As autumn settles in, change is quite literally in the air. The chill on the breeze and a little less sunlight each day signals plants and animals to make preparations for winter. Near the Mississippi River numerous bird species can be seen migrating along the river corridor as they use it to guide their journey south. Dropping temperatures cause insects to slow down and finish their work for the season. However, the most pronounced feature about the autumn season is the display of beautiful colors. Outside, the world takes on a different tone as the vibrant greens of summer give way to the warm yellows, oranges, reds and browns of fall.
How and why does this impressive color change happen? Since spring when they leafed out, plants have been very busy using sunlight to convert water and carbon dioxide into sugar and oxygen in a process called photosynthesis. This process wouldn’t be possible without chlorophyll, which is a green pigment produced by leaf cells that allows them to absorb energy from light. During the long, bright sunny days of summer chlorophyll is produced in abundance and so the leaves are green. However, chlorophyll is not the only pigment present in leaves; it’s simply the most dominant until this time of year when the other underlying pigments get their chance to show through. These pigments are: xanthophylls for yellows, carotenoids for oranges and tannins for browns.
As the amount of sunlight decreases each day through late summer and into autumn, trees and shrubs begin preparations for winter. Each leaf is connected to the branch through an opening in the bark in order for resources to be transported between the leaves and the rest of the tree. If leaves were just pulled off, the vein-like connective structures would be open wounds, leaving the tree vulnerable to infections. Instead, the tree needs to slowly create a protective barrier, like a scab, where leaves have been attached all growing season before releasing them for the year. As the connection between the leaves and tree is blocked off the production of chlorophyll slows until it eventually stops completely. Without chlorophyll masking the other pigments they are revealed until they too break down and only the brown tannins remain.
Typically red and purple pigments are not present in leaves during the growing season. These colors come from anthocyanins, which are manufactured from sugars produced in the leaf during sunny fall days, but then trapped in the leaf by the drop in temperature at night. The brilliance of red fall colors varies year to year depending on weather conditions such as temperature, precipitation and cloud cover. The yellow and orange colors tend to be fairly constant, since those pigments are always present in leaves and the amount is not impacted by weather.
After their spectacular fall display, leaves make themselves useful by dropping to the base of the tree, which helps to create better growing conditions for next year. These fallen leaves act as a sponge to soak up rain water and snow melt that would otherwise build up around the tree and instead release it slowly. Additionally, as they break down, leaves add nutrients and minerals to the surrounding soil.
Hike with us to find more signs of the season from 1:30-2:30 p.m. on Saturday, October 21. The Kroening Interpretive Center at North Mississippi Regional Park is open all year and has lots of fun, free activities for all ages. Join us for a public program on the weekend, check out a free nature activity backpack about birds, or borrow a pair of binoculars and see what you encounter in nature.
Explore the outdoors with your little one by registering for our new play group series – Nature Nuts – for children under age five and an accompanying adult. Each series includes four sessions on Fridays from 11:30 a.m.-12:30 p.m.
Need a fun way for kids ages 6-12 to spend MEA break? Then, sign them up for Gross Nature to investigate the slimy, creepy and grotesque parts of nature. This program runs Wednesday through Friday October 18-20 from 8 a.m.-4 p.m.
October Free Public Programs (all ages): Oct. 7 – Early Birding: Search for birds while on a naturalist-guided hike. 9-10:30 a.m.; Oct. 14th – Mississippi River Flyway: Learn about bird migration and look for birds along the river. 1:30-3 p.m.; Oct. 21 – Signs of the Season: Hike to find the ever-changing elements of autumn. 1:30-2:30 p.m.; and Oct. 29 – Family Funday Autumn Arts &Animals: Meet MN animals and let nature inspire creation of autumn themed artwork. 1-3 p.m.
Find registration for these programs and more at minneapolisparks.org or call 612-370-4844 for more details. Like us on Facebook to stay in the loop about what is happening at your park!
This article was written by Kelley Ekstrom, Minneapolis Park & Recreation Board Naturalist at North Mississippi Regional Park