Fall Color Change in Ohio

Across much of the United States, autumn is marked by a series of chemical changes in leaves, resulting in them changing colors from green to red, orange, yellow, or gold. This change in leaf color combined with their subsequent drop is part of the tree’s ability to survive winter weather (Chaney, 1997).

Depending on the location, leaves can begin to change color as early as September and continue through December. In Ohio, fall colors typically peak between mid- to late October. This fact sheet explains the processes that cause fall leaf-color change in hardwood and softwood trees.

Leaf Pigments

Three main pigment types are found in leaves:

Chlorophyll
Chlorophyll gives tree leaves their dominant green color and is necessary for photosynthesis (the process through which trees convert sunlight, water, and carbon dioxide into sugar). Chlorophyll is present in leaves year-round.

Carotenoids
These pigments produce yellow, orange, and brown colors in leaves. As with chlorophyll, carotenoids are present in leaves year-round (Figures 1 and 2) (Kramer & Kozlowski, 1979).

Anthocyanins
These pigments produce red and purple colors in leaves. Unlike the other pigments, most anthocyanins are produced in the fall in response to warm sunny days, cool nights, and extra sugars found in the leaves (Figure 3) (USDA Forest Service, n.d.).

Factors Influencing Fall Color Change

As the amount of daylight decreases in the fall, the production of chlorophyll decreases and eventually ceases. This loss of chlorophyll unmasks the carotenoid and anthocyanin pigments, allowing them to briefly dominate before the leaf dies.

The timing of color change and colors exhibited by individual tree species is somewhat genetically controlled. Some species, such as sourwood in southern Ohio, will turn bright red late in the summer, while the leaves of many oak species won’t turn color until after many other species have already lost their leaves. Table 1 lists tree species found in Ohio and their typical fall colors (Burns & Honkala, 1990b; USDA Forest Service, n.d.).

Length of Daylight

Shorter days in late summer and early fall trigger changes in leaves, thus initiating the color change process. Veins that carry water and sugars from the leaves to the tree (and back again) become clogged, trapping some sugars in the leaves. This allows for the production of anthocyanin pigments in some species (Chaney, 1997).

Abscission Layer

Tissues connecting the leaf to the stem allow for water and sugars to pass through the leaf to the tree. When the flow of water and sugars in the fall begins to decline, the tree produces waxes to clog up these connective tissues. The point where this happens is called the abscission layer. The development of the abscission layer signals that the leaf is ready to fall off the tree (Kramer & Kozlowski, 1979).

Why Do Some Leaves Stay on the Tree All Winter?

Certain species of trees retain some of their leaves throughout the winter. Members of the beech family (beech and oak) are the most common Ohio species exhibiting this trait. Such trees are said to be marcescent. The leaves remain on the trees due to the abscission layer not completely forming between the leaf and branch it is attached to (Kramer & Kozlowski, 1979). This is commonly seen in Ohio with beech trees in the understory showing their golden-bronze-colored leaves all winter (Figure 4). New leaf buds growing in the spring force the marcescent leaves off the tree.

How Does Weather Affect Fall Colors?

Since anthocyanins are produced in the fall, weather conditions directly impact the degree of these pigments’ development. Fall weather doesn’t have much of an impact on carotenoids, which are present in the leaves all season long.

Fall weather conditions that favor more vibrant red leaf colors are warm sunny days followed by cool nights. Freezing nighttime temperatures reduce the intensity of red and purple fall colors. Rainy or cloudy days near the time of peak color will also reduce the intensity of red and purple fall colors. Favorable weather during the growing season supports leaf development and will in turn increase the production of carotenoid pigments within the leaves. Soil moisture also plays a part in fall leaf-color changes and varies greatly not only within the year but also between years as well. In general, warm spring weather with plenty of rainfall combined with warm sunny days and cool nights in the fall should provide the most vivid fall colors.

Why Do Most Conifer Needles Not Change Color Like Hardwood Leaves?

Hardwood leaves change color and eventually fall off Ohio’s trees every autumn. But why don’t evergreens, such as pine and spruce, lose their needles the same way? We can see that some needles die and fall off the trees each year as they collect under the trees and persist for fairly long periods of time. This may restrict grass growth. Yet this needle drop happens at different times of the year according to species.

Pigments in Conifer Needles

The needles of most Ohio conifers are missing two of the three main pigment types found in hardwood leaves. The needles contain chlorophyll but are missing carotenoids and anthocyanins. When the needles die, they simply turn from green to brown due to the breakdown of chlorophyll within the needles and then eventually drop to the ground under the canopy.

Needle Age and Genetic Control of Needle Drop

Conifer needles age, but unlike hardwood leaves, they do not all die and fall off every year. As the needles age, they become less efficient at producing sugars for the tree through photosynthesis. In addition, over time, older needles become shaded by new limbs and needles, further reducing their ability to photosynthesize. As these older needles die, they turn from green to brown and eventually fall off the tree. As healthy conifer trees grow, newer needles are found at the end of branches, while older needles remain on the interior of branches and the crown for a period of time according to the species (Figure 5).

The length of time needles remain alive on trees varies by species. Table 2 lists the life span of needles of coniferous evergreen tree species found in Ohio (Burns & Honkala, 1990a).

Deciduous Conifers are Exceptions

Eastern larch (Larix laricina) is a native deciduous conifer found on poorly drained sites, primarily in northeast Ohio. A member of the pine (Pinaceae) family, eastern larch needles change color from green to gold in the fall and drop every year like the leaves of deciduous hardwoods (maple, oak, etc.) This needle-drop habit of eastern larch is believed to be an adaptation to growing on poor sites and aids in water and nutrient retention in the tree. As with deciduous hardwood leaves, new needles are grown every spring.

Other deciduous conifers commonly found in Ohio include dawn redwood (Metasequoia glyptostroboides) and bald cypress (Taxodium distichum). The needles of both trees turn orange, red, or brown in the fall. As with deciduous hardwoods, new needles appear every spring on a healthy tree.

What Happens to the Leaves and Needles That Fall to the Ground?

Leaves that fall to the ground may be eaten by arthropods, such as earthworms and insects, or decomposed by fungi and bacteria in the soil. These processes add organic matter to the soil and recycle nutrients so they can be utilized by soil organisms and taken back up by the trees.

Conifer needles, unlike hardwood leaves, are resinous and have a waxy coating while they’re on the tree that helps protect them during cold and dry weather. Once they’ve fallen from the tree, these resins and waxes make the needles more resistant to decomposition. It can take many years for conifer needles to decompose. These needles can be left in place to decompose, raked up to be used for landscaping mulch, or simply be removed from the site.

A common myth is that conifer trees and the needles they drop make the soil more acidic. It is true that soils commonly found under conifer trees are acidic. But that acidity is not caused by the tree. The tree is growing there because the soils are acidic. As conifer needles on the ground decompose, the microorganisms breaking them down neutralize their natural acidity.

References

Burns, R. M., & Honkala, B. H. (1990a). Silvics of North America. Volume 1: Conifers. USDA Forest Service. Agriculture Handbook 654. Retrieved from
srs.fs.usda.gov/pubs/misc/ag_654/volume_1/vol1_table_of_contents.htm

Burns, R. M., & Honkala, B. H. (1990b). Silvics of North America. Volume 2: Conifers. USDA Forest Service. Agriculture Handbook 654. Retrieved from
srs.fs.usda.gov/pubs/misc/ag_654/volume_2/vol2_table_of_contents.htm

Chaney, W. R. (1997). Why do leaves change color - the physiological basis. Purdue University Extension publication FNR-FAQ-5. Retrieved from
extension.purdue.edu/extmedia/fnr/fnr-faq-5.pdf

Kramer, P. J., & Kozlowski, T. T. (1979). Physiology of woody plants (1st ed.). Academic Press.

United States Department of Agriculture (USDA) Forest Service. (n.d.). Science of Fall Colors. Retrieved from
fs.usda.gov/visit/fall-colors/science-of-fall-colors