We Minnesotans know that as colder weather approaches, the leaves of deciduous trees change color and eventually fall to the ground. But how and why do these changes occur?
As days grow shorter and nights cooler, trees produce less chlorophyll (the green pigment responsible for photosynthesis). Other pigments — such as carotenoids (think carrots), anthocyanins and tannins — are present all summer but are masked by chlorophyll. As the latter diminishes, the former become visible, producing the oranges, reds, yellows and browns of autumn.
In general, fall leaf color will be most vibrant when days are sunny and mild and nights are cool. The Minnesota DNR has additional fall color change information on their website, including an up-to-date fall color map and a page on why leaves change color.
Shortening days and reduced temperatures trigger color changes in leaves.
Eventually hormones in the tree trigger abscission (think scissors), the three-step process that causes leaves to drop. The first step is the resorption of nutrients from the leaves into other parts of the tree, where they are stored for next year’s growth. Steps two and three may occur in either order depending on the tree species. These steps involve the formation of a protective layer where the leaf stem attaches to the tree and the breaking down or bursting of cells at the abscission zone that causes the leaf to drop.
The Arboretum’s trees are displaying beautiful fall colors, but will soon be dropping their leaves.
When I was in fifth grade I wrote a report titled “Seeds and How They Travel.” I’m reminded of that report after taking a small detour into the woods to avoid a muddy spot on Green Heron Trail. I emerge with small seeds stuck all over my clothing — primarily beggartick and stickseed.
Many plants produce seeds that catch on the fur or clothing of passersby (a process called epizoochory), ensuring that the seeds will be carried away from the original plant to grow anew elsewhere. Other seed-dispersal techniques include wind (think dandelions and milkweed) and fruit-eating birds (the flesh of the fruit nourishes the birds and the seeds are excreted in their droppings).
Many seeds hitch rides on fur or clothing, a process called epizoochory.
White-throated sparrows are singing “Oh-sweet-Canada-Canada”. A familiar bird in northern Minnesota during the summer, in the Metro we primarily see them only during migration. Song sparrows are also calling and singing. Found throughout the state during the breeding season, most are now heading south for the winter.
Song sparrows and white-throated sparrows are abundant right now at the Arboretum but will soon head south.
A flock of wild turkeys makes its way through the woods, then breaks into smaller groups to stroll down a path and across a bridge. Minnesota’s largest game bird, turkeys have excellent eyesight and hearing and can fly up to 55 mph and run up to 25 mph. They roost in trees at night. The turkey nearly became our national bird, losing out to the bald eagle in 1782 by just one vote.
Wild turkeys are Minnesota’s largest game bird and nearly became our national bird.
Many changes will take place in the coming weeks — days will grow shorter and cooler, leaves will continue to change color and drop, seeds will disperse and many birds will leave the state. All in preparation for winter and, ultimately, spring, when life will once again carry itself forward.
Holly Einess is a Minnesota Master Naturalist Volunteer. More information about the program is available on the Minnesota Master Naturalist website.