Leaves change color in the fall when they start to break down chlorophyll, which gives them their green color. Chlorophyll is used for photosynthesis, a process by which plants synthesize glucose using the energy of sunlight; photosynthesis also removes CO2 from the atmosphere and generates oxygen. Chlorophyll breakdown recycles precious building blocks that trees use as they shut down their metabolism in preparation for winter. Once chlorophyll levels drop, we can start to see the yellow and orange pigments from the carotenoids. Chlorophyll and carotenoids are always present in leaves but trees also synthesize a new red pigment called anthocyanin in the fall. Anthocyanin is thought to protect fall leaves from high light intensities, acting as a sort of sunscreen. Neither carotenoids nor anthocyanins contain a lot of the building blocks trees scavenge from chlorophyll. The resulting varying levels of chlorophyll, carotenoids and anthocyanin result in a color palette that ranges from pale green to yellow, orange, red and dark maroon, beautiful to behold in the fall.
This color display of broad-leaf trees is not equally stunning each year, nor does it happen at exactly the same time. Plants sense what season it is by measuring both the length of the day and the ambient temperature. Foliage changes are mainly triggered by the shortening of day length in the fall. Importantly, anthocyanin biosynthesis requires sunlight and sugars, so the display of red is most stunning in sunny falls. Cool nights (low 40s) also contribute to increased anthocyanin production by trapping sugars in the leaves. The optimal combination of short, sunny, days and cool night means that the majority of the trees in the Northeast produce anthocyanin, while very few do so in the Southern or Western parts of the country. Leaf color displays may be less stunning if the climate changes, for example by causing warmer fall nights in the Northeast and Midatlanic regions. Another factor that can impinge on the fall foliar color display is premature leaf drop triggered by summer droughts, storms in early fall with high winds and rain, or early frost.
Why do some trees -such as broad-leaf trees- drop their leaves in the fall to begin with? Clearly many trees do not drop their leaves including conifers (pine trees or firs) or trees with tough small leaves (like California live oaks). Leaves lose a lot of water to the atmosphere during photosynthesis, with broad, fleshy, leaves incurring much larger water losses than pine needles or small scaly leaves. The lost water is replenished from the soil via water uptake in the roots and water transport through the tree trunk and branches to the leaves. If photosynthesis were triggered in broad-leaf trees on a sunny winter day, the water lost not could not be replenished if the ground and the water in the tree is frozen. This would cause tree embolism, presence of air bubbles in the water transport system that block water transport even after a thaw. Therefore, broad leaf trees play it safe and drop their leaves in the fall to resume their cycle of growth come spring. Prolonged droughts, as have been experienced in California in recent years, also causes tree embolisms, again water is lost from leaves that cannot be replenished from soil water, leading to loss of foliage and even tree death.
For more coverage on this topic see 11/24/2021 PennToday Article: