Leaf color change is one of the most iconic events that define the fall season. In a matter of days, the green color gives way to bright yellow, orange, and red hues. The appearance of these colors not only increases the beauty of our forests but also helps the growth of our orchards and backyard trees. These fall colors are created by pigments such as ß-carotene, anthocyanin, or lutein. Some may wonder, “why would trees spend energy in creating these pigments right when they are about to drop their leaves?” The answer is that the strategy is not about spending but saving for the next season.
These pigments were already in the leaves throughout spring and summer. However, there was so much chlorophyll (the pigment that makes leaves green) that all the other ones were masked by the chlorophyll. These pigments can absorb light at wavelengths different than chlorophyll, thus, making leaves more efficient at doing photosynthesis and, at the same time, they can also protect the photosynthesis machinery from stressful events such as ultraviolet radiation, cold tolerance, or drought stress.
In fall, when days become shorter and cooler and before trees drop their leaves, chlorophyll becomes less important and starts degrading. This degradation makes these other pigments more visible but, most importantly, allows the tree to recycle its components. For instance, a very valuable mineral nutrient, nitrogen, which is part of the molecule of chlorophyll, is taken back to the tree, stored in the twigs, branches, and roots during winter for its future use at the beginning of the spring, when trees start to bloom and sprout. Similarly, high quantities of other nutrients such as phosphorus, potassium, and sulfur are also resorbed. This means that any environmental or human factor that induces a premature leaf drop, such as early freezes, droughts, or even pruning trees before leaves have naturally fallen, would significantly reduce the nutrient reserves that support tree growth in the next season. For more information on these compounds and details on these biochemical processes, visit HGIC 1029, Color Changes in Autumn Leaves.