On The Trails: Fall foliage and more questions
- Mary F. Willson
- Sep 28
- 3 min read
By Mary F. Willson
Here in the coniferous forest, we don’t get the spectacular display of fall foliage colors that occurs in the eastern deciduous forest. But our cottonwoods and willows do produce bright yellows and golds. We begin to see those yellow tones when the green pigments (chlorophylls) of summer leaves start to break down, exposing yellow pigments in the leaves. The yellow pigments help with photosynthesis by absorbing some wavelengths that the green pigments don’t handle.
Some local plants (fireweed, highbush cranberry) commonly produce foliage of reddish hues and blueberries sometimes do too. Those reddish pigments are formed using sugars produced by the plants’ photosynthesis. During the summer, sugars produced in the leaves move readily from the leaf to the tree. But in fall, as the green pigments essential to photosynthesis begin to fail, an abscission layer develops at the base of the leaf, preparing the leaf for dropping. This layer reduces the transport of the remaining sugars out of the leaf and some remain there.
In most plants that make reddish autumn leaves, those colors are due to anthocyanins made with those excess sugars, especially in good sunlight. In addition, respiration by the still-living leaf produces carbon dioxide, which may accumulate in the leaves, in some cases increasing the acidity of the leaf and enhancing the red hues.
What might be the function of red pigments? They can serve as a protective screen from UV radiation. And they have antioxidant properties that can protect a leaf from reactive forms of oxygen. But why do that in fall, when the leaves will soon drop from the tree?
In September, I noticed a few willow trees that bore a branch or two with red leaves, most unusual for willows. These trees stood out from the surrounding yellow-leaved willows and the red-leafed branches stood out from the rest of the tree, which had the usual yellow leaves. Clearly, willows CAN make red leaves, but only do so in certain circumstances. That observation raises the question, of course, of what accounts for the rare occurrence of red leaves, just on some branches of some individuals?
Sometime in mid-September, I walked past a rusty Menziesia shrub with a few conspicuous black spots on some of its leaves. A bit farther on, the leaves of one such shrub bore numerous black spots. Of course, I wanted to know a little about that. Upon inquiry, I learned that the black spots are a fungus in the genus Rhytisma. This genus has many species, most of which have quite specific host requirements, using a limited array of host species. For instance, there’s one that specializes on maples; the local one may specialize on rusty Menziesia. Members of this genus also infect the leaves of blueberries and huckleberries. The common name for this fungus is "tar spot," although some other fungi share that name.
Rhytisma disperses spores in spring, from fallen leaves that were infected. The spores are released in wet or foggy conditions. Then wind and splashing rain may disperse the spores for relatively short distances, opportunistically landing on new, moist leaves. The hyphae spread into the leaf, initially causing yellow spots that later develop thick black areas, beginning the cycle again.
This leaf fungus must interfere with the reception of light and photosynthesis and it presumably uses some of the plant’s resources. Nevertheless, it reportedly has little effect on the host plant. Perhaps only when many leaves are infected???
Deer, moose, and elk eat Menziesia leaves, although they are apparently not very palatable and are not a favored food. I wonder the fungus would affect them if it were ingested or if they avoid the fungus-infected leaves. And do generalist herbivores such as porcupines eat the leaves? As usual, lots of questions!
Thanks to Jenifer Shapland for the fungus ID.
• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent.
