This question isn’t as silly as it might sound at first. Unlike the orange and yellow pigments that are present in a leaf all summer (but are revealed only when green chlorophyll starts to break down and fade), the red pigments, anthocyanins, aren’t produced until autumn.
It takes a significant amount of energy to produce anthocyanins, however — and at a time when the tree most needs to conserve it. This fact led plant physiologists to wonder what advantage red leaves must confer to make the investment worthwhile.
Researchers found that in some tree species, anthocyanins act much like a sunscreen. While chlorophyll is breaking down and photosynthesis is slowing, a plant’s ability to absorb light diminishes, and excess light can damage its leaves.
Anthocyanins protect those leaves, letting the tree extend the period of time during which it can make and store nutrients. In addition, more recent studies have confirmed that when anthocyanin production is genetically blocked, leaves become vulnerable to sunlight, slowing transport of nutrients to the tree’s roots.
But not all trees turn equally red. So what accounts for these vast differences in anthocyanin levels? Again, nutrients play a role. In 2003, researchers found that nutrient-poor leaves, particularly those low in nitrogen, may trigger early and more intense red color in sugar maples (Acer saccharum).
So, vibrantly red leaves may be a sign that the tree — just like humans and other animals at this time of year — is attempting to prolong its summer activities and squirrel away those last few morsels of food for the approaching winter.