SIOUX CITY, Iowa (KCAU) —Every year as fall arrives, the days get shorter, the nights get longer, and temperatures slowly, but surely get cooler. As those changes take place, another event gets underway.

The fresh, vibrant green hues seen in the leaves and nature during the warm, sunny days of summer are replaced with various warm shades of reds, oranges, yellows, and golds, just before those very leaves are shed and plants die off or go dormant in preparation for the long, cold winter months.

But why, exactly do the leaves change colors in the fall?

Three main factors play a role in this: leaf pigments, weather, and night length.

Of these, day and night length play arguably the biggest role, as do the various pigments within leaves and plants.

There are three basic types of pigment within plants and leaves: carotenoids, anthocyanin, and chlorophyll. Carotenoids are responsible for yellow, orange, and brown coloring, like in corn or bananas. Anthocyanin are a water soluble pigment, found within the watery liquids of plant cells, which gives the familiar coloring of things like cherries, cranberries, plums, and blueberries, so your shades of reds and purples/blues.

Finally, chlorophyll, which many are more than likely most familiar with. Chlorophyll is the distinctive green pigment, seen in plants and leaves, especially during the spring and summer months. Chlorophyll is what allows plants to absorb light (mainly sunlight) and convert it to energy, also known as photosynthesis.

During peak growing season, the spring and summer months, with correspond with longer days and shorter nights, plants continuously produce chlorophyll. Since there is ample sunlight and far more intense sunlight during the spring and especially summer months, that chlorophyll is continuously being broken down as well as continuously being produced. As a result, leaves and plants appear green continuously.

However, this is where day/night length comes into play in a big way. As we go into the fall months, the days get shorter and nights get longer, meaning plants and leaves are gradually receiving less and less sunlight, as well as less intense sunlight as the sun angle changes also.

As plants and leaves receive less sunlight and night length continues to increase, overall sunlight exposure is decreased so chlorophyll production slows and then stops altogether. The plants and leaves process the remaining chlorophyll left, and this is what we see as leaves gradually shifting from green to other shades.

Like chlorophyll, carotenoids are present within plants during the growing season. And as chlorophyll production slows and eventually stops, those carotenoid pigments are gradually revealed as shades of yellow, orange, brown, and gold and/or combinations of those colors.

Some plants and leaves also produce anthocyanin in the fall in response to certain factors and conditions within plants in autumn. This production of pigments and reduction of chlorophyll and the classic spring/summer green pigments, results in shades of reds and purples/plum colors.

So, in short, as chlorophyll production is slowed/stopped due to less sunlight, the other pigments within leaves and plants, whether present but concealed by chlorophyll’s green hue or produced within the fall months (but similarly concealed by chlorophyll) are revealed as the classic shades of golds, reds, oranges, and browns.

In trees, certain species can produce different shades and hues. For example, oak trees typically produce shades of red and/or brown while dogwood trees’ fall foliage present in a purplish-red hue.

Even different types of the same species of tree can have varying shades and hues. Red maples tend to have brilliant scarlet leaves at their peak in the fall, while black maples show off glowing yellow foliage.

Meanwhile, some like elm just shrivel and fall while oaks hold their green hues much longer than other tree species, changing their color well after other trees change color and shed their leaves.

Other factors like late summer/early fall weather conditions also impact colors and how vibrant they are, with temperature and moisture the primary factors.

For ideal color and the most vibrant shades and hues, a stretch of warm, sunny days and cool and crisp (but not freezing) nights tend to result in the best, most optimal fall colors. This is because the warmth and sunshine result in leaves and plants producing lots of sugars and then at night, the cool temperatures and veins of the leaves and plants already in the slow process of closing results in the sugars getting stuck/clogged and in turn aids in the production of anthocyanin. That anthocyanin production and the decreased chlorophyll production eventually reveals hues and shades of red and blues/purples.

In addition, fall color can be offset by several weeks with the presence of a late spring or with a severe summer drought. A stretch of warm weather in the fall can also act as a damper for the vibrancy and brilliance of fall foliage colors.

For optimal fall colors, ideal conditions include a warm, wet spring, adequately warm and wet summer and finally warm & sunny fall days paired with cool and crisp fall nights leading up to peak fall color season.

As those in Siouxland know though, weather from year to year across the region can vary quite dramatically, so no two fall foliage displays will be the same. Here in Siouxland, peak fall colors are just a couple weeks away, expected between October 8th through October 14th. And some parts of the area are already seeing colors beginning to change as of the Iowa DNR’s September 25th update! So you’ll definitely want to keep your eye on the trees and forest canopies across Siouxland the next few weeks!

After peak fall colors occur, leaves colors fade and the leaves fall. Those leaves decompose and add valuable nutrients to the surrounding soil and also become food for organisms found within those soils and forests. Quite the circle of life within the forests and the ecosystem!