By Eleanor Lin
With shortening days and cooling temperatures, a stunning show comes to temperate forests every autumn. The foliage of deciduous trees—those that lose their leaves—turns red, orange, yellow, and even purple before being cast off.
The falling of leaves, called autumn senescence, is such an integral part of "fall" that it gives the season its name. But autumn senescence is just as vulnerable to disturbances from humans as any other process in the intricately woven fabric of Earth's biosphere. Scientists are still teasing out the combination of biochemical, evolutionary, and ecological factors which gives rise to fall colors, but one thing is clear: humans are altering those factors, including through climate change.
Why do trees literally drop everything in the fall? The key is nutrient reabsorption. Trees invest lots of energy and nutrients into their leaves during the growing season. But the thin, delicate tissues of deciduous tree leaves are not sturdy enough to survive freezing winter temperatures. So in fall, deciduous trees begin to recycle their leaves, remobilizing key nutrients such as nitrogen, phosphorus, and sulfur from the dying leaf tissues to the rest of the tree.
Thus, trees face a trade-off. The earlier they begin senescence, the more efficiently they can reabsorb nutrients from their leaves. On the flip side, losing leaves earlier means less time for photosynthesis, the process which harnesses solar energy to produce food, i.e. sugars and starches, from carbon dioxide and water. Without leaves, a tree must rely on its internal energy stores until spring, so the timing of senescence is key. Still, none of this explains why some species' leaves change color before they drop.
Before diving into hypotheses about the adaptive value of autumn leaf color change, it's worthwhile to review the pigments making up the fall color palette. During the spring and summer, tree leaves appear green due to high concentrations of chlorophyll, the main pigment necessary for photosynthesis. Another class of pigments, carotenoids (which are also responsible for orange carrots), are usually masked by high chlorophyll concentrations. But as chlorophyll breaks down in senescing fall leaves, the carotenoids are unmasked, causing yellow and orange colors to appear.
The presence of carotenoids during the growing season makes sense, because they are known to play a photoprotective role. When the photosynthetic machinery of a leaf becomes overloaded due to excessive light, reactive, oxygen-containing, and therefore potentially damaging compounds form. Carotenoids act as antioxidants. They help neutralize and prevent the formation of these compounds.
In contrast to yellow and orange, which are already present and simply waiting to be revealed in fall, red and purple are caused by anthocyanins, a type of pigment which must be newly manufactured beginning in autumn. Why do some species of trees produce anthocyanins and turn their leaves red, given that they will soon lose these leaves anyways? Although both have a ways to go in terms of being experimentally and observationally confirmed, the two most accepted hypotheses are the photoprotection hypothesis and the coevolution hypothesis.
The photoprotection hypothesis holds that anthocyanins shield senescing leaves from the damaging effects of excess light, both by physically blocking light and by neutralizing reactive oxygen-containing chemicals, much like carotenoids do. Photoprotection becomes especially important in fall, because a thinning forest canopy lets in more light, and because chlorophyll breakdown in the leaves leads to less protective shading. Photoprotection is advantageous to the tree, because intact leaves allow more efficient reabsorption of nutrients.
The coevolution hypothesis also proposes that the red color of anthocyanins is protective, but against migratory insects rather than light. Over time, insects would have evolved to associate red with toxic, non-nutritious, or soon-to-fall leaves. Trees would have coevolved chemical defenses, flagged by red autumn leaf coloration, to avoid being eaten and infected with viruses, fungi, and bacteria by the insects. Some have even suggested that "weak trees display stronger autumn colours because they are the ones with more need to avoid insects." However, many insects cannot see the color red, implying that they are instead sensing associated chemicals given off by the trees.
While evolution has encoded the patterns of autumn senescence internally in some tree species' genes, external environmental factors also play a role in determining how trees change color. According to the U.S. Forest Service, "A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays," while a warmer fall can lead to less intense colors, and summer droughts can delay senescence by weeks. Through climbing carbon dioxide levels and increasing temperatures in all seasons, climate change could be delaying autumn senescence. One study found that autumn senescence has been getting later by a rate of 3 to 4 days per decade "in European and North American temperate forests since 1982." Like the rest of the biosphere, these trees are being subjected to large environmental changes on an extremely short time scale compared to their long evolutionary histories.
At Columbia's Lamont-Doherty Earth Observatory, the newly installed PhenoCam is tracking those changes through a 24/7 visual feed of the surrounding forest in Palisades, New York. Trees are an important source of food and shelter for organisms from bugs to birds, so scientists expect changes in the life rhythms of trees to have implications for the entire forest. The PhenoCam at Lamont-Doherty is part of a larger international network, tracking global shifts in climate and nuances in climate change between regions.
When contemplating fall colors, it's easy to default to an anthropocentric point of view, focusing on their stunning beauty. But there's so much more to a falling leaf than meets the eye. Autumn senescence is the result of complex interactions between organisms and their environment on a vast, evolutionary timescale, interactions which are becoming precariously unbalanced in the anthropocene epoch.