The following article was written by Rich Norby and originally published in the Volume 19, Number 3 issue of UT Arboretum Society Journal, The Leaflet.

In articles about fall color, the flashy trees always get top billing—maples with brilliant reds and oranges, sumacs that turn purple, birches that are bright yellow. These are the trees you see in every Vermont tourist brochure. While there is no denying the splendor of these trees, the image of fall that is most evocative for me is that of an oak forest. The colors are usually muted—often just brown—but somehow they seem more substantial, the color appearing as if painted on. The leaves stay on oak trees longer than on their neighboring flash-in-the-pan maple or (dare I mention?) Bradford pear. Oaks often form the canvas on which other trees make their display. When I conjure up an image of walking a trail in late October, it’s oak leaves that I imagine rustling in the breeze or being kicked along by my feet.

Oaks are not without their own display of color, some with different shades of brown, others more golden, and a few with splotches of red, orange, or purple. The standout among the oaks for fall color must be scarlet oak, the featured tree for this issue of the leaflet. The name of this tree is well chosen for its leaves start out bright red in the spring, and after a summer of green, turn into brilliant shades of dark red. Scarlet oak makes a good choice for a shade tree or in an ornamental planting in large part because of its attractive color display, but also because of its open crown and rapid growth rate. It can become a large tree—typically 80 feet tall and it is sensitive to soil compaction, so it might not be the best choice for a small yard.

Scarlet oak (Quercus coccinea) is a close relative of the northern red oak and other trees in the red oak group (including the featured tree of the previous Leaflet, the black oak). It is found from southwestern Maine to central Alabama and Georgia. Scarlet oak has a scattered presence on the dry upper slopes and ridges in the Appalachians where it can maintain its dominance because of its rapid growth and drought tolerance. It is very intolerant of shade, however, and is out competed by other species on lower slopes. It occurs up to 5,000 feet in the southern Appalachian Mountains but is most common below 3,000 feet. In the Smoky Mountains, scarlet oak is most frequent in middle and lower slope positions centered around 2,500 feet in elevation. Growing a variety of soil types, scarlet oak is found more often in dry, sandy or gravelly soils. Its wood is not especially valuable and is usually mixed in with other red oaks; its acorns, can be valuable to wildlife.

Returning to the subject of fall color, why do some trees dazzle us so much more than others? It’s all a matter of chemistry, and the chemistry reflects a range of strategies of how trees interact with their environment. The red color of scarlet oak leaves comes from compounds called anthocyanins. Anthocyanins form in leaf cells by reactions between sugars and other ring-like compounds. They are soluble in the cell sap, and their color depends on the acidity of the sap—they are red in acid solution and become blue as the acidity decreases (the same story as with hydrangeas). The color from anthocyanins often mix with that from other compounds such as carotenoids and xanthophylls that are responsible for the bright yellow color of some species. The color of many oaks is dominated by the brown of tannin. Although new anthocyanins may be produced in the fall if there are enough sugars in the leaf, many of these compounds are present all summer long. They can play an important role in protecting the leaf from the damaging effects of too much light or ultraviolet radiation (natural sunscreen). Their colors, however, are masked in the summer by the strong green color of chlorophyll. In the fall, chlorophyll breaks down and no more is synthesized as the need for photosynthesis declines. Various proteins associated with chlorophyll and the process of photosynthesis also are broken down, enabling the valuable nitrogen-containing compounds to be reabsorbed into perennial tissue. (The leaves of alder and black locust trees, you might notice, stay green right up until frost. These species are nitrogen-fixers and do not need to conserve nitrogen as much as other trees.) So the characteristic fall color of different species results from various combinations of the formation of red pigments (anthocyanin), the unmasking of yellow pigments (xanthophyll and carotenoids), and the disintegration of the green chlorophyll. Scarlet oak leaves, as well as red and sugar maple, sumac, sweetgum, sassafras, and a few others, form large amounts of anthocyanin, and thereby make the top ten list for fall color.

Every year the local weather man, and occasionally a tree physiologist, is asked whether it will be a good year for fall color. Any environmental factor that favors the production of sugars or the conversion of insoluble carbohydrates to soluble one will favor the production of anthocyanin and bright autumn colors. Low temperatures (but above freezing) stimulate anthocyanin, but severe early frosts will retard it. Bright sunny days will also favor bright colors, and leaves that are hidden from the sun will not develop bright colors (try this as an experiment!). People often think that lots of rainfall in the autumn should be good for color, but this is not so. Drought actually stimulates anthocyanin production, and lots of rain means not very much sunshine. When leaves are damaged, by an insect for example, the process of resorption [movement] of nitrogen compounds out of the leaf is disrupted, and a portion of the leaf may stay green, resulting in a splotchy appearance (another experiment you could try!).

Scarlet oak provides a special bonus for our autumn enjoyment. Its leaves stay attached to the tree into the winter. Normally, leaves of deciduous trees are shed in the fall after a special layer of cells forms at the base of the petiole (the abscission zone). With a little wind, these thin cells break and the leaf falls. In so-called marcescent (leaves withering but not falling off) species, including many oaks and beech, the abscission zone doesn’t begin to form until winter, and the leaves are not shed until winter or spring. One writer speculates that the pilgrims landing at Plymouth Rock must have appreciated scarlet oak as the only tree in the forest that still had leaves. While the veracity of this story seems tenuous, the idea that scarlet oak is a species to enjoy is solid as an oak.

Scarlet oak is found growing throughout the wooded areas and along all the nature trails in the Arboretum. About 200 feet south of the intersection of Arboretum Drive and the Tulip Poplar Trail, an outstanding specimen can be found. This tree has a diameter of 33 inches (9-feet in circumference) and is 120 feet tall. It is marked with a blue ribbon and a sign identifying it as the “Featured Tree”. Ask at the office or take a look at its position on the trail map if you need further directions in finding this specimen.

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