

Let the learner adjust day length and temperature sliders to watch chlorophyll levels drop and other pigments emerge, exploring what actually triggers the change.

• Shorter days trigger chlorophyll breakdown
• Cool nights favor anthocyanin production
• Light and temperature interact, not just cold alone

Explain why some species produce brilliant reds while others stop at yellow: anthocyanins are actively synthesized in autumn, possibly as a sunscreen or antioxidant while the leaf recovers nutrients.

• Reds require active pigment production
• Anthocyanins may protect the leaf during shutdown
• Species and conditions determine the final palette

Present a changed context: tropical trees that lose their leaves in dry seasons rather than cold ones, and ask whether the same pigment story applies.

• Apply the pigment framework to a new climate
• Predict color behavior where there is no cold autumn
• Recognize the underlying mechanism is reusable

Directly resolve the opening question: leaves change color because chlorophyll breaks down as days shorten, unmasking pigments that were always there, and because some species actively build new red pigments in autumn.

• Chlorophyll breakdown unmasks yellows and oranges
• Red pigments are newly produced
• The change is triggered by day length and temperature, not simply by cold