The mystery of fall colors

We all enjoy the vibrant colors of fall. But why do some trees develop such brilliant colors in the fall?

Pigments are not just pretty; they often perform many important functions. In flowers and fruits bright anthocyanins and other flavonoid pigments attract the attention of pollinators or animals that will eat the fruit and disperse their seeds. Chlorophyll, probably the most abundant pigment on earth, captures the sun’s light to be used in photosynthesis. Carotenoids give us the oranges and yellows that characterize the fall colors of hickories, grapes and sassafras. Carotenoids are found in leaves throughout the year, but their colors only show through as the chlorophyll breaks down and is reabsorbed by the tree (trees are great recyclers). Xanthophylls are specialized carotenoids that absorbing excess radiation that can damage the chloroplasts.

Outside of their production in fruit and flowers, anthocyanins are only produced as the chloroplasts begin to break down in the fall, and that is a mystery… why would a tree begin to manufacture a relatively expensive compound in foliage that is about to fall from the tree? Anthocyanins show up in the foliage of a minority of tree species, but they are responsible for the bright reds and purples in tupelo, dogwood, maples, sourwood, sweetgum, sumacs and oaks. Several hypotheses have been put forward to explain the red colors.

Red poison ivy. Poison ivy can also be yellow in the fall.

Red maples and sourwoods showing their reds above the yellow foliage of witch hazel.

The distinctive oranges of sassafras.

 

Red leaves warn insect herbivores: The most popular right now is that the bright red or purple warns insect pests that the foliage is not edible. On face value this is a particularly weak hypothesis. The costs of herbivores feeding on autumn leaves would seem to be very low, after all this is tissue that the tree is about to be jettisoned to the forest floor.

Proponents of this hypothesis, expounded by the great D.W. Hamilton just prior to his death, note that anthocyanins are costly; indeed, anthocyanins are hydrocarbons, the energy currency of life on earth. But how do the trees benefit?

Proponents of Hamilton’s theory argue that aphids exert a substantial negative effect on tree fitness, transmitting debilitating viruses and removing vital nutrients. Aphid biologists know that aphids are attracted to green and yellow, but not to red. To “test” their claims, proponents of the “warning” hypothesis compared aphid production on red foliaged apple varieties to non-reddening varieties. Aphids fared better on green and yellow apple foliage than red. So we know that aphids don’t do as well on red foliage and we know that aphids are not attracted to red foliage. But is it worth the investment in red warning pigments to ward off populations of aphids?

What proponents of the “warning” hypothesis have not demonstrated is the benefit of the warning foliage to the trees producing them. They take it for granted that aphids are a strong selective force for trees.

There is no doubt that aphids removing sugars and nutrients from the phloem is a net negative. However, trees are much more resilient than we often give them credit for. Most trees can support enormous populations of aphids without a detectable decline in health.

We can’t completely discount aphids as a selective force for trees. The hemlock wooly adelgid and the grape phylloxera, insects that are related to aphids and feed in an almost identical manner, have had enormous impacts on hemlock and grape populations, respectively.

The most damaging aspect of aphids is their ability to transmit viruses. Although annual vegetables are often devastated by virus infections, little is known about viral diseases affecting trees. Professionals that work in tree health are familiar with fungi, bacteria and insects that attack trees, but viruses affecting tree health are not discussed much. Perhaps we don’t know enough about viruses affecting forest trees. But the argument that trees with red foliage are avoiding viruses transmitted by aphids is pretty weak. 

Many aphids have a special association with trees in the fall. After feeding on herbaceous plants all summer some aphids seek out trees in the fall, not to feed but to lay their eggs near the buds of these trees. When the young tender foliage bursts forth in the spring, the aphid eggs will hatch and begin to multiply. There are several reasons aphids arriving in fall would be very inefficient at transmitting viruses. These aphids are not likely to feed and if they did, the leaves they are infecting will very likely fall to the forest floor before a virus could be moved into the tree..

However, aphid populations feeding on young foliage may be particularly costly for the tree. The tree is still relying on energy reserves in the early spring and anything sucking out large quantities of nutrients can weaken the tree. By dissuading aphids from laying eggs in the fall, trees with bright colored leaves may avoid the stress of supporting aphid populations in spring.

The sunscreen hypothesis: Anthocyanins can function as a sunscreen, protecting tissues from radiation. Anthocyanins absorb blue-green and ultraviolet wavelengths, protecting cells from high-light stress. In fact, it is noted that brilliant fall colors are associated with brighter, sunnier days and cooler temperatures. But why protect cells in leaves that are about to fall to the forest floor?

The competition hypothesis: Studies of maple foliage with high anthocyanin content have revealed that these leaves can inhibit germination of seeds of other plant species. This is an attractive hypothesis. The trees are investing in a selective herbicide to keep competitors at bay. However, this effect is weak in maples and has not been reported for other species with anthocyanin-rich fall foliage.

None of these hypotheses have very persuasive evidence to support them and so we will have to wait for a satisfactory explanation. Until then, we can enjoy the fall colors!