During the long days of spring and summer, trees convert the sunlight that hits their leaves into energy — a process that requires water. But cold temperatures freeze water in the soil, so it's not available for uptake. And the short winter days means that there simply isn't enough sunlight energy to maintain those leaves. But instead of just tossing them away like used-up candy wrappers, trees break down the leaves’ chlorophyll, transport the nutrients to their roots, and then recycling them in the spring.
The process of moving out the chlorophyll reveals the yellow and orange of other leaf pigments. They were there all along, just masked by the chlorophyll.
Timing is critical. Trees have to maximize leaf-time to capture energy, but avoid risks of freezing before completing that nutrient recycling process.
So how do trees tell seasonal time without a calendar? You might think that the air temperature is the trigger. But the answer is photoperiod — the balance of daytime and nighttime, a far more reliable indicator of seasonal change.
What I think is cool is that trees don’t assess the length of the day — rather, they measure the duration of the night!
A pigment called phytochrome is the key. It has two interconvertible forms. One is activated by red light – which leaves get during the day. The other is activated by far-red light, which dominates at night. The proportion of these two forms is the trigger that signals it’s time for the leaves to fall.
So, when you check out our colorful mountains, remember that it's not the temperature that triggers the show — it’s the length of the night.
